Biochemical Genetics 1 Flashcards
Aminoacidopathies, urea cycle disorders, and organic acidemias
How is phenylalanine hydroxylase deficiency detected on NBS
Tandem mass spec after 24hrs of age
Hyperphenylalanemia manifests as a time-dependent increase of Phe in the blood (false -‘s)
What is seen on brain MRI in 90% of individuals with PAH deficiency
progressive white matter dz on brain MRI even w/out evidence of neurologic deterioration
How is phenylalanine hydroxylase deficiency detected on biochemical analysis
Elevated Phe levels > 120umol/L with low Tyr levels; Normal BH4/Pterin levels
Not typically useful in dx bc PAH is a liver enzyme
Those with classic PKU have levels >1200umol/L
How is phenylalanine hydroxylase deficiency detected on single gene testing
Sequence analysis of PAH FIRST followed by gene-targeted del/sup
When should diet be initiated in relation to molecular genetic analysis for patients with phenylalanemia
Low Phe diet should be initiated prior to results of pterin or molecular genetic studies
What are the clinical features associated with classic PKU if left untreated
epilepsy, ID/behavior problems, Parkinson features (in adults), musty body odor, eczema, decreased skin/hair pigmentation, variable microcephaly, osteopenia, and B12 deficiency
What causes the distinct body odor and eczema in PKU
Excess Phe
What causes the decreased skin/hair pigmentation in PKU
Low Tyr
What are pathognomonic features associated with PKU
Musty/mousy body odor, Parkinson features in adults, decreased skin and hair pigmentation
What are the clinical features associated with hyperphenylalanemia if left untreated
Those with < or equal to 600Umol/L are not at increased risk for intellectual, neurological, and neuropsychiatric impairment than those without PAH deficiency
What are the molecular features associated with the PAH gene
less severe of the 2 variants determines dz severity
majority of the PVs in PAH are missense, nonsense, frameshift, and splice variants
PVs that confer the most severe phenotypes abolish PAH activity (null variants of various types
Missense variants typically retain some residual enzyme activity
What are some DD for PAH deficiency
BH4 deficiency: accounts for 2% of elevated Phe levels in most pops; will also have abnormal pterin levels in urine/blood (NOT SEEN IN PKU). Also have recurrent hyperthermia without infections, swallowing issues, hypersalivation
What is the treatment regimen for classic PKU
AVOID ASPARTAME
measurement of blood Phe levels weekly for the first yr of life, biweekly until 13yo, monthly thereafter
monitor Phe blood levels 2-3hrs after eating
Sapropterin (Kuvan): 30% decrease in Phe plasma aa analysis; majority with mild or moderate PKU may be responsive while up to 10% with classic PKU show a response
What is the treatment regimen for non-classic hyperphenylalanemia
experts believe dietary tx is unnecessary
<600umol/L and >360umol/L tx is controversial
<360umol/L and >120umol/L no tx
How is tyrosinemia type 1 detected on NBS
presence of succinylacetone from blood spot via tandem mass spec on NBS is pathognomonic
What do increased tyr or met levels in the blood suggest
Liver disease
Infants with type 1 tyrosinemia could have slightly elevated/normal blood levels when the first NBS is collected
Increased Tyr could be transient tyrosinemia in a newborn, tyrosinemia type 2/3, or other liver disease
Increased met could be liver dysfunction, defects in met metabolism, or homocystinuria
How is tyrosinemia type 1 detected on biochemical analysis
Increased succinylacetone, increased tyr, met, and phe; increased urinary conc. of tyr metabolites and S-ALA
How is tyrosinemia type 1 detected on single gene testing
Sequence analysis of FAH first, then gene-targeted del/dup analysis
What are the clinical features associated with tyrosinemia type 1
Severe liver involvement or later in in the first yr of life with liver dysfunction, significant renal involvement, odor of “boiled cabbage/ rotten mushrooms”, growth failure, neurologic crises, hepatocellular carcinoma, and rickets
When is the typical onset of tyrosinemia type 1
presents in young infants or within the first year of life
Describe the liver involvement associated with tyrosinemia type 1
Before age 6mo typically have acute liver failure which can progress to ascites (fluid buildup in the abdomen), jaundice, and gastrointestinal bleeding
Untreated, may die from liver failure w/in wks or mos of symptoms
Describe the renal involvement associated with tyrosinemia type 1
Renal tubular involvement beginning after 6mo, includes Fanconi syndrome (affects how the kidneys reabsorb certain essential substances causing increased urination, bone pain, muscle weakness), and renal tubular acidosis
Describe the neurological crises associated with tyrosinemia type 1
changes in mental status, abdominal pain, peripheral neuropathy (weakness, numbness and pain, usually in the hands and feet), and/or respiratory failure requiring mechanical ventilation
Describe the cancer risks associated with tyrosinemia type 1
Without nitisinone tx and low tyr diet, there are significantly increased risks of developing and succumbing to hepatocellular carcinoma
What are the molecular features associated with FAH
Targeted analysis for the p. Pro261Leu variant accounts for >99% in AJs
c.1062+5G>a (IV612+5 G>A) accounts for 88% of variants in the French Canadian pop
Pseudodeficiency alleles exist- leads to decreased FAH enzyme activity but adequate FAH mRNA
PVs (missense, nonsense, and splice site variants) result in LOF of FAH activity leading to cellular damage + apoptosis
What are some DD for tyrosinemia type 1
Tyrosinemia type 2: defect in tyrosine aminotransferase (TAT) with high levels of ONLY tyr; associated with painful, nonpruritic, and hyperkeratotic plaques on the soles and palms, ophthalmologic involvement, and DD
Tyrosinemia type 3: very rare and ill defined; deficiency of P-hydroxyphenylpyruvic acid dizoygenase; ID/ataxia, NO LIVER INVOLVEMENT, skin and ocular changes
Homocystinuria
Acute intermittent porphyria
What is the treatment regimen for tyrosinemia type 1
Respiratory support, fluid management, and blood products for bleeding in liver failure
Nitisinone (Orfadin): blocks the second step of the tyrosine degradation pathway and prevents FAA accumulation that would turn into succinylacetone
Low tyr diet: nitisinone increases blood concentrations of tyr, which can lead to tyr crystals forming in the cornea
Measure carnitine levels due to skeletal muscle weakness from renal tubular Fanconi anemia
Osteoporosis and rickets tx with 25-hydroxy-vitamin D, Ca++. and phosphate
When is liver transplantation appropriate for patients with tyrosinemia type 1
Those with severe liver failure at presentation and don’t respond to nitisinone tx
Have documented evidence of malignant changes in hepatic tissue
What kind of special genetic mechanism can occur in tyrosinemia
Gene reversion can occur (explains clinical variability in families/those w same genotype) in which the spontaneous self correction of a germline PV to the normal sequence occurs in somatic cell division
The “normal” cells have a selective growth advantage bc they are no longer at risk for apoptosis from the accumulation of FAA
How is maple syrup urine disease detected on NBS
Based on quantification of the ratios of (leu +isoleu) to ala and phe on dry blood spots
positive screen requires follow-up w quantitative plasma aa and alloisoleucine analyses
NEONATES WITH ISOLATED HYDROXYPROLINEMIA WILL SCREEN + BUT CONFIRMATORY AA ANALYSIS WILL ONLY SHOW INCREASED HYDROXYPROLINE
When should testing for MSUD begin
Immediately after positive NBS either by quantification of aa ratios or plasma AA/ alloisoleucine levels
How is MSUD detected on genetic testing
Identification of biallelic PVs in BCKDHA, BCKDHB, or DBT
Use of a multigene panel w/ del/dup is recommended
What are the clinical features associated with classic MSUD
Maple syrup odor in cerumen (earwax) after birth (~12hrs) and in urine @5-7do; acute metabolic intoxication (leucinosis) with risk for cerebral edema and death, and neurologic deterioration from net protein degradation by infection, sx, injury, psychological stress, iatrogenic essential aa deficiency, recurrent thrush infections
83-100% chance of anxiety, depression, or panic disorder by 35yo; 50% chance for ADHD
How is MSUD detected on biochemical testing
increased levels of BCAAs and alloisoleucine; urinary excretion of BCKDs and BCKAs in infants >48-72hrs old; ketonuria, absence of hypoglycemia/hyperammonemia
What does leucinosis cause in patients with MSUD
nausea, anorexia, altered consciousness, acute dystonia, ataxia
What does neurologic issues cause in patients with MSUD
cognitive impairment, hyperactivity, sleep disturbances, hallucinations, mood swings, focal dystonia, choreoathetosis, and ataxia
What is the single strongest predictor of neurocognitive disability in patients with MSUD
Prolonged neonatal encephalopathy (lethargy, intermittent apnea, opisthotonus, “fencing and bicycling” movement) between 4/5do
What features are associated with iatrogenic essential aa deficiency in patients with MSUD
Anemia, hair loss, growth failure, due to chronic deficiency of leucine, isoleucine, and valine
What clinical features are associated with intermediate MSUD
may have maple syrup smell in neonatal period and abnormal labs
present with feeding issues, poor growth, DD in infancy or later with apparent nonsyndromic ID (majority are caught on NBS)
