Inborn errors of metabolism

Question 1

Introduction

Answer 1

• Inherited Characteristics:
  
  • Determined by about 50,000 gene pairs.
  • Genes arranged on 23 pairs of chromosomes.
  • One chromosome of each pair from the father, one from the mother.
• Genotype Diversity:
  
  • Introduced by:
    
    • Random selection during meiosis.
    • Recombination during meiosis.
    • Occasional mutation.
• Genetic Variants:
  
  • May be incompatible with life (extreme case).
  • May produce biochemical differences detectable only by special techniques (milder cases).
  • Many variations exist between the extremes.
  • Between the two extremes there are many variations that produce functional abnormalities or inborn errors of metabolism (IEM).
• Inborn Errors of Metabolism (IEM):
  
  • Functional abnormalities or inborn errors of metabolism.
  • Incidences range from 1 in 100 to 1 in 200,000.
  • Variation depends on the disorder and population involved.

Question 2

Some metabolic consequences of genetic defects

Answer 2

• Inherited Inborn Disorders:
  
  • Can involve any peptide or protein.
  • Most obvious when there is an enzyme abnormality.
• Deficiency of a Single Enzyme in a metabolic pathway may produce its effects in several ways (example scenario):
  
  • Substance A is acted on by enzyme X to produce substance B.
  • Substance C is on an alternative pathway.
  • Enzyme X deficiency can cause:
    
    • Deficiency of product B of the enzyme reaction: e.g., cortisol deficiency in congenital adrenal hyperplasia.
    • Accumulation of substance A: e.g., phenylalanine in phenylketonuria (PKU).
    • Diversion through an alternative pathway: some products of the latter (C) may accumulate and rpoduce effects e.g., accumulation of androgens causing virilization in congenital adrenal hyperplasia.
• ** The effects of the last two types of abnormalitt will be aggravated if the whole metabolic pathway is controlled by negative feedback from the final product:**
  
  • Example: In congenital adrenal hyperplasia, cortisol deficiency reduces negative feedback, increases steroid synthesis, leading to androgen accumulation causing virilization in females.
• Modification of Clinical Effects of some IEM by Physiological or Environmental Factors:
  
  • E.g. Iron Loss:
    
    • Occurs during menstruation and pregnancy.
    • Women with hereditary haemochromatosis accumulate iron less rapidly than men with the condition and rarely present clinical features before menopause.
• Patients with Cholinesterase Variants:
  
  • Symptoms develop only if muscle relaxant suxamethonium is given.

Question 3

Clinical importance of inborn errors of metabolism

Answer 3

• Some Inborn Errors (Probably Harmless):
  
  • Produce effects that may lead to misdiagnosis.
    
    • Examples: Renal glycosuria, Gilbert’s syndrome.
• Importance of Diagnosis (Even Without Effective Treatment):
  
  • Recognition in early infancy is crucial.
  • Early treatment can prevent irreversible clinical consequences or death.
    
    • Examples of important diseases:
      
      • Phenylketonuria (PKU)
      • Galactosaemia
      • Maple syrup urine disease

Question 4

Neonatal screening

Answer 4

• Screening Programs:
  
  • Implemented in many countries for newborn infants.
  • Aim: Detect inherited metabolic disorders or congenital defects.
• Criteria for Screening:
  
  • Disease should not be clinically apparent at screening time.
  • Disease should have a relatively high incidence in the population screened.
  • Disease should be treatable or early treatment should improve the outcome.
  • Results of the screening test should be available before irreversible damage occurs.
  • Screening test should be simple, reliable, and cost-effective and prevent future costs for long time care.
• UK Screening Process:
  
  • Conducted at 5-8 days old with a heel prick blood sample (taking a small capillary blood sample from a heel prick)
  • Blood spots placed on a paper card, sent to the regional laboratory for assay.
• Conditions Screened in the UK:
  
  • Neonatal hypothyroidism
  • Phenylketonuria (PKU)
  • Other conditions (in some regions):
    
    • Cystic fibrosis
    • Sickle cell disease or thalassaemia
    • Medium-chain acyl coenzyme A dehydrogenase deficiency (MCADD)
    • Glucose-6-phosphatase deficiency
    • Galactosaemia
    • Congenital adrenal hyperplasia
• Future Screening Techniques:
  
  • Increased use of DNA technology and tandem mass spectroscopy in antenatal screening.

