1219899822 Lesson 7.1 Inborn Errors of Metabolism
Transcript of 1219899822 Lesson 7.1 Inborn Errors of Metabolism
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Lesson 7.1 : Metabolic Diseases
Inborn Errors Of Metabolism (IEM)
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A primer on metabolic disease in the neonate...
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What is a metabolic disease?
Inborn errors of metabolism
inborn error : an inherited (i.e. genetic) disorder
metabolism : chemical or physical changes undergone by
substances in a biological system
any disease originating in our chemical individuality
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What is a metabolic disease?
Garrods hypothesis
product deficiency
substrate excess
toxic metabolite
A
D
B C
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What is a metabolic disease?
Small molecule disease
Carbohydrate
Protein
Lipid
Nucleic Acids
Organelle disease
Lysosomes
Mitochondria
Peroxisomes
Cytoplasm
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How do metabolic diseases present in the
neonate ?? Acute life threatening illness
encephalopathy - lethargy, irritability, coma
vomiting
respiratory distress
Seizures, Hypertonia
Hepatomegaly (enlarged liver)
Hepatic dysfunction / jaundice
Odour, Dysmorphism, FTT (failure to thrive), Hiccoughs
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How do you recognize a metabolic disorder ??
Index of suspicion
eg with any full-term infant who has no antecedent maternal fever or PROM
(premature rupture of the membranes) and who is sick enough to warrant a blood
culture or LP, one should proceed with a few simple lab tests.
Simple laboratory tests
Glucose, Electrolytes, Gas, Ketones, BUN (blood urea nitrogen), Creatinine
Lactate, Ammonia, Bilirubin, LFT
Amino acids, Organic acids, Reducing subst.
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Index of suspicion
Family History
Most IEMs are recessive - a negative family history is not
reassuring!
CONSANGUINITY, ethnicity, inbreeding
neonatal deaths, fetal losses
maternal family history
males - X-linked disorders
all - mitochondrial DNA is maternally inherited
A positive family history may be helpful!
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Index of suspicion
History
CAN YOU EXPLAIN THE SYMPTOMS?
Timing of onset of symptoms
after feeds were started?
Response to therapies
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Index of suspicion
Physical examination
General dysmorphisms (abnormality in shape or size), ODOUR
H&N - cataracts, retinitis pigmentosa
CNS - tone, seizures, tense fontanelle
Resp - Kussmauls, tachypnea
CVS - myocardial dysfunction
Abdo - HEPATOMEGALY
Skin - jaundice
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A parting thought ...
Metabolic diseases are individually rare, but as a group are not
uncommon.
There presentations in the neonate are often non-specific at the
outset.
Many are treatable.
The most difficult step in diagnosis is considering the possibility!
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INBORN ERRORS OF METABOLISM
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Inborn Errors of Metabolism
An inherited enzyme deficiency leading to the disruption of normal
bodily metabolism
Accumulation of a toxic substrate (compound acted upon by an
enzyme in a chemical reaction)
Impaired formation of a product normally produced by the
deficient enzyme
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Type 1: Silent Disorders
Do not manifest life-threatening crises
Untreated could lead to brain damage and developmental
disabilities
Example: PKU (Phenylketonuria)
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PKU
Error of amino acids metabolism
No acute clinical symptoms
Untreated leads to mental retardation
Associated complications: behavior disorders, cataracts, skin
disorders, and movement disorders
First newborn screening test was developed in 1959
Treatment: phenylalaine restricted diet (specialized formulas
available)
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Type 2: Acute Metabolic Crisis
Life threatening in infancy
Children are protected in utero by maternal circulation which
provide missing product or remove toxic substance
Example OTC (Urea Cycle Disorders)
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Type 3: Progressive Neurological Deterioration
Examples: Tay Sachs disease
Gaucher disease
Metachromatic leukodystrophy
DNA analysis show: mutations
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Mutations
Nonfunctioning enzyme results:
Early Childhood - progressive loss of motor and cognitive skills
Pre-School non responsive state
Adolescence - death
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Other Mutations
Partial Dysfunctioning Enzymes
-Life Threatening Metabolic Crisis
-ADH
-LD
-MR
Mutations are detected by Newborn Screening and Diagnostic
Testing
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Treatment
Dietary Restriction
Supplement deficient product
Stimulate alternate pathway
Supply vitamin co-factor
Organ transplantation
Enzyme replacement therapy
Gene Therapy
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Children in School
Life long treatment
At risk for ADHD
LD
MR
Awareness of diet restrictions
Accommodations
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Inborn errors of metabolism
Definition:
Inborn errors of metabolism occur from a group of rare genetic disorders in which the body
cannot metabolize food components normally. These disorders are usually caused by defects in the enzy
mes involved in the biochemical pathways that break down food components.
