Inborn errors of Metabolism Approach to Diagnosis and management

Defining inborn errors of metabolism Broad definition: Any defect related to the inappropriate metabolism of substrates essential for an organism’s survival Defects in energy metabolism Defects in protein synthesis or catabolism leading to intoxication with toxic substrates and/or absence of essential metabolic substrates Synthetic defects – inability to produce an essential structural substrate Defects in transport of essential components – cystic fibrosis, lysinuric protein deficiency Defects in detoxifying specific substances Generalized global tissue dysfunction (mitochondria)

Inborn errors of metabolism: General points Incidence in heterogeneous population 1:750-1000 However, incidence higher if we include hyperlipoproteinemias Heterozygote familial hypercholesterolemia has an incidence of 1:500 Much more common in inbred populations and consanguineous couples Signs and symptoms of metabolic disease are usually non-specific Can present at all ages Underdiagnosis of metabolic diseases but improving with new genetic techniques and improved mass screening

Mortality related to metabolic diseases 120 children aged 0-14 y died in a ten year period of inborn errors of metabolism This was 3.5% of total child mortality and 9.5% of non-acquired (genetic) mortality

Complexity of inborn errors of metabolism

Diagnosing metabolic disease

Approach to diagnosing metabolic disease Genetic testing for specific at risk groups Prevention Screening approach Testing entire population using relatively inexpensive (price/unit) techniques High throughput (180,000 samples/yr/IL) Clinical, biochemical, and molecular analysis of symptomatic patients

Populations at risk Specific population groups are at greater risk for a disease Founder effect Inbreeding Prevention Premarital testing Prenatal testing Preimplantation genetic diagnosis

Basis for newborn screening Cost-effective Easy/inexpensive method for detecting IEM High throughput methods Rapid turnaround High sensitivity (detect most cases) High specificity (low false positive rate) Effective notification system Possibility for intervention Minimal harm to patient

Newborn screening First introduced for PKU (Guthrie) Ames test Extended to several other diseases including galactosemia, hypothyroidism, sickle cell, thallasemia, congenital adrenal hyperplasia, and homocystinuria More countries and states including Israel are now using tandem mass spectrometry for more extensive evaluation

WHO criteria for screening

Early methods for screening for inborn errors of metabolism Ames test Paper impregnated with ferric chloride which reacts with phenylalanine Guthrie test Place blood drop on agar plate containing B. subtilis and an inhibitor B-2-thienylalanine High levels of phenylalanine in blood spot overcome inhibition and there is growth

Tandem mass spectrometry (MS/MS)

Approximately .3/1000 detection rate

Phenylketonuria Phenylalanine hydroxylase

Phenylketonuria In 1934, Folling discovered that children with mental retardation had high levels of phenylalanine in blood Classic: deficiency in phenylalanine hydroxylase Abnormalities in tone, myoclonic seizures, mental retardation, disorders of pigmentation Incidence: 1;10,000-15,000, Turkey: <1:3000 TX: Dietary – reduction in phenylalanine, provision of tyrosine

CONTROL PKU MSUD

Problems with newborn screening Not all diseases are screened False positive – without disease False negative – disease undetected Some disorders are not disorders Asymptomatic children Unclear treatment recommendations Difficulty in follow-up Privacy issues

Disorders not detected Disorders of carbohydrate metabolism Mitochondrial disorders Steroid disorders and hyperlipidemias Some urea cycle disorders Tissue specific disorders with no blood markers

CASE STUDY: Short chain acyl dehydrogenase (SCAD) deficiency Associated with CNS disease Question of whether this gene should be screened for US vs. Europe High prevalence of 319 C>T mutation among Ashkenazic Jews 1:15 carrier rate, 1:900 homozygotes Should be 2700 cases in Israel but only a handful of symptomatic individuals What type of intervention?