Question 5

Prenatal screening

Answer 5

• Prenatal Screening (High-Risk Groups only):
  
  • Performed for some disorders to plan dselivery methods or offer termination if early diagnosis is acceptable.
  • Demonstrates the metabolic defect in cultured fetal fibroblasts.
    
    • Obtained by:
      
      • Amniocentesis (early second trimester).
      • Chorionic villus sampling (first trimester).
• Indications for Screening:
  
  • Women with a previously affected infant.
  • Ethnic groups with a high incidence of the carrier state.
    
    • Example: Tay-Sachs disease in Ashkenazi Jews.
• Pre-Conception Screening:
  
  • Performed in high-risk populations for genetic advice and prenatal diagnosis to be offered to couples who are carriers if they become pregnant.
• Selective Screening (Examples):
  
  • Cystic fibrosis gene defect known in the parent of an affected infant.
  • Then Selective screening of subsequent pregnancies carried out using molecular biological techniques.
• Prenatal Screening for Congenital Disorders may also be peformed: Examples: Neural tube defects, chromosomal abnormalities.

Question 6

When to suspect an inborn error of metabolism

Answer 6

• Consider inherited metabolic defect if:
  
  • Unusual, unexplained clinical features(examples in next flashcard)
  • Abnormal laboratory findings in infancy/early childhood especially if More than one infant in the family affected or there is Consanguineous marriage present
• Screening tests:
  
  • Interpret with caution
  • Confirm suspected diagnosis with specific techniques in specialized labs
• Inborn errors presenting acutely (usually due to enzyme abnormality):
  
  • Indirect demonstration: detecting high concentration of substance metabolized by the enzyme or low concentration of the product
  • Direct demonstration: detecting low enzyme activity in appropriate tissue or blood cells (these assays available only in special centers)
• Examples of indirect screening methods:
  
  • Estimation of plasma ammonia concentration for urea cycle disorders or organic acidurias, in which it accumulates.
  • Chromatography of plasma & urine for amino acid to detect amino acid metabolism disorders
  • Detection of organic acids in urine for branched-chain amino acid metabolism disorders and organic acidurias
• If strongly suggestive clinical signs and symptoms:
  
  • Use specific measurements (e.g., urinary glycosaminoglycan excretion, white cell enzymes for MPSs, mucopolysaccharidoses)
• Tandem mass spectroscopy:
  
  • Useful in investigating various Inborn Errors of Metabolism (IEM)
• Genetic tests:
  
  • Increasingly used and likely to become more frequent

Question 7

Some clinical features suggestive
of an inborn error of metabolism

Answer 7

Some clinical findings suggestive of an inborn error of metabolism:

Early
* Hypoglycaemia
* Metabolic acidosis
* Failure to thrive
* Vomiting
* Fits or spasticity
* Hepatosplenomegaly
* Prolonged jaundice
* A peculiar smell, or staining, of the nappies
* Death of child in family and positive family history
* Cataracts or retinitis pigmentosa

Late
* Intellectual disabilities
* Refractory rickets
* Renal calculi
* Neuropathy
* Short stature
* Dysmorphic features

Question 8

Possible laboratory investigation of a suspected inborn error of metabolism

Answer 8

Possible laboratory investigation of a suspected inborn error of metabolisma

• Full blood count
• Serum electrolytes, bicarbonate and blood gases for
  acid-base status
• Renal function tests, including plasma urea and
  creatinine
• Liver function tests
• Plasma ammonia
• Blood glucose
• Urine ketones
• Serum cholesterol and triglyceride
• Plasma lactate
• Plasma uric acid
• Thyroid function tests
• Porphyrins

Further specialist tests
* Plasma and sometimes urine amino acids
* Urine orotic acid
* Urine organic acids
* Plasma carnitine
* Metabolites in urine or plasma by tandem mass
spectroscopy
* Specific enzyme assays
* DNA analysis of leucocytes or fibroblasts
* Histological studies of affected tissue

• This is best carried out in conjunction with a specialist
  paediatric metabolic laboratory. Many patients
  present with at least one of the following: metabolic
  acidosis, hypoglycaemia or hyperammonaemia. The
  laboratory tests may include those listed above.

