Mutations in the AMT gene are associated with Glycine encephalopathy, also known as nonketotic hyperglycinemia (NKH), which is an inborn error of glycine metabolism defined by deficient activity of the glycine cleavage enzyme and, as a consequence, accumulation of large quantities of glycine in all body tissues including the brain. The majority of glycine encephalopathy presents in the neonatal period (85% as the neonatal severe form and 15% as the neonatal attenuated form). Of those presenting in infancy, 50% have the infantile attenuated form and 50% have the infantile severe form. Overall, 20% of all children presenting as either neonates or infants have a less severe outcome, defined as developmental quotient greater than 20. A minority of patients have mild or atypical forms of glycine encephalopathy. The neonatal form manifests in the first hours to days of life with progressive lethargy, hypotonia, and myoclonic jerks leading to apnea and often death. Surviving infants have profound intellectual disability and intractable seizures. The infantile form is characterized by hypotonia, developmental delay, and seizures. The atypical forms range from milder disease, with onset from late infancy to adulthood, to rapidly progressing and severe disease with late onset. Glycine encephalopathy is suspected in individuals with elevated glycine concentration in blood and CSF. An increase in CSF glycine concentration together with an increased CSF-to-plasma glycine ratio suggests the diagnosis. Enzymatic confirmation of the diagnosis relies on measurement of glycine cleavage system (GCS) enzyme activity in liver obtained by open biopsy or autopsy. The majority of affected individuals have no detectable enzyme activity. The three genes in which biallelic mutations are known to cause glycine encephalopathy are: GLDC (encoding the P-protein component of the GCS complex and accounting for 70%-75% of disease), AMT (accounting for ~20% of disease), and GCSH (encoding the H-protein component of the GCS complex and accounting for <1% of disease). About 5% of individuals with enzyme-proven glycine encephalopathy do not have a mutation in any of these three genes and have a variant form of glycine encephalopathy.
