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Inborn Errors of Metabolism
Dr B.Vahabi
Lecture outcomes
• Understand the general pathophysiology underlying
the inborn errors of metabolism (IEMs)
• Review some important IEMs
• Understand the genetic inheritance of IEMs
• Review the general diagnostic methods used for
detection of IEMs
• Discuss the current treatment options for people
suffering from IEMs
Metabolism
• Metabolism is the sum of all the chemical
reactions in the body
• Some chemical reactions are involved in
breaking down molecules, others are
involved in building up (synthesis)
• A metabolic pathway consists of several
stages involved in the conversion of one
metabolite to another.
Metabolism
Food
Enzyme A
Amino acids
Carbohydrates
Lipids
Nucleic acids
Enzyme B
Protein
Carriers
Energy
Biomolecules
Errors in Metabolism
• If an error occurs in the gene that codes for the
enzyme a FAULT occurs.
• Subsequently the enzyme is not produced and the
pathway breaks down.
• These are called INBORN ERRORS OF
METABOLISM (IEMs).
• IEMs are uncommon but complicated medical
conditions involving abnormalities in complex
biochemical and metabolic pathways
Transporters
Metabolite D
Enzyme 1
Enzyme 2
Substrate
Metabolite A
Accumulation of
substances
present in small
amount
Deficiency of
critical
intermediary
products
Metabolite B
Deficiency of
specific final
products
• The concept of inborn errors of
metabolism (IEM) was first
introduced by Archibald Garrod in
1908.
Incidence
•More than 1000 human diseases are
known today that are caused by IEM
•Overall prevalence of 1 in 5000
•However the prevalence of each disease
has many variables
•Certain IEMs have a race related
prevalence
e.g Tay-Sachs in Ashkenazi Jews
Inheritance
•Majority IEMs are autosomal recessive
•Some IEMs are X-linked (Mothers are
carriers)
•Mitochondrial diseases have also been
detected
Categories of IEMs
•Amino acid metabolism disorders
•Carbohydrate metabolism disorders
•Lysosomal storage disorders
•Fatty acid oxidation disorders
•Urea cycle defects
•Peroxisomal disorders
•Mitochondrial disorders
Amino acid metabolism disorders
• A heterogeneous group of disorders
• Block at early step of metabolic pathway
resulting in accumulation of amino acids
•Block at later stages of metabolic pathway
resulting in accumulation of metabolites
•Defect in transport mechanism of amino acids
resulting in decreased intestinal transport and
increased urinary excretion
Amino acid metabolism disorders
Examples:
•Phenylketonuria- phenylalanine
• Homocysteinuria- methionine
•Maple syrup urine disease- Leucine, isoleuscine
and valine
•Tyrosinaemia- Tyrosine
Phenylketonuria (PKU)
• Most prevalent disorder caused by inborn errors
of amino acid metabolism
• Caused by mutations in phenyalanine hydroxylase
(PAH) gene
•PAH converts phenyalanine into tyrosine and
requires the cofactor tetrahydrobiopterin (BH4),
molecular oxygen and iron
•Loss of PAH activity  increased concentrations of
phenyalanine in blood an d toxic concentrations in
the brain
Molecular genetics and
classification
• The PAH gene consists of 13 exons
• PKU arises when both alleles are mutated
(548 separate mutations)
• Some mutations only partly inhibit the
enzyme activity mild PKU
• About 1-2% of cases of PKU are due to
mutations in genes coding for enzymes
involved in BH4 biosynthesis
Molecular genetics and
classification contd…
• PKU is classified by the severity of
hyperphenylalaninaemia:
– Blood Phenylalanine concentrations of:
• 50-110 µmol/Lnormal
• 600-1200µ Mol/L Mild
• >1200 µMol/L classic PKU
• Classification difficult in newborn babies
• Classification can also be made on the basis
of tolerance for dietary phenylalanin
Pathophysiology of PKU
• Phenylalanine’s entry into the brain is mediated
by the large neutral aminoacid carrier Laminoacid transporter (LAT1)
• Raised phenylalanine concentration can induce
damage in the brain by:
– Reducing formation of myeline in brain’s white matter
– Inhibition of LAT1 carriers and neutral amino acids
from entering the brain
– Reduced activity of pyruvate kinase
– Disturbed glutamatergic neurotransmission
– Reduced activity of the enzyme 3-hydroxy-3methylglutaryl coenzyme reductase.
Presentation of PKU
• Developmental Delay
• Behavioural abnormalities and motor
dysfunction
• Reduced IQ levels
• Autism
• Hypopigmentation (decreased melanin)
• Musty odour
• Detected by newborn screening (heel prick
test)
• Can be dietary controlled.
Carbohydrate metabolism
disorders