Question 9

PRINCIPLES OF TREATMENT OF INBORN ERRORS OF METABOLISM

Answer 9

• Treatment strategies for inborn errors:
  
  • Limiting dietary intake of precursors in the affected metabolic pathway (e.g., phenylalanine in PKU, lactose in galactosaemia)
  • Supplying the missing metabolic product (e.g., cortisol in congenital adrenal hyperplasia)
  • Removing or reducing the accumulated product (e.g., ammonia in urea cycle disorders)
• Experimental treatments treatments may be tried for some disorders with particularly poor prognoses :
  
  • Enzyme replacement by bone marrow transplantation. Results have sometimes been disappointing and come with complications
  • Insertion of the missing or defective gene for some disorders like adenosine deaminase deficiency
    
    • Many disorders still lack treatment unless they respond to the measures listed

Question 10

DISEASES DUE TO INBORN ERRORS
OF METABOLISM

Answer 10

Only a few of the known IEM are discussed in this chapter. Some
of these conditions are mentioned briefly in the relevant chapters in this book.

For the sake of convenience, nine general categories of IEM are arbitrarily defined:

1. Urea cycle defects
2. Disorders of amino acid metabolism, for example amino acidurias
3. Lysosomal storage defects
4. Disorders of carbohydrate metabolism, for example glycogen storage disorders, gluconeogenesis and carbohydrate intolerance defects
5. Lipid, fatty acid oxidation defects and organic
   acidurias
6. Mitochondrial disorders
7. Peroxisomal disorders
8. Abnormalities of drug metabolism
9. Miscellaneous causes

Question 11

1. Urea Cycle disorders

Answer 11

• Urea cycle defects:
  
  • Important cause of hyperammonaemia
  • May cause raised urinary orotic acid concentration (an intermediate metabolite of pyrimidine synthesis derived from carbamyl phosphate)
  • Urea cycle defects can present with:
    
    • Severe hyperammonaemia
    • Respiratory alkalosis
    • Low plasma urea concentration
• Carbamyl phosphate synthetase (CPS) deficiency:
  
  • A urea cycle disorder
  • Unlike other defects in this pathway, urinary orotic acid is not raised
• Ornithine transcarbamylase deficiency:
  
  • Probably the most common urea cycle defect
  • Sex-linked inheritance

Question 12

2, Disorders of amino acid metabolism

Answer 12

• Disorders of amino acid metabolism:
  
  • Characterized by raised plasma concentrations of one or more amino acids with overflow amino aciduria
• Main metabolic pathway for aromatic amino acids:
  
  • Outlined in Figure 27.3 (includes known enzyme defects)
• Tyrosine:
  
  • Normally produced from phenylalanine
  • Precursor of several important substances

Question 12

Phenylketonuria

Answer 12

• Phenylketonuria (PKU):
  
  • Autosomal recessive disorder
  • Caused by abnormality of the phenylalanine hydroxylase system
  • Incidence in the UK: approximately 1 in 10,000
  • Phenylalanine hydroxylase is the enzyme most commonly affected
  • About 3% of cases involve abnormal enzymes for synthesizing the cofactor tetrahydrobiopterin
• Therefore several different inherited deficiencies may have very similar biochemical and clinical consequences.
  
  • Phenylalanine cannot be converted to tyrosine
  • Accumulates in plasma and is excreted in urine with metabolites (e.g., phenylpyruvic acid (a phenylketone) )
• The Clinical features of this:
  
  • Intellectual disabilities developing between 4 and 6 months, with psychomotor irritability
  • Reduced melanin formation because of reduced production of tyrosine (many patients are pale-skinned, fair-haired, and blue-eyed)
  • Irritability, feeding problems, vomiting, and fits during the first few weeks of life
  • Often generalized eczema
• Diagnosis:
  
  • Measuring phenylalanine concentration in blood from a heel prick
  • Microbiological Guthrie test was previously used to assay phenylalanine
  • Now chromatography methods or tandem mass spectroscopy
  • In newborns and especially preterm infants,the
    enzyme system may not be fully developed so false-positive results likely if tested too early
  • Positive results should be repeated later to allow for enzyme development
  • Phenylalanine concentrations may be greater than 240 µmol/L
• Heterozygotes:
  
