Pitfalls in the diagnosis of glycine encephalopathy (non‐ketotic hyperglycinemia)

Non-ketotic hyperglycinemia (NKH), also termed glycine encephalopathy (MIMa 605899), is an autosomal recessive inborn error of glycine degradation which leads to severe neurological symptoms and profound psychomotor disability. In NKH, glycine accumulates in all body fluids and tissues, including the CNS. The biochemical hallmark of NKH is increased glycine concentration in the plasma and to an even greater extent in the CSF, leading to an elevation of the CSF:plasma glycine ratio (C:PGR) to above 0.08 (normal <0.04). The fundamental defect is in the glycine cleavage system (GCS), a multienzyme complex located in the inner mitochondrial membrane of the liver, kidney, brain, and placenta. It consists of four individual protein components termed P (a pyridoxal phosphate-dependent glycine decarboxylase), H (a lipoic acid-containing hydrogen carrier protein), T (a tetrahydrofolate-dependent protein), and L (a lipoamide dehydrogenase). In more than 80% of patients the defect is in the P protein (MIM 238300), but defects in the T (MIM 238310) and H (MIM 238330) proteins have also been described. The pathogenesis of NKH is related to the properties of glycine as an excitatory neurotransmitter acting via the N-methyl-Daspartate receptor in the cortex and an inhibitory neurotransmitter in the brainstem and spinal cord.1–3 Classically, NKH presents in the early neonatal period with progressive lethargy, hypotonia, myoclonic jerks, hiccups, and apnea, usually leading to total unresponsiveness, coma, and death unless the patient is supported through this stage with mechanical ventilation. Survivors almost invariably display profound neurological disability and intractable seizures. In a minority of NKH cases the presentation is atypical with a later onset and features including seizures, developmental delay and/or regression, hyperactivity, spastic diplegia, spinocerebellar degeneration, optic atrophy, vertical gaze palsy, ataxia, chorea, and pulmonary hypertension.4–20 Atypical cases are more likely to have milder elevations of glycine concentrations and C:PGR with residual GCS activity. A handful of transient NKH cases have been reported21–26 with neonatal onset and characteristic EEG and biochemical abnormalities which, however, return to normal by 2 months of age, usually with complete clinical resolution. Such cases have been attributed to delayed maturation of the hepatic and cerebral GCS. NKH is generally considered to be a rare disease, but relatively higher incidences have been reported in Northern Finland,27 British Columbia,28,29 and Israel.30–32 In our own laboratory, one of four in Israel performing amino acid analyses, we have diagnosed 15 new unrelated cases of NKH in the past 3 years. Ideally, the diagnosis of NKH should be confirmed by demonstrating deficient GCS activity using the [1-14C] glycine decarboxylation assay, together with assay of the individual components of the complex.33 This necessitates a liver biopsy, as NKH activity is not expressed in fibroblasts nor untransformed lymphocytes. An open liver biopsy is necessary in order to obtain the required amount of tissue for the complete evaluation. In most cases, the treating physician and/or family are unwilling to perform this procedure, particularly in a critically ill infant. Furthermore, the assay is available in only a few centres worldwide necessitating transport of a limited and crucial sample under stringent conditions. Enzymatic diagnosis of NKH due to P-protein deficiency using Epstein-Barr virus transformed lymphoblasts has been reported.34 However, the normal GCS activity in lymphoblasts is low, and some other laboratories have been unable to reproduce these results consistently.35 Furthermore, lymphoblast GCS activity may be normal despite unequivocally deficient hepatic GCS activity.17 At present, molecular diagnosis of NKH by mutation analysis is not a practicable alternative for most sporadic cases. With the exception of a common mutation in the P-protein gene among Finnish patients36 representing a founder effect, there have been few reports of recurring mutations in the Por T-protein genes in unrelated patients.37–39 Thus, A nntation