Dominant, toxic gain-of-function mutations in gars lead to non-cell autonomous neuropathology.

Stuart J Grice,Ji-Long Liu,Robert W Burgess,M Zameel Cader,Kevin Talbot,James N Sleigh,William W Motley

doi:10.1093/hmg/ddv176

Abstract

Charcot–Marie–Tooth (CMT) neuropathies are collectively the most common hereditary neurological condition and a major health burden for society. Dominant mutations in the gene GARS, encoding the ubiquitous enzyme, glycyl-tRNA synthetase (GlyRS), cause peripheral nerve degeneration and lead to CMT disease type 2D. This genetic disorder exemplifies a recurring motif in neurodegeneration, whereby mutations in essential, widely expressed genes have selective deleterious consequences for the nervous system. Here, using novel Drosophila models, we show a potential solution to this phenomenon. Ubiquitous expression of mutant GlyRS leads to motor deficits, progressive neuromuscular junction (NMJ) denervation and pre-synaptic build-up of mutant GlyRS. Intriguingly, neuronal toxicity is, at least in part, non-cell autonomous, as expression of mutant GlyRS in mesoderm or muscle alone results in similar pathology. This mutant GlyRS toxic gain-of-function, which is WHEP domain-dependent, coincides with abnormal NMJ assembly, leading to synaptic degeneration, and, ultimately, reduced viability. Our findings suggest that mutant GlyRS gains access to ectopic sub-compartments of the motor neuron, providing a possible explanation for the selective neuropathology caused by mutations in a widely expressed gene.

Highlights

Mutations in several aminoacyl-tRNA synthetase (ARS) genes have been linked to different forms of Charcot–Marie–Tooth (CMT) disease [1,2,3,4,5,6], a heterogeneous set of conditions characterized by progressive distal muscle wasting, weakness and sensory dysfunction [7]
To mimic the dominant GARS mutations that cause CMT2D, and to better understand the selective neuronal pathology seen in the disease, we expressed the cytoplasmic isoform of wild-type (UAS-garsWT, which produces GlyRSWT) and mutant (UAS-garsP234KY, which produces GlyRSP234KY) Drosophila gars transgenes using the UAS-GAL4 system [18]
This work reveals a novel mechanism for the pathology associated with the peripheral neuropathy CMT2D, and identifies a complex series of events leading to the characteristic selective neuromuscular toxicity

Summary

Introduction

Mutations in several aminoacyl-tRNA synthetase (ARS) genes have been linked to different forms of Charcot–Marie–Tooth (CMT) disease [1,2,3,4,5,6], a heterogeneous set of conditions characterized by progressive distal muscle wasting, weakness and sensory dysfunction [7]. GARS encodes the non-redundant, homodimeric enzyme, glycyl-tRNA synthetase (GlyRS), which covalently links the amino acid glycine to its cognate tRNA, making it essential for protein translation fidelity. Two GARS translational start sites result in the production of mitochondrial and cytoplasmic GlyRS isoforms. Two mouse models of CMT2D, GarsNmf249/+ and GarsC201R/+, result from dominant amino acid substitutions and display features akin to the human condition, including muscle weakness and peripheral axon degeneration [8,9]. Mouse phenotypes do not correlate with aminoacylation function [8,9,10,11], a 50%

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Conclusion