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BIOCHEMICAL GENETICSBIOCHEMICAL GENETICS
Greg Enns, MB, ChB, FAAPProfessor of Pediatrics
Director, Biochemical Genetics ProgramLucile Packard Children’s Hospital
October 22, 2015
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Learning GoalsLearning Goals
Develop abilities to recognize signs and symptoms associated with inborn errors of metabolism
Become aware of clues to an underlying inborn error of metabolism by using simple lab tests
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Lecture OutlineLecture Outline
Inborn Errors of Metabolism◦Signs and Symptoms◦Diagnostic Tests
Routine labs Biochemical profiling
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So what exactly is a Biochemical Geneticist anyway?
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Inborn Errors of MetabolismInborn Errors of Metabolism “Metabolic Disorders” Inherited disorders of intermediary metabolism
◦ Autosomal recessive◦ Autosomal dominant◦ X-linked◦ Maternal (mitochondrial) inheritance
Metabolic imbalance most commonly secondary to an enzyme deficiency Toxic metabolites Deficiency states Altered energy production
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Who, What, & How?Who, What, & How? Common presentations
◦ Neurologic decompensation◦ Multi-organ system involvement◦ Lysosomal storage disorders
Types of tests◦ Basic◦ Complex◦ Newborn screening
Test interpretations◦ Common mistakes◦ Case examples
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Who to TestWho to TestClinical FeaturesClinical Features
Neurologic symptoms◦ Altered Mental Status◦ Developmental regression
Mental retardation Psychiatric illness
◦ Autism? Failure to thrive SIDS Non-immune hydrops
Eye findings◦ Cataracts◦ Pigmentary retinopathy
Hepatosplenomegaly Cardiomyopathy Bone/joint disease
◦ Contractures◦ Dysostosis multiplex◦ Osteonecrosis
Unusual odor
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Signs and Symptoms of Inborn Errors of MetabolismSigns and Symptoms of Inborn Errors of Metabolism
Appear after an interval of good health (hours, days) in a usually previously healthy full-term infant
Unmasked by “stressors” that lead to catabolism (e.g. infection, fasting, dehydration, excessive protein, birth)
Non-specific (limited response to stress in neonates)
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Signs and Symptoms of Inborn Errors of MetabolismSigns and Symptoms of Inborn Errors of Metabolism
Failure to thrive, feeding difficultiesRespiratory distressHypotonia, seizures, lethargyHypothermia (especially neonates)
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Signs and Symptoms of Inborn Errors of MetabolismSigns and Symptoms of Inborn Errors of Metabolism
◦Ocular findings◦Hepatosplenomegaly◦Cardiomyopathy◦Renal tubular disease◦Abnormal odor◦Unusual skin, hair◦Dysostosis multiplex◦Dysmorphic features
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Signs and Symptoms of Inborn Errors of MetabolismSigns and Symptoms of Inborn Errors of Metabolism
Typically patients who have metabolism disorders display a constellation of features
Isolated failure to thrive, cardiomyopathy, etc. in the absence of other organ system involvement or other clinical features (e.g. developmental delay) would be unusual
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Signs and Symptoms of Inborn Errors of MetabolismSigns and Symptoms of Inborn Errors of Metabolism
MAY PRESENTAT ANY AGE
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Case #1Case #1
A 4-day-old boy is taken to the emergency room on Christmas eve because of poor feeding and lethargy. He is hypothermic and is tachypneic. He has elevated (3+) ketones in his urine and a metabolic acidosis (anion gap 25 with normal lactate). His ammonia is 735 M (normal 5-35).
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Case #1Case #1
This presentation is most suggestive of …?A) Maple syrup urine diseaseB) Urea cycle defectC) Fatty acid oxidation defectD) Glycogen storage diseaseE) Organic acidemia
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Your answer?Your answer?
