Inborn errors of protein metabolism

inborn errors of metabolism

1. Inborn errors of protein metabolism Presenting by T.hemanth

2. Genetic disorder fall into mainly three categories Chromosomal disorders e.g. down syndrome and Klinefelter`s syndrome. Monogenic disorders, “ inborn errors of metabolism (IEM) comes under this group . Complex disorders (multifactorial disorders): here, genetic factors and others factors are involved in the pathogenesis. Charaka the father of Indian medicine wrote that “diseases are three types; inborn, exogenous and psychological.( charaka Samhita). Background

3. Sir Archibald Garrod (1857- 1936) coined the term of “inborn errors of metabolism” in 1909. Garrod’s tetrad are alkaptonuria, albinism, pentosuria and cystinuria. Some inborn errors are completely, or almost completely harmless. They are important because they produce effects that may lead misdiagnosis or alarm the patient. example are glycosuria, alkaptonuria, and gilbert’s disease.

4. 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 enzymes involved in the biochemical pathways that break down food components.

5. INTRODUCTION :- Moderately an active man consuming about 300g of carbohydrates, 100g fats and 100g proteins. Protein contains carbon, hydrogen, oxygen and nitrogen as the major components while sulfur and phosphorus are minor components. Nitrogen is characteristics of proteins. On an average, the nitrogen content of ordinary proteins is 16% of weight. All proteins are polymers of amino acids. 95% of nitrogen is eliminated by the kidneys and the remaining 5% excrete through the feces.

6. INBORN ERRORS OF UREA CYCLE Normal values of urea :- The normal level in blood plasma is 20- 40mg/dl . Indians take less proteins, hence normal level in Indians varies from 15-40 mg/dl. Deficiency of any of the urea cycle enzymes would result in hyperammonemia . Deficiencies of later enzymes results accumulation of other intermediates which are less toxic. Disorders of urea cycle characterized by encephalopathy and respiratory alkalosis.

7. Symptoms of newborns with urea cycle defects Normal appearance at birth Somnolence progressing to lethargy then coma Loss of thermoregulation (hypothermia) Feeding disruption (increases catabolism) Neurologic posturing (from cerebral edema) Seizures Hyperventilation and then hypoventilation

8. Based on the enzyme deficiency divided into five types Hyperammonemia type-I Hyperammonemia type-II Citrullinemia. Argininosuccinic aciduria. Hyper argininemia.

9. HYPERAMMONEMIA TYPE-I A familiar disorder, enzyme deficiency carbamoyl phosphate synthase 1 , produces Hyperammonemia and symptoms of ammonia toxicity. CO2 + NH3 +2 ATP Carbamoyl phosphate. A variant of the condition is seen in N- acetylglutamate synthetase.

10. HYPERAMMONEMIA TYPE-II X-linked inheritance. Ornithine Citrulline Enzyme deficiency ornithine transcarbamoylase , Increased ammonia in blood Increased glutamine in blood, CSF, and urine. Orotic aciduria due to channelling of carbamoyl phosphate into pyrimidine synthesis.

11. CITRULLINEMIA It is an autosomal recessive disorder. Enzyme deficiency is Argininosuccinate synthatase . Citrulline + Aspartate Arginosuccinate Clinically :- Deficiency is characterized by hyperammonemia, cirullinemia and cirullinuria (1-2 g/day). CSF citrulline levels are also elevated. Feeding arginine in the patients enhance citrulline excretion.

12. ARGININOSUCCINIC ACIDURIA Autosomal recessive disorder. Enzyme deficiency Argininosuccinate lyase Argininosuccinate Arginine + Fumarate Clinically :- enzyme deficiency leads to Argininosuccinic aciduria and therefore metabolic acidosis. The enzyme deficiency has been identified in brain, liver, kidney and RBC

13. HYPER ARGININEMIA Enzyme deficiency is Arginase. Arginine Ornithine + Urea Clinically:- Hyperammonemia, developmental delay and progressive spasticity Urine :- increased urinary excretion of lysine, cystine, ornithine and Arginine. Low protein diet result in lowering of plasma ammonia levels and disappearance of urinary lysinecystinuria pattern.

