Reciprocal congenics defining individual quantitative trait loci for sedative/hypnotic sensitivity to ethanol

Background: We have identified four major genes or quantitative trait loci (QTLs) that determine duration of loss of righting reflex (LORR), induced by sedative doses of ethanol: Lore1, Lore2, Lore4, and Lore5. Together these genes explain more than 50% of the phenotypic variance for sensitivity to the sedative/hypnotic effects of ethanol between the Inbred Long Sleep (ILS) and Inbred Short Sleep (ISS) strains of mice. The derivation of these strains is reviewed here. Methods: Each QTL has been bred onto the opposite background (ILS or ISS) through 10 rounds of backcrossing by using OTL-marker-assisted counter selection to produce reciprocal congenic strains. Mice were genotyped for markers that flanked each of the QTLs. Selection for the donor at the desired QTL, and against donor markers at the other four QTLs, allowed rapid fixation of the genetic background. Phenotypic assessment in the ISS-recipient congenic strains was conducted throughout the backcross. Results: By the N-5 generation, phenotypic assessments failed to detect significant effects in some sublines; these sublines were discarded and positive lines split to create new replicate sublines. In the N-10 all sublines retained the phenotypic difference between heterozygotes and ISS homozygotes; however, the expected additive effect was not found in the Lore1 congenics. On the ILS background, each Lore was captured, as shown by the expected differential LORR. Two strains on the ILS background, and one on the ISS, exhibited the differential effect on blood ethanol concentration associated with the donor strain. Conclusions: Congenic strains represent an important resource for confirmation of previously identified QTLs, for identification and mapping of additional phenotypes, and for exclusion of candidate genes. QTL-marker-assisted selection rapidly stabilized the genetic background within four generations (based on phenotypic assessments); however, phenotypic selection during the backcrossing to generate congenic strains did not contribute to the successful capture of the ISS QTLs.