Biochemical Genetics Flashcards
Defect in the synthesis of cortisol from pregnenolone via 21-hydroxylase
17-hydroxyprogesterone -x-> 11-deoxycortisol
excess 17-hydroxyprogesterone converts to androgens (testosterone) causing virilization in utero
Lack of cortisol results in salt imbalances in the kidneys
CAH (congenital adrenal hyperplasia)
What metabolic tests are commonly ordered?
amino acids organic acids acylcarnitine profiles enzyme assays gene sequencing
Phenylalanine
amino acid abnormality found on quantitative analysis associated with PKU (nl ~70 umol/L; PKU > 1200)
Leucine
amino acid abnormality found on quantitative analysis associated with MSUD (nl ~100 umol/L; MSUD > 700)
Methionine, homocysteine
amino acid abnormalities found on quantitative analysis associated with homocystinuria
Citrulline
amino acid abnormality found on quantitative analysis associated with urea cycle disorders
What is important to remember when ordering amino acid panels?
Can be done on blood OR urine
Best to do a fasting specimen (because levels go up after a meal)
Slight elevations are common, significant elevations (3-10 times normal limit) indicate concern for disease
What is important to remember when ordering organic acid panels?
Done most often on urine
Can be qualitative or quantitative
Slight elevations are common, making it occasionally difficult to interpret
Branched chain ketoacids
organic acid abnormality found on analysis associated with MSUD
What testing should be ordered for children with:
developmental delays OR
unexplained acute illnesses OR
poor growth (“failure to thrive”).
amino acid and organic acid screening
What is important to remember when ordering an acylcarnitine profile?
Can be done on blood OR urine (but blood is most common)
By detecting carnitines you can reflect intracellular acyl-CoA’s
C3 elevation (acylcarnitine profile)
propionlycarnitine (propionic or methylmalonic acidemia)
C8 elevation (acylcarnitine profile)
octanoylcarnitine (MCAD)
C14:1 elevation (acylcarnitine profile)
14 carbons, one double bond (VLCAD)
What is important to remember when ordering enzyme assays?
done on blood (enzymes that appear in WBC or serum or are stable enough for dried blood spot for NBS) or biopsy specimens (enzymes that are tissue specific such as liver or muscle or require fibroblasts which require skin biopsy and culture)
try to minimize freeze/transport because this can affect results
What is important to remember when ordering testing for inborn errors in metabolism in general?
gene sequencing is often preferred to enzyme assay (since most have known genetic loci) - since DNA is often more accessible (rather than biopsy) and there is less problems from transport artifacts
Not the possibility of VUS, though
List the causes of methylmalonic aciduria (elevated C3 on acylcarnitine profile).
Methylmalonyl-CoA mutase deficiency (most common and most severe) Cobalamin defects (Cbl A, Cbl B, Cbl C, etc.; can't activate B12) Pernicious Anemia/Vitamin B12 deficiency (because it is a cofactor of the methylmalonyl-CoA mutase reaction)
List the causes of homocystinuria.
Classical- Methionine metabolism defects (cystathionine beta-synthase)
Defects in folic acid metabolism OR vitamin B12 metabolism (homocysteinemia)
Most common cause- Folate defect (MTHFR)
NOTE- this can present very similarly to Marfan syndrome with a significant intellectual disability aspect (not seen in Marfan)
List the causes of propionic acidemia.
Defects in propionyl-CoA carboxylase enzyme (has two subunits with two different genetic loci on different chromosomes - same symptoms)
Biotin (cofactor for PCC) defects (causes different symptoms than PCC defects)
What are the general symptoms/principles that would lead you to suspect an IEM in an acutely ill infant?