• A heterogeneous group of disorders
• Caused by inability to metabolize
specific sugars, aberrant glycogen
synthesis or disorders of
gluconeogenesis
• Manifest with hypoglycemia,
hepatosplenomegaly, lactic acidosis or
ketosis
Carbohydrate metabolism
disorders
Examples:
•
•
•
•
Glycogen storage diseases
Galactosemia
Fructose intolerance
Fructose 1,6-diphosphate deficiency
Glycogen storage diseases
(GSDs)
• Characterized by abnormal inherited
glycogen metabolism in the liver,
muscle and brain.
• Lead to build up of glycogen in tissues
• Categorised numerically (0-X)
(e.g. Type II, Type III etc.)
Pathways of liver glucose
production
Von Gierke disease (GSD
type I)
• Caused by defective liver glucose 6phosphatase activity
• Mutations can either be in:
– Gene coding for the liver glucose-6phosphatase
– Gene coding for endoplasmic reticulum
substrate
– Product transport proteins of the
glucose-6-phosphatase system
Presentation of Von Gierke 1a
disease
• Initial symptoms are due to hypoglycaemia and
include:
– Tremor, irritability, hyperventilation, apnea, convulsions, paleness,
sweating, cerebral edema, coma and death
• Older infants may present with:
– Doll-like facial appearance, frequent lethargy, difficult arousal from
sleep, overwhelming hunger, protuberant abdomen, relatively thin
extremities.
• With ageing the patient presents:
– Poor growth, short stature, and rachitic changes
• Most striking laboratory findings:
– Hypoglycaemia, lactic acidosis, hyperlipidemia, hyperuricaemia,
mosaic pattern of the liver, pale staining of the tissue and swollen
hepatocytes
Presentation of Von Gierke 1b
disease
• In addition to clinical symptoms seen in GSD1a:
–
–
–
–
–
–
–
Recurrent infections
Neutropenia
Neutrophil dysfunction
Inflammatory bowl disease
Fever
Diarrhea
Perioral and anal ulcers
Lysosomal storage disorders
• Genetic disorder inherited in an autosomal
recessive fashion
• Result from defective lysosomal acid
hydrolysis of endogenous macromolecules
accumulation of glycoproteins, glycolipids or
glycosaminoglycans within lysosomes in
various tissues
• Usually present later in infancy with
organomegaly, facial coarseness and
neurodegeneration
• Show progressively degenerative course
Lysosomal storage disorders
Examples
• Tay-Sachs
• Niemann-Pick disease
• Gaucher’s disease
Tay-Sachs disease
• An autosomal recessive disorder with an overall
prevalence of 1:300
• More prevalent in Ashkenazi Jews and Ferench
Canadians
• Lack of lysosomal β-hexosaminidase A (Hex-A)
enzyme activity
• Mutations in the α-subunit of Hex-A are
responsible for Hex-A deficiency
• Hex-A breaks down a fatty acid substance called
GM2 ganglioside in nerve cells
• Accumulation of GM2-ganglioside has a toxic
effect on cells neuronal deterioration  mental
and motor retardation
Tay-Sachs disease contd..
• The severity of the disease is inversely
proportional to the amount of residual Hex-A
activity:
– No Hex-A activity classic/infantile form early
age onset of disease death early childhood
– Some residual Hex-A activity  childhood/
Juvenile/ adult forms late onset Less severe
• More than 100 mutations of the alpha-subunit
have been described
Presentation of Tay-Sachs
disease
• Infant appears normal at birth but within few weeks may become less
visually attentive, hypotonic and easily startled by sound, light or touch
• By 6-8 months developmental delay becomes obvious
• Fundiscopic examination of retina reveals a whitish surrounding lipid
deposition
• By 1 year  marked reduction in purposeful movement, child becomes
spastic and lethargic
• Vision deteriorates
• Frequent seizures
• By age 2 years the child is in a vegetative state and requires constant
care
• Feeding difficulty
• A light cherry red spot in the middle of the eye
• The brain increases in weight and size but shows
generalized atrophy and reduction in nerves
and white matter
• Deafness
• Usually death before age of 5.
Diagnosis and Management
There are 3 important steps in the diagnosis
and management of IEM:
1. Suspicion
2. Evaluation
3. Treatment
Suspicion
• An important key to diagnosing
IEM is thinking about the
possibility in the first place
• The symptoms are very common
and non-specific
• Screening allows for the
differential diagnosis
Usual clinical presentation of
IEMs
Neonates
Young Children
•
•
•
•
•
•
•Recurring vomiting
•Dysmorphic features
(characteristic facial
expression, slant of eyes)
•Developmental delay
(milestones)
•Seizures
•Mental retardation
Poor feeding
Vomiting
