Whole genome analyses reveal no pathogenetic single nucleotide or structural differences between monozygotic twins discordant for amyotrophic lateral sclerosis

Karyn Meltz Steinberg, Thomas J. Nicholas, Daniel C. Koboldt, Bing Yu, Elaine Mardis, Roger Pamphlett

The contribution of genetic and environmental factors to the pathogenesis of sporadic amyotrophic lateral sclerosis (ALS) remains unclear. To investigate the genetic component of the disease, we performed whole genome sequencing on ALS discordant monozygotic twins. Illumina whole genome sequencing on white blood cell DNA of five ALS-discordant monozygotic twin pairs (10 samples in total) yielded ∼30x coverage per individual. All single nucleotide variants, indels, and structural variants (copy number variants, inversions and translocations) were called and evaluated for functional consequence, evolutionary conservation, population frequency and overlap with known ALS associated variants and genes. Results showed that no validated discordant coding or regulatory single nucleotide variants or indels were found, and nor were any genome-wide discordant structural variants detected. Concordant variants of particular interest were: 1) two rare, highly-conserved heterozygous non-synonymous variants in SYT9 and EWSR1, genes previously associated with ALS (out of 2044 rare heterozygous variants detected); 2) three rare homozygous missense variants; and 3) three novel copy number deletions that overlapped genes. In conclusion, no convincing coding or regulatory nucleotide or genome-wide structural differences were found between ALS discordant monozygotic twins. The results suggest that more work is needed to elucidate possible environmental, epigenetic, oligogenic and somatic genetic factors that could underlie susceptibility to sporadic ALS.