Molecular identification of aspartate N-acetyltransferase and its mutation in hypoacetylaspartia

The brain-specific compound N-acetylaspartate (NAA) occurs almost exclusively in neurons, where its concentration reaches approximately 20 mM. Its abundance is determined in patients by magnetic resonance spectroscopy to assess neuronal density and health. The molecular identity of the N-acetyltransferase that catalyses NAA synthesis has remained unknown, because this enzyme is membrane-bound and difficult to purify. Database searches indicated that, among the putative N-acetyltransferases (i.e., proteins homologous to known N-acetyltransferases, but with uncharacterized catalytic activity) encoded by the human and mouse genomes, two, NAT8L and NAT14, were almost exclusively expressed in brain. Transfection studies in HEK293T cells indicated that NAT8L, but not NAT14, catalysed the synthesis of NAA from L-aspartate and acetyl-CoA. The specificity of NAT8L, its KM for aspartate and its sensitivity to detergents are similar to those described for brain aspartate N-acetyltransferase. Confocal microscopy analysis of CHO cells and neurons expressing recombinant NAT8L indicates that it is associated with the endoplasmic reticulum, but not with mitochondria. Mutation search in the NAT8L gene in the only patient known to be deficient in NAA disclosed the presence of a homozygous 19-bp deletion resulting in a change in reading frame and the absence of production of a functional protein. We conclude that NAT8L, a neuron-specific protein, is responsible for NAA synthesis and is mutated in primary NAA deficiency (hypoacetylaspartia). The molecular identification of this enzyme opens perspectives for the clarification of the function of this most abundant amino acid derivative in neurons and for the diagnosis of hypoacetylaspartia in other patients