Nonketotic hyperglycinemia (NKH) is an autosomal recessive disorder of glycine metabolism caused by a defect in the glycine cleavage system and characterized by neurological disease. Severity of presentation varies considerably and, to date, prognosis is entirely based on extent of symptoms. We present a molecular study of 35 patients diagnosed with NKH. Mutations in either the glycine decarboxylase (GLDC) or the aminotransferase (AMT) protein of the cleavage system were identified in all 35 patients, many of which were novel mutations. We propose here that prognostic outcome in NKH patients is rooted in the nature of the mutations in two key genes. Through molecular analysis of the AMT and GLDC genes in both patients and parents, we can utilize genotypic information to help predict severity of outcome. In concordance with this goal, we have also designed a method of deletion detection for exon 3 of GLDC. Deletions encompassing in this exon have been identified in many NKH patients. However, compound heterozygosity precludes these deletions from detection by direct sequencing. Our detection system is based on real-time, quantitative PCR and utilizes the pseudogene to GLDC as an internal control. This technique has allowed identification of 8 individuals possessing a large deletion in GLDC. Pedigree and Southern blot analysis verify these results. An increased ability to detect mutation and predict phenotype will aid in identifying patients who would benefit from available treatment. Since decisions to abort or continue pregnancy may depend on predicted severity, molecular analysis will greatly aid in prenatal testing options as well.
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