Oligogenicity, C9orf72 expansion, and variant severity in ALS

"Oligogenic inheritance" is used to describe cases where more than one rare pathogenic variant is observed in the same individual. While multiple variants can alter disease presentation, the necessity of multiple variants to instigate pathogenesis has not been addressed in amyotrophic lateral sclerosis (ALS). We sequenced ALS-associated genes in C9orf72-expansion-positive and negative ALS patients, alongside unaffected controls, to test the importance of oligogenicity and variant deleteriousness in ALS. We found that all groups had similar numbers of rare variants, but that variant severity was significantly higher in C9orf72-negative ALS cases, suggesting sufficiency of C9orf72 expansion to cause ALS alone.