14. DISORDERS OF AROMATIC AMINO ACIDS { PHE, TYR & TRP } Phenylketonuria Alkaptonuria Tyrosinemia Albinism Hartnup’s disease

15. PHENYLKETONURIA Autosomal recessive metabolic genetic disorder Mutation in the gene for phenylalanine hydroxylases (PAH) Gene located on 12 th chromosome. A carrier does not have symptoms of the disease, but can pass on the defective gene to his or her children. Deficiency of the enzyme phenylalanine hydroxylase.

16. BIOCHEMICAL ABNORMALITIES Phenyl alanine could not be converted to tyrosine. So phenylalanine accumulates in blood. So alternate minor pathways are opened, phenyl ketone, phenyl lactate, phenyl acetate are excreted in urine. Type 1 due to p henylalanine hydroxylase deficiency Type II and III due to dihydrobiopterin reductase . Type IV and V due to enzyme synthesizing biopterin

17. CLINICAL CONDITIONS : Mental retardation Failure to walk/talk. Failure of growth. This maybe because phenylalanine interferes with neurotransmitter synthesis. The child often has hypopigmentation explained by the inhibition of tyrosinase. Phenyllactic acid in sweet may lead to mousy body odor.

18. Screening Every state now screens the blood phenylalanine level of all newborns at about 3 days of age. Laboratory diagnosis Blood phenyl alanine normal level is 1mg/dl. In PKU the level is >20mg/dl. This is identified by chromatography. Ferric chloride test Urine of the patient contains phenyl ketones, about 500-3000mg/dl.

19. ALKAPTONURIA Alkaptonuria is an autosomal recessive condition with an incidece of 1 in 2,50,000 births. Black urine disease or black bone disease is an inborn error of amino acid metabolism.

20. CAUSE Mutation or defect in HGD gene which causes lack of the enzyme homogentisate dioxygenase (HGD). This causes a build up of homogentisic acid (HGA) in the bones, cartilage and urine. HGA is an intermediate in the degradation pathway of the amino acids ( Phe & Tyr ) to the Krebs cycle.

21. SYMPTOMS Urine becomes black when exposed to air. Osteoarthritis (mainly spine, hips, shoulders and knees). Black spots in the sclera of the eye ( Ochronosis ). Discolored ear and dark earwax. Heart valves are affected by the accumulation of HGA. Blue-black speckled discoloration of the skin. Kidney, prostate and bladder stones due to the buildup of HGA in the genito -urinary tract, during urine production.

22. DIAGNOSIS Urine test - addition of ferric chloride to the urine will change it’s color to black. Gas chromatography to look for traces of HGA in urine. DNA testing - to check for mutated HGD gene. It is generally done by analyzing blood sample. Prenatal tests (amniocentesis or chorionic villus sampling) can be done to screen a developing baby for this condition if the genetic change has been identified.

23. ALBINISM It is an autosomal recessive disease with an incidence of 1 in 20,000 birth. Defect is tyrosinase enzyme leads complete absence of melanin synthesis The ocular fundus is hypopigmented and iris may be grey or red. They will be associated photophobia and decreased visual acuity.

24. The skin has low pigmentation and so skin is sensitive to UV rays. The hair is also white

25. HYPERTYROSINEMIAS It is due to deficiency of phenylacetoacetate hydrolase Symptoms :- the first six months of life and death occurs rapidly. Cabbage like odor and hypoglycemia are seen. Urine contains tyrosine, p-hydroxy phenyl pyruvic acid and phenyl latic acid; and serum shows tyrosine and methionine.

