Inborn error of metabolism screening in neonates.
Inborn errors of metabolism (IEM) form a large class of genetic disorders which occur as a result of gene defects. The majority of them are due to defects of single genes coding for enzymes. [1-3] Newborn screening of inborn error of metabolism refers to the coordinated and comprehensive way of detecting disorders which includes knowledge, awareness, screening, follow-up of abnormal test results, confirmatory testing, diagnosis, treatment and evaluation of periodic outcome, and efficiency, for example, early detection of phenylketonuria and various other disorders helps in significant decrease in morbidity and helps in prevention from mental retardation. [4-7] Screening refers to the various biochemical and clinical tests done on asymptomatic neonates for the sake of decrease in morbidity and mortality rates and improving the efficiency outcome of better and healthy living of neonates. The identification of IEM as a disorder in neonates was described in the early 20th century. First of all, the disease known as alkaptonuria was discovered by Archibald Garrod, in 1908 [8,9] followed by a research in 1917 regarding the advice of less intake of the milk by the galactosemic infants, but the treatment of various disorders of IEM changed in the 1950s with phenylketonuria. 
Successful treatment outcome depends on early and rapid diagnosis and early therapeutic implementation in IEM disorders of neonates. Neonate suffering from IEM disorder is suspected as a result of acute clinical symptoms.  Sometimes, non-specific clues also exist, like previous unexplained death of neonate in few families showing the risk of IEM disorders in the baby. These disorders are detected through newborn screening programme though in India awareness of the program and lethal consequences of IEM disorders are not paid proper attention which may be due to lack of knowledge about the disease spectrum among the population and lack of funds to meet the screening expenses.
MECHANISTIC BIOCHEMISTRY AND ENZYME DEFECTS
Errors in amino acid metabolism conclude some correlations between biochemical and pathological conditions, for example, alkaptonuria, an inherited metabolic disorder is caused by the absence of enzyme homogentisate oxidase due to which accumulation of homogentisate occurs and is excreted in urine, which turns dark black on standing due to oxidation. [12-14] In maple syrup urine disease (MSUD), the oxidative decarboxylation of a-keto acids derived from valine, leucine, and isoleucine gets blocked, leading to mental and physical retardation. Phenylketonuria, another disorder of IEM, is caused by an absence of the deficiency of phenylalanine hydroxylase, leading to accumulation of phenylalanine as it cannot be converted into tyrosine. Following is the list of various IEM disorders of protein, fat, carbohydrate, nucleic acid, and hemoglobin metabolism [Table 1]. 
CURRENT STATUS IN INDIA
It is nearly 60 years gone for newborn screening for IEM. In course of this long span of time, our country faced many challenges with regard to its startup, including awareness among masses and its implementation in the form of pilot projects for few of the metabolic disorders. Various studies have been done in India at different times which concluded the importance of screening of IEM in neonates. In India, the prevalence of IEM is quite high. Distinct religions, communities, ethnic groups, etc., are responsible for wide variation and prevalence of IEM in these groups.  Hence, there is a need to do research in variation of IEM among different groups and look forward for the risk or aggravating factors of IEM in particular groups. [16,17] Many foreign countries recommend newborn screening mandatory because as per their guidelines delay in detection of few of these disorders such as metabolic errors, endocrinological disorders, and hearing loss will all lead to significant morbidity and mortality. [18,19] Andhra Pradesh is the fifth largest state of India with infant mortality rate of 66.  A study was done in Andhra Pradesh regarding IEM and a database was generated for 43 IEM observed in newborns.  Furthermore, in India, the incidence of congenital hypothyroidism (CH) is 2.1  and the rate of glucose 6 phosphate deficiency is 2-7.8%.  In a study which was undergone over a period of 4 years in West Bengal using gas chromatography in the urine and tandem mass spectrometry for the detection of aminoacidurias concluded 15% newborns positive of IEM,  but their final confirmation needs either enzymatic analysis or genetic studies. A study done on 98,256 newborn showed the prevalence of homocysteinemia, hyperglycemia, MSUD, phenylketonuria, hypothyroidism, and G6PD deficiency. Another expanded study started in 2000 in Hyderabad for amino acid disorders, CH, congenital adrenal hyperplasia (CAH), G6PD deficiency, biotinidase deficiency, galactosemia, and cystic fibrosis, revealed high prevalence of CH followed by CAH and G6PD deficiency.  The prevalence was noticed 1 in every 1000. A Newborn Screening Pilot project concluded disorders such as homocysteinemia, hyperglycemia, MSUD, phenylketonuria, hypothyroidism, and glucose-6-phosphate dehydrogenase deficiency were found to be the common errors in the neonates.  Another pilot study in Hyderabad revealed high prevalence of disorders such as congenital adrenal hyperplasia, G-6-PD deficiency, and aminoacidopathies as the cause of IEM. 
