Inborn Errors of Metabolism Flashcards

1
Q

Define inborn errors of metabolism

A

A single gene defect resulting in disruption to metabolic pathways (blocked pathways)

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2
Q

What causes the symptoms faced in IEM

A
  • Toxic accumulation of substrates
  • Toxic accumulation of intermediates from
    alternative metabolic pathways
  • Defects in energy production/use due to
    deficiency of products
  • Combination of above

Can vary in age of onset and clinical severity

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3
Q

Describe the discovery of the idea of IEM

A

Archibold E Garrod 1857 - 1936

  • Father of IEM
  • Croonian lectures to the
    Royal College of Physicians
    in June 1908
  • Published in the Lancet July
    1908
  • Reprinted as a book ‘Inborn errors of metabolism’
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4
Q

What disorders did Garrod study

A
  • Alkaptonuria
  • Cystinuria
  • Albinism
  • Pentosuria
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5
Q

What did Garrod propose about the disorders

A
  • Congenital (present at birth)
  • Inborn (transmitted through the gametes)
  • Followed Mendel’s laws of inheritance
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6
Q

What is Alkaptonuria

A
  • Urine turns black on standing (and
    alkalinisation)
  • Black ochrontic pigmentation of cartilage & collagenous tissue
  • Homogentisic acid oxidase deficiency
  • Autosomal recessive disease
  • Congenital
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7
Q

What is the one gene - one enzyme concept

A

Beadle and Tatum 1945 (Nobel prize 1958)

  • All biochemical processes in all organisms are under
    genetic control
  • Biochemical processes are resolvable into a series of
    stepwise reactions
  • Each biochemical reaction is under the ultimate control of
    a different single gene
  • Mutation of a single gene results in an alteration in the
    ability of the cell to carry out a single primary chemical
    reaction
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8
Q

What is the molecular disease concept

A

Pauling et al 1949, Ingram 1956

  • Direct evidence that human gene mutations produce an alteration in the primary structure of proteins
  • Inborn errors of metabolism are caused by mutations in genes which then produce abnormal proteins whose functional activities are altered
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9
Q

What are the mechanisms of inheritance for IEM

A
  • Autosomal recessive
  • Autosomal dominant
  • X-linked
  • Mitochondrial

An accurate family history required to establish
pattern of inheritance

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10
Q

Describe Autosomal Recessive inheritance of IEM

A
  • Both parents carry a mutation affecting the same gene
  • 1 in 4 risks each pregnancy
  • Consanguinity increases the risk of autosomal recessive
    conditions
  • Examples: PKU, alkaptonuria, MCADD
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11
Q

Describe the Autosomal Dominant Inheritance of IEM

A
  • Only one parent needs to have a dominant allele
  • 2 in 4 risks each pregnancy
  • Rare in IEMs
  • Examples: Marfan’s, acute intermittent porphyria
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12
Q

Describe X-Linked Inheritance of IEM

A

Recessive X linked conditions passed through the
maternal line:

  • condition appears in males
  • condition carried in females
  • Female carriers may manifest condition
    –Lyonisation (random inactivation of one of the X
    chromosomes)
  • Examples: Fabry’s disease, Ornithine carbamoyl
    transferase deficiency
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13
Q

Describe mitochondrial inheritance of IEM

A
  • Mitochondrial gene mutation
  • Inherited exclusively from mother

Affects both male and female offspring:

  • Eg. MERFF -Myoclonic epilepsy and ragged red fibre disease:
    deafness, dementia, seizures
  • Eg. MELAS – Mitochondrial encephalopathy with lactic
    acidosis and stroke-like episodes
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14
Q

What does heteroplasmy mean in mitochondrial inheritance

A

Cell contains varying amounts of normal mt DNA and also mutated mt DNA

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15
Q

What are the characteristics of mitochondrial inheritance

A
  • Distribution of affected mitochondria determines
    presentation
  • Mitochondrial disease can vary in symptoms, severity,
    age of onset - Varying penetrance
  • High energy-requiring organs are more frequently affected
  • Recent debate on three parent babies
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16
Q

