Age-Based Risk Estimates for C9orf72RE-related Diseases: Theoretical Developments and Added Value for Genetic Counselling

The C9orf72 hexanucleotide repeat expansion is the most common genetic cause of amyotrophic lateral sclerosis (ALS) or frontotemporal dementia (FTD). In genetic counseling, children of mutation carriers are often told that they have a 50% risk of carrying the mutation, but this figure does not take into account the fact that penetrance is age-related, with a unimodal distribution of disease onset around 58 years of age. Using a Bayesian approach, we developed a theory to calculate the probability of carrying the mutation for asymptomatic relatives (children/siblings and grandchildren/niblings) as well as the probability of developing ALS/FDT within a given time frame, based on their age. Usi