  • Clinically normal but can be detected by biochemical tests
  • A variant - persistent hyperphenylalaninaemia - without intellectual disabilities has been described.
  • Infants exposed in utero to high phenylalanine concentrations of undiagnosed or poorly controlled PKU mothers may have intellectual disabilities , although
    they themselves do not have detectable PKU (maternal
    phenylketonuric syndrome).
• Treatment:
  
  • Lower plasma phenylalanine concentrations with a low-phenylalanine diet. Monitoring essential, especially if planning to conceive or pregnant
  • Dietary restriction recommended to be lifelong with supervision by a metabolic physician and expert dietitian
  • Remember aspartame (artificial sweetener) is metabolized to phenylalanine

Question 13

Tyrosimaemia

Answer 13

• Tyrosinaemia:
  
  • Presents with:
    
    • Renal tubular dysfunction
    • Hypoglycaemia
    • Severe liver disease with very raised plasma alkaline phosphatase concentration
• Defect:
  
  • Due to abnormal fumarylacetoacetase
  • Leads to raised levels of:
    
    • Tyrosine
    • Succinylacetone
    • Hydroxyphenylpyruvate
• Diagnosis:
  
  • Showing raised urinary succinylacetone concentration
  • Showing assay of fumarylacetoacetase in cultured leucocytes or fibroblasts
• Treatment options:
  
  • Dietary management
  • Liver transplantation
  • Nitro-trifluoromethylbenzoyl cyclohexanedione (thought to reduce accumulation of toxic metabolites)

Question 14

Alkaptonuria

Answer 14

• Alkaptonuria:
  
  • Autosomal recessive disorder
  • Associated with deficiency of homogentisic acid oxidase
  • Homogentisic acid accumulates in tissues and blood, and is passed in urine
• Oxidation and polymerization of homogentisic acid:
  
  • Produces pigment alkapton (similar to polymerization of dihydroxyphenylalanine producing melanin)
  • Deposition of alkapton in cartilages which causes darkening is called ochronosis
  • Results in visible darkening of the cartilages of the ears and often arthritis in later life
• Conversion of homogentisic acid to alkapton:
  
  • Accelerated in alkaline conditions
  • Most obvious abnormality in alkaptonuria is darkening of urine as it becomes more alkaline on standing
• Homogentisic acid reducing substance that reacts with Clinitest tablets

Question 15

Albinism

Answer 15

• Albinism:
  
  • A deficiency of tyrosinase in melanocytes causes one form of albinism
  • Inherited as an autosomal recessive disorder.
• Characteristics:
  
  • Reduced pigmentation of skin, hair, and iris.
  • Eyes may appear pink.
  • Photosensitivity due to reduced pigmentation of the iris.
  • Increased incidence of certain skin cancers due to decreased skin pigmentation.
• The tyrosinase involved in catecholamine synthesis is a different isoenzyme controlled by a different gene. Consequently adrenaline (epinephrine) metabolism remains normal.

Question 16

Homocystinuria

Answer 16

• Homocystinuria:
  
  • Autosomal recessive disorder
  • Due to deficiency of cystathionine synthase
  • Pathways involve sulphur-containing amino acids
• Clinical features:
  
  • Progressive central nervous system (CNS) dysfunction
  • Thrombotic disease
  • Eye disease, including cataracts
  • Cardiovascular problems
• Diagnosis:
  
  • Based on presence of raised urinary and plasma homocysteine with low plasma methionine concentrations
  • The defective enzyme can be assayed in cultured skin fibroblasts.

Question 17

Maple Syrup Urine Disease

Answer 17

• Maple Syrup Urine Disease:
  
  • Inherited as an autosomal recessive condition
  • Deficient decarboxylation of oxoacids resulting from deamination of three branched-chain amino acids: leucine, isoleucine, and valine
  • Amino acids accumulate in plasma and are excreted in urine with their corresponding oxoacids
  • Urine has a sweet smell similar to maple syrup, hence the name
• Presentation:
  
  • Disease presents during the first week of life
  • If not treated, severe neurological lesions develop, causing death within weeks or months
• Treatment:
  
  • Diet low in branched-chain amino acids can lead to normal development
• Diagnosis:
  
  • Made by demonstrating raised concentrations of branched-chain amino acids in plasma and urine
  • Low plasma alanine concentration
  • Confirmed by demonstrating the enzyme defect in leucocytes