tolerate more leucine and require less nutritional support
What is the residual enzyme activity associated with MSUD
3-30%
What clinical features are associated with intermittent MSUD
tolerate normal leucine intake, typically normal or mildly elevated BCAAs
MAY ESCAPE DETECTION ON NBS
During infections/physiological stress, can develop all features of classic MSUD
What is the mechanism of disease in patients with MSUD
caused by decreased activity of BCKD, a multi-enzyme complex in the mitochondria
skeletal muscle is the major site of transamination and oxidation of BCAAs
BCKD is expressed in the brain
mechanism of dz is decreased function or LOF
What are the molecular features of MSUD
BCKDHA has PV missense in 3’UTR; DBT has PV missense in 3’UTR which are higher than expected
Notable PVs include c.548G>C in BCKDHB a founder variant in the AJ pop; c.1196C>G in DBT a founder variant in the Malay pop
BCKDHA accounts for 45% of PVs, BCKDHB accounts for 35% of PVs; DBT accounts for 20% of PVs
What are some DD for MSUD
Need to exclude: birth asphyxia, hypoglycemia, status epilepticus (seizures >5min), meningitis, and encephalitis
Urea cycle defects
Hyperketosis syndromes
Glycine encephalopathy
Propionic acidemia/isolated methylmalonic acidemia
What are pathognomonic findings of MSUD that sets it apart from other similar conditions
Unique for odor and + urine dinitrophenylhydrazine test
What is the treatment regimen for MSUD
requires supervision of metabolic dietitian (age appropriate tolerance of leu, iso, val, AND avoiding deficiencies of essential aas, fatty acids, and micronutrients)
Dietary indiscretion causes elevated BCAAs but RARELY results in encephalopathy and acute decompensation (lethargy, encephalopathy, seizures, or progressive coma should be managed with generous caloric support in a hospital)
Orthotopic liver transplants
Plasma levels should be:
Leu 150-300umol/L
Iso ~= to Leu
Val at LEAST 2X >Leu
When should MSUD tx begin if the syndrome is suspected
IMMEDIATELY
What does an orthotopic liver do for a patient with MSUD
metabolic “cure” for classic MSUD
does not reverse cognitive disability but may arrest progression of neurocognitive impairment
What considerations are needed for individuals who are pregnant and have MSUD
Pregnancy possible for pts with classic MSUD
Increased Leu in pregnant person likely teratogenic
Keep mat levels of BCAAs b/wn 150-300umol/L for safe delivery
Postpartum is dangerous for mom – refer to metabolic center
How is the dx of nonketotic hyperglycemia established using laboratory findings
Isolated elevation of levels of glycine in plasma and CSF (obtained simultaneously) by quantitative aa analysis
abnormal CSF to glycine ratio (elevation of CSF glycine is more important than the ratio)
Urine organic acid profile is expected to be normal
What findings on brain MRI would be suspicious for nonketotic hyperglycemia
most consistent abnormalities noted on diffusion weighted imaging in the first 3mo of life
ALL INFANTS with diffuse restriction of white matter that can recede after 3mo but can be recognized elsewhere between 3-14mo
The corpus callosum can be thin and shortened but is NOT absent
Small group of infants develop hydrocephalus
Atrophy present in older individuals with severe NKH
How is the dx of nonketotic hyperglycemia established using genetic testing
Biallelic PVs in GLDC, AMT, and (proposed to be involved) GCSH
A multigene panel including the above that also includes del/dup analysis is recommended
What does the analysis of the glycine cleavage enzyme system (GCS) entail for the dx of NKH? How much enzyme activity do these individuals have? Those with variants in the AMT gene?
requires analysis of a liver bx, usually obtained by sx endoscopy as a wedge bx or at autopsy
Vast majority have no detectable GCS activity
Those w defects in AMT tend to have GCS activity up to 25% of normal values
What are the clinical features associated with severe NKH
No developmental progress and intractable epilepsy
Never make developmental gains, some will regain few skills
Spontaneous bottle feeding, looking, and smiling when tx with benzoate (around 3-4mo they can lose these skills)
Before 6mo, develop progressive spasticity and cortical blindness
increasingly difficult to tx seizures in the 1st yr; EEG with multifocal spikes
scoliosis/hip dislocation; occasionally can have cleft palate, clubfeet, secondary microcephaly
Recurrent and long episodes of unexplained severe crying
What type of condition is NKH? What is the life expectancy?
Inborn error of metabolism resulting in accumulation of large quantities of glycine in all body tissues including the brain
Typically will pass away within the first few weeks of life, but can live up to 3-5yo
What are the clinical features associated with attenuated NKH
variable developmental progress with treatable or no epilepsy (relatively easy to tx with benzoate or dextromethorphan alone)
Hyperactivity common, can be severe
Many have choreic movements
Intermittent episodes of lethargy triggered by fever and infection
Can have poor, intermediate, or good outcomes
What are the clinical features associated with poor attenuated NKH
developmental quotient <20 with epilepsy
Manifestations between attenuated and severe forms
Learn how to grasp objects, usually able to sit, have limited interaction with some signs
What are the clinical features associated with intermediate attenuated NKH
developmental quotient 20-50 with easily treatable or no epilepsy
walk and communicate with some speech and sign language
grasp items and eat independently
choreatic movements and pronounced ADHD
What are the clinical features associated with good attenuated NKH
Developmental Quotient >50 with no epilepsy
make substantial developmental progress
half do not present with symptoms until after 3mo
have ADHD; episodes of severe lethargy with infections
What percent of infants with neonatal onset NKH have the severe form? The attenuated form?
85%; 15%
What percent of infants with infantile onset NKH have the severe form? The attenuated form?
defined as greater than 2 weeks
50%; 50%
What percent of infants with late onset NKH have the severe form? The attenuated form?
defined as onset >3mo
all have attenuated NKH
What is the neonatal presentation of NKH
progressive lethargy into profound coma and marked hypotonia in the first hours to days of life
80% of infants ventilatory drive slows, leading to prolonged apnea and often death (20% maintain spontaneous ventilation)
Majority of infants regain spontaneous respiration within the first three wks of life, some show spontaneous improvement in alertness in the first mo of life
Myoclonic jerks and hiccups are often a sign of epilepsy
initial EEG often shows a burst suppression pattern
What is the infantile presentation of NKH
hx of hypotonia early one, present with DD and infantile-onset seizures that can be mild or increasingly difficult to tx
What is the late presentation of NKH
rare, associated with the attenuated form, involves DD and possible mild seizures
Is there intrafamilial variability for nonketotic hyperglycemia
No, the phenotype of severe vs attenuated NKH is consistent within families
What are the prognostic predictors for dx of severe NKH
presence of a very thin and shortened corpus callosum
Presence of hydrocephalus
development of clear pyramidal tract signs (hyperreflexia, weakness, spasticity, and a Babinski sign) before 6mo
cleft palate and clubfeet when present
persistent burst suppression pattern
What are the prognostic predictors for dx of attenuated NKH
glycine/creatine ratio is higher in severe than in attenuated NKH
presence of choreatic movements
What are some of the differential diagnoses for nonketotic hyperglycemia
GCS cofactor deficiencies (pyridoxine dependent epilepsy)
Abnormal regulation of GCS (intracellular cobalamin metabolism)
Glycine transport defects (GLYT1 encephalopathy)
Inhibition of GCS activity (organic acidemias- will have abnormal urine organic acids)
What is the management for patients with nonketotic hyperglycemia
MRI of the brain in neonates (brain atrophy in severe form)
EEG
Developmental assessment
Neurologic assessment
Orthopedic eval
Pulmonary eval
Ophthalmologic assessment
GI eval
What are the current treatment methods for severe NKH
Glycine-lowering therapy is not effective in improving the affected individuals development, even when initiated at birth (does however decrease frequency and severity of seizures)
Improves attentiveness and resolves neonatal apnea
What are the current treatment methods for attenuated NKH
Reduction of plasma concentration of glycine tx with sodium benzoate (improves alertness, reduces or eliminates episodic lethargy, and may also improve behavior) and blockade of NMDA receptors
Early, aggressive tx of children with PVs associated with residual GCS activity who are likely to develop attenuated NKH resulted in improved neurodevelopmental outcome and reduced propensity for epilepsy
Why is a glycine-restricted diet not necessarily indicated in the tx of patients with NKH
contribution of dietary glycine is small compared to the excess in endogenous glycine synthesis versus endogenous catabolism of glycine
been associated with protein malnutrition
What are the agents to avoid in a patient with NKH
Valproate- it raises blood and CSF glycine concentrations and may increase seizure frequency