E) Organic acidemia
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THE ANION GAPTHE ANION GAP
Na+ - [Cl- + HCO3-]
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THE ANION GAPTHE ANION GAP
Total serum cations:Na + UC
Total serum anions:Cl + HCO3 + UA
So…NA + UC = UA + Cl + HCO3
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THE ANION GAPTHE ANION GAP
UA – UC = Na+ - [Cl- + HCO3-]
= the anion gap
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Concentrations of Concentrations of ““unmeasuredunmeasured”” cations and anions( cations and anions(mEq/LmEq/L))
Unmeasured Cations
K 4.5Ca 5.0Mg 1.5Total 11
Unmeasured Anions
Protein 15
PO4 2
SO4 1
Organic acids 5
Total 23
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Metabolic Acidosis with Increased Metabolic Acidosis with Increased Anion GapAnion Gap
Increased Unmeasured Anion:Methanol, metforminUremiaDiabetic ketoacidosisParaldehyde, phenforminInborn errors of metabolismLactic acidemiaEthanol, ethylene glycolSalicylates, solvents, strychnine
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Metabolic Acidosis with Increased Anion GapMetabolic Acidosis with Increased Anion Gap
Decreased Unmeasured Cation*:HypokalemiaHypocalcemiaHypomagnesemia
*Possible in theory, but rarely encountered
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Metabolic Acidosis with Normal Anion GapMetabolic Acidosis with Normal Anion Gap
• Renal tubular acidosis• Intestinal loss of bicarbonate
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A B C DATA EAB EBC ECD
F G
Negative Feedback
A, B, C, D – substrate and products of major pathwayF, G – products of minor pathwayT – transporter ; E - enzymes
Prototypical Metabolic Pathway
Cell membrane
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The Laboratoryspecimen
results
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Organic AcidsOrganic Acids(GC-MS)(GC-MS)
Urine only Qualitative screen 100’s of compounds 3 hour prep 30 min run
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Methylmalonic AcidemiaTotal Ion Chromatogram
Ion
Abu
ndan
ce
Time (min)
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Case #1Case #1Initial methylmalonic acid (MMA) level 1,200 uM
(nl <0.3 uM)Treated with:◦High dextrose intravenous fluids + insulin◦ Intravenous “nitrogen scavenging” medications
Sodium benzoate + sodium phenylacetate◦ Intravenous carnitine◦ Intramuscular hydroxocobalamin
Dialysis is often needed to decrease MMA and ammonium levels, but avoided in this case
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TC IIOH-Cbl
TC IIOH-Cbl
OH-Cbl+3 Cbl+3
Me CblHomocysteine Methionine
Methionine Synthase
Cbl+2
L-Methylmalonyl CoA
SuccinylCoA
Ado CblCbl+1
TC II
OH-Cbl+3
MethylmalonylCoA Mutase
F
A,HB
C,D
E,G
The Cobalamin PathwayThe Cobalamin Pathway
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Methylmalonic AcidemiaMethylmalonic Acidemia
Common signs/symptoms:◦ lethargy◦ intermittent vomiting◦ dehydration◦ failure to thrive◦ respiratory distress◦ hypotonia
Matsui et al. NEJM 308:857, 1983
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BRAIN INJURY IN ORGANIC ACIDEMIAS
Pediatr Res 40:404-9, 1996
caudate andputamenhyperintensity
delayed myelination
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Case #2Case #2
You are called about a 55-year-old male who became confused and lethargic a few days after endoscopic sinus surgery to remove nasal polyps that were causing chronic sinusitis. He had been discharged home on steroids to prevent swelling.His wife took him to the emergency room after he became more confused and disoriented. He progressed to a coma and was placed on mechanical ventilation in the intensive care unit.
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Case #2Case #2
The ICU physician upon reviewing the initial laboratory studies noted a respiratory alkalosis and normal complete blood count, glucose, and liver enzymes. Cerebrospinal fluid studies and urinalysis are normal. Blood, urine, and cerebrospinal fluid cultures are obtained. A urine toxicology screen was also normal. Initial head CT scan and CXR were normal.
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Case #2Case #2
Your differential diagnosis includes which of the following?A) Steroid psychosisB) Occult infectionC) IntoxicationD) Inborn error of metabolismE) All of the above
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Your answer?Your answer?
E) All of the above
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Case #2Case #2 Although most metabolic studies take days to weeks
to obtain results, you recall that ammonia is a central respiratory stimulant and urea cycle disorders often are associated with tachypnea. Moreover, unlike many metabolic disorders, a metabolic acidosis is typically not present initially in urea cycle disorders.
You look at the lab results again and notice the blood urea nitrogen is low.
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Case #2Case #2
So…you check an ammonia (obtained on ice and sent to the lab quickly) and find an abnormal elevation.
The ammonia level is 281 M (normal 5-35).
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Case #2Case #2
The most likely diagnosis is:A) Urea cycle disorderB) Mitochondrial diseaseC) Propionic acidemiaD) Medium-chain acyl-CoA dehydrogenase deficiencyE) Proteus mirabilis urinary tract infection
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Your diagnosis?Your diagnosis?
A) Urea cycle disorder
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Quantitative Amino AcidsQuantitative Amino Acids(HPLC)(HPLC)
Plasma, urine, CSF 40 amino acids (sometimes more) 10 minute prep 3 hour run per sample
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Case #2Case #2
Metabolic labs:
glutaminecitrulline 3 uM (8-47)orotic acid <1 g/mg creatinine (<4.7)
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Urea Cycle DisordersUrea Cycle DisordersClinical Features◦Term birth◦~DOL 2 poor feeding, vomiting, lethargy progressing
to coma, apnea◦Seizures may occur with cerebral edema◦Often mistaken for sepsis◦Later onset with ataxia, altered mental status (post-
partum)
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Late-Onset Urea Cycle DisordersLate-Onset Urea Cycle DisordersBehavioral problems◦Agitation◦Delirium◦Aggression◦Confusion
AtaxiaHeadacheHemiparesis
Nassogne et al. JIMD 28:407-14, 2005
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Lysosomal Storage DisordersLysosomal Storage DisordersFamily of > 40 disorders; collective prevalence
