inborn errors of metabolism pathways for mbbs students

for medical students

1. Inborn Errors of Metabolism Dr. Apeksha Niraula Assistant Professor Clinical Biochemistry Institute of Medicine Maharajgunj

2. What are IEMs? What are the different types of IEMs? What is the relative frequency of these IEMs? What is the inheritance pattern of these IEMs? How can these IEMs be diagnosed in a timely manner? What is the role of newborn screening (NS) programs in early diagnosis and prevention of morbidity and mortality? Objectives

3. Inborn Errors of Metabolism IEMs: group of heritable genetic disorders interfering with metabolic pathways in different ways, leading to inadequate functioning of a particular pathway This interference in the normal enzymatic or metabolic pathway has varying consequences, including deficiency of a particular end product or excessive accumulation of a substrate that may be toxic Either of these two scenarios leads to significant morbidity and mortality by hampering normal functioning of a particular metabolic pathway.

4. History IEMs have been known for the approximately the past 100 years Term being first used by Sir Archibald Garrod in 1902 The initial disorders described were Alkaptonuria, Benign pentosuria, Albinism, and Cystinuria at that time Followed by a description of one of the major IEMs, namely, Phenylketonuria (PKU), by Folling in 1934 Since that time, advances in medicine have uncovered more than 500 IEMs

5. Garrod’s Hypothesis product deficiency substrate excess toxic metabolites A B D C Enzyme defect

6. Categories of IEM 1. Disorders of protein metabolism 2. Disorder of carbohydrate metabolism 3. Lysosomal storage disorders 4. Disorder of Lipid metabolism 5. Mitochondrial disorders 6. Peroxisomal disorders 7. Trace metal disorders

7. Types

8. Frequency of occurrence

9. Inheritance Pattern It is important to detail a three- to four-generation pedigree to evaluate the mode of inheritance accurately Autosomal recessive (AR) inheritance is the most common mode [ Both parents of an affected person are carriers; not typically seen in every generation] X-linked recessive inheritance may also be seen [ Males are more frequently affected; affected males often present in each generation] Mitochondrial mode of inheritance is also seen [ Can affect both males and females, but only passed on by females because all mitochondria of all children come from the mother; can appear in every generation]

10. Clinical signs and symptoms IEM may present early in the newborn period, later on in early or late childhood or much later in adulthood A high index of suspicion needs to be maintained Symptomatology of these disorders is often nonspecific Lead to workup for other medical conditions

11. Clinical presentation attributable to IEMs may be subclassified into a few broad categories Early-onset disorders : presents in newborn further classified

12. 1. Silent Disorders Do not cause life threatening crisis in infancy Later presents in early childhood with mental retardation and developmental delay PKU, Congenital hypothyroidism

13. 2. Disorder presenting with acute metabolic encephalopathy Urea cycle disorders, Organic acidemias, Aminoaciduria May present with metabolic disturbances caused by accumulations of precursors or metabolites Symptoms: poor feeding, lethargy, persistent vomiting, seizures, hypotonia, apnea, respiratory distress, tachypnea and tachycardia Toxic effect of metabolites on CNS: causing a picture of metabolic encephalopathy Biochemical features are significant for metabolic acidosis, hyperammonemia or other metabolic abnormalities

14. 3. Disorders presenting metabolic acidosis Organic acidemias Neonates exhibit severe metabolic acidosis with an increased anion gap along with elevated organic acid: lactate Lactic acidosis is present in disorders of pyruvate metabolism: pyruvate dehydrogenase deficiency, defect in gluconeogenesis, pyruvate carboxylase deficiency, mitochondrial disorders

15. 4. Disorders presenting with hyperammonemia Defect in the urea cycle, organic acidemias, and Transient hyperammonemia of the newborn (THAN) Presents with metabolic encephalopathy and hyperammonemia

16. 5. Disorders presenting later on in childhood Lysosomal storage disorders, Tay -Sachs disease, Gaucher’s disease, Metachromatic leukodystrophy Generally presents with progressive neurologic deterioration

17. Classifications Pathophysiologically , IEMs can be divided into three groups: IEMs: causing intoxication because of defects in the intermediary metabolic pathway, resulting in the accumulation of toxic compounds proximal to the metabolic block IEMs: resulting in energy deficiency and include mitochondrial respiratory chain defects IEMs: resulting in defects in the synthesis or the catabolism of complex molecules in certain cellular organelles , such as Lysosomal storage disorders

18. Clinical features After an initial symptom-free period, neonates with IEMs can start deteriorating for no apparent reasons and do not respond to symptomatic therapies The interval between birth and clinical symptoms may range from hours to weeks , depending on the enzyme deficiency Neonates with IEMs can present with 1 or more of the clinical groups Nearly every metabolic disease has several forms that vary in age of onset, clinical severity, and often mode of inheritance