IMPORTANCE OF IEM AMONG NEONATES
IEM are the most important cause of the neonatal illness and many of these disorders are treatable if diagnosed in early phase; therefore, there is a need of IEM screening in newborns. [11,28] In other various countries, IEM screening has expanded quite well. A pilot study was done by Shawky et al.,  in 2015, which included around 40 neonates with various reasons of abnormal behavior such as poor suckling, poor crying, and convulsions and was suspected to have IEM and concluded that around 32.5% of selected neonates for the case study were diagnosed with IEM who have sepsislike symptoms. Another study was done by Shawky et al.  in which the screening of mentally retarded children was done by paper chromatography and various other tests such as ferric chloride test and nitroprusside test resulting in 11.3% of neonates with confirmed diagnosis of IEM. In Brazil, a study was conducted on 101 hypoglycemic neonates having metabolic acidosis, jaundice, diarrhea, vomiting, hepatomegaly or splenomegaly, cataract, apnea, and convulsions. Around 63.3% of 101 were diagnosed as IEM.  In China, a study was conducted by Huang et al.  on 11,060 neonates, of which only 62 were diagnosed as IEM. The symptomatic neonates were presented with metabolic acidosis, jaundice, hepatosplenomegaly, recurrent vomiting, hypoglycemia, convulsions, and unconsciousness. In German study,  106 neonates were diagnosed as IEM out of 250,000 neonates. In Taiwan, the newborn screening at the national level revealed phenylalanine metabolism defect as the most common defect of IEM followed by MSUD. [34-36] IEM screening should be done for the betterment of any country's health and wealth, but it is still lacking due to various hurdles coming in its way like financial constraints as it is quite expensive, so every individual person or country cannot afford it and also there is a lack of education and awareness among the citizens of one's country regarding the importance of IEM or its role in the well-being of the child in near future.
Individually rare kind of disorders, IEM manifest due to partial and full enzymatic defects leads to the accumulation of toxic metabolites in the body. To manage its morbid and mortal effects, early and timely diagnosis and management is essential. The newborn screening program one of the important ways to provide early and pre-symptomatic diagnosis. The approach is proved to be a boon for innocent infants suffering from IEM disorders who can live a normal life if properly managed.
The authors thankfully acknowledge the management of Santosh Medical College and Hospital, for supporting the work.
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Preeti Sharma (1), Shivani Gupta (2), Pradeep Kumar (1), Rachna Sharma (3), Mahapatra T K (1), Gaurav Gupta (4)
(1) Department of Biochemistry, Santosh Medical College and Hospital, Ghaziabad, Uttar Pradesh, India, (2) MBBS Student, TSM Medical College and Hospital, Lucknow, Uttar Pradesh, India, (3) Department of Biochemistry, TSM Medical College and Hospital, Lucknow, Uttar Pradesh, India, (4) Department of Biochemistry, Government Medical College, Badaun, Uttar Pradesh, India
Correspondence to: Preeti Sharma, E-mail: firstname.lastname@example.org
Received: December 14, 2018; Accepted: January 08, 2019
Source of Support: Nil, Conflict of Interest: None declared.
