In the current study five subcongenic strains with overlapping and non-overlapping wild-derived genomic regions were developed to fine map a previously identified obesity QTL.
Previously a congenic strain, B6.Cg-Pbwg1, was developed with a 44.1 Mb genomic region derived from the Philippine wild mouse, Mus musculus castaneus, on the genetic background of a common inbred strain, C57BL/6J (B6). The M. m. castaneus mouse has only 60% of the body weight of the B6 mouse. Subsequent QTL analysis in an F2 intercross between B6.Cg-Pbwg1 and B6 revealed several closely linked QTLs affecting body weight gain, lean body weight and obesity within the 44.1 Mb region harboring Pbwg1, a growth QTL on mouse chromosome 2 [J:121981].
In the current study recombinants from an intercross of B6.Cg-Pbwq1 x B6 mice were mated to fix the following wild derived genomic regions on Chromosome 2:
30.54-61.54 Mb in subcongenic B6.Cg-Pbwg1-(D2Mit33-D2Mit472)<M. m. castaneus>/(SR13)Aishk;
65.15-70.48 Mb in subcongenic B6.Cg-Pbwg1-(D2Mit205-D2Mit56)<M. m. castaneus>/(SR12)Aishk;
38.09-61.54 Mb in subcongenic B6.Cg-Pbwg1-(D2Mit7-D2Mit472)<M. m. castaneus>/(SR8)Aishk;
30.45-52.76 Mb in subcongenic B6.Cg-Pbwg1-(D2Mit33-D2Mit270)<M. m. castaneus>/(SR3)Aishk;
30.45-31.20 Mb in subcongenic B6.Cg-Pbwg1-(D2Mit33-D2Mit64)<M. m. castaneus>/(SR4)Aishk.
Obesity traits: body weight, lean body weight, inguinal fat pad weight, gonadal fat pad weight, retroperitoneal fat pad weight and total fat pad weight were compared in the five subcongenics (273 mice) and their parental strains (182 mice) on a low fat standard chow diet and on a high fat diet at 13 weeks of age.
In the comparison of strains fed the standard chow a locus that affected obesity was identified within the region common to B6.Cg-Pbwg1-(D2Mit33-D2Mit472)<M. m. castaneus>/(SR13)Aishk and B6.Cg-Pbwg1-(D2Mit7-D2Mit472)<M. m. castaneus>/(SR8)Aishk. Similar to the standard chow diet, the B6.Cg-Pbwg1-(D2Mit7-D2Mit472)M.m.castaneus/(SR8)Aishk strain fed a high fat diet showed significantly lower body weight and obesity-related traits.
When the data from the standard chow and the high-fat diets were combined and reanalyzed the obesity locus was physically defined within an approximately 8.8 Mb region between D2Mit270 and D2Mit472. The region, derived from the wild mouse, was resistant to high fat diet and contained the positions of peak LOD scores for 4 previously identified fat pad QTLs, Pbwg1.5, Pbwg1.6, Pbwg1.7 and Pbwg1.8, J:121981. [Figure 1.] It was not possible to conclude whether the four previously identified QTLs and the newly identified QTL were the same or closely linked loci. MGI curators have named the obesity resistant locus mapped in the current study, Obrq21, obesity resistant QTL 21.
Since the B6.Cg-Pbwg1-(D2Mit7-D2Mit472)M.m.castaneus/(SR8)Aishk subcongenic showed significant reductions in body weight and obesity-related traits , the serum levels of glucose, triglyceride, total cholesterol and high density lipoprotein were analyzed. Compared with B6 and B6.Cg-Pbwg1-(D2Mit33-D2Mit64)<M. m. castaneus>/(SR4)Aishk, a positive control, glucose and triglyceride levels were significantly lower in both sexes than in B6 mice. The serum level of high density lipoprotein was also lower in SR8 males, but not in females, than in males of the B6 strain.
The 8.8 Mb region of Obrq21 contained 29 genes (UCSC Genome Browser). Microarray analysis revealed 4 genes in the liver and seven genes in the gonadal fat pad that were differentially expressed between B6.Cg-Pbwg1 and B6 strains. Gpd2 was upregulated in both the liver and fat of B6.Cg-Pbwg1 mice, Acrv1c was upregulated in fat only. Upp2, MArch7, and Rbms1 were downregulated in the liver and fat of the B6.Cg-Pbwg1 mice.