Term babies Good APGARs No dysmorphic features Normal for first 24-48 hours (prodrome) Problems/distress related to feeding Lethargy Emesis Seizures Hepatomegaly Breathing pattern (hyperammonemia - rapid, shallow; acidosis - deep/Kussmaul breathing)/respiratory distress Anion gap Hypoglycemia Lactic acidemia (more common in mito) Ammonia level (high- 10-20x normal) Ketonuria/high blood ketones Blood amino acid abnormalities Urine organic acid abnormalities Carnitine/acylcarnitine profiles
Provide a genetics Ddx for the following acute illness symptoms:
emesis
lethargy
coma
Urea cycle defects
Galactosemia (with milk feedings)
Maple Syrup Urine disease (MSUD)
Organic acidemias (propionic, methylmalonic, isovaleric)
Provide a genetics Ddx for the following acute illness symptoms:
acidosis with blood pH < 7.1 (nl 7.4) and/or blood bicarb < 10 (nl 25)
Organic acidemias (propionic acidemia, methylmalonic acidemia, isovaleric acidemia) Primary lactic acidosis (electron transport chain/mito defects, pyruvate dehydrogenase deficiency)
Provide a genetics Ddx for the following acute illness symptoms:
respiratory distress
neonatal “pneumonia” (without actual infection)
Urea cycle defects (hyperpnea - rapid breathing) Organic acidemias (Kussmaul - deep breathing) Maple syrup urine disease (respiratory depression) Nonketotic hyperglycinemia (hiccuping and/or apnea)
Provide a GENETICS Ddx for the following acute illness symptoms (note that this is common in newborns and is usually not due to an enzymopathy):
hypoglycemia
Congenital adrenal hyperplasia (CAH) Fatty acid oxidation defects Galactosemia Propionic acidemia Gluconeogenic defects Glycogen storage disorders
Provide a genetics Ddx for the following acute illness symptoms:
hepatomegaly
Galactosemia (also have liver failure)
Tyrosinemia (usually later)
Fatty acid oxidation defects
Lysosomal storage disorders (usually later)
Provide a genetics Ddx for the following acute illness symptoms;
seizures
Nonketotic hyperglycinemia
Urea cycle defects (hyperammonemia)
Organic acidemias (hyperammonemia, hypoglycemia)
Gluconeogenic defects (hypoglycemia)
Lysosomal storage disorders (usually later)
acute hyperammonemia (NI ammonia ~30)
tachypnea (respiratory alkalosis), emesis, lethargy
NOT ACIDOTIC
neonatal comas (associated with ID)
Urea cycle defects
How can urea cycle defects be treated?
dialysis
medications (especially ammonia scavengers)
organ transplant
List the amino acids that carry ammonia in the urea cycle.
Ornithine Citrulline Argininosuccinate Arginine NOTE- all of these are measurable by amino acid analysis (can tell us which enzyme may be deficient)
acute, severe neonatal ketoacidosis (pH < 7.1)
emesis, hyperpnea, lethargy
Organic acidemias
How do you treat organic acidemias?
some respond to diet/vitamin treatment (B6 in homocystinuria; B12 in methylmalonic aciduria)
dialysis for acute illness
some may need kidney + liver transplant (methylmalonic acidemia)
severe ketoacidosis
sweaty feet odor
isovaleric acid in urine organic acids
elevated C5 acylcarnitine
isovaleric acidemia (isovaleryl-CoA dehydrogenase deficiency)
acute ketoacidosis with sweet smell lethargy hypotonia respiratory depression seizures emesis elevated leucine on serum amino acids branched chain acids found on urine organic acids
Maple syrup urine disease (MSUD)
How are fatty acid oxidation defects treated?
some can respond to avoidance of prolonged fasting (especially MCAD)
How can you treat MSUD?
responds well to diet
vitamin Rx (Thiamin)
treat acute illness with dialysis (removes leucine)
How is Galactosemia treated?
responds well to diet treatment
NOTE- infants improve when on IV fluids (glucose), then get worse when fed milk again
Prodrome period fasting (>12 hours) hypoglycemia liver disease/transient hepatomegaly myopathy lethargy emesis weakness cardiac degeneration
fatty acid oxidation defects (MCAD, VLCAD, LCHAD)
fasting intolerance, hypoglycemia, lethargy, poor response to metabolic stress (cortisol deficiency)
circulatory collapse, salt wasting (low sodium, high potassium due to aldasterone deficiency)
prenatal virilization (masculinization) in females (excessive androgens)
elevated 17-hydroxyprogesterone
poor response in ACTH stimulation test (blunted rise in cortisol)
Congenital adrenal hyperplasia (CAH)- defects in cortisol synthesis (21-hyrdoxylase deficiency most common)
What kinds of testing would you do to distinguish between conditions that present with hepatomegaly?
electrolyte analysis (looking for an anion gap- high anion gap indicates an organic acidemia)
blood glucose
lactate
CBC (looking for anemia and low platelets- may indicate spleen involvement)
urine MPS screen (glycoaminoglycans)
skeletal survey (complete X-ray series)
liver/bone marrow biopsy histology (looking for glycogen granules in liver cells -not necessarily pathognomonic for a genetic glycogen storage disease- and looking for Gaucher cells in bone marrow)
Describe the types of Gaucher disease.
