Genetic dissection of photosensitivity and its relation to idiopathic generalized epilepsy.

Photosensitivity or photoparoxysmal response (PPR) is a common and highly heritable electroencephalographic trait characterized by an abnormal visual sensitivity of the brain in reaction to intermittent photic stimulation. PPR occurs frequently associated with idiopathic generalized epilepsies (IGEs). The present genomewide linkage scan was designed to map susceptibility loci for PPR and to explore their genetic relationship with IGE. The study included 60 families with at least two siblings displaying PPR. To dissect PPR-specific and IGE-related susceptibility loci, we defined two distinct family subgroups, comprising 19 families with predominantly pure PPR and photosensitive seizures (PPR-families) and 25 families, in which PPR was strongly associated with IGE (PPR/IGE-families). MOD score analyses provided significant evidence for linkage to the region 6p21.2 in the PPR-families (empirical p = 0.00004) and suggestive evidence for linkage to the region 13q31.3 in the PPR/IGE families (p = 0.00015), both with a best-fitting recessive mode of inheritance. In the PPR/IGE-families, linkage evidence was even stronger (p = 0.00003) when the trait definition was broadened by IGE traits. Our study shows two PPR-related susceptibility loci, depending on the familial background of IGE. The locus on 6p21.2 seems to predispose to PPR itself, whereas the locus on 13q31.3 also confers susceptibility to IGE.