Apnoea (breathing disorder)
Irritability
Abnormal tone
Seizures
Developmental delay
Evaluation-1
Once the possibility of an IEM is suspected, how should it
be evaluated?
• History
 An important clue is a history of deterioration after
an initial period of good health
 Developmental delay
 Change in diet and unusual dietary preferences
Family history
 Most IEMs are autosomal recessive: any other
siblings with the same condition?
 Consanguineous marriages
increases the incidence
of recessive disease
Evaluation-2
• There are two different types of testing for
metabolic conditions: screening tests and diseasespecific diagnostic testing
• Initial screening tests
– Prenatal tests
Ability to detect IEMs prenatally has
increased
Biochemical methods
 Detection of metabolites in amniotic fluid
 Enzyme assays
DNA analysis
 Detection of genetic mutations
Prenatal tests:
Choice of sample can be dictated by which
disorder is to be tested for.
Amniocentesis
Best carried out at 15-16 weeks
Used for analysis of specific metabolites by gas
chromatography with mass spectroscopy, tandem
mass spectroscopy, etc
Used for detection genetic defects using DNA
technology
Intended for diagnosis of some amino acid
disorders, lysosomal storage disorders etc.
Prenatal tests:
Cultured amniotic fluid cells
Used for measurement of specific enzyme
activity using various enzyme assays
Used for the study of various metabolic
pathways
Major disadvantage is the delay in waiting for
sufficient number of cells to grow
Chorionic villus sampling (CVS)
Offers a greater advantage over
amniocentesis
Samples are taken at around 11-week gestation
Used for determination of enzyme activity using
various enzyme assays
Prenatal tests:
Foetal blood and Foetal tissue
Foetal blood is rarely used
Sample taken late in pregnancy
Used when there has been a failure in amniotic fluid
analysis
Liver biopsies are used when enzyme deficiencies are not
expressed in CVS
Very risky
Used for diagnosis of conditions where enzyme deficiency
is expressed in the liver
Testing of Pre-implantation embryos
Postnatal Tests
•The investigation of IEM should begin with simple urine and
blood analysis.
•Screening tests allow you to detect the presence of a
particular class of conditions and includes:
Serum electrolytes (looking for evidence of acidosis), glucose & ammonia
levels
Blood and urine amino acids for disorders of amino acid metabolism
Urine organic acids for disorders of organic acid metabolism,
Acylcarnitine profile for disorders of fatty acid
Blood lactate and pyruvate for disorders of carbohydrate metabolism and
mitochondrial disorders
Odours attributed to IEMs
Phenylketonuria (PKU)
Musty, mousy
Tyrosinemia
Musty, Cabbage like
Maple syrup urine disease
Sweet, Maple syrup
Isovaleric acidemia
Multiple carboxylase deficiency
Sweaty feet
Cat urine
Examples of screening tests
Tandem mass spectroscopy
• Used for measurement of amino acids and acylcarnitines
in blood
•Used for detection of disorders of amino-acid, organic
acid and fatty-acid metabolism.
•Potential of simultaneous multi-disease screening
•Blood taken from newborn babies are absorbed by filter
paper (can also be used in the Guthrie test).
•A punched sample from the dried blood spot is extracted
with solvent containing appropriate isotopes
•The extracted metabolites are identified and quantified
with electrospray ionisation
Disease specific diagnostic
tests
• Key to exclusion or inclusion of an
IEM and include:
– MRI (Magnetic resonance imaging)
can be used for detection of
demyelination/neuron loss in the
brain
– MRS (Magnetic resonance
spectroscopy) can be used for
detection of lactate levels in
individuals with mitochondrial
disorder
– Study of cells and tissues obtained
via biopsies to establish the nature
of accumulated material, organelle
alterations and specific markers
Brain in Tay-Sachs disease
•
Treatments/ Management of IEMs
Treatment depends on the clinical manifestation and type of
metabolites accumulated
•
The basic principal for treatment is reduction of the substrate
that accumulates due to deficient enzyme activity
•
This can be mediated by an increasing number of therapeutic
approaches:
1) Prevent Catabolism
2) Limit the intake of the offending substance
3) Increase excretion of toxic metabolites
4) Enzyme-replacement therapy
5) Increase the residual enzyme activity
6) Reduce substrate synthesis
7) Replacement of the end products
8) Transplantation and gene therapy
Treatments/ Management of IEMs
1) Prevent Catabolism

