Genetic landscapes in neuromuscular disorders: The influence of next-generation sequencing analysis

Inherited neuromuscular disease (NMDs) are a large group of diseases involving muscles, motor neurons, neuromuscular junctions and nerves; within a given disease group genetic and clinical heterogeneity is the hallmark. The genetic diagnosis in rare diseases is now becoming mandatory for the inclusion in emerging therapeutic trials and the heterogeneous genetic landscape of NMDs raises challenges regarding the definition of a molecular diagnosis. The next generation sequencing approach in NMDs patients is therefore aimed at improving the diagnostic definition through clinical gene panel analysis (diagnostic field) and is also aimed at the identification of novel causative genes through WES (whole exome sequencing analysis, research field). Within the EU neuromics project we performed WES analysis in 6 “families of four” and 2 “trios”: 3 of them with congenital myopathy/dystrophy, 1 with spastic paraplegia, 2 with ataxia, 1 with myofibrillar myopathy, and 1 with AV block and LGMD. WES analysis unraveled the genetic cause of 5 out of 7 families. We identified 3 known gene mutations (RYR1, ISPD and STIM1) and 2 novel causative genes, functionally validated. In the remaining 3 families, a few candidates were identified and functional assays are in progress. Moreover we performed WES analysis in 2 families with Bethlem myopathy and in 2 patients affected by Ullrich muscular dystrophy with no identified mutation in COL6 genes. The WES output data were prioritized on the basis of a list of 115 candidate genes involved in collagen VI myopathy (clinical exome). All the prioritized variants were validated by Sanger and functional validation by pathway analysis is ongoing. Finally, for diagnostic purpose, we studied two families with hereditary neuropathy and hereditary ataxia, orphan of genetic definition, with a clinical gene-panel approach. In both families we identified the causative mutation in known genes (ATP7A and AFG3L2).