Glycyl-tRNA synthetase mutations cause Charcot-Marie-Tooth disease type 2D and distal spinal muscular atrophy type V: A potentially novel disease mechanism for human peripheral neuropathies

Charcot-Marie-Tooth disease type 2D (CMT2D) and distal spinal muscular atrophy type V (dSMA-V) are inherited axonal neuropathies with a more prominent phenotype in the upper extremities. Knowledge of the gene(s) mutated in patients with these axonal neuropathies would help to dissect the pathogenesis and to define a path for developing potential therapies. We pursued three specific aims to advance this area of research: (1) identify and characterize the gene(s) responsible for CMT2D and dSMA-V; (2) establish the molecular pathology associated with the identified mutations; and (3) investigate the role of the implicated gene in neuronal function. Our genetic analyses revealed that mutations in the glycyl-tRNA synthetase (GARS) gene account for both CMT2D and dSMA-V. GARS is a ubiquitously expressed alpha2 homodimer responsible for charging tRNA molecules with glycine by an aminoacylation reaction. Biochemical analyses of mutant GARS and a model of human GARS mutations in the yeast ortholog GRS1 revealed that disease-associated mutations give rise to a loss of enzyme function. Analysis of cultured neurons showed that wild-type GARS is present in granules in neurite projections, while a subset of mutant GARS proteins are not. Based on these analyses, our current hypothesis is that the CMT2D and dSMA-V phenotypes arise from: (1) a loss of GARS enzyme function; (2) poor localization of GARS within granules required for tRNA charging in the axon periphery; or (3) a combination of these phenomena. We propose that a key factor in CMT2D and dSMA-V pathogenesis is a decrease in GARS activity within peripheral nerve axons. Specifically, peripheral nerves in the upper extremities may be more sensitive to defects in protein translation or to decreased levels of proteins bearing glycine at key residues. Our results represent the first example of a defect in an aminoacyl-tRNA synthetase being directly associated with a human genetic disease and suggest a link between protein synthesis defects and inherited peripheral neuropathies.