The first objective is to confirm pathogenicity of a muscle-specific IARS haploinsufficiency as observed in a cohort of three siblings. The second objective is to investigate epimutations as a cause of aberrant IARS expression.

Despite recent advancements in next-generation DNA-sequencing technologies, at least 25% of myopathy patients remain without a genetic diagnosis. In order to increase diagnostic yield, we combined omic technologies to identify novel gene mutations for functional studies in a cohort of 17 patients with rare undiagnosed myopathies. Our first sub-cohort consists of three siblings with a mild congenital muscular dystrophy. Exome and RNA-sequencing analysis identified a maternally inherited missense variant and an allelic imbalance suggesting degradation of the paternal allele in a muscle-specific manner in the gene aminoacyl-tRNA synthetase (IARS).

The impact of muscle-specific IARS downregulation on muscle function and structure was assessed in C. elegans. Specifically, we created a muscle specific knock-down using IARS siRNA (R11A8.6) and a transgenic strain of C. elegans (NR350) sensitized for RNAi solely in muscle cells. The effect of epimutations on IARS expression will be investigated in fibroblasts derived from the patients.

Locomotion impairment, lifespan, and synaptic transmission in IARS depleted worms will be compared to controls (empty vector) using video microscopy, a lifespan assay, and an aldicarb-induced paralysis assay respectively. Analysis of muscle structure integrity, evaluated by fluorescence microscopy, has already revealed that IARS depleted worms demonstrate a disruption in the organization of the sarcomere as compared to negative controls.

While next-generation sequencing techniques have provided much evidence to support the pathogenicity of the muscle-specific allelic imbalance identified in these patients, verification by functional analysis is still required. While confirmation of pathogenicity will have a direct impact on the affected individuals and their families, it will also help inform disease progression, risk management and prognosis.