Type I (Adult) - no neurological involvement, responds well to ERT, AJ background more common (N370S) Type II (Infantile) - severe neurological involvement in infants, early death from rapid neurological deterioration, does not respond to ERT Type III (Neuropathic) - slowly progressive neurological disease, disturbances of upward gaze, Swedish background (L444P)
most present in adulthood (may present in childhood) - variable manifestations (even within families) splenomegaly anemia, low platelets (cytopenia) bone pain, pathogenic fractures "Ehrlenmeyer flask" deformity osteopenia SOB, exercise intolerance, hypoxemia
Type I Gaucher
hepatosplenomegaly, distended abdomen
hypotonia, weakness, difficulty with exercise, gait disturbances
neurologic deterioration, vision problems
poor growth
Neimann-Pick disease (general)
Describe the types of Neimann-Pick disease.
Type A (sphingomyelinase deficiency) - Infantile, common in AJ, death by age 3 from neurologic symptoms Type B (sphingomyelinase deficiency) - allelic with Type A, later presentation, fewer neurologic symptoms Type C (intercellular cholesterol trafficking defect) - presents in childhood with ataxia, palsy of upward gaze, different gene from A and B
hepatosplenomegaly
corneal clouding
“coarse” facial features
skeletal dysplasia, joint stiffness (claw hand deformity, spinal “gibbus”, difficulty raising arms above shoulders, bent over posture at the hips)
“dystosis multiplex” on XR (multiple bones involved, spindle shaped bones in hands, “hooking” in vertebral bodies, broad ribs)
+/- progressive neurological impairment
obstructive/restrictive pulmonary disease
hearing loss
cardiac valve dysfunction
expression of glycosaminoglycans in urine
Mucopolysaccharidoses (general)
Describe the types of MPS.
MPS I/MPS V (Hurler, Scheie, Hurler/Scheie) - L-alpha-iduronidase deficiency; spectrum of severity MPS II (Hunter) - X-linked, iduronidate sulfatase deficiency, no corneal clouding MPS III (Sanfilippo) - 4 enzymes, more neurological symptoms, typically presents in early childhood with delays and typically pass in adolescence MPS IV (Morquio) - 2 enzymes, specific skeletal findings, less neurological symptoms MPS VI (Maroteaux-Lamy) - arylsulfatase B deficiency, similar to MPS1 symptoms (differ in skeletal findings on XR) MPS VII (Sly)- most rare, characterized by organomegaly and severe skeletal dysplasia
weakness pain, cramping muscle breakdown after exercise hepatomegaly hypoglycemia during fasting lactic acidemia hyperlipidemia hyperuricemia poor growth \+/- myopathy
Hepatic Glycogen storage diseases (general) - there are 6 types (More severe - GSD I > GSD III > GSD VI - Less severe)
hepatosplenomegaly failure to thrive progressive cirrhosis liver failure myopathy/cardiomyopathy
GSD IV (Andersen disease) - glycogen brancher deficiency
gangliosidosis
loss of skills/milestones at beginning at 6 months and full loss by 12 months
seizures
“cherry red spot” in retina
loss of visual acuity (leading to unusual eye movements)
enlargement of head in second year
NO organomegaly
swallowing difficulties
leukodystrophy (white matter changes) on brain MRI
death due to respiratory deficiency by age 2-4 y/o
Tay-Sachs (hexosaminidase A deficiency)
weakness
stiffness
scissoring of legs
opisthotonic posturing (head back, back arched)
irritability, excessive startle to loud noise
leukodystrophy (white matter changes) on brain MRI
NO organomegaly
Normal eye findings
death by age 18 months
Krabbe disease (galactocerebrosidase deficiency)
weakness, floppiness cardiomegaly elevated muscle enzymes (CK) in blood short PR interval on EKG NO hypoglycemia NO hepatomegaly progressive heart and respiratory failure death in first year
Pompe Disease/ GSD II (acid alpha-glucosidase/ acid maltase deficiency)
How is Pompe disease treated?
ERT (improved survival if started before ventilator support -NBS possible but not currently utilized- , but antibody development is possible in CRM- patients so immune modulation may be required)
How is Tay-Sachs disease treated?
life can be prolonged by tube feedings, but it does not change the outcome
How is Krabbe disease treated?
HSCT/BMT- but poor results after symptoms have started and better outcomes possible in the late onset forms
exercise intolerance (with cramping, pain, and myoglobinuria)
NO hypoglycemia
NO hepatomegaly
McArdle disease/ GSD V (muscle phosphorylase deficiency
How is McArdle disease treated?
high carb diet may help
Describe how the time between meals and hypoglycemia can give you an indication of what the likely cause is.