26. HYPERTYROSINEMIA-2 It is due to deficiency of tyrosine amino transferase Symptoms :- Mental retardation, keratosis of palmar surface and photophobia are seen. There is increased excretion of tyrosine, tyramine in urine.

27. Hartnup’s disease It is a hereditary disorder of tryptophan metabolism the clinical symptoms include dermatitis and ataxia. The pellagra like symptoms are due to the deficiency of niacin derived from tryptophan. The diagnosis is based on aminoaciduria and increased excretion of indole compounds detected by the

28. Obermeyer test Hartnup’s is characterized by low plasma level of tryptophan and other neutral amino acids and their elevated urinary excretion.

29. GLYCINE Glycine is a non-essential optically inactive and glycogenic amino acids. Glycine is actively involved in the synthesis of many specialized products in the body(Heme, purines, creatinine).

30. METABOLIC DISORDERS OF GLYCINE : Glycinuria :- This is rare disorder, due to defect in the glycine cleavage system. Glycine level is increased in blood and CSF. Very high amount of it is excreted in urine. Glycinuria characterized by increased tendency for the formation of oxalate stones.

31. PRIMARY HYPEROXALURIA :- Increased excretion of oxalates observed upto 600mg/day compared to a normal of 50mg/day. Primary hyperoxaluria is due to defect in glycine transaminase coupled with impairment in Glyoxalate oxidation to formate . Glycine Glyoxalate In vit-B6 deficiency, urinary oxalate is elevated it can be corrected by B6 supplementation.

32. SULPHUR CONTAINING AMINO ACIDS Sulfur containing amino acids :- Methionine, Cystein and Cystine. The other sources of sulfur in the body are sulfur containing vitamins are the thiamin, biotin and lipoic acid. Disorders :- Cystinuria, Cystanosis, Homocysteinurias (I, II, III), Hyper methioninemias.

33. Cystinuria :- It is one of the most inherited disease with a frequency of 1 in 7,000 births. Defect :- it is considered to be due to a renal transport defect in that re-absorption of the four amino acids, lysine, arginine, and ornithine A single re-absorptive site is involved.

34. Complications :- Cystine is relatively insoluble amino acids which may precipitate in renal tubules uterus and bladder to form “cystine calculi”. Cystine stones account for 1-2 % of all urinary tract calculi. It forms a major complication of the disease.

35. Cyanide nitroprusside test :- It is a screening test urine is made alkaline with ammonium hydroxide and sodium cyanide is added cystine if present reduced to cysteine. Then added sodium nitroprusside to get a megenta red colored complex. Specific amino aciduria may be conformed by chromatography.

36. CYSTINOSIS Defective enzyme is cystine reductase. It is familial disorder characterized by the wide spread deposition of cystine crystals in the lysosomes. Cystine accumulates in the liver, spleen, bone marrow and lymph nodes.

37. Microscopy of blood shows cystine crystals in WBC. Treatment policies are to give adequate fluid intake so as to measured output, alkalization of urine by sodium bicarbonate as well as administration of D- Penicillamine.

38. Hypermethioninemias : CAUSES Impaired utilization Excessive remethylation of homocysteine Oasthous syndrome is due to malabsorption of methionine, in such children excrete methionine, aromatic amino acids and branched amino acids in urine.

39. HOMOCYSTINURIA TYPE-I These are a group of metabolic disorders due to a defect in the enzyme cystathionine synthase. Accumulation of homocysteine results in the various complications like thrombosis, mental retardation etc. The deficiency of cystathionine is associated with damage to endothelial cells.

40. TYPE-II N5N10 methylene THF reductase TYPE-III N5N10 methyl THF homocysteine methyl transferase. This is mostly due to impairment in the synthesis of methylcobalamin . TYPE-IV • N5 Methyl thf homocysteine methyl transferase, due to defect in intestinal absorption of vit-B12.