Table 1: Various IEM disorders IEM disorders S. No. Hemoglobinopathies 1 Beta-thalassemia 2 Sickle cell anemia (HB SS) 3 Sickle cell disease (Hb S/C) 4 Variant hemoglobinopathies (C, D, H, Bart band), including HbE Endocrinology 5 Congenital hypothyroidism 6 Congenital adrenal hyperplasia Endocrinology 7 Cystic fibrosis 8 G6PD deficiency 40 Iminoglycinuria 41 2-Ketoadipic aciduria 42 Sacchropenuria 43 Hydroxylysinuria 44 Cystathionuria 45 Hyperprolinemia 46 Hyperprolinemia type II 47 Hyperhydroxyprolinemia 48 5-Oxoprolinuria 50 Hypersarcosinemia 51 Imidazole aminoaciduria 52 Formiminoglutamic aciduria 85 Lactose intolerance Fatty acid oxidation disorders 86 SCAD 87 MCAD 88 LCAD 89 VLCAD 90 Short/medium-chain 3-hydroxy-CoA dehydrogenase deficiency 91 Long-chain 3-hydroxy-CoA dehydrogenase deficiency 92 Mitochondrial trifunctional protein deficiency 93 Carnitine transport defect 94 Multiple CoA dehydrogenase deficiency 95 Medium-chain ketoacyl-CoA dehydrogenase deficiency Test done on urine samples Amino acid disorders 9 Phenyl ketonuria 10 Defect in biopterin cofactor biosynthesis 11 Defects in biopterin cofactor regeneration 12 GTPCH deficiency 13 Dihydropteridine reductase deficiency 14 Benign H-PHE 15 Tyrosinemia type I 16 Tyrosinemia type II 17 Tyrosinemia type III 18 Transient tyrosinemia in infancy 19 Tyrosinemia caused by liver dysfunctions 20 MSUD 21 Carbamoyl phosphate synthetase-1 deficiency 22 OTC deficiency 23 Citrullinemia 24 Citrullinemia type II 25 Argininosuccinic aciduria 26 Argininemia 27 Hypermethioninemia 28 Homocystinuria 29 Alkaptonuria 30 Tryptophanuria with dwarform 31 Xanthurenic aciduria 32 Valinemia 33 Hyperleucinemia 34 Dihydroptoyl dehydrogenase deficiency 35 3-Hydroxylbutyryl-CoA deacylase deficiency 36 Histidinuria 37 Hartnup disease 38 Lysinuric protein intolerance 39 Familial renal iminoglycinuria 53 Serum carnosinase deficiency 54 Glutathionuria 55 Hyperpipecolatemia 56 3-Aminobutyric aciduria 57 Histidinemia Organic acid disorders 58 Propionic academia 59 Multiple carboxylase deficiency 60 Methylmalonic acidemia 61 Methylmalonyl-CoA mutase deficiency 62 Methylmalonic aciduria 63 Malonic acidemia 64 Biobutyryl-CoA dehydrogenase deficiency 65 Methylbutyryl-CoA dehydrogenase deficiency 66 Methylmalonic semialdehyde dehydrogenase deficiency 67 B-Ketothiolase deficiency 68 Isovaleric acidemia 69 3-Methylcrotonyl-CoA carboxylase deficiency 70 3-Methylglutaconic aciduria 71 3-Hydroxy-3-methylglutaric aciduria 72 Glutaric aciduria type-II 73 Glutaric aciduria type-l 74 Mevalonic acidemia 75 3-Methyl-3-hydroxybutyric aciduria 76 4-Hydroxybutyric aciduria Carbohydrate disorders 77 Galactosemia 78 Galactokinase deficiency 79 Galactose epimerase deficiency 80 Transient galactosemia 81 Fructosuria 82 D-glyceric aciduria 83 Fructose-1, 6-diphosphatase deficiency 84 Endogenous sucrosuria Peroxisomal disorders 96 Zellweger syndrome 97 Neonatal adrenoleukodystrophy 98 Infantile Refsum's disease 99 Zellweger-like syndrome 100 Primary hyperoxaluria Disorders of purine pyrimidine metabolism 101 Adenosine deaminase deficiency 102 Lesch--Nyhan syndrome 103 Partial deficiency of hypoxanthine adenine phosphoribosyltransferase 104 Adenine phosphoribosyltransferase deficiency 105 Xanthinuria 106 Orotic aciduria 107 Thymine-uraciluria 108 Dihydropyrimidinase deficiency 109 Hyperuric acidemia Lactic acidemia, hyperpyruvic acidemia 110 Pyruvate dehydrogenase deficiency 111 Pyruvate dehydrogenase phosphatase deficiency 112 Pyruvate carboxylase deficiency 113 Pyruvate decarboxylase deficiency 114 Leigh syndrome Other IEM 115 Biotinidase deficiency 116 Canavan deficiency 117 Fumarate hydrolase deficiency 118 HHH syndrome Miscellaneous genetic condition 119 Neuroblastoma MSUD: Maple syrup urine disease, H-PHE: Hyperphenylalaninemia, SCAD: Short-chain CoA dehydrogenase deficiency, MCAD: Medium-chain CoA dehydrogenase efficiency, LCAD: Long-chain CoA dehydrogenase deficiency, VLCAD: Very long-chain CoA dehydrogenase deficiency, HHH: Hyperornithinemia-hyperammonemia-hyperhomocitrullinemia, GTPCH: GTP cyclohydrolase, OTC: Ornithine transcarbamylase, IEM: Inborn errors of metabolism