19. Hepatic manifestations Hepatomegaly Cholestatic jaundice Liver failure presenting with jaundice, coagulopathy, elevated transaminases, hypoglycemia, ascites

20. Abnormal urine odor

21. Disorders of Amino Acids Argininemia Argininosuccinic aciduria Citrin deficiency Citrullinemia type I Homocystinuria Maple syrup urine disease Phenylketonuria and pterin defects Tyrosinemia Type Ia Tyrosinemia Type II

22. Phenylketonuria Caused by primary deficiency in Phenylalanine hydroxylase , or rare cases By defect in the synthesis or recycling of essential cofactor Tetrahydrobiopterin (BH4)

23. Discovery PKU was the first disorder to be routinely diagnosed through widespread newborn screening Robert Guthrie  introduced the newborn screening test for PKU in the early 1960s. Nobel and Pulitzer Prize winning author  Pearl S. Buck  had a daughter named Carol who lived with PKU before treatment was available, and wrote a moving account of its effects in a book called  The Child Who Never Grew

24. Autosomal recessive inheritance pattern PKU is characterized by  homozygous  or  compound heterozygous   mutations  in the gene for the hepatic enzyme  P henylalanine hydroxylase  (PAH), rendering it nonfunctional  PAH gene is located on  chromosome 12  in the bands 12q22-q24.1 >500 mutations-most common: R408W & R241C Incidence of 1: 10,000 to 1: 20,000 live births

25. Pathophysiology

26. Classification Defects in PAH ( Type I, classic PKU ) Defect in dihydrobiopterin reductase ( Types II and III ) Defect in dihydrobiopterin biosynthesis ( Types IV and V ): variant PKU or non-PKU hyperphenylalaninemia

27. Clinical manifestations Normal at birth Microcephaly Delayed development Hypopigmentation Seborrheic or eczematoid rash Musty or mousy odor of urine - unpleasant odor of phenylacetic acid Neurologic signs : seizures, spasticity, hyperreflexia, tremors Prominent maxillae with spaced teeth, enamel hypoplasia Growth retardations/Shunted growth Profound mental retardation develops if untreated

28. Dihydrobiopterin reductase deficiencies Infants shows signs of neurological involvement-despite adequate diet control of phenylalanine concentrations Impairment of tyrosine and tryptophan hydroxylase reduces synthesis of neurotransmitters- dopamine and serotonin, with severe neurologic consequences BH4 ‘cofactor’ for nitric oxide synthase which catalyzes the generation of nitric oxide from arginine

29. Maternal PKU Phenylalanine at high concentration is teratogenic : depending on concentration and period of exposure during pregnancy will cause Increased risk of spontaneous abortion “ Maternal PKU syndrome ”: microcephaly, mental retardation, facial dysmorphism and congenital heart abnormalities in fetus Adverse outcomes are minimized by maintaining a level: of less than 360 μ mol /L Thus, dietary control of blood Phenylalanine must begin before conception, and be maintained throughout pregnancy

30. Diagnosis New born screening Relatively inexpensive method developed by USA Bacterial Guthrie test – 1 st method used Tandem mass spectrometry: identifies all forms of phenyalanemia with low false positive rate Phenylalanine/ tyrosine molar ratio DNA sequencing of the causative gene

31. Management Especial dietary treatment Lifelong supplementation with neurotransmitter precursors Plasma phenylalanine concentrations should be monitored (60-360 μ mol/L)

32. Tyrosinemia Type I Deficiency of Fumarylacetoacetate Hydrolase (FAH) , which functions in the catalytic pathway of tyrosine FAH deficiency results in the accumulation of fumarylacetoacetate and its derivative succinylacetone , both of which form glutathione adducts thereby rendering cells susceptible to free radical damage In addition, fumarylacetoacetate is an alkylating agent that has a widespread effect on cellular metabolism resulting in cell death

33. Clinical manifestation Children with Tyrosinemia type I can present during early infancy with Vomiting Diarrhea, blood stool Poor weight gain Extreme sleepiness, irritability Boiled cabbage-like odor: urine , skin Hepatomegaly hypoglycemia sepsis liver failure with coagulopathy ascites Jaundice renal tubulopathy

34. Diagnosis Biochemical abnormalities include: Elevated urine succinylacetone Tyrosine metabolites (p-hydroxyphenylpyruvate, p- hydroxyphenyllactate , and p- hydroxyphenylacetate ) Elevated tyrosine and methionine in plasma Serum a-fetoprotein is markedly elevated Diagnosis can be confirmed by enzyme assay and molecular genetic testing for the FAH gene

35. Management Nitisinone (NTBC) (1–2 mg/kg/d divided in 2 doses) blocks hydroxyphenylpyruvate dioxygenase , the second step in the tyrosine degradation pathway, and prevents the accumulation of fumarylacetoacetate and its derivative succinylacetone Low tyrosine diet Liver transplant

