Dissection of a genetically complex cluster of growth and obesity QTLs on mouse chromosome 2 using subcongenic intercrosses

In a previous study we characterized the B6.CAST-(D2Mit329-D2Mit457)N(6) (B62D) congenic strain, which possesses CAST/EiJ (CAST) chromosome 2 donor alleles from 74 to 180 Mbp on a C57BL6/J (B6) background. This strain exhibited significant decreases in body weight and adiposity attributable to the weight gain 2 (Wg2) quantitative trait locus (QTL). To refine the location of Wg2, we used a two-stage genetic dissection strategy consisting of a B62D × B6 backcross, which mapped Wg2 to the proximal portion of the B62D donor region, followed by the development of seven overlapping subcongenic F₂ intercrosses targeting the Wg2 genomic interval. Surprisingly, five of the seven intercrosses displayed significant differences, dependent on genotype, in body weight and/or fat pad mass. These effects were the result of at least four independent QTLs that were named Wg2a, b, c, and d. In contrast to the lean and low body weight phenotype of the B62D parental strain, mice homozygous for CAST congenic alleles (cast/cast) at Wg2a were significantly heavier at 6 and 9 weeks of age, while cast/cast mice at Wg2c had higher levels of total fat. Consistent with the prior observed effects of Wg2, cast/cast mice at Wg2b displayed significant decreases in 6- and 9-week body weight as well as a decrease in total fat pad mass. All of the QTLs had additive effects on body composition except Wg2d, which displayed underdominance for total fat mass. Significant differences in weight and adiposity were also observed in genetically identical b6/b6 homozygous mice across the panel of subcongenics, suggesting either maternal or paternal contributions to body composition. These data represent a significant advancement toward the identification of mouse chromosome 2 growth and obesity quantitative trait genes.