What is the prevalence of IEM

A

Individually rare (e.g PKU 1:10,000)

Collectively common (1:800 to 1:2500):

  • High mortality within the first year of life
  • Significant contribution to children of school age with physical
    handicap and children with severe learning difficulties
17
Q

How are IEMs classified

A

Toxic accumulation:

  • Protein metabolism
  • Carbohydrate intolerance

Deficiency in energy production/utilisation:

  • Fatty acid oxidation
  • Carbohydrate utilisation/intolerance
  • Mitochondrial disorders

Disorders of complex molecules involving organelles:

  • Lysosomal storage disorders
  • Peroxisomal disorders
18
Q

How does IEM present in patients

A

Neonatal to adult-onset depending on the severity of
metabolic defect:

  • Neonatal presentation often acute
  • Often caused by defects in carbohydrate intolerance
    and energy metabolism

Late-onset due to accumulation of toxic molecules:

  • Patients have residual enzyme activity allowing slower
    accumulation of toxins
  • Symptoms appear at adulthood
  • Present with organ failure, encepalopathy, seizures
19
Q

What are the clinical scenarios faced in neonatal presentation

A
  • Poor feeding, lethargy, vomiting
  • Epileptic encephalopathy
  • Profound hypotonia – ’floppy’ baby
  • Organomegaly e.g. cardiomyopathy, hepatomegaly
  • Dysmorphic features
  • Sudden unexpected death in infancy (SUDI)
20
Q

What are the biochemical abnormalities faced in neonatal IEM

A
  • Hypoglycaemia
  • Hyperammonaemia
  • Unexplained metabolic acidosis / ketoacidosis
  • Lactic acidosis
21
Q

What could be the causes for neonatal IEM

A
  • Consanguinity
  • FH of similar illness in siblings or unexplained deaths
  • Infant who was well at birth but starts to deteriorate for
    no obvious reason
22
Q

What routine laboratory investigation can we carry out to test for IEM

A
  • Blood gas analysis
  • Blood glucose and lactate
  • Plasma ammonia
23
Q

What specialist investigation can we carry out to test for IEM

A
  • Plasma amino acids
  • Urinary organic acids + orotic acid
  • Blood acyl carnitines
  • Urinary glycosaminoglycans
  • Plasma very long chain fatty acids
  • CSF tests e.g. CSF lactate/pyruvate, neurotransmitters
24
Q

What confirmatory investigation can we carry out to test for IEM

A

Enzymology:

  • Red cell galactose-1-phosphate uridyl transferase for galactosaemia
  • Lysosomal enzyme screening for Fabry’s

Biopsy (muscle, liver)
Fibroblast studies
Mutation analysis – whole genome sequencing

25
Q

How can newborn screen help with treatment for IEM

A
  • Carry out bacterial inhibition assay
  • Early identification of life-threatening disease in pre-symptomatic babies
  • Earlier initiation of medical treatment
  • Reduction of morbidity and mortality
26
Q

What are the criteria for screening

A
  • Condition should be an important health problem
  • Must know incidence/prevalence in screening population
    The natural history of the condition should be understood
  • Availability of a screening test that is easy to perform and interpret
  • Availability of an accepted treatment for the condition
  • Diagnosis and treatment of the condition should be cost-effective
27
Q

What is the newborn blood spot screening

A

Blood samples taken from days 0 to 5 to test for various conditions in newborns

28
Q

What does the blood spot screening test for

A
  • PKU
  • Congenital hyperthyroidism
  • Sickle cell disease
  • Cystic fibrosis
  • Medium-chain acyl-CoA dehydrogenase deficiency (MCADD)
29
Q

What does the tandem mass spectrometry test screen for after being introduced in 2015

A
  • Maple syrup urine disease (MSUD)
  • Homocystinuria (pyridoxine unresponsive) (HCU)
  • Isovaleric acidaemia (IVA)
  • Glutaric aciduria type 1 (GA1)