~1:8,0001
Enzyme deficiency causes lysosomes to become engorged1
Each disease is a consequence of type of substrate and where it accumulates1
Progressive accumulation of substrate may result in irreversible damage2
1. Meikle P et al. JAMA. 1999;281:249-254.2. Wraith JE et al. J Pediatr. 2004;144:581-588.
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MPS ENZYME BLOCKSMPS ENZYME BLOCKS
iduronate sulfatase
-L-iduronidaseHURLER (MPS I)
COOH
COOH
S-O
O
NAc
O
COOH
O
NAc
O
S-O
O-S
HUNTER (MPS II)
O
S-O
NAc
O
COOH
O
S-O
NAc
O
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Normal Cell Abnormal Cell
Lysosomal Storage DisordersLysosomal Storage Disorders
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CNS InvolvementCNS Involvement
Metachromatic Leukodystrophy
8%
Sanfilippo A7%
Hunter Severe5%
Krabbe6%
Sandhoff2%
GM 1 Gangliosidosis
2%
Mucolipidosis type II / III2%
Niemann-Pick A2%
Niemann-Pick C4%
Tay-Sachs4%
Sanfilippo B4%
Gaucher type 2 & 31%
Niemann-Pick B2%
Maroteaux-Lamy3%
Cystinosis4%
Morquio5%
Pompe5%
Hurler/Scheie (MPS I)4%
Gaucher type I13% Scheie (MPS I)
1%
Hurler (MPS I)4%
Hunter Mild1%
Fabry7%
Significant or severe CNS involvement(~ 54%)
No or minimal CNS involvement(~ 46%)
Adapted from Meikle P et al. JAMA. 1999;281:249-254.
Other2%
-Mannosidosis
Sanfilippo D
1%
1%
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Mucopolysaccharidosis type IMucopolysaccharidosis type IPathology Inheritance-L-iduronidase enzyme deficiencyAccumulation of glycosaminoglycans (GAGs)
Autosomal Recessive
Onset PrevalenceSevere form: first 6 months after birthAttenuated form: 3 to 8 years of age
≈1:90,0001
ProgressionOften life threateningSevere cases life span < 10 yAttenuated cases life span ≈ normal
Disease-at-a-Glance
Disease-at-a-Glance
1. Meikle P et al. JAMA. 1999;281:249-254.
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MPS I MPS I
Umbilical/inguinal hernia
Cardiovascular disease
Obstructive airway diseaseCorneal clouding
Hepatosplenomegaly
Chronic rhinitis/otitis
Joint stiffness
Developmental delay
Hearing loss
Enlarged tongue
Look for unusual symptoms or clusters of more common symptoms
Carpal tunnel syndrome
Skeletal deformities
Signs &Symptoms
Signs &Symptoms
Macrocephaly
Neufeld EF, Muenzer J. In: Scriver C, Beaudet A, Sly W, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. New York, NY: McGraw-Hill; 2001:3421-3452.
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Disease Progression: Severe MPS IDisease Progression: Severe MPS I
10 months 12 months
22 months 34 months
39 months
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Disease Progression: Mild MPS IDisease Progression: Mild MPS I
3 years 4 years
6 years 8 years
11 years
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“Scheie” MPS I S
“Hurler” MPS I H
All patients typically have < 1% of normal enzyme levels,but only MPS l H involves the CNS
“Hurler-Scheie” MPS I HS
MPS IMPS I Clinical Heterogeneity
Clinical Heterogeneity
SevereAttenuated
Courtesy of Emil Kakkis, MD.
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Peripheral Nervous System InvolvementPeripheral Nervous System Involvement
• Carpal tunnel syndrome – nerve entrapments/
nerve compressions• Most patients lack
typical symptoms until severe compression occurs
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Contractures of the ArmContractures of the Arm
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Skeletal AbnormalitiesSkeletal Abnormalities
(Clarke LA, 1997) Photo reproduced by permission of Hodder/Arnold Publishers.
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Gibbus in MPS IGibbus in MPS I
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Dimethylmethylene Blue Binding Assay For Glycosaminoglycans
Blank 5 µg/ml 17.5 µg/ml 35 µg/ml 50 µg/ml
Urine + DMB reagent Blue product
Courtesy T. Cowan
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NormalMPS IHurler
MPS IScheie
MPS IIHunter
MPS IIISanfilippo
MPS IVMorquio
MPS VIMaroteaux-
Lamy
MPS VIISly
Dermatan,Heparan
Dermatan,Heparan
Dermatan,Heparan
Dermatan,Heparan
Heparan Keratan Dermatan
Thin-layer chromatography
Courtesy of T. Cowan
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SUMMARYSUMMARY
High index of suspicion – consider IEM in parallel with more common conditions
Presentations are non-specific Simple screening tests yield critical diagnostic clues
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InterventionalGeneticist
Anesthesiologist
Primary Care Physician
Neurologist
Otorhinolaryngologist
Genetic Counselor
Cardiologist
Gastroenterologist
Surgeon
Pulmonologist
Dentist
Ophthalmologist
Orthopedist
Hematologist
Nephrologist
Palliative Care Specialist
Metabolic DiseaseTreatment Team