Amino Acid Disorders Flashcards
1
Q
What are Physiological Fucntions of Amino Acids?
A
Building blocks of proteins
Intermediates in metabolism
Key compounds e.g. Neurotransmitters, haem
2
Q
What are clinical manifestation of Inborn Errors of Metabolsim?
A
- Toxic accumulation of substrates before the blocks or of intermediates from alternative metabolic pathways
- Defects in energy production or utilisation caused by a deficiency of products after the block
3
Q
What are causes of Toxic Accumulation associated Inborn Errors of metabolism?
A
- Disorder of protein metabolism (e.g amino acidopathies, organic acidopathies, urea cycle defects)
- Disorders of carbohydrate intolerance
- Lysosomal storage disorders
4
Q
What are causes of disorders of energy production, utilization associated Inborn Errors of metabolism?
A
- Fatty acid oxidation defects
- Disorders of carbohydrate utilization, production (i.e. Glycogen storage disorders, disorders of gluconeogenesis and glycogenolysis)
- Mitochondrial disorders
5
Q
What are features of Amino Acid Disorders?
A
- Clinically and biochemically heterogeneous
- Age of onset variable
- Characterised by Pathological accumulation of physiological metabolites and Presence of non-physiological metabolites
6
Q
What are types of Primary Amino Acids Disorders?
A
- Phenylketonuria
- Tyrosinaemia (I/II/III)
- Maple syrup urine disease
- Homocystinuria
- Non-ketotic hyperglycinaemia
- Hyperprolinaemia (I/II)
- Sulphite oxidase deficiency
- Urea cycle disorders (OTC deficiency, CPS deficiency, Citrullinaemia, Argininosuccinic aciduria, Argininaemia, NAG deficiency)
- Hyperornithinaemia - hyperammonaemia homocitrullinaemia (HHH)
7
Q
What are some examples of Primary Renal Amino Acid disorders?
A
Cystinuria
- Cystine, Ornithine, Arginine, Lysine
Hartnup disease
- Neutral amino aciduria
Lysinuric protein intolerance
- Lysine, Ornithine, Arginine
Iminoglycinuria
- Proline,hydroxyproline,glycine
8
Q
What are secondary causes of Aminoacidopathies?
A
Generalised aminoaciduria
- Fanconi syndrome
- Galactosaemia
- Tyrosinaemia type I
- Cystinosis
Increases in urine/plasma
- Glycine – immaturity, anticonvulsants
Increases in plasma
- Alanine: Lactic acidosis
- Glutamine: Hyperammonaemia
- Methoinine/Tyrosine: Liver diseases
- Isoleucine/Leucine/Valine: Ketosis
9
Q
What is Phenylketonuria?
A
- Autosomal recessive leading deficiency of phenylalanine hydroxylase
If untreated, develop
- Severe mental retardation
- Microcephaly
- Possibly epilepsy;
- Older patients have behavioural problems,
- Some suffer from psychiatric illness
- Movement disorders with extrapyramidal signs
10
Q
How is Phenyketonuria screened?
A
- Increased blood phenylalanine levels
- NHS Newborn Bloodspot Screening Programme
- Phenylketone detected in the urines
11
Q
What is the management of Phenylketonuria?
A
- Phe restricted diet
- Requiring dietetic and laboratory monitoring
- Recommendation: Clinical referral by 14 days of age and dietary treatment initiated by 21 days of age
- UK guidelines: Phe 360μmol/L up to 5 yrs, 480μmol/L 5-10 yr; <700μmol/L thereafter
12
Q
How is Phenylketonuria monitored through life?
A
- Maternal PKU syndrome: fetal anomalies including cardiac and skeletal defect, microcephaly, developmental delay and low birthweight if exposed to high level of phe in-utero
- Require strict dietary control pre-conception
13
Q
What is Tyrosinaemia Type 1?
A
- Deficiency of enzyme fumarylacetoacetate hydrolase leading to accumulation of toxic compound succinylacetone
- Presents during infancy or during first year
14
Q
What are symptoms of Tyrosinaemia Type 1?
A
- Liver failure: Jaundice, Increased transaminases, Pronounced coagulopathy (increased prothrombin time + thrombocytopenia), Alpha fetoprotein; Renal tubulopathy – hypophosphataemic ricket
- Porphyria like neurologic crisis due to SuAc inhibiting porphobilinogen synthase in erythrocytes
15
Q
What are complications of Tyrosinaemia Type 1?
A
Hepatic carcinoma
16
Q
How is the diagnosis of Tyrosinaemia Type 1 made?
A
Biochemical and Haematological
Specialised metabolic tests:
- Increased tyrosine (+ methionine) in plasma amino acids
- Succinylacetone (pathognomonic!) on urine organic acids or on DBS
Mutation analysis: Rarely required for diagnosis but useful for prenatal testing
17
Q
How is Tyrosinaemia Type 1 treated?