36. Maple syrup urine disease

37. History In 1954, Menkes reported a family: 4 infants died within the first 3 months of their lives due to neurodegenerative disorder The urine of these infants has an odor resembling Maple syrup (burnt sugar) In the following year, Dancis identified the pathogenic compound as branched amino acids and their corresponding ketoacids In 1960, Dancis demonstrated that the enzymatic defect in MSUD was at the level of decarboxylation of branched-chain amino acids

38. Pathophysiology Branched chain Ketoacid Dehydrogenase enzyme complex Catalyzes decarboxylation of alpha-ketoacid to acyl- coA Enzyme complex a/w inner mitochondrial membrane E1: TPP-dependent branched-chain -ketoacid decarboxylase E2: Dihydrolipoyl transacylase (contains lipoamide) E3: Dihydrolipoamide dehydrogenase (FAD)

39. Clinical manifestation Neonates with classic MSUD typically present in the first week of life with Poor feeding Irritability Ketosis Maple syrup odor of urine and cerumen Lethargy Opisthotonus Stereotyped movements (fencing and bicycling) Coma and apnea

40. Diagnosis MSUD can be diagnosed biochemically by Identification of elevated plasma alloisoleucine and the BCAAs with perturbation of the normal 1:2:3 ratio of isoleucine: leucine: valine Ketoacids and hydroxy acids can be detected in urine organic acid analysis or the Dinitrophenylhydrazine (DNPH) test Enzyme activity and molecular testing for the genes coding BCKAD subunits (BCKDHA, BCKDHB, and DBT) are available

41. Management Holding protein intake and suppressing catabolism with glucose and insulin infusions Medical formula, low protein diet Hemodialysis can be considered for rapid correction of hyperleucinemia Thiamine, a cofactor for BCKAD, can be tried for 4 weeks at a dosage of 10 mg/kg/d Long-term management requires a BCAA-restricted diet

42. Homocystinuria 1 st described-1962 Latest in the series of IEM Most common form affects at least 1 in 200,000 to 335,000 people worldwide High incidence in Qatar(1:1800) AR Mutations in the CBS, MTHFR, MTR, MTRR and MMADHC genes cause homocystinuria .

43. MTHFR Gene Mutation Methylene tetrahydrofolate reductase : converts N5,N10-methylene THF to 5-methyl THF Homocysteine increased and some excreted in urine, Methionine is reduced Variations in the MTHFR gene may increase the risk of NTD by changing the ability of MTHF to process folate- behavioral change, vascular abnormality Folate supplement beneficial

44. Clinical manifestation Healthy and normal at birth Overtime causes: growth and learning delays affect eyes, bones, heart, and blood vessels Delay Growth and learning: noticed between ages of 1-3 Eyes: develop nearsightedness after 1 year of age if untreated: ectopia lentis Bone and skeletons: osteoporosis Heart and blood vessels: clots resulting in heart disease/ stroke

45. Diagnosis Classic homocystinuria- elevated Met conc; the presence of disulfide homocysteine Defects in remethylation- very low conc of Met Cyanide nitroprusside test will be positive in urine Dx: Immunoassays, HPLC, or tandem MS Definitive dx: DNA testing Complementation studies in fibroblast

46. Management Low methionine diet Special medical foods and formula Vitamin B6 supplement Supplements: Betaine, vitamin B12, folic acid

47. For evaluating an IEM the following five important aspects should follow: History/ Family History : born to consanguineous parents, previous history of older siblings like fetal deaths or miscarriages or genetically affected siblings, note the pedigree for about two generations Physical examinatio n like dermatitis, alopecia, facial dysmorphia, cataracts etc Initial screening tests include Complete blood count, Electrolytes level, Glucose, Ammonia, Lactate, Lactate/ Pyruvate ratio, Reducing substances, organic acids, Amino acids, Ketones

48. 4. Advanced screening tests : The test is performed on the basis of clinical context which includes long chain fatty acids, MPS separation and specification, quantitation of amino acids, organic acids, carbohydrates and other metabolites 5. Definitive diagnostic tests : To confirm the disorder detected, a specific enzyme assays in leucocytes, plasma/serum or red cells; immunoassays and DNA/Mutation based analysis will be tested

49. Common considerations in determining whether to screen for disorders are: A disease that can be missed clinically at birth. A high enough frequency in the population. A delay in diagnosis will induce irreversible damages to the baby. A simple and reasonably reliable test exists A treatment or intervention that makes a difference if the disease is detected early

50. Collection Heel Prick sampling procedure of the neonates

51. Techniques High Performance Liquid Chromatography (HPLC) Gas Chromatography-Mass Spectrophotometry (GC-MS) Partition chromatography Fully automated cellulose acetate electrophoresis Tandem Mass Spectrometry (TMS)

52. Thank You