Congenic strains reveal effects of the epilepsy quantitative trait locus, El2, separate from other El loci

Congenic mouse strains made by transferring epilepsy predisposing alleles El1, El2, and El3 from the EL/Suz strain to the ABP/Le recipient were tested for seizure frequency following gentle rhythmic stimulation. Mice homozygous for El2, but not El1 or El3, experienced seizures much more frequently than ABP controls, while respective El1 homozygotes and El2 heterozygotes had only a modest increase over ABP, and El3 homozygotes showed no increase. Association between marker genotypes and seizure frequency in small intra-strain crosses showed that the phenotypic effects of El2 map to the selected interval, and that segregation of El2 accounts for virtually all genetic effects. However, in separating El2 from other EL susceptibility alleles, the seizure frequency phenotype was weaker and less heritable than in crosses between parental strains. These results confirm El2 as an important QTL and show that it has significant phenotypic effects in the absence of other EL-derived alleles, including El1. In addition, the present localization of El2 on Chr 2 suggests several potential candidate genes for El2, including the beta subunit of phospholipase-C. The approach to dissecting complex traits by making congenic strains for individual QTL is discussed.