A
- NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione)
- Inhibitionof4-hydroxyphenylpyruvatedehydrogenase
- Liver transplant
18
Q
What are Urea Cycle Defects?
A
- Enzyme defects in this pathway all contribute to hyperammonaemia
- Causes Orotic aciduria
- All autosomal recessive except for OTC which is X-linked
19
Q
What is the Urea Cycle?
A
- Urea cycle occurs primarily in the liver for removal of excess nitrogen
- Converts toxic ammonia and other nitrogenous compound into urea (non-toxic) which is excreted in urine
20
Q
What are features of Ornithine transcarbamylase deficiency?
A
- X-linked
- Present first few days of life with lethargy, anorexia, respiratory alkalosis, vomiting, irritability, seizure
- Male hemizygotes usually present in infancy, neonatal presentation is catastrophic
- Female carriers often asymptomatic however may have symptoms in childhood e.g. after excessive protein intake, or metabolic stress e.g. fasting or severe illness; severe hyperammonaemia causing brain damage
- Patient with this condition may appear to be faddy eaters
21
Q
How is diagnosis of Ornithine transcarbamylase deficiency?
A
- High ammonia
- Low blood urea nitrogen level
- Elevated glutamine and alanine, low citrulline and arginine
- Elevated urinary orotic acid
22
Q
How Ornithine transcarbamylase deficiency managed?
A
- Restricted protein intake
- Adequate essential AAs with supplementary dietary carbohydrates and lipids
- “Ammonia scavengers” such as sodium benzoate, sodium phenylbutyrate
- Arginine supplementation
- Nasogastric tube or gastrostomy tube feeds
- Liver transplant
23
Q
What is Maple Syrup Urine Disease?
A
- Autosomal recessive disorder.
- Deficiency of the branched chain alpha-keto acid dehydrogenase (BCKAD) complex, which catalyses the catabolism of the BCAAs leucine, isoleucine and valine.
- BCKAD has four subunit components: E1a, E1b, E2 and E3, encoded by at least 4 genes. Molecular genetic testing available
- Distinct clinical variants based on age of onset, severity of symptoms, and response to thiamine treatment
24
Q
What are types of MAPLE SYRUP Urine Disease?
A
- Classical MSUD
- Intermediate MSUD
- Intermittent MSUD
- Thiamine-responsive form, E3-deficiency MSUD
25
What are features of Classical MSUD?
* Early onset, manifest 12-24 hr after birth, or at 2-3 days previously well
* Poor feeding, vomiting, poor weight gain, lethargy
* Neurological signs develop rapidly – changed muscle tone, seizures, encephalopathy
* Ketosis and characteristic odour of maple syrup in the urine
26
What are features of Intermediate MSUD?
* Residual BCKAD activity
* Diagnosed between 5 months and 7 years
27
What are features of Intermitent MSUD?
* Normal growth and intellectual development throughout infancy and early childhood
* Only present with classic clinical and biochemical features of MSUD during physiological stress e.g. infection
28
How is MSUD diagnosed?
* Increased plasma branched chain amino acids, presence of alloisoleucine diagnostic
* Branched chain Ketoacids
* Abnormal urinary organic acids showing hydroxyisovalerate, lactate, pyruvate, and alpha-ketoglutarate
29
How is MSUD Screened?
* Newborn screening on DBS detection of leucine/isoleucine of \>600umol/L
* Specific enzyme studies on lymphocytes or cultured fibroblasts, not required for diagnosis but supports prenatal diagnosis if required
30
How is MSUD managed?
* Long-term dietary restriction of BCAA
* Emergency regimen during acute episodes of metabolic decompensation: Avoid protein catabolism and AA release by high energy intake (glucose infusions with insulin to promote anabolism) with BCAA-free amino acid supplements; haemodialysis may be necessary
31
What are features of Homocysteinuria?
* Autosomal recessive disorder of methionine metabolism, leading to accumulation of homocysteine and its metabolites (homocystine, homocysteine- cysteine complex etc)
* Most common cause is deficiency of beta cystathionine synthase (classical Hcy); there are pyridoxine-responsive or unresponsive forms.
* Other points of defect are in the transsulphuration pathway or re-methylation pathway
32
What are symptoms of Homocysteinuria?
* Mental retardation
* Severe thrombotic tendencies
* Signs and symptoms of strokes in any vascular distribution: hemiplegia, aphasia, ataxia, pseudobulbar palsy
33
What are types of homocysteinura?
**_Classic Homocystinuria CBS Deficiency_**
* Normal at birth, at age around 20 years with increased likelihood of suffering a thromboembolic event
**Defective Methylcobalamin Synthesis or Defective Tetrahydrofoliate Metabolism**
* May present in early infancy, differentiated from CBS deficiency by presence of methylmalonic acidaemia
34
How is diagnosis of Homocysteinuria made?