Vigabatrin resulted in rapid loss of function of the glycine cleavage enzyme system
What is the mechanism of disease for Nonketotic hyperglycemia
Loss of function. Residual enzyme activity correlates with the outcome of the dz
What are some molecular considerations when taking into account the diagnosis of NKH
GLDC analysis is complicated by the presence of a processed full-length pseudogene with 98% homology to the true gene
20% of pathogenic GLDC variants are large deletions as a result of nonallelic homologous recombination of Alu repeats
How is the diagnosis of homocystinuria made biochemically
total homocysteine levels above 100 umol/L when accompanied by high or borderline high methionine makes the dx very likely
ENZYME ACTIVITY TESITNG IS NO LONGER AVAILABLE IN THE US
How is the dx of homocystinuria made on NBS
Detected in SOME affected individuals
screens for hyperMETHIONINEMIA using tandem mass spec
If initial result exceeds cutoff of methionine, follow up testing is required: 1. repeat dried blood specimen OR 2. quantitative plasma aa analysis and analysis of plasma total homocysteine (dependent on screening program recommendation)
Since NBS is for methionine and not homocysteine, other causes of elevated total homocysteine may not be detected bc the methionine level in these disorders is reduced or normal
What are the MAJOR clinical findings of homocystinuria
Ectopia lentis and/or severe myopia
Skeletal abnormalities (excessive height, long narrow limbs (dolichostenomelia), scoliosis, pectus excavatum)
Thromboembolism (vascular abnormalities)
DD/ID
NO JOINT HYPERMOBILITY (SPECIFIC TO MARFAN)
How is the diagnosis of homocystinuria made with molecular genetic testing
Sequence analysis of CBS FIRST then gene-targeted del/dup analysis if only one or no PVs is found
Targeted analysis is performed only in the Qatari pop in which a single PV (p.Arg336Cys, c.1006C>T) is present in 93% of the pop
What type of challenge testing is required immediately after dx of homocystinuria
Pyridoxine (B6) challenge test
Affected individual (in the neonate or beyond) is given 100mg of pyridoxine daily for two consecutive days, concentrations of plasma total homocysteine and aas are measured in 48hrs after the first dose
Reduction of 30% or more in plasma total homocysteine and/or plasma methionine concentration suggests B6 responsiveness
What are the clinical characteristics of homocystinuria
B6-responsive pts typically have milder presentations than non-responsive individuals (BUT NOT ALWAYS)
Myopia followed by ectopia lentis (after one yo but before 8yo)
Tall and slender, marfanoid habitus, prone to osteoporosis
Scoliosis, high-arched palate, arachnodactyly, pes cavus, pectus excavatum or carinatum, genu valgum (knock knees)
Thromboembolism is the major cause of morbidity and early death (problems typically appear in young adults)
DD is often the first abnormal sign in individuals with homocystinuria
Seizures in 21% of individuals
Psychiatric problems (personality disorder, anxiety, depression, OCD, psychotic episodes
Hypopigmentation of the hair and skin, malar flush, livedo reticularis (skin to appear reddish-violet and blotchy, forming a net-like pattern), and pancreatitis
What are the genotype-phenotype correlations in homocystinuria
presence of a single p.Gly307Ser allele predicts B6 non-responsiveness
presence of a single p.Ile278Thr allele usually predicts B6 responsiveness
In what populations is homocystinuria most commonly present
- Qatar
- Norway
- Germany
- Ireland
What are some of the differential diagnoses associated with homocystinuria
Marfan
Tyrosinemia type 1 or galactosemia (secondary hypermethioninemia)
What is the recommended treatment for individuals with homocystinuria
methionine restricted diet continued indefinitely. A methionine free aa formula supplying the other aas is provided
Folate and vitamin B12 optimize the conversion of homocysteine to methionine by methionine synthase, thus helping to decrease the plasma homocysteine concentration
Tx with betaine provides an alternate remethylation pathway to covert excess homocysteine and methionine may help to prevent thrombosis
What is the recommended surveillance for patients with homocystinuria
Infants should be monitored monthly for the fist six months of life and bimonthly until age 1yr, then every 3mo until age 3yrs
Semiannual monitoring through the remainder of childhood and annual monitoring in adolescence and adulthood are indicated
Regular ophthalmology assessments
DXA scans should be performed q3-5yrs to monitor for osteoporosis
What agents/circumstances should individuals with homocystinuria avoid
Oral contraceptives increase coagulability and should be avoided in females
Sx should also be avoided if possible bc increase in plasma homocysteine concentrations elevates the risk of a thromboembolic event
What is the pregnancy management for patients with homocystinuria
Prophylactic anticoagulation in the third tri and post partum recommended (low molecular weight heparin), low dose aspirin
does NOT appear to have teratogenic effects on a fetus
What is a counseling consideration that must be addressed with the parents of a patient with homocystinuria
It is possible (though unlikely) that a parent has classic homocystinuria but has remained asymptomatic (high clinical variability of the condition), it is appropriate to perform an exam and plasma homocysteine and aa analysis in both parents (affected women at increased risk for thromboembolic events during pregnancy
What are the type of PVs seen in patients with homocystinuria
Most often missense variants, some nonsense; the rest are various in/dels, and splice variants
Where are the two most common variants found in patients with homocystinuria
p.Gly307Ser and p.Ile278Thr are found in exon 8
p.Gly307Ser is the leading cause of homocystinuria in Ireland (71% of PVs)
p.Arg336Cys is present in 93% of affected individuals in the Qatari pop
How do the urea cycle disorders occur and what are they
group of rare disorder affected the urea cycle, a series of biochemical processes in which nitrogen is converted into urea and removed from the body through the urine
failure to break down nitrogen results in the abnormal accumulation of nitrogen (in the form of ammonia) in the blood
What are the signs and symptoms of severe neonatal onset NAGS (N-acetylglutamate synthetase deficiency)
refusal to eat and poor feeding habits, progressive lethargy, recurrent vomiting, diarrhea, irritability, and hepatosplenomegaly
Seizures, confusion, respiratory distress, and cerebral edema
in some cases, may progress to coma due to high levels of ammonia in the blood (resulting in neurological abnormalities including DD, learning disabilities, ID)
What are the signs and symptoms of late onset NAGS (N-acetylglutamate synthetase deficiency)
failure to thrive, poor growth, avoidance of protein from diet, ataxia, lethargy, vomiting, hypotonia
Can still experience hyperammonemic coma and life threatening complications
When do the symptoms of NAGS start in the severe neonatal form and what causes the symptoms
occur within 24-72hrs after birth
caused by accumulation of ammonia in the blood (nitrogen is a waste product of protein metabolism)
When do the symptoms of NAGS start in the late onset form
Occurs later in infancy or childhood or even adulthood in some cases
What causes NAGS and what is the function of the defective enzyme
biallelic mutations in NAGS
NAGS is an activator of another enzyme of the urea cycle (carbamyl phosphate synthetase (CPS))
What are some differential diagnoses for NAGS
Other urea cycle disorders
Reye syndrome: liver failure, encephalopathy, hypoglycemia, hyperammonia, occurs following a viral infection
Organic acidemias (affect the urea cycle as a secondary phenomenon)
What are some common clinical features among all urea cycle disorders
lack of appetite, vomiting, drowsiness, seizures, and/or coma
hepatosplenomegaly in some cases
in severe cases, life threatening complications may result
What are the common clinical features among all organic acidemias
hypotonia, poor feeding, lethargy, vomiting, and seizures
can progress to coma and life-threatening complications
What characteristic biochemical/laboratory findings should be indicative of a NAGS diagnosis
excessive amounts of ammonia in the blood (can also indicate other disorders like organic acidemias, congenital lactic acidosis, and fatty acid oxidation disorders)
URINARY ORGANIC ACIDS SHOULD BE NORMAL
What is the recommended treatment for a patient with NAGS
prevent excessive ammonia during a hyperammonemic episode
Carbaglu (carbamylglutamate) tablets to reduce blood ammonia levels
Dietary restrictions (limit the amount of protein intake) and receive supplemental arginine
Sodium phenylacetate and sodium benzoate (nitrogen scavengers, allow an alternate route for the metabolism of nitrogen)
Hemodialysis may be nexessary if there is no improvement/hyperammonemic coma develops
PROMPT TX IF A PT HAS EXTREMELY HIGH AMMONIA LEVELS TO AVOID HYPERAMMONEMIC COMA
What is the surveillance required for pts with NAGS
administer carbaglu
periodic blood tests to determine the levels of ammonia in the blood
receive periodic tests to measure the amount of aas such as glutamine in the blood (elevated levels of glutamine often precede the development of hyperammonemia by weeks-days)
What is the clinical course for a patient with neonatal onset carbamoyl phosphate synthetase 1 deficiency (CPS1)
