Reduced penetrance of gene variants causing amyotrophic lateral sclerosis

Background: amyotrophic lateral sclerosis overlaps etiologically and genetically with frontotemporal dementia and occurs in both familial and apparently sporadic forms. The most commonly implicated genes are C9orf72, SOD1, TARDBP and FUS. Penetrance of disease-causing variants in these genes is known to be incomplete but has not been wellstudied at population level.Objective: we sought to determine the population-level penetrance of pathogenic and likely pathogenic variants in genes commonly causing amyotrophic lateral sclerosis.Methods: published epidemiological data for amyotrophic lateral sclerosis and frontotemporal dementia were used to calculate expected frequencies of disease-causing variants per gene at population level. Variant data from gnomAD and ClinVar databases were used to ascertain observed numbers of disease-causing variants and to estimate populationlevel penetrance per gene. Data for C9orf72 were obtained from published literature. Results: maximum population penetrance for either amyotrophic lateral sclerosis or frontotemporal dementia was found to be 33% for C9orf72 (95% CI [20.9, 53.2]), 54% for SOD1 (95% CI [32.7, 88.6]), 38% for TARDBP (95% CI [21.1, 69.8]) and 19% for FUS (95% CI [13.0, 28.4]).Conclusion: population-level penetrance of amyotrophic lateral sclerosis disease genes is reduced. This finding has implications for the genetic testing and counselling of affected individuals and their unaffected relatives