2-4 hours (meal digested, absorbed)- galactosemia (reactions in this period caused by reactions to components of food)
6-8 hours (liver glycogen broken down) - Glycogen storage disorders
8+ hours (glucose maintained by gluconeogenesis) -Fatty acid oxidation defects/Disorders of gluconeogenesis
hepatomegaly (permanent) muscle weakness (uncommon) severe hypoglycemia after short fast (can bottom out to zero) lactic acidemia (may be severe) hyperlipidemia, hyperuricemia liver adenomas/hepatoma (can progress to liver cancer) glomerulosclerosis behavioral problems
von Gierke’s disease/ GSD Type I (glucose-6 phosphatase deficiency)
Ia- primary enzyme defect
Ib- transported defect (associated with low neutropenia and late onset IBD)
How do you treat von Gierke’s disease?
frequent feedings (night time NG or G-tube drip feeds) cornstarch q6h
hepatomegaly severe hypoglycemia (bottoms out at 40- but no symptoms of hypoglycemia) lactic acidemia hyperlipidemia, hyperuricemia progressive hypertrophic cardiomyopathy liver symptoms may resolve by adulthood
Cori disease/ Forbes disease/ GSD Type III (glycogen debrancher deficiency)
hepatomegaly
fasting hypoglycemia
mild lactic acidemia
mild hyperlipidemia
GSD VI, VII, IX (X-linked), etc. (hepatic phosphorylase/phosphorylase activation system - kinases - defects)
SIDS extreme lethargy/coma emesis hypoglycemia normal except when fasting (fasting intolerance)
MCAD (medium chain acyl-CoA dehydrogenase deficiency)
may have muscle and/or cardiac involvement such as cardiomyopathy/cardiomegaly, exercise intolerance
fasting intolerance/hypoketotic hypoglycemia
lipid myopathy
VLCAD (long chain acyl-CoA dehydrogenase deficiency)
hypoketotic hypoglycemia
cardiomyopathy
retinopathy (some)
Female hets at risk for HELLP syndrome (hypertension, elevated liver enzymes, low platelets) during pregnancy
LCHAD (long chain hydroxyacyl-CoA dehydrogenase deficiency - trifunctional enzyme deficiency)
asymptomatic at birth and for first few months
developmental delays
seizures
musty body odor
Phenylketonuria (deficiency of Phenylalanine Hydroxylase; can also be cased by tetrahydrobiopterin metabolism defects)
How is PKU treated?
PKU diet (elimination of phenylalanine) if primary cause is PAH deficiency started in the first week of life
may need other additional treatments if caused by defects in tetrahydrobiopterin metabolism (Kuvan)
MUST be treated prior to onset of symptoms
Elevated maternal PHE is teratogenic (microcephaly, heart malformations) - must be well controlled prior to conception
asymptomatic at birth feeding intolerance (lactose in all mammalian milk) emesis liver failure, jaundice, hepatomegaly blood clotting abnormalities predisposition to infections E. coli sepsis
Galactosemia (GaliPUT/GALT deficiency)
How is Galactosemia treated?
removal of milk (lactose) from diet
How is CAH treated?
cortisol
Florinef (mineralocorticoid) + NaCl
Surgical management of virilization in females
Describe heteroplasmy vs homoplasmy
heteroplasmy means there are two different populations of mtDNA present in a given cell or tissue (e.g. wt and mutant)
homoplasmy means that there is only (100%) one population of mtDNA present in the cell (e.g. only wt or only mutant)
stroke-like lesions without vascular pattern basal ganglia Ca hyperintensity Encephalopathy provoked by valproate Epilepsia partialis continua Myoclonus Ataxia Cardiomyopathy (hypertrophic + arrhythmia; dilated + myopathy; any + disproportionate lactic acidosis) WPW Heart block Retinal degeneration (night blindness, color vision loss, pigmentary retinopathy) Pediatric opthalmoplegia Valproate-induced liver failure Liver steatosis Dysmotility Pseudoobstruction Hypotonia Failure to thrive Acidosis Exercise intolerance Anesthesia hypersensitivity
“Red flag” symptoms for mitochondrial diseases
Bilateral, painless subacute visual failure that develops during young adult life (blurring of central visual field in one eye - 25%of cases bilateral at onset - , similar symptoms appear in the other eye approximately 2-3 months later, VA worsens to counting fingers)
Degeneration of the retinal ganglion cell layer and optic nerve (optic disc and valvular changes)
cardiac arrhythmias
Tremor, preipheral neuropathy, myopathy, movement disorders
MS-like picture in women
Leber Hereditary Optic Neuropathy (caused by mtDNA mutations- typically homoplasmic in blood in 85-90%) REDUCED PENETRANCE (50% of males develop vision loss, 10% of females)
How is Leber Hereditary Optic Neuropathy treated?