41. Branched chain Amino acids VALINE LEUCINE ISOLEUCINE

42. MAPLE SYRUP URINE DISEASE: The urine of effected individuals smells like maple syrup or burnt sugar. Enzyme defect is α-keto acid dehydrogenase, which causes a blockade in conversion of α-keto acid to the respective acyl CoA thioesters. Elevated levels of branched aa & their ketoacids in plasma & urine, so known as branched chain ketonuria

43. Biochemical complications & symptoms Impairment in transport of other aa Protein biosynthesis is reduced The disease results in acidosis, mental retardation, coma & finally leads to death within one year of birth.

44. ISOVALERIC ACIDEMIA Specific inborn error of leucine metabolism. Due to defect in enzyme CoA isovaleryl dehydrogenase Isovaleryl CoA methylcrotonyl is impaired. Symptoms - acidosis & mild mental retardation.

45. Hypervalinemia Increased plasma concentration of valine while leucine and isoleucine remain normal. The transamination of valine alone is selectively impaired

46. HISTIDINE Histidinemia :- defect in enzyme histidase Increased excretion of imidazole pyruvate & histidine in urine Symptoms – Defect in speech & mental retardation.

47. PROLINE . Hyperprolinemia type I Defect in enzyme proline oxidase. ARGININE : Hyperargininaemia is due to defect in enzyme arginase

48. Tests for metabolic disorders in new-born These diseases may be demonstrated indirectly by detecting high concertation of the substrate normally metabolized by the enzyme or low concentration of the product. The ultimate specific diagnosis of inherited metabolic disease generally requires the demonstration of primary biochemical abnormality, such as a specific enzyme deficiency or mutation that have been shown to cause disease

49. A useful first step in helping to focus the laboratory investigation of possible inherited metabolic diseases is to try to determine whether the disease is due to a defect in the metabolism of water soluble intermediates such as amino acids, organic acids or likely due to an inherited defect in lysosomal, mitochondrial, or peroxisomal metabolism.

50. Prenatal diagnosis About 2 % life births associate with a genetic defect in addition, genetic disorders are also a major cause of pregnancy loss as well as perinatal mortality and morbidity. Genetic counselling : this process involves an attempt by the trained persons to help the individual or family to. Amniocentesis : prenatal diagnosis of IEM can be made by enzymatic assays of cultured aminocytes . E.g. if the couple already had a child affected by inherited disorder, if one of the parents is affected an autosomal or X- linked.

51. 3. Chorionic villi sampling : the most common indications for CVS or advanced maternal age, or biochemical or genetic disorders indicated by molecular markers. 4. Cordocentesis : fetal blood sampling can be performed at 20 weeks gestation. 5. Cytogenetics and molecular genetics : cytogenetics analysis may be done with fluorescence in situ hybridization (fish), for common chromosomal aneuploidies involve in chromosomes 13,18,21,X,and Y.

52. PRENATAL SCREENING Prenatal screening of high-risk groups can be done to plan place and method of delivery or even to offer termination of pregnancy. Termination is commonly done by demonstrating metabolic defect in cultured fetal fibroblasts obtained by amniocentesis.

53. Maternal serum screening : prenatal screening has become standard obstetric practice in all pregnancies having a risk factor or abnormal ultrasonographic (USG) findings. Five analytes namely alpha fetoprotein (AFP), human chorionic gonadotropin ( hCG ), unconjugated estriol (uE3), inhibin, and pregnancy associated plasm protein A (PAPPA) are estimated. NTD’s, trisomy 21 and trisomy 18 are detected prenatally by these measurements.

54. Screening during the first trimester : double marker test: consists of PAPPA and hCG . These are measured in maternal serum between 10 weeks, it is an indicator early pregnancy failure and complications. Persistently lower level of PAPPA in second trimester is indicative of trisomy 18.

55. The triple test and quadruple test The test is done at 18 weeks along with a detailed USG for any fetal anomalies. Triple test include AFP, uE3, and hCG . While the quadruple marker contains inhibin.