healthy at birth but after a few days begin to manifest with lethargy and unwillingness to feed
Severe hyperammonemia with vomiting, hypothermia, hypotonia, seizures, coma, which can lead to death
What is the gene that causes carbamoyl phosphate synthetase 1 deficiency, where is it primarily located, and what is its’ function
CPS1, in the hepatocytes, controls the first step of the urea cycle where the ammonia is converted into carbamoyl phosphate but excess nitrogen is NOT converted to urea for excretion by the kidneys
What is the biochemical profile you would expect for someone with CPS1 deficiency
severe hyperammonemia
very low plasma levels of citrulline and arginine
high plasma levels of glutamine
Increased transaminases
Low/normal levels of orotic acid in the urine
What is the recommended management/tx for a patient with CPS1 deficiency
life-long diet low in natural protein, supplements of essential aa’s, citrulline and arginine as needed, sodium benzoate and/or sodium or glycerol phenylbutyrate
Nutritional support to avoid catabolic stress
early liver transplantation for those with neonatal onset CPS1 deficiency can correct metabolic abnormalities, does not reverse neurological complications
What is the recommended management/tx for a patient in a hyperammonemic coma with CPS1 deficiency
plasma ammonia levels must be lowered (by hemodialysis or hemofiltration), ammonia scavenger therapy, catabolism reversed (glucose and lipid infusions), special care to reduce the risk of neurological damage
What should be avoided in patients with CPS1 deficiency
Valproic acid
What is the expected prognosis for a patient with neonatal CPS1 deficiency
considered bleak
episodes of hyperammonemic coma of long duration are associated with a poor neurological outcome
How is OTC diagnosed on NBS
Primarily based on the quantification of citrulline on dried blood spots
Values outside the range are considered positive and require follow-up biochemical testing (plasma ammonia, plasma aa profile, urine organic acid profile, urine orotic acid quantification)
Current NBS methods for OTC vary greatly in sensitivity/specificity
How do newborn males with OTC typically present
Normal at birth, development of reduced oral intake w poor latching/suck, acute neonatal encephalopathy (lethargy, drowsy) with hyperventilation and hypothermia
How do child, adolescent, or adults males and females with OTC typically present
Encephalopathic/psychotic episodes (erratic behavior, clouded consciousness, delirium)
Recent stressor
Hx of recurrent vomiting
Migranes
Reye-like syndrome (swelling in the brain and liver damage)
True protein avoidance
Unexplained “cerebral palsy”
What are the prelim lab findings in a pt with OTC
elevated plasma ammonia concentration
abnormal plasma amino acid analysis (high glutamine and a VERY low citrulline level)
Elevated orotic acid on urine analysis
Blood gas findings suggestive of respiratory alkalosis in an individual who is hyperventilating
Unexpectedly low blood urea nitrogen (may suggest reduced urea production)
How is the diagnosis of OTC established in a male proband
Hemizygous PV in OTC
Markedly abnormal increase of orotic acid excretion
Decreased OTC enzyme activity in the liver
How is the dx of OTC established in a female proband
Heterozygous PV in OTC
Markedly abnormal increased of orotic acid excretion
LIVER BX IS NOT RECOMMENDED IN FEMALES DUE TO THE POSIBILITY OF FALSE NEGATIVE RESULTS
What is the order of molecular testing for a patient with suspected OTC and what are the types of variants we are looking for
Sequence analysis is performed first to detect missense, nonsense, and splice site variants and small intragenic indels
Deep intronic PVs have been reported
Therefore, sequencing methods that can detect splice donor and acceptor variants should be included
What is an allopurinol test used for and what genetic condition is it necessary for
to determine the amount of orotic acid over a 24hr period
OTC
When is OTC enzyme activity measured by liver bx
when an OTC PV is not found in a male with a high clinical suspicion for OTC deficiency or if an allopurinol challenge is inconclusive
FOR FEMALES: bx may not represent the true total OTC activity bc of the X chromosome inactivation pattern
What are the clinical features associated with neonatal onset OTC
ASYMPTOMATIC AT BIRTH
Become symptomatic in the first week of life (2-3do) with poor suck, reduced intake, and hypotonia followed by lethargy progressing to lethargy and extreme fatigue
Hyperventilate and may have subclinical/electroencephalographic seizures
What is the typical presentation when a neonate with OTC comes to care
typically catastrophically ill with low body temp (hypothermia), severe encephalopathy, and respiratory alkalosis
What can occur in a neonate with OTC post rescue from neonatal hyperammonemic coma
Can easily become hyperammonemic again despite low protein diet and tx with oral ammonia scavenger. Even on max ammonia scavenger therapy a neonate may only tolerate the bare minimum protein intake needed to grow
Typically liver transplant is required to prevent life threatening hyperammonemic episodes
What are the clinical features associated with partial (post-neonatal-onset) OTC deficiency
become symptomatic in infancy when switched from breast milk to formula/whole milk
show episodic vomiting, lethargy, irritability, FTT, DD
Show true protein avoidance
Become encephalopathic (erratic behavior, combativeness, and delirium)
How can female patients with OTC present
Range from asymptomatic to significant symptoms with recurrent hyperammonia and neurologic compromise depending on X inactivation in her liver cells
What are the neuropsychological and neurologic complications seen in patients with OTC
ADHD; half are described as withdrawn, depressed, and/or anxious
Impulsivity and immaturity can lead to inappropriate behaviors
During hyperammonemic coma, EEG shows low voltage with slow waves and may include a burst suppression pattern
Seizures are common
Neonates who survive after prolonged coma may have ventriculomegaly, diffuse brain atrophy, and low density white matter defects
What are the GI complications seen in patients with OTC
Liver enzymes are typically moderately elevated and PT and PTT may be prolonged
Severe elevations of liver enzyme and coagulopathy consistent with acute liver failure
Risk of developing progressive growth failure over time
What are the general phenotype/genotype correlations in OTC
PVs that are missense that affect residues for catalysis, substrate binding and folding severely impair or completely abolish OTC activity and result in the neonatal onset form of the dz
Nonsense, in/dels, that cause frameshift or ORF and SNVs in intronic splice sites result in complete absence of functional OTC and neonatal onset form of dz
AA substitutions that decrease OTC enzyme activity or stability may result in a post-neonatal onset phenotype
What are some DDs for OTC
Late onset NAGS, CPS1, ASS, ASL deficiencies
Generalized liver dysfunction resulting in hyperammonemia
What is the recommended management for a pt with OTC
Consultation with metabolic physician/biochemical geneticist and specialist metabolic dietician
Lab testing
Developmental assessment
Neurologist
Consultation with psychologist and/or social worker
Genetic counseling
What is the recommended tx for a patient with OTC in acute distress
Renal replacement therapy, nitrogen scavenger therapy, and supplemental citrulline to address hyperammonemia
Provide calories from glucose and fat to decrease protein catabolism
EEG surveillance to tx seizures and assess risk for neurologic damage
What is the recommended long-term tx for a patient with OTC
Protein intake restricted to amt necessary for growth and prevention of catabolism; use of an aa medical food (high glutamine concentrations are interpreted as evidence of poor metabolic control)
Nitrogen scavengers to provide alternative routes for nitrogen disposal and allow more protein intake
Consider liver transplant (removes risk for hyperammonemic episodes)
Tx with ASM as directed by neurologist for seizure disorder
What is the utility of a liver transplant for a patient with OTC? When is it typically performed?
No matter how mild, stressors can precipitate a hyperammonemic crisis that becomes life-threatening
Remains the most effective means of preventing further hyperammonemic crises and neurodevelopmental deterioration typically performed by 6mo
Considered when an affected individual is unstable and has frequent hyperammonemic episodes
What agents/circumstances should someone with OTC avoid
valproate, haloperidol, fasting, stress, systemic corticosteroids bc they cause catabolism
What should be considered in the pregnancy of someone with OTC
heterozygous females are at risk of becoming catabolic during pregnancy and especially in the postpartum period
Caution should also be taken for those who are asymptomatic since they can deteriorate due to stress in the peripartum or postpartum periods
What is the de novo rate in OTC? Any other considerations?
Germline mosaicism has been reported
De novo rate of 26% in males
Spontaneous mutation rate of 67% in females
What is the life expectancy for a male with neonatal onset OTC
Prospective tx as soon as the child is born and improved rescue therapy followed by liver transplant now allow some males to reach reproductive age and reproduce
What is the molecular pathogenesis of OTC? Mechanism of dz?
