Familial cosegregation of rare genetic variants with disease in complex disorders

Family-based designs are increasingly being used for identification of rare variants in complex disorders. This paper addresses two questions related to the utility of these designs. First, under what circumstances are rare disease-related variants expected to cosegregate with disease in families? Second, under what circumstances is a disease-variant association expected to be greater in studies restricted to familial cases than in studies of unselected cases? To investigate these questions, we developed a probability model of disease causation involving two loci. To address cosegregation, we examined the probability that an affected first-degree relative of a variant-carrying proband would also carry the variant. We find that this probability increases with increasing odds ratio (OR) for the variant, but declines with increasing sibling recurrence risk ratio (λs). For example, under reasonable assumptions, the 15q13.3 microdeletion in idiopathic generalized epilepsy, with an OR estimate of 68 in large case-control studies, is expected to be present in >95% of affected first-degree relatives of variant-carrying probands. However, for a variant with OR=5, the probability an affected relative has the variant ranges from 82% (when λs=2) to 58% (when λs=50). We also find that restriction of a study to familial cases does not necessarily increase a rare variant's association with disease, especially if λs is high and the variant contributes little to overall disease familial aggregation. These findings provide guidance for the design of family-based studies of rare variants in complex disorders.