Alternative Names:
Galactosemia - nutritional considerations; Fructose intolerance - nutritional considerations;
Maple sugar urine disease (MSUD) - nutritional considerations; Phenylketonuria (PKU) - nutritional co
nsiderations; Branched chain ketoaciduria - nutritional considerations
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Background:
Inborn errors of metabolism (IEMs) individually are rare but collectively are
common. Presentation can occur at any time, even in adulthood.
Diagnosis does not require extensive knowledge of biochemical pathways or individual
metabolic diseases.
An understanding of the broad clinical manifestations of IEMs provides the basis for
knowing when to consider the diagnosis.
Most important in making the diagnosis is a high index of suspicion.
Successful emergency treatment depends on prompt institution of therapy aimed at
metabolic stabilization.
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A genetically determined
biochemicaldisorderin which aspecific enzymedefect produces a
metabolic block that may have
pathologic
consequences at birth (e.g.,
phenylketonuria) or in later life (e.g.,
diabetes mellitus); called also
enzymopathyand genetotrophic
disease.
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Metabolic disorders testable on Newborn Screen
Congenital Hypothyroidism
Phenylketonuria (PKU)
Galactosemia
Galactokinase deficiency
Maple syrup urine disease
Homocystinuria
Biotinidase deficiency
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Classification
Inborn Errors of Small molecule Metabolism
Example: Galactosemia
Lysosomal storage diseases
Example: Gaucher's Disease
Disorders of Energy Metabolism
Example Glycogen Storage Disease
Other more rare classes of metabolism error
Paroxysmal disorders
Transport disorders
Defects in purine and pyrimidine metabolism
Receptor Defects
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Categories of IEMs are as follows:
Disorders of protein metabolism (eg, amino acidopathies, organic acidopathies, and
urea cycle defects)
Disorders of carbohydrate metabolism (eg, carbohydrate intolerance disorders, glycogen
storage disorders, disorders of gluconeogenesis and glycogenolysis)
Lysosomal storage disorders
Fatty acid oxidation defects
Mitochondrial disorders
Peroxisomal disorders
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Pathophysiology:
Single gene defects result in abnormalities in the synthesis or catabolism of proteins,
carbohydrates, or fats.
Most are due to a defect in an enzyme or transport protein, which results in a block
in a metabolic pathway.
Effects are due to toxic accumulations of substrates before the block, intermediates
from alternative metabolic pathways, and/or defects in energy production and utilization caus
ed by a deficiency of products beyond the block.
Nearly every metabolic disease has several forms that vary in age of onset, clinical
severity and, often, mode of inheritance.
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Frequency:
In the US: The incidence, collectively, is estimated to be 1 in 5000 live births.
The frequencies for each individual IEM vary, but most are very rare. Of term infants who develop symptoms of sepsis without known risk factors, as many as 20% may have an I
EM.
Internationally: The overall incidence is similar to that of US. The frequency
for individual diseases varies based on racial and ethnic composition of the population.
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Mortality/Morbidity:
IEMs can affect any organ system and usually do affect multiple organsystems.
Manifestations vary from those of acute life-threatening disease to subacute
progressive degenerative disorder.
Progression may be unrelenting with rapid life-threatening deterioration
over hours, episodic with intermittent decompensations and asymptomatic intervals, o
r insidious with slow degeneration over decades.
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Purine metabolism
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Adenine phosphoribosyltransferase deficiency
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The normal function of adenine phosphoribosyltransferase
(APRT) is the removal of adenine derived as metabolic waste from the
polyamine pathway and the alternative route of adenine metabolism to
the extremely insoluble 2,8-dihydroxyadenine, which is operative when
APRT is inactive. The alternative pathway is catalysed by xanthine oxi
dase.
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Hypoxanthine-guanine phosphoribosyltransferase (HPRT,
EC 2.4.2. 8)
HGPRTcatalyses the transfer of the phosphoribosyl
moiety of PP-ribose-P to the 9 position of the purine ring of
the bases hypoxanthine and guanine to form inosine monop
hospate (IMP) and guanosine monophosphate (GMP) respe
ctively.
HGPRT is a cytoplasmic enzyme present in virtually
all tissues, with highest activity in brain and testes.
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The importance of HPRT in the normal interplay between
synthesis and salvage is demonstrated by the biochemical and clinical co
nsequences associated with HPRT deficiency.