Catalyzes formation of citrulline from ornithine and carbamylphosphate in the liver and small intestine
Loss of function
How is Citrullinemia flagged on NBS? What is the f/u after identification of suggestive findings
Quantification of citrulline on dried blood spots (high values are considered positive)
F/u includes biochemical testing for plasma ammonia concentration (high), plasma quantitative aa analysis (citrulline- high, argininosuccinic acid- absent, arginine and ornithine- low to normal, lysine, glutamine, and alanine- increased)
Urinary organic acid analysis may show elevated orotic acid
What medical interventions are necessary immediately following + Citrullinemia on NBS
Evaluate neonate for GI involvement (poor feeding, vomiting, signs of liver dz), respiratory distress, neurologic involvement
Initiate management- protein restriction, nitrogen scavengers, arginine supplementation
How is the Citrullinemia dx established through molecular genetic testing for patients with abnormal NBS results and other symptomatic individuals
Sequence analysis of ASS1 to detect missense, nonsense, splice variants, and small indels. F/u w gene-targeted del/dup analysis for exon and whole gene deletions or duplications
Urea cycle disorders/hyperammonemia multigene panel
Sequence analysis identifies 96% of PVs
Why is the ASS enzyme activity measurement not helpful in the dx of Citrullinemia
Predominantly expressed in the liver (and some in the kidneys)
Clinical presentation and relatively specific pattern of metabolites found in affected individuals are sufficient to establish the dx
What is the expected clinical presentation of someone with neonatal (classic) Citrullinemia
infant appears normal at birth
one to a few days, infant becomes progressively more lethargic, feeds poorly, may vomit, develops signs of liver failure, cerebral edema
Longest one can live without tx is 17days
Significant neurologic deficits in a manner corresponding to prior # and degree of exposures to elevated ammonia levels
Gross motor skills tend to be less well developed than expressive language skills
What is the expected clinical presentation of someone with non-classic Citrullinemia
Neurologic findings may be more subtle bc of the older age of the affected individuals
intense headaches, scotomas (blind spot in eye), migraine-like episodes, ataxia, slurred speech, lethargy
respiratory alkalosis and tachypnea
Without intervention, intracranial pressure increases, increased neuromuscular tone, spasticity, ankle clonus seizures, loss of consciousness, and death
What is the expected clinical presentation of someone with non-classic Citrullinemia in the post partum/pregnancy period
Hyperammonemic coma shortly after birth
Implicated in postpartum psychosis
What are the genotype/phenotype correlations associated with Citrullinemia
severe classic form typically results from 22 PVs; PV in exon 15, p.Gly390Arg, remains the most prevalent
mild form is associated with 12 PVs
What are some DD for Citrullinemia
Citrullinemia type 2 (citrin deficiency), ASL deficiency
Secondary hyperammonemia caused by an organic acidemia
What initial evaluations should a pt with Citrullinemia undergo following dx
Consultation with metabolic specialist and specialist metabolic dietician
Metabolic control (plasma ammonia levels, blood gases, electrolytes, plasma aas)
consultation with neurologist
developmental assessment
consultation w psychologist and/or social worker
consultation with PT, OT, and speech
What is the only known curative therapy for pts with Citrullinemia
Liver transplant, eliminates need for dietary restriction
What are the routine txs that a person with Citrullinemia must have
protein restriction
nitrogen scavenger meds
arginine supplementation
secondary carnitine deficiency- supplement with carnitine
What are the tx needed for an acute inpatient for a person with Citrullinemia
withhold all protein intake for 24-48hrs
Pharmacologic nitrogen scavenger therapy
dialysis most effective means of decreasing plasma ammonia rapidly
Administer high energy fluids, consider carnitine supplementation, address electrolytes and pH imbalances
Control intracranial pressure
What are the agents/circumstances to avoid for pts with Citrullinemia
excess protein intake
prolonged fasting
obvious exposure to communicable diseases
Why is it important for at risk relatives for Citrullinemia to be evaluated asap
the long term prognosis is correlated with initial and peak plasma ammonia concentration
What is the molecular pathogenesis of Citrullinemia? What is the mechanism of dz
catalyzes an essential rx in the biosynthesis of urea, causing the condensation of citrulline and aspartate to argininosuccinic acid in the cytosol. Transcription starts near the 5’ end of exon 3; At least 14 ASS pseduogenes are known
Loss of function
How is ASL deficiency detected on NBS? What is it based on?
Based on quantification of the analyte citrulline on dried blood spots
confirmation of the dx requires f/u testing to detect elevated plasma or urine concentration of argininosuccinic acid or its anhydride compounds
What are the biochemical findings you would expect to see in a patient with ASL deficiency
elevated citrulline, argininosuccinic acid, and ammonia
low to normal arginine
normal to high glycine, glutamine, alanine, and orotic acid
What are the molecular genetic testing options for a pt with suspected ASL deficiency
Single gene testing: detects small indels and missense, nonsense, and splice site variants. PERFORM SEQUENCE ANALYSIS FIRST, then perform gene-targeted deldup analysis
Most appropriate when the dx is made based on results of biochemical testing
For this disorder a multigene panel that also includes del/dup analysis is recommended
Multigene panel may be considered when the presentation is with hyperammonemia and confirmatory biochemical dx has not been performed/is unavailable
What are the clinical features associated with the severe neonatal form of ASL deficiency
hyperammonemia within the first few days after birth but healthy for the first 24hrs
vomiting, lethargy, and refusal to accept feeds; tachypnea and respiratory alkalosis are early findings; seizures, coma, and death
hepatomegaly and trichorrhexis nodosa
What are the clinical features associated with the late-onset form of ASL deficiency
range from episodic hyperammonemia to cognitive impairment, behavioral abnormalities, and/or learning disabilities in the absence of any documented episodes of hyperammonemia
SYMPTOMS ARE UNRELATED TO THE SEVERITY OR DURATION OF HYPERAMMONEMIC EPISODES
What are the features of ASL deficiency compared to those with other UCDs
higher incidence of DD and neurologic abnormalities than did those with OTC deficiency
increased incidence of ADHD, ID, behavioral abnormalities, and/or learning disability, seizures compared to those with other UCDs
many individuals who are tx with protein restriction and supplemental arginine have normal cognition and development
What are some DD for the severe neonatal and late onset forms of ASL
severe neonatal: shares the phenotype of the typical acute neonatal hyperammonemia
Late onset: similar to late-onset OTC, late onset citrullinemia type 1 (elevation of argininosuccinate is characteristic
What is the recommended management for those with ASL deficiency
consult with metabolic physician and specialist metabolic dietician
neurocognitive assessment
baseline eval for evidence of hepatic involvement (hepatomegaly, hepatitis, and signs of liver failure)
plotting of systolic and diastolic BP on centile charts based on age and stature
What is the recommended tx for those with ASL deficiency in acute hyperammonemic episodes
discontinue oral protein intake
supplemental IV lipids, glucose, and insulin if needed to promote anabolism
IV nitrogen-scavenging therapy. L-arginine
prompt institution of hemodialysis if ammonia levels are not decreasing
What is the recommended tx for those with ASL deficiency in long term management
dietary restriction of protein, arginine supplementation, oral nitrogen-scavenging therapy, orthotopic liver transplant. salt restriction and use of antihypertensives (for HTN). potassium supplementation for hypokalemia, special ed services as needed
What are the agents/circumstances someone with ASL should avoid
excess protein intake, large boluses of protein or aas, less than recommended intake of protein, prolonged fasting or starvation, obvious exposure to commmunicable dz’s, valproic acid, IV steroids, and hepatotoxic drugs in those with hepatic involvement
What is the molecular pathogenesis of ASL deficiency? Mechanism of disease?
Sources of arginine are exogenous from the diet and endogenous from the breakdown of proteins and synthesis from citrulline
Its main role in the urea cycle is conversion of argininosuccinate into arginine and fumarate
occurs through loss of ASL enzyme function. Residual activity may be present
What are ASL-specific laboratory considerations during analysis
Complicated by a pseudogene
abnormal ASL alleles typically found in affected individuals with late-onset ASL deficiency had either high residual ASL enzyme activity or two mutated alleles that exhibited complementation
How is arginase deficiency tested for on NBS? What has to be considered about infants that test positive?
Based on quantification of arginine on dried blood spots
Some infants with arginase deficiency may have f/u arginine levels in the normal range, and thus infants who continue to have elevated arginine-to-ornithine ratios and arginine toward the upper limit or normal should undergo additional diagnostic testing
What are the preliminary laboratory findings you would expect to see in someone with arginase deficiency
Elevated plasma arginine three to fourfold the upper limit of normal
Intermittent elevation of ammonia; acute hyperammonemia is uncommon
Urinary orotic acid is elevated
What molecular genetic testing approaches should be taken for an individual with suspected arginase deficiency
Perform sequence analysis first (identifies >98% of variants) then do de/dup analysis to detect intragenic del/dups
In individuals in the French Canadian populations, the c.57+1G>A founder variant may be tested for first
How can arginase enzyme activity be measured in an individual with suspected arginase deficiency
most affected individuals have no detectable (<1%) enzyme activity in RBC extracts
Liver and RBC arginase activity correlate well- it is not necessary to perform liver bx when enzyme activity can be measured from a blood sample
What are the clinical characteristics associated with arginase deficiency? What is the prognosis
age 1-3, linear growth slows (children demonstrate growth deficiency; microcephaly is common; feeding issues may develop
age 1-3, normal cognitive development slows or stops, lose developmental milestones; if untx, progresses to severe ID with neurologic findings
Progressive neurological signs typically include the development of severe spasticity with loss of ambulation and complete loss of bowel and bladder control; between 2-4, often misdx with cerebral palsy; can have joint contractures and lordosis; seizures in 60-75% (mostly generalized tonic-clonic); cortical atrophy
hyperammonemia is rarely severe enough to be life threatening
Hepatic dysfunction if present is usually mild (some have developed hepatocellular carcinoma
Vast majority appear to survive and live long handicapped lives
What are some genotype-phenotype correlations seen in Arginase deficiency
Severe dz associated with:
Homozygosity/compound heterozygosity for LOF variants like c. 466-2A>G, c.77delA, c.263_266delAGAA, and c.647_648ins32
Missense changes such as p.Ile8Lys or p.Gly106Arg when homozygous or in combo with another severe allele
Where is arginase deficiency most prevalent?
