Charcot-Marie-Tooth–Linked Mutant GARS Is Toxic to Peripheral Neurons Independent of Wild-Type GARS Levels

Abstract

Charcot-Marie-Tooth disease type 2D (CMT2D) is a dominantly inherited peripheral neuropathy caused by missense mutations in the glycyl-tRNA synthetase gene ( GARS). In addition to GARS, mutations in three other tRNA synthetase genes cause similar neuropathies, although the underlying mechanisms are not fully understood. To address this, we generated transgenic mice that ubiquitously over-express wild-type GARS and crossed them to two dominant mouse models of CMT2D to distinguish loss-of-function and gain-of-function mechanisms. Over-expression of wild-type GARS does not improve the neuropathy phenotype in heterozygous Gars mutant mice, as determined by histological, functional, and behavioral tests. Transgenic GARS is able to rescue a pathological point mutation as a homozygote or in complementation tests with a Gars null allele, demonstrating the functionality of the transgene and revealing a recessive loss-of-function component of the point mutation. Missense mutations as transgene-rescued homozygotes or compound heterozygotes have a more severe neuropathy than heterozygotes, indicating that increased dosage of the disease-causing alleles results in a more severe neurological phenotype, even in the presence of a wild-type transgene. We conclude that, although missense mutations of Gars may cause some loss of function, the dominant neuropathy phenotype observed in mice is caused by a dose-dependent gain of function that is not mitigated by over-expression of functional wild-type protein.

Author Summary

Mutations in the glycyl-tRNA synthetase gene ( GARS) cause Charcot-Marie-Tooth disease type 2D, a disease characterized by neuronal axon loss in the arms and legs, resulting in weakness and sensory problems. The GARS protein is essential for protein synthesis in every cell, and it has been difficult to determine whether the mutations result in disease because they impair this function or whether GARS somehow becomes toxic when it is mutated. We generated mice that overexpress normal GARS and mated these to two different mouse models of the disease to determine whether a restoration of normal function could prevent the disease. These crosses demonstrated that the mutant forms of GARS are toxic, and this toxic effect increases as the amount of mutant protein increases. Furthermore, this toxicity cannot be reduced or prevented by providing additional normal GARS. Therefore, these results suggest that, for most patients, therapies need to specifically target the mutant form of GARS or the toxic function.