Gross uric acid overproduction results from the inability to
recycle either hypoxanthine or guanine, which interrupts the inosinate c
ycle producing a lack of feedback control of synthesis, accompanied by r
apid catabolism of these bases to uric acid. PP-ribose-P not utilized in th
e salvage reaction of the inosinate cycle is considered to provide an additi
onal stimulus to de novo synthesis and uric acid overproduction.
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The defect is readily detectable in erythrocyte
hemolysates and in culture fibroblasts.
HGPRT is determined by a gene on the long arm of the
x-chromosome at Xq26.
The disease is transmitted as an X-linked recessive trait.
Lesch-Nyhan syndrome
Allopurinal has been effective reducing concentrations
of uric acid.
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PP-ribose-P acts as an allosteric regulator of the first specific
reaction of de novo purine biosynthesis, in which the interaction of glu
tamine and PP-ribose-P is catalysed by amidophosphoribosyl transfer
ase, producing a slow activation of the amidotransferase by changing it
from a large, inactive dimer to an active monomer.
Purine nucleotides cause a rapid reversal of this process,
producing the inactive form.
Variant forms of PRPS have been described, insensitive to
normal regulatory functions, or with a raised specific activity. This res
ults in continuous PP-ribose-P synthesis which stimulates de novo puri
ne production, resulting in accelerated uric acid formation and overex
cretion.
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The role of PP-ribose-P in the de novo synthesis of IMP and adenosine
(AXP) and guanosine (GXP) nucleotides, and the feedback control normally
exerted by these nucleotides on de novo purine synthesis.
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Purine nucleoside phosphorylase (PNP, EC 2.4.2.1)
PNP catalyses the degradation of the nucleosides inosine,
guanosine or their deoxyanalogues to the corresponding base.
The mechanism appears to be the accumulation of purine nucleotides
which are toxic to T and B cells.
Although this is essentially a reversible reaction, base
formation is favoured because intracellular phosphate levels normally
exceed those of either ribose-, or deoxyribose-1-phosphate.
The enzyme is a vital link in the 'inosinate cycle' of the purine
salvage pathway and has a wide tissue distribution.
Purine nucleotide phosphorylase deficiency
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The necessity of purine nucleoside phosphorylase (PNP) for the normal catabolism and
salvage of both nucleosides and deoxynucleosides, resulting in the accumulation of dGTP,
exclusively, in the absence of the enzyme, since kinases do not exist for the other nucleosi
des in man. The lack of functional HGPRT activity, through absence of substrate, in PNP
deficiency is also apparent.
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The importance of adenosine deaminase (ADA) for the catabolism of dA, but not A, and
the resultant accumulation of dATP when ADA is defective. A is normally salvaged by ad
enosine kinase (see Km
values of A for ADA and the kinase, AK) and deficiency of ADA i
s not significant in this situation
Adenine deaminase deficiency
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The role of AMPDA in the deamination of AMP to IMP, and the recorversion of the
latter to AMP via AMPS, thus completing the purine nucleotide cycle which is of par
ticular importance in muscle.
Myoadenylate deaminase (AMPDA) deficiency
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Purine and pyrimidine degradation
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PRPP synthesis
bophosphate pyrophosphokinase 3=phosphori
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Salvage pathway of purine
Purine ribonuc
purine PPi
PRPP Adenylate
(AMP)
Mg 2+
APRTase
d by adenine phosphoribosyl transferase (A
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IMP and GMP interconversion
nthine + PRPP Inosinat
( IMP
Mg 2+
HGPRTase
+ PRPP Guanylate
(GMP)
Mg 2+
HGPRTase
Tase = Hypoxanthine-guanine phosphoribosyl transf
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i d
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purine reused
1=adeninephosphoribosyltransferase
2=HGPRTase
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Formation of uric acid from hypoxanthine and xanthine catalysed
by xanthine dehydrogenase (XDH).
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Intracellular uric acid crystal under polarised light (left) and under non-polarised light
(right)
With time, elevated levels of uric acid in the blood may lead to deposits around
joints. Eventually, the uric acid may form needle-like crystals in joints, leading to a
cute gout attacks. Uric acid may also collect under the skin as tophi or in the urinar
y tract as kidney stones.
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Additional Gout Foot Sites: Inflamation In Joints Of Big Toe, Small Toe And Ankle
Gout-Early Stage: No Joint Damage
Gout-Late Stage: Arthritic Joint
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Disorders of pyrimidine metabolism
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The UMP synthase (UMPS) complex, a bifunctional protein comprising the enzymes
orotic acid phosphoribosyltransferase (OPRT) and orotidine-5'-monophosphate decarboxyl
ase (ODC), which catalyse the last two steps of the de novo pyrimidine synthesis, resulting
in the formation of UMP. Overexcretion formation can occur by the alternative pathway in
dicated during therapy with ODC inhibitors.