French Canadian pop*
Turkey
Brazil
China
What are some DD for arginase deficiency
Other causes of hyperammonemia from a UCD
Cerebral palsy (static spastic diplegia)
What are the recommended evals following dx of arginase deficiency
obtain plasma ammonia, aa profile, guanidinoacetate, and liver function tests
gastro/nutrition/feeding team eval
developmental assessment
neurologic eval
musculoskeletal eval
How are patients with arginase deficiency typically managed
Less prone to episodes of hyperammonemia and when present, more likely to respond to conservative management like IV fluid administration
Arginine supplement obviously contraindicated
Restriction of dietary protein
Administration of oral nitrogen scavenging drugs
seizures- standard ASM depending on seizure type
spasticity- orthotics, wheelchairs, walkers, etc.
hepatic fibrosis and cirrhosis- liver transplant
joint contractures- PT
How are patients with arginase deficiency typically managed in an acute inpatient setting (with hyperammonemia)
mild to moderate: increase protein free caloric intake
severe: same as above plus nitrogen scavengers; dialysis is rarely needed
What should be avoided in a pt with arginase deficiency
arginine
valproic acid (exacerbates hyperammonemia)
What is the protocol for a pregnant pt with arginase deficiency? What are the fetal outcomes?
generally continue dietary protein restriction and ammonia-scavenging meds based on clinical course before pregnancy (protein restriction is challenging given the complications like nausea and vomiting in pregnancy)
no well controlled studies of the fetal effects of nitrogen scavengers (sodium benzoate, phenylacetate, phenylbutyrate); however generally advised against since animal studies have shown teratogenic effects
What kind of diagnostic prenatal testing can and cannot be done for a fetus with suspected arginase deficiency
May be possible to measure arginase enzyme activity in fetal RBCs from percutaneous umbilical blood sampling after 18wks gestation
Amniocytes and CV cells do not have arginase activity naturally, and therefore cannot be used for testing
What causes propionic acidemia
deficiency in the mitochondrial multimeric enzyme propionyl-CoA carboxylase; enzyme is composed of 6 alpha subunits and 6 beta subunits encoded by PCCA and PCCB respectively
What testing on NBS is done to flag for Propionic acidemia
Acylcarnitine analysis by tandem mass spec on dried blood spots – elevated propionylcarnitine (C3)
methionine, C3/C2 ratios, and C3/C16 ratios increase dx accuracy
What biochemical findings are consistent with Propionic acidemia
Elevated propionylcarnitine (C3) on PLASMA ACYLCARNITINE PROFILE
On URINE ORGANIC ACIDS: elevated 3-hydroxypropionate, presence of methylcitrate, tiglyglycine, propionylglycine, and lactic acid
on PLASMA AAs: elevated glycine
What are common lab abnormalities seen in individuals with Propionic acidemia in acute decompensation
High anion gap metabolic acidosis
Lactic acidosis
Elevated plasma and urinary ketones
Low to normal blood glucose concentration
Hyperammonemia
Neutropenia, anemia, and thrombocytopenia
How is the dx of propionic acidemia established through molecular genetic testing
Identification of biallelic PVs in PCCA or PCCB
Neither gene is more common and there are no characteristic findings to distinguish between PCCA or PCCB associated PA
Sequence analysis first, followed by gene targeted del/dup if only one or no PV is found
What are the molecular features associated with propionic acidemia
Exon dels account for ~20% of PCCA dz causing alleles
PCC catalyzes the conversion of propionyl CoA to D methymalonyl-CoA which eventually enters the Krebs cycle as succinyl-CoA
Most PCCA PVs are private; CNV variants are responsible for 18% of reported variants
Most PVs in PCCA cause protein instability
Most PVs in PCCB are predicted to alter the active site and reduce the enzymatic activity
What is another approach to the dx of propionic acidemia other than molecular, biochemical/laboratory, or NBS
enzyme assay for activity of propionyl-CoA carboxylase in lymphocytes or cultured skin fibroblasts followed by molecular dx
What are the characteristic features (clinical and lab) of neonatal onset propionic acidemia
Clinical
poor feeding, vomiting, irritability, lethargy, progressive encephalopathy, seizures, coma, respiratory failure
Lab
high anion gap metabolic acidosis, ketonuria, hyperammonemia, hypoglycemia, increase in 3-OH propionic acid and methylcitric acid, hyperglycinemia, anemia, leukopenia, thrombocytopenia
What are the characteristic features (clinical and lab) of late onset propionic acidemia
May remain asymptomatic and suffer a metabolic crisis under catabolic stress or experience a more insidious onset
Clinical
encephalopathy, coma and/or seizures precipitated by catabolic stress, vomiting, protein intolerance, FTT, hypotonia, developmental regression, movement disorders, isolated cardiomyopathy
Lab
with or without metabolic acidosis or hyperammonemia
increased 3-OH propionic acid and methylcitric acid, hyperglycinemia
MRI abnormalities including basal ganglia lesions
What can occur in metabolic decompensation of a pt with propionic acidemia
Acidosis, hyperammonemia, pancreatitis, metabolic stroke, cardiomyopathy, bone marrow suppression, seizures, and encephalopathy
Infectious complications (sepsis, bacterial meningitis) often accompany metabolic crisis and are major contributors to mortality
What are the neurologic manifestations that can be seen in pts with propionic acidemia
DD, developmental regression, ID, seizures, hypotonia, spasticity, and movement disorders
predisposed to basal ganglia lesions- may precede an acute “stroke-like” episode as altered mental status, dystonia, choreoathetosis, or hemiplegia
acute psychosis in older individuals typically the presenting feature
can also see delayed myelination, white matter changes, basal ganglia abnormalities, cerebellar hemorrhage, and cerebral atrophy on MRI
What are the cardio, gastro, and hematological manifestations seen in propionic acidemia
Cardiomyopathy- both dilated and hypertrophic, avg age of onset 7yo; early manifestations can include tachypnea, hepatomegaly, hypotension, tachycardia, or bradycardia
prolonged QT interval commonly detected leading to syncope, arrhythmia, and cardiac arrest
pancreatitis may be recurrent and could lead to insulin-dependent diabetes; poor feeding and lack of appetite common; liver issues: hepatomegaly, hypoalbuminemia, abnormal liver function tests
anemia. leukopenia, and thrombocytopenia are common
What are the immune, ophthalmologic, hearing, and musculoskeletal manifestations seen in propionic acidemia
high frequency of recurrent infections seen in 60-80% of affected individuals; bone marrow suppression, immune dysfunction, frequent hospitalizations, and potential nutritional deficiencies caused by diet could contribute
eye findings include: dyschromatopsia, optic atrophy, scotomas, abnormal electroretinogram; optic neuropathy in 11-25% of pts
SNHL has been reported
Severe osteopenia has been described
What are the genotype phenotype correlations seen in pts with propionic acidemia
null variants and out of frame small indels result in LOF alleles that are associated with more severe PA
homozygous missense variants in which partial enzyme activity is retained have been associated with a less severe phenotype
What are the founder variants/populations for propionic acidemia
homozygous PCCB variant in Amish and Mennonite pops, p.Asn536Asp, is associated with high residual PCC activity. Low risk of developing metabolic crisis, but can lead to late onset cardiomyopathy
Highest birth incidence in Greenlandic Inuits (1:1000)
What are some DD for propionic acidemia
methylmalonic acidemia
biotinidase and holocarboxylase synthetase deficiencies
Maternal B12 deficiency
Mitochondrial disorder
What clinical evaluations should follow the initial dx of propionic acidemia
assessment of growth parameters, ability to feed, need for G-tube placement, neurologic status
lab assessment of nutritional status, CBC to monitor for cytopenias
clinical eval for cardiomyopathy and arrhythmia with EKG, 24 hr Holter monitor, echo
EEG and brain MRI in symptomatic individuals
Developmental eval
Dilated eye exam
Hearing eval
Immunology consult
Maintain a high index of suspicion for endocrine, immune, and renal problems and address accordingly
How should a pt with propionic acidemia in acute decomp be supported? In patient management?
promotion of anabolism and removal of toxic intermediates; medical emergency and requires transfer to a center with biochemical genetics and ability to support urgent hemodialysis
Tx precipitating stress factors; reverse catabolism by giving IV glucose and lipids; manage protein intake to reduce propionic precursors, avoidance of protein transiently may be required; nitrogen scavenger meds, oral n-carbamoylglutamate; hemodialysis, carnitine supplementation
How should propionic acidemia be managed at home?
