A Nonketotic Hyperglycinemia Mouse Shows Wide-Ranging Biochemical Consequences of Elevated Glycine, Reduced Folate One-Carbon Charging, and Serine Deficiency.

Abstract

Nonketotic hyperglycinemia is a severe neonatal epileptic encephalopathy caused by deficient glycine cleavage enzyme activity, for which currently no effective treatment exists. Incomplete understanding of brain biochemistry represents a major knowledge gap to develop new treatments. We examined the biochemistry in blood, liver, cortex, hippocampus, and cerebellum of a mouse model homozygous for the Gldc variant p.Ala394Val. Glycine was increased in all compartments and caused increased brain neurotoxic metabolites guanidinoacetate and methylglyoxal, and also N-acetylglycine and cystathionine. The glycine extruding transporter Slc6a20 was increased. There was reduced one-carbon folate charging with secondarily reduced methionine in the cortex, and reduced alternative one-carbon donors L-serine and formate. Serine deficiency was associated with reduced amounts of sphingosine, sphingomyelin, and ceramide species important for myelination, but not phosphatidylserines. There was a region-specific deficiency of D-serine in the cortex and hippocampus. This difference, also present in humans, was strain- and age-related, most evident in young J129X1/SvJ mice, reflecting symptomatology. There was no evidence of oxidative stress or a bioenergetic defect. The biochemistry of the nonketotic hyperglycinemia mouse model can be traced to three components: increased glycine, reduced folate one-carbon charging, and decreased L- and D-serine. These changes will need to be addressed in new therapeutic approaches.