Hereditary orotic aciduria
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Dihydropyrimidine dehydrogenase (DHPD) is responsible for the catabolism of the end-products of
pyrimidine metabolism (uracil and thymine) to dihydrouracil and dihydrothymine. A deficiency of DH
PD leads to accumulation of uracil and thymine. Dihydropyrimidine amidohydrolase (DHPA) catalyses
the next step in the further catabolism of dihydrouracil and dihydrothymine to amino acids. A deficienc
y of DHPA results in the accumulation of small amounts of uracil and thymine together with larger am
ounts of the dihydroderivatives.
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The role of uridine monophosphate hydrolases (UMPH) 1 and 2 in
the catabolism of UMP, CMP, and dCMP (UMPH 1), and dUMP an
d dTMP (UMPH 2).
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CDP-choline phosphotransferase catalyses the last step in the synthesis
of phosphatidyl choline. A deficiency of this enzyme is proposed as the
metabolic basis for the selective accumulation of CDO-choline in the er
ythrocytes of rare patients with an unusual form of haemolytic anaemi
a.
CDP-choline phosphotransferase deficiency
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WHAT IS TYROSINEMIA?
Hereditary tyrosinemia is a genetic inborn error of metabolism
associated with severe liver disease in infancy. The disease is inherited in an
autosomal recessive fashion which means that in order to have the disease,
a child must inherit two defective genes, one from each parent. In families
where both parents are carriers of the gene for the disease, there is a one in
four risk that a child will have tyrosinemia.
About one person in 100 000 is affected with tyrosinemia globally.
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HOW IS TYROSINEMIA CAUSED?
Tyrosine is an amino acid which is found in most animaland plant proteins. The metabolism of tyrosine in humans takes p
lace primarily in the liver.
Tyrosinemia is caused by an absence of the enzyme
fumarylacetoacetate hydrolase (FAH) which is essential in the me
tabolism of tyrosine. The absence of FAH leads to an accumulati
on of toxic metabolic products in various body tissues, which in t
urn results in progressive damage to the liver and kidneys.
WHAT ARE THE SYMPTOMS OF TYROSINEMIA?
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WHAT ARE THE SYMPTOMS OF TYROSINEMIA?
The clinical features of the disease ten to fall into two categories, acute and chronic.
In the so-called acute form of the disease, abnormalities appear in the first month of life.
Babies may show poor weight gain, an enlarged liver and spleen, a distended abdomen, s
welling of the legs, and an increased tendency to bleeding, particularly nose bleeds. Jaundi
ce may or may not be prominent. Despite vigorous therapy, death from hepatic failure freq
uently occurs between three and nine months of age unless a liver transplantation is perfor
med.
Some children have a more chronic form of tyrosinemia with a gradual onset and less
severe clinical features. In these children, enlargement of the liver and spleen are prominent, the abdomen is distended with fluid, weight gain may be poor, and vomiting and diarrh
oea occur frequently. Affected patients usually develop cirrhosis and its complications. Th
ese children also require liver transplantation.
Methionine synthesis
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Methionine synthesis
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Homocystinuria
H ti i
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Homocystinuria
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Figure 1: the structures of tyrosine, phenylalanine and homogentisic acid
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Phenylketonuria
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Maple syrup urine disease
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Maple syrup urine disease
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Albinism
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This excess can be caused by an increase in production by the body, by under-eliminationof uric acid by the kidneys or by increased intake of foods containing purines which are m
etabolized to uric acid in the body. Certain meats, seafood, dried peas and beans are partic
ularly high in purines. Alcoholic beverages may also significantly increase uric acid levels
and precipitate gout attacks.
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Hereditary hemolytic anemia
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Blood film: PK deficiency:
Characteristic "prickle cells" may be seen.