At home detection and monitoring of urine ketones
Diet modification under the direction of the metabolic team
any injury, hospitalization, or sx procedure should involve consultation with the metabolic team
seizures are a frequent complication requiring ASM; use of valproic acid in organic acidemias is avoided
dermatologic manifestations (persistent dermatitis or eczema) warrant nutritional reassessment
What are the steps needed to prevent primary manifestations of propionyl acidemia
modification of diet to control the intake of propiogenic substrates (isoleucine, valine, methionine, and threonine)
Levocarnitine
Antimicrobial therapy via oral metronidazole has been shown to reduce propionic acid production by intestinal gut flora
Biotin supplementation
Orthotopic liver transplant may be indicated for those who still experience frequent metabolic decompensations, uncontrollable hyperammonemia, and poor growth (NOT CURATIVE- does not completely protect against metabolic stroke, hyperammonemia, or metabolic decomp)
What agents/circumstances should individuals with propionic acidemia avoid
prolonged fasting, catabolic stressors, and excessive protein intake
Lactated Ringer’s solution not recommended
Avoid meds that can prolong QT interval
Neuroleptic antiemetics (promethazine) can mask symptoms of progressive encephalopathy
How is methylmalonic acidemia identified on NBS
primarily based on the quantification of propionylcarnitine (C3) on dried blood spots
ratios of C3/C2, C3/C0, C3/C16, C3/glycine, or C3/methionine are recommended for “positive” NBS acylcarnitine analysis by mass spec
aa analysis of blood spot will show normal methionine and elevated C3/methionine ratio
What medical interventions are necessary immediately following recepit of a positive NBS for MMA? What testing should you consider?
eval for prevention of hyperammonemia and metabolic ketoacidosis
Daily intramuscular vitamin B12 administration (Hydroxocobalamin)
initiation of low protein diet
carnitine supplement
Testing for methylmalonic acid, 2 methylcitrate, and total homocysteine
After positive NBS, what biochemical results will confirm dx of methylmalonic acidemia
elevated plasma methylmalonic acid
elevated levels of urine MMA, presence of 3-OHpropionate, 2-methylcitrate, and tiglyglycine on urine organic acids
elevated concentrations of glycine, possibly alanine and normal methionine
elevated propionylcarnitine (C3) on plasma acylcarnitine profile
elevated plasma ammonia, metabolic ketoacidosis, pancytopenia, lactic acidosis, hypoglycemia
normal serum B12 and plasma homocysteine
What are the preliminary laboratory findings someone with late onset methylmalonic acidemia would have
severe ketoacidosis and lactic acidosis
hyperammonemia
anemia, neutropenia, and/or thrombocytopenia on CBC
isolated renal tubular acidosis or chronic renal failure in untreated infants and children
What are the molecular causes of methylmalonic acidemia
complete (mut0) or partial (mut-) deficiency of methylmalonyl-CoA mutase, encoded by MMUT
Diminished synthesis of methylmalonyl-CoA mutase enzyme cofactor 5’ deoxyadenosylcobalamin, associated with cblA, cblB, or cblD-MMA complementation groups caused by biallelic PVs in MMAA, MMAB, or MMADHC, respectively
deficient activity of methylmalonyl-coenzyme A epimerase encoded by MCEE
What are the molecular genetic testing approaches for methylmalonic acidemia
include use of a multigene panel (MCEE, MMAA, MMAB, MMADHC, MMUT)
most commonly variants in MMUT
What type of responsive test should be given to patient with methylmalonic acidemia ? what does it entail
In vivo responsiveness to vitamin B12 should be determined for all individuals
given hydroxocobalamin followed by assessment of production of MMA and related metabolites by serial urine organic acid analyses and/or measurement of plasma concentrations of MMA, propionylcarnitine, and homocysteine
reduction in plasma or urine mean methylmalonic acid concentration indicative of responsiveness
Describe the clinical features associated with infantile/non B12 responsive methylmalonic acidemia
mut0 enzymatic subtype, cblB
can result in death despite aggressive intervention, can also present with an acute neonatal crisis
presents during infancy; can be normal at birth but develop lethargy, tachypnea, hypothermia, vomiting, and dehydration on initiation of protein-containing feeds
rapidly progress to coma due to hyperammonemic encephalopathy
severe high anion gap metabolic acidosis, ketosis and ketonuria, hyperammonemia and hyperglycemia
dialysis may be needed
thrombocytopenia and neutropenia can be seen
Describe the clinical features associated with partially deficient or B12 responsive phenotypes (mut-, cblA, cblB [although rare], cblD-MMA)
can occur in the first few months or years of life
exhibit feeding problems (anorexia, vomiting), FTT, hypotonia, DD
until dx is established, infants are at risk for catastrophic decomp
child may die despite intensive intervention of prompt tx is not initiated
can have devastating injury to the basal ganglia
can also present with isolated renal tubular acidosis or chronic renal failure
Describe the clinical features associated with MCEE deficiency
complete absence of symptoms to severe metabolic acidosis with increased MMA and 2-methylcitrate and ketones in the urine at initial presentation
metabolic ketoacidosis, hypoglycemia, seizures, DD, and spasticity
not responsive to B12 supplementation
What are the secondary complications associated with methylmalonic acidemia
mut0 and cblB subtypes have a higher rate of mortality, neurologic, and other multisystem complications than those with mut- and cblA subtypes
ID may or may not be present even in those with severe dz
all individuals are at risk of developing renal insufficiency that can progress to end stage renal dz requiring kidney transplant (creatinine is a late marker of renal dysfunction)
some develop “metabolic stroke” which can produce an incapacitating movement disorder
acute pancreatitis is a well recognized complication
growth failure is frequent and multifactorial, result of severe chronic illness
functional immune impairment results in an increased susceptibility to severe infections
can exhibit pancytopenia with bone marrow hypoplasia and/or dysplasia that most frequently reverts to normal with tx
late-onset optic atrophy associated with acute/subacute vision loss
liver u/s can show hepatomegaly; have been reports of hepatoblastoma and hepatocellular carcinoma
What are genotype/phenotype correlations seen in pts with methylmalonic acidemia
MMAB
c.556C>T (p.Arg186Trp): most common PV in 29-33% of alleles from European and North American cohorts (homozygous individuals present in the neonatal period and not responsive to hydroxocobalamin tx
those with at least one c.700C>T (p.Gln234Ter) PV have more variable, generally later age of presentation/dx, some demonstrate a biochemical response to hydroxocobalamin therapy located in the last exon resulting in partially functional protein
MMUT
people with 2 truncating PVs usually have the mut0 subtype
mut- enzymatic subtype PV usually plays a dominant role when in compound heterozygous state with a mut0 enzymatic subtype PV
What are the recommended initial evals someone with methylmalonic acidemia should go through
consultation with metabolic physician and specialist metabolic dietician
assessment of vitamin B12 responsiveness
screening lab tests (serum B12, serum chem panel including renal function and liver enzymes, CBC w/differential, iron status, and folate, arterial/venous blood gas, plasma ammonium and lactic acid concentration, urinalysis and urine ketone measurement, quantitative aas, urine organic acids, serum methylmalonic acid and methylcitrate levels, measurment of free and total carnitine, pancreatic anzymes (amylase and lipase), serum albumin, total protein, and prealbumin
cardiac eval (BP, EKG, echo)
measurement of growth parameters
baseline bone age and density (DXA)
consultation with neurologist
ophthalmology eval
audiology eval
consultation with social work/psychologist
What is the recommended tx for patients with methylmalonic acidemia
vitamin B12 supplementation (if responsive)
restriction of natural protein (propiogenic aa precursors, while maintaining a high calorie diet)
address feeding difficulties, recurrent vomiting, and growth failure
lifelong carnitine supplementation
reduction in propionate production from gut flora (metronidazole)
What is the recommended tx for patients in the acute inpt setting with methylmalonic acidemia
administration of high energy IV fluids
lipid emulsion to provide sufficient calories
decrease/omit protein for 24-48hrs
carnitine supplementation
N-carbamylglutamate (Carbaglu)/nitrogen scavengers for hyperammonemia; hemodialysis may be necessary
neurologic consultation/Brain MRI
consider giving granulocyte-colony stimulating factor for BM failure
What agents/circumstances should people with methylmalonic acidemia avoid
fasting, stress, increased dietary protein, supplementation with valine and isoleucine (increase toxic metabolite load), nephrotoxic meds (ibuprofen), agents that prolong QTc in the EKG
What is the molecular pathogenesis for methylmalonic acidemia? The mechanism of dz?