Tay-Sachs Hexoaminidase A deficiency Cerebral and retinal degeneration Cherry red spot (macular) Hypotonia, motor weakness, developmental delay Carrier rate 1:30 in Ashkenazic Jews Screening has led to 90% reduction in Jewish population

Signs of metabolic disease If the child: Looks bad Smells bad Feels bad Tastes bad Sounds bad Think Metabolic

Manifestations of metabolic diseases CNS Neuroanatomic malformations Leigh’s disease Developmental delay, mental retardations Movement disorders Hypotonia, spasticity Altered mental status Seizures Visual disturbances Pigmented retinopathy Deafness

Manifestations of metabolic diseases Cardiac Cardiomegaly (Pompe’s disease) Dilated cardiomyopathy Rhythm Disturbances Pulmonary Proteinosis (lysinuric protein intolerance) Insufficiency Frequent pneumonia Tachypnea, Kussmaul breathing (acidosis)

Manifestations of metabolic diseases GI Dysphagia Pseudo-obstruction, diarrhea Bloating Liver Insufficiency Inflammation (increased transaminases) Hyperbilirubinemia Cirrhosis Hepatomegaly (glycogen or fats)

Manifestations of metabolic diseases Renal Insufficiency Storage disease RTA Musculoskeletal Malformations Osteopenia/osteoporosis Weakness Myoglobinuria Contractures

Manifestations of metabolic diseases Skin Nearly all skin disorders including seborrhea, ichthyosis Hematological Anemia (all types) Thrombocytopenia/Leukopenia Pancytopenia

Manifestations of mitochondrial disease Cardiomyopathy / myopathy Stroke-like episodes, neuroanatomic changes Lactic acidosis, Leigh syndrome Seizures, myoclonues, dystonia, Parkinsonism Diabetes Deafness Chorea, ataxia, encephalomyelopathy Sudden infant death syndrome Ophalmoplegia, optic neuropathy, pigmented retinopathy Nelson, Textbook of Pediatrics, 19th edition, p. 508

Manifestations of metabolic diseases Biochemical Hypoglycemia Endocrine disturbances Metabolic acidosis Respiratory acidosis/alkalosis Lactic acidosis Hyperammonemia Hypoalbuminemia Altered coagulation profile Hepatitis profile

Evaluation of infants with suspected metabolic disorders Primary evaluation Arterial blood gas Glucose CBC and Differential Lactate/pyruvate Urinary reducing substances Serum ammonia Liver function tests PT/PTT NOTE: Very important at this step to rule out other non-metabolic related causes for clinical manifestations

Evaluation of infants with suspected metabolic disorders Secondary evaluation Urine organic acids Serum amino acids Total and free plasma carnitine Very long chain fatty acids Fed and fasting lactate and pyruvate Cardiac echo Opthalmologic exam EEG Head CT and/or MRI

Evaluation of infants with suspected metabolic disorders Tertiary evaluation Depends on results of initial evaluation Specific enzyme assays in lymphocytes, fibroblasts or tissue Specialized tests, e.g., CSF glycine or lactate, acylcarnitine profile, glycoprotein electrophoresis Histology, e.g., muscle biopsy, histological staining Mitochondrial studies Activity O2 Uptake DNA analysis Challenge tests – fasting, protein loading

Genetic analysis Mutational analysis Reverse genetics When specific defect is known Reverse genetics Arrays, chips, genomic scanning, SNPs, Cost effective Excellent coverage Does not always identify correct genetic defect or sole defect Advantages over the traditional approach

Treatment of metabolic diseases Enzyme replacement Detoxification Urea cycle defects Nutritional support Bypass defect NTBC in tyrosinemia Cofactor treatment Transplantation Genetic therapy Bone marrow Stem cell

Galactosemia Classic disease caused by galactose-1-phosphate uridyl transferase Symptoms after lactose formula or breast feeding Hepatomegaly and jaundice Gram- sepsis Cataracts if untreated Tx: non-lactose containing formula

Tyrosinemia Type 1 Defect in Fumarylacetoacetate hydrolase Hepatic decompensation, coagulopathy, low albumin, cirrhosis, RTA DX: elevated tyrosine, characteristic urine metabolites including succinylacetone Long term – hepatic carcinoma TX: Low tyrosine/phenylalanine diet, treatment with NTBC, a herbicide, which inhibits hydroxyphenyl-pyruvate hydroxylase