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Drug induced hemolytic anemia
Glycogen storage disease
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Glycogen storage disease
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Case Description
A female baby was delivered normally after an
uncomplicated pregnancy. At the time of the infants
second immunization, she became fussy and was
seen by a pediatrician, where examination revealed
an enlarged liver. The baby was referred to a
gastroenterologist and later diagnosed to have
Glycogen Storage Disease Type IIIB
GlycogenosesDisorder Affected Tissue Enzyme Inheritance Gene Chromosome
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Disorder Affected Tissue Enzyme Inheritance Gene Chromosome
Type 0 Liver Glycogen synthase AR GYS2[125] 12p12.2[121]
Type IA Liver, kidney, intestine Glucose-6-phosphatase AR G6PC[96] 17q21[13][94]
Type IB Liver Glucose-6-phosphate transporter (T1) AR G6PTI[57][104] 11q23[2][81][104][155]
Type IC Liver Phosphate transporter AR 11q23.3-24.2[49][135]
Type IIIA Liver, muschle, heart Glycogen debranching enzyme AR AGL 1p21[173]
Type IIIB Liver Glycogen debranching enzyme AR AGL 1p21[173]
Type IV Liver Glycogen phosphorylase AR PYGL[26] 14q21-22[118]
Type IX Liver, erythrocytes, leukocytes Liver isoform of -subunit of liver and muscle
phosphorylase kinase
X-Linked PHKA2 Xp22.1-p22.2[40][68][162][165]
Liver, muscle, erythrocytes,
leukocytes
-subunit of liver and muscle PK AR PHKB 16q12-q13[54]
Liver Testis/liver isoform of -subunit of PK AR PHKG2 16p11.2-p12.1[28][101]
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Glycogen
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Type 0Glycogen Storage Diseases
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Type I
Type II
Type IV
Type VII
Glycogen Storage Disease
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Glycogen Storage Disease
Type IIIb
Deficiency of debranching enzyme in the liver needed to
completely break down glycogen to glucose
Hepatomegaly and hepatic symptoms
Usually subside with age
Hypoglycemia, hyperlipidemia, and elevated liver transaminases
occur in children
GSD Type III
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Type III
D b hi E
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Debranching Enzyme
Amylo-1,6-glucosidase
Isoenzymes in liver, muscle and heart
Transferase function
Hydrolytic function
G ti H th i
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Genetic Hypothesis
The two forms of GSD Type III are caused by different mutations
in the same structural Glycogen Debranching Enzyme gene
A l 1 6 Gl id G
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Amylo-1,6-Glucosidase Gene
The gene consists of 35 exons spanning at least 85 kbp of DNA
The transcribed mRNA consists of a 4596 bp coding region and a
2371 bp non-coding region
Type IIIa and IIIb are identical except for sequences in non-
translated area
The tissue isoforms differ at the 5 end
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Wh M t ti O
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Where Mutation Occurs
The GDE gene is located on chromosome
1p21, and contains 35 exons translated into
a monomeric protein
Exon 3 mutations are specific to the type
IIIb, thus allowing for differentiation
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Inheritance
Inborn errors of metabolism
Autosomal recessive disorder
Incidence estimated to be between 1:50,000 and 1:100,000
births per year in all ethnic groups
Herling and colleagues studied incidence and frequency in
British Columbia
2.3 children per 100,000 births per year
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Inheritance
Single variant in North African Jews in Israel shows both liver and muscle
involvement (GSD IIIa)
Incidence of 1:5400 births per year
Carrier frequency is 1:35
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Inheritance
normal
carrier
GSD
Baby
Clinical Features
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Clinical Features
Hepatomegaly and fibrosis in childhood
Fasting hypoglycemia (40-50 mg/dl)
Hyperlipidemia
Growth retardation
Elevated serum transaminase levels(aspartate aminotransferase and alanine aminotransferase > 500 units/ml)
Commonpresentation
Clinical Features
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Splenomegaly
Liver cirrhosis
Clinical Features
Less Common
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Galactosemia is an inherited disorder that affects the way the body breaks down
certain sugars. Specifically, it affects the way the sugar called galactose is broken down.
Galactose can be found in food by itself. A larger sugar called lactose, sometimes called
milk sugar, is broken down by the body into galactose and glucose. The body uses gluc
ose for energy. Because of the lack of the enzyme (galactose-1-phosphate uridyl transfe
rase) which helps the body break down the galactose, it then builds up and becomes toxi
c. In reaction to this build up of galactose the body makes some abnormal chemicals. Th
e build up of galactose and the other chemicals can cause serious health problems like a
swollen and inflamed liver, kidney failure, stunted physical and mental growth, and cata
racts in the eyes. If the condition is not treated there is a 70% chance that the child coul
d die.
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Lysomal storage diseases
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The pathways are shown for the formation
and degradation of a variety of sphingolipids, with the hereditary metabolic diseases indicated.
Note that almost all defects in sphingolipidmetabolism result in mental retardation and
the majority lead to death. Most of the disea
ses result from an inability to break down sphingolipids (e.g., Tay-Sachs, Fabry's diseas
e).
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