results from failure to convert methylmalonyl-CoA into succinyl-CoA in the mitochondrial matrix
variants have been reported in MCEE and MMUT that are intronic, may not be detected by routine panels/WES
LOF
How is isovaleric acidemia detected on NBS
based on quantification of isovalerylcarnitine (C5-carnitine) in dried blood spots by tandem mass spec
additional testing of urinary organic acids is required to establish the dx
How is the dx of isovaleric acidemia established
C5-carnitine metabolites, isovalerylglycine, and 3-hydroxyisovaleric acid on analysis or urinary organic acids by gas chromatography-mass spec
variants in IVD by molecular genetic testing
What type of molecular testing is recommended for isovaleric acidemia
single gene sequencing analysis of IVD, then perform gene-targeted del/dup
What are the clinical features associated with isovaleric acidemia
early onset acute metabolic decomp (within the first two weeks or later in life; vomiting, poor feeding, lethargy, hypotonia, seizures, and a distinct odor of sweaty feet), epilepsy, DD, ID and/or impaired cognition, movement disorder, poor weight gain, growth deficiency
can present with metabolic decomp before NBS is available
more favorable prognosis than other organic acidemias
What is a similar condition to isovaleric acidemia and how can it be differentiated
PV p.Ala311Val commonly identified on NBS and is associated with attenuated IVA either as homozygous or compound homozygous
present with mild elevations of C5-Carnitine with normal cognition, no dietary tx or meds, should be protected from overtreatment. If they have prolonged catabolism, tx with IV glucose, rehydrate with normal saline, consider temporary use of carnitine
What are some DD associated with isovaleric acidemia
other organic acidemias, urea cycle disorders, NAGS
Describe the recommended evals following initial dx of isovaleric acidemia
consult with metabolic physician and specialist metabolic dietician
nutrition/feeding assessment
consultation with neurologist
developmental assessment
consultation with PT, OT, speech
What are the recommended tx methods for those with isovaleric acidemia
goal is to reduce leucine and enhancing physiologic detox of isovaleryl-coenzyme A
reduce total natural protein (leucine) intake
conjugation of isovaleryl-coenzyme A using carnitine or combined carnitine and glycine
carnitine supplementation
glycine supplementation
Daily:
low leucine diet in infants, protein-controlled diet in adolescents and adults
PT
What is the acute impatient tx for someone with isovaleric acidemia
for increased catabolism: temporary stop of protein intake, IV glucose to restore anabolism, normal saline for rehydration
consider use of buffers for metabolic acidosis
nitrogen scavengers for hyperammonemia
What are the agents/circumstances that those with isovaleric acidemia should avoid
excess dietary protein/protein malnutrition inducing catabolic state
prolonged fasting
catabolism during illness
What is the molecular pathogenesis and mechanism of dz for isovaleric acidemia
results in reduced enzyme activity in the mitochondrial isovaleryl-conenzyme A dehydrogenase and, subsequently, in accumulation of isovaleryl- conenzyme A and its metabolites
LOF
How is biotinidase deficiency detected on NBS
based on fluorescent or colorimetric tests for biotinidase activity on dried blood spots
confirmational testing requires f/u measurement of biotinidase activity in serum/plasma
need to start tx before confirmational testing is done if NBS is +
What are the associated laboratory findings for biotinidase deficiency
metabolic ketolactic acidosis
organic aciduria (although urinary organic acids can be normal in those who are symptomatic)
hyperammonemia (mildly elevated)
How can the dx of biotinidase deficiency be established (molecular and enzymatic)
detection of deficiency biotinidase enzyme activity in serum/plasma (ACMG established these standards- profound biotinidase deficiency: >10% activity; partial biotinidase deficiency: 10-30% activity)
identification of biallelic PVs in BTD
How is biotinidase deficiency detected on molecular genetic testing? What is the mechanism of dz
sequence analysis first then del dup analysis
for those of Amish ancestry, can start with c.1330G>C (p.Asp444His) and c.1368A>C (p.Gln456His)
LOF
What are the clinical features associated with profound biotinidase deficiency
Those who are dx before symptoms and are tx have normal development
symptoms appear between 1wk-10yrs
Seizures, hypotonia, ataxia, DD, SNHL in 76%; optic atrophy, scotoma, skin rash, alopecia, recurrent viral or fungal infection, hyperventilation, laryngeal stridor, central apnea
What are the clinical features associated with partial biotinidase deficiency
may develop symptoms only when stressed
one child had hypotonia, skin rash, and hair loss during a stressful event but symptoms resolved after biotin tx
What is the prognosis of pts with biotinidase deficiency
symptomatic children can experience reversal of all their symptoms with biotin tx
biochemical abnormalities and seizures rapidly resolve after biotin tx then improvement of the cutaneous abnormalities
hair growth returns
optic atrophy and hearing loss may be resistant to therapy
some treated children have rapidly achieved developmental milestones
normal life span is expected as long as they continue lifelong biotin supplementation
What are the genotype-phenotype correlations that exist for biotinidase deficiency
Profound: most PVs cause complete loss or near complete loss of biotinidase enzyme activity, more likely to be deletions, insertions, or nonsense PVs; increased risk for earlier onset of symptoms
Partial: those who have one p.Arg444His and one profound allele still have enzymatic function (20-25%); if homozygous, expected to have 45-50% enzyme activity and do NOT require therapy
In which populations is biotinidase deficiency most commonly seen
Generally higher in pops with a high rate of consanguinity (Turkey, Saudi Arabia)
Increased in the Hispanic pop
What are the recommended evals following initial dx of biotinidase deficiency on NBS
consultation with metabolic physician and specialist metabolic dietician
measurement of growth parameters
neurologic eval
audiologic eval
What are the recommended evals following initial dx of biotinidase deficiency in a symptomatic individual
consultation with metabolic physician and specialist metabolic dietician
measurement of growth parameters
nutrition/feeding team eval
neurologic eval
ophthalmologic eval
full skin and mucosal exam
clinical assessment for breathing issues
developmental assessment
audiologic eval
assess for hx of recurrent viral or fungal infections
What is the recommended tx for biotinidase deficiency
Biotin therapy is lifelong
should be tx with oral biotin in the free form as opposed to the bound form
protein restricted diet not necessary
if in metabolic decomp, IV hydration and bicarbonate for acidosis
What agents/circumstances should someone with biotinidase deficiency avoid? What should a pregnant person do if their fetus has biotinidase deficiency
raw eggs should be avoided bc they contain avidin, an egg white protein that binds biotin, thus decreasing its availability
consideration of biotin supplementation for the mother
How is glutaric acidemia type 1 detected on NBS
measuring glutarylcarnitine (C5DC) in dried blood spots (96% sensitivity)
requires f/u testing with EITHER urine organic analysis OR quantitative glutaric and 3-OHglutaric acid (preferable)
How is the dx of glutaric acidemia established
identification of biallelic PVs in GCDH via single gene testing; sequence analysis generally performed first, then del/dup
detection of significantly reduced activity of enzyme glutaryl-CoA dehydrogenase in cultured fibroblasts or leukocytes (shortcomings- difficulty distinguishing from affected individuals, which is particularly true for the dominant negative variant; large blood volumes are required)
What are the clinical features commonly seen in pts with infantile onset GA-1
if untx, 80-90% of children will experience an acute encephalopathic crisis in the first 2yrs of life
disability and mortality are high after acute crises
progressive macrocephaly
seizures
What are the clinical features commonly seen in pts with late onset GA-1
onset after 6yo
chronic headaches, macrocephaly, epilepsy, tremor, dementia
frontotemporal hypoplasia, abnormal signal of the white matter
chronic kidney dz may occur
dystonia due to basal ganglia injury
Describe genotype/phenotype correlations seen in glutaric acidemia type 1
Most GCDH variants are missense variants
two subtypes: high excreters and low excreters
high: no or very low glutaryl-CoA dehydrogenase activity (0-3%) sensitivity on NBS is 100%
low: up to 30% residual glutaryl-CoA dehydrogenase activity (at least one hypomorphic allele), sensitivity on NBS is 84%
What are some DD for glutaric acidemia type 1
Canavan dz, Leigh syndrome, Isolated methylmalonic acidemia, propionic acidemia
What are the recommended evals for someone following initial dx of glutaric acidemia type 1
consultation with metabolic physician and specialist metabolic dietician
swallow study for gastro/feeding concerns
developmental assessment
consultation with neurologist
consultation with psychologist and/or social worker
consultation with PT, OT, and speech therapist
What is the recommended tx for someone with glutaric acidemia type 1
the main goal is to reduce lysine oxidation and enhance physiologic detoxification of glutaryl-CoA
low lysine diet
carnitine supplementation
emergency tx during episodes with the goal of averting catabolism and minimizing CNS exposure to lysine and its toxic metabolic byproducts
maintenance of adequate trp levels (depletion may cause severe neurologic deficits)
referral to neurologist for ongoing dystonic movement disorders
What is the recommended acute inpatient tx for someone with glutaric acidemia type 1
administer high energy fluids, decrease/omit natural protein for 24hrs, increase carnitine supplementation for catabolism
can present with myalgia, muscle tenderness, and/or urinary discoloration with increased CK
neurologic consultation (may need ASM), MRI of the brain
judicious use of IV sodium bicarbonate for metabolic acidosis
What are agents/circumstances individuals with glutaric acidemia type 1 should avoid
excessive dietary protein or protein malnutrition inducing catabolic state
prolonged fasting
catabolic illness
inadequate caloric provision during other stressors, especially when fasting is involved (sx)
given increased risk of subdural hemorrhage, avoid or have extreme caution with contact sports and physical activities that involve high risk for minor head injuries
What is the molecular pathogenesis of glutaric acidemia type 1
caused by insufficiency or absence of functional glutaryl-CoA dehydrogenase (GCDH); results in accumulation of upstream byproducts
accumulation of glutaric acid and 2-OH-glutaric acid causes neurotoxicity
results from LOH of GCDH
What are some notable PVs in glutaric acidemia type 1
c.91+5G>T (Ojibway-Cree First Nation founder variant) and others are low excreter phenotype and more difficult to detect conclusively on biochemical testing
p.Gly185_ser190del (c.553_570del18) is a suspected dominant negative allele