* Cyanide nitroprusside test for sulphur-containing amino acids (false –ve and +ve)
* Plasma total homocystine alongside with a plasma amino acids quantitation
* Methylmalonic acid in plasma or urine
* MTHFR Mutation on chromosome 1
35
How is Homocyteinuria screened?
* Newborn Screening (UK Expanded Screen) available – Increased methionine on DBS
* Without screening patients remain asymptomatic and usually not diagnosed until 2-3 years of age
36
How is Homocysteinuria managed?
* Level of homocysteine controlled by giving Vitamin B6 (pyridoxine)
* For the non responders they are treated with low protein diet, with a methionine free formula.
* Folic acid also given, B12 and betaine
* Monitoring of total homocystine and amino acids measurement
37
What are features of Cystinuria?
* Autosomal recessive disorder
* Can lead to Renal calculi, renal impairment
* Cystine crystals precipitate or aggregate around urinary debris e.g. bacteria, blood cells or other solid matter. Obstruction for urinary tract, haematuria, pain and renal failure
* Defective transport of cystine and dibasic amino acids (ornithine, arginine, lysine) in the renal proximal tubule and in the apical brush- border membrane of jejunum in the small intestine
* Homozygosity and heterozygosity
38
How is Cystine affected by the pH?
Cystine solubility reduces at urinary pH levels of 5-7 (pKa 8.3).
At pH 7.8 and 8, the solubility of cystine is doubled and tripled respectively
39
How is Cystinuria diagnosed?
Diagnosed by urinary quantitation of cystine and the dibasic amino acids
40
How is Cystinuria managed?
* Hydration therapy
* Alkalinisation of urine using potassium citrate (pH 7.5 or above)
* Chelating agents e.g. D-penicillamine, alpha-MPC or captopril to form disulphide bond with cysteine to form a more soluble compound for excretion
* Lithotripsy for removal of renal calculi
41
What is Nonketotic Hyperglycinamiea (NKH)?
* Defective glycine degradation resulting in large quantities of glycine accumulate in all body tissues, including the CNS
* Majority of patient have neonatal phenotype, presenting first few days of life with lethargy, hypotonia and myoclonic jerks, progressing to apnoea and often to death; those who regain spontaneous respiration develop intractable seizures and profound mental retardation
* Infantile form develop seizures and mental retardation after a symptom-free interval and seemingly normal development for up to 6 months.
42
How is diagnosis of Nonketotic Hyperglycinamiea (NKH) made?
* Diagnosis is established by measuring CSF to plasma glycine ratio if greater than 0.04 is diagnostic
* Confirmed by measurement of activity of the glycine cleavage system in liver tissue; nowadays mutation analysis is possible
43
What are some analytical techniques?
* Spot tests – Urine
* Qualitative screening
* Quantitative analysis
44
What are some spot tests used with urine?
**Ferric Chloride**
* Reacts with phenols
**2,4 Dinitrophenylthydrazine**
* Reacts with branch-chain ketoacids and phenylketones
* MSUD, PKU
**Cyanide Nitroprusside**
* Reacts with sulphur containing amino acids
* Homocystinuria, cystinuria
45
What are some Qualitative screening?
* **Thin layer chromatography (TLC)** – High voltage electrophoresis(HVE)
46
What are some Quantitative analysis used?
* High pressure liquid chromatography (HPLC)
* Ultra-high pressure liquid chromatography (UPLC)
* Ion exchange chromatography (IExC)
* Tandem mass spectrometry (MS/MS)
47
What are advantages and disadvantages of Spot Tests?
**Advantages**
* Low cost
* Easy
* No expensive equipment required
**Disadvantages**
* Prone to interference
* Neither sensitive or specific
* May mislead investigation
* H&S issues
48
What are types of Qualitative Analysis methods?
**2-Dimensional Thin layer chromatography**
* Ninhydrin stain
* Selective staining increases numbers of compounds identified
**High voltage electrophoresis**
49
What are advantages and disadvantages of Qualitatitve screening?
ADVANTAGES
* Cheap
* Can be used to pre- screen samples before referring
LIMITATIONS
* Significant staff time
* Technically demanding
* Interpretation requires experience
* Does not identify all compounds of interest
* May only detect gross abnormalities
50
What advantages and disadvantages of Quantitative analysis-TMS?
**ADVANTAGES**
* Established in IEM field
* Simultaneous measurement other compounds of interest on same injection
* Simple sample prep
* Rapid results
* Ideal for targeted screen
**LIMITATIONS**
* Expensive capital cost
* Expertise in technology required
* Isobaric/isomeric compounds require separation
51
What are some advantages and disadvantages of Ion exchange chromatography?
**ADVANTAGES**
* Dedicated instrument
* Specific for amino acids
* Will identify all compounds of interest
**LIMITATIONS**
* Long run times
* Significant maintenance
* Often running at capacity
* Urgent cases need rapid results