regular cardiac, ophthalmology, and neurology evaluations
avoidance of mitochondrial toxins
normal early development with first signs usually in childhood (age 2-10) or delayed onset (age 10-40) seizures headaches emesis proximal limb weakness exercise intolerance encephalomyopathy lactic acidosis stroke-like episodes (characterized by transient hemiparesis and cortical blindness that lead to impaired motor abilities and cognitive function over time) short stature hearing loss dementia myopathy
MELAS (mtDNA mutations; 80% to the MT-T1 gene; tRNA leucine mutations)
myoclonic epilepsy
ragged red fibers
MERRF (mtDNA point mutations in tRNA lysine)
anemia
exocrine pancreas dysfunction
fatal in infancy
Pearson Syndrome (mtDNA deletion)
typically de novo
Deletion- detected in blood
retinitis pigmentosa Progressive external ophthalmoplegia cardiac conduction block increased CSF protein cerebelar ataxia short stature, growth hormone deficiency hearing loss dementia diabetes mellitus limb weakness hypoparathyroidism onset between early childhood and young adulthood
Kearns-Sayer Syndrome (mtDNA deletion)
typically de novo
Deletion- detected in CNS and muscle
ptosis opthalmoplegia (lose ability to move eyes laterally) \+/- generalized myopathy variable degrees of SNHL axonal neuropathy ataxia depression Parkinsonism hypogonadism cataracts onset between young adulthood to late adulthood
Progressive External Opthalmoplegia (mtDNA deletion OR POLG AR/AD mutation)
typically de novo
Deletion- detected in muscle
childhood-onset
progressive and severe encephalopathy
intractable epilepsy
liver failure
Aplers-Huttenlocher syndrome (mtDNA POLG mutation)
infantile or childhood onset developmental delays or dementia lactic acidosis myopathy failure to thrive liver failure renal tubular acidosis pancreatitis cyclic emesis hearing loss
Childhood myocerebrohepatopathy spectrum (mtDNA POLG mutation)
epilepsy
myopathy
ataxia
NO opthalmoplegia
Ataxia neuropathy spectrum (mtDNA POLG mutation)
How do you treat mitochondrial disorders?
Exercise
Treat symptoms
Minimize infections (vaccinations critical)
Avoid fasting
one-size-fits-all “supplement cocktails” that target mitochondrial enzymes and stress…
Increase free CoQ pool (carnitine, pantothenate)
Enzyme co-factors (vitamins B1 or B2)
Metabolite therapies (arginine, folinic acid, creatine)
Enzyme activators (dichloroacetate)
Antioxidants (vitamins C or E, lipoic acid, coenzyme Q)
Mitochondrial Replacement Therapy (NOT AVAILABLE IN US)
What are the symptoms of fasting metabolic conditions?
Children are healthy when fed (infants eat every few hours, so they are typically healthy)- brought on by fasting
Lethargy, emesis
(Transient) hepatomegaly
Lactic acidemia (NOT in fatty acid disorders)
Hypoketosis (especially with the fatty acid disorders)
+/- hepatomegaly
urine organic acids normal (except for lactate)
fasting hypoglycemia associated with lactic acidemia
Disorders of gluconeogenesis
(pyruvate carboxylase deficiency and fructose-1,6-bisphosphatase deficiency)
RARE
How do you diagnose Liver type Glycogen Storage Disorders?
Glucose, lactate, uric acid, and triglyceride levels
Fasting study, glucagon response (in a very controlled environment)
Liver biopsy (histology and enzyme assay)
Gene Panels (most practical)
How do you diagnosed Muscle type Glycogen Storage Disorders?
Muscle biopsy with enzyme assay Gene panels (most practical)
What is the Duarte variant?
N314D variant in GALT that results in false positive NSB because, though it causes low enzyme activity, individuals with one classic galactosemia variant and the Duarte variant do not require treatment (very mild)
Name the drugs used to treat these conditions:
Gaucher
Tyrosinemia
Miglustat
Orfadin
How is MPS treated?
ERT
hematopoietic stem cell transplant