Controlling the administration of calories: used to prevent
endogenous protein breakdown and induction of anabolism
2) Limit the intake of offending substance

Restriction of certain dietary components.

E.g. restriction of intake of galactose and fructose to prevent
galactosaemia and fructose intolerance

E.g. Neonates with PKU should be given protein substitute that is
phenyalanine-free.
3) Increase the excretion of toxic metabolites

Rapid removal of toxic metabolites can be achieved by exchange
transfusion, peritoneal dialysis, haemodialysis, forced diuresis etc.

E.g. Haemodialysis is considered mandatory for hyperammonaemia
Treatments/ Management of IEMs
4) Enzyme replacement therapy

Replacement of the deficient enzyme

E.g.Human alpha glucosidase enzyme is used for treatment of
pompe’s disease
5) Increase the residual enzyme activity (if possible)

Usually accomplished by administration of pharmacological doses
of vitamin cofactor for the defective enzyme
6) Reduce substrate synthesis

Inhibiting the synthesis of a substrate that can not be converted
to the end products

E.g used for treating lysosomal storage disorders in order to
reduce the rate of glycosphingolipid breakdown.
Treatments/ Management of IEMs
7) Replacement of end products

Replacement of a product due to an enzyme defect

E.g. in patients with glycogen storage disease, hypoglycaemia is
prevented with frequent feeds during the day and nasogastric
feeding during night in infants and young children.
8) Transplantation and gene therapy

Bone marrow transplantation (BMT) has been used as effective
therapy for selected IEMs

Mainly Lysosomal storage diseases and peroxisomal disorders are
treated by BMT.

The main rationale is based on provision of correcting enzymes by
donor cells within and outside the blood compartment.

In most gene therapy procedures a "normal" gene is inserted into
the genome to replace an "abnormal," disease-causing gene
Diagnosis and treatment of PKU
Prenatal diagnosis

Prenatal diagnosis is less commonly performed for PKU due to
good prognosis on treatment

Few have been undertaken by DNA analysis
Postnatal diagnosis

Gutherie test using the ability of phenylalanine to facilitate
bacterial growth in a culture medium with an inhibitor.



The Guthrie assay is sensitive enough to detect serum phenylalanine
levels of 180-240 μmol/L (3-4 mg/dL). In healthy normal people,
phenylalanine levels are usually under 120 μmol/L.
Tandem mass spectroscopy

Have a sensitivity of 3umol/l for phenyalanine

Discrimination is further enhanced by simultaneous
measurement of tyrosine.
Defects in BH4 synthesis should also be checked
Treatment of PKU
• Treatment from birth with a low phenylalanine diet largely
prevents the deviant cognitive phenotype
• Present treatment relies on a diet low in phenylalanine
• Tyrosine supplementation in the diet
• Enzyme replacement therapy is being investigated
• Pharmacological doses of exogenous BH4
• Drug based therapeutics using sapropterin dihydrochloride which
is a synthetic cofactor for PAH.
• Gene therapy is being used in preclinical trials to deliver the PAH
gene into liver
Summary
• Individually rare but collectively
important
• Present a wide variety of metabolic
disorders
• Can be present at different stages of
development
• Can be fatal!!
Key references
• Blau, N. et al (2010) Phenyketonuria. The Lancet. 376:1417-1427
• Martins, A.M. (1999) Inborn errors of metabolism: a clinical overview.
Sao Paulo Med J/Rev Paul Med. 117:251-65
• Myerowitz, R (1997) Tay-Sachs Disease-Causing Mutations and Neutral
Polymorphisms in the Hex A Gene. Human Mutation. 9:195-208
• Bayraktar, Y. (2007) Glycogen storage diseases:New perspective. World
J Gastreonterol. 13:2541-2553
• Shin, Y.S. (2006) Glycogen Storage Disease: Clinical, Biochemical, and
Molecular Heterogeneity. Semin Pediatr Neurol. 13: 115-120
• Low, L.C.K. (1996) Inborn errors of metabolism: clinical approach and
management. HKMJ. 2:274-281
• Saudubray, J.M. et al (2002) Clinical approach to inherited metabolic
disorders in neonates: an overview. 7:3-15
• Besley, G.T.N in Walker, J.M & Rapley, R. (Eds 2001) Medical
biomedthods handbook. Humana press Inc. Totowa, N.J.
• Burchell, A (2003) Von Gierke disease. Encyclopedia of Genetics. 21202122