A deletion containing a CTCF-element in intron 8 of the Bbs7 gene is partially responsible for juvenile obesity in the Berlin Fat Mouse.

The Berlin Fat Mouse Inbred (BFMI) line is a model for juvenile obesity. Previous studies on crosses between BFMI and C57Bl/6N (B6N) have identified a recessive defect causing juvenile obesity on chromosome 3 (jObes1). Bbs7 was identified as the most likely candidate gene for the observed effect. Comparative sequence analysis showed a 1578 bp deletion in intron 8 of Bbs7 in BFMI mice. A CTCF-element is located inside this deletion. To investigate the functional effect of this deletion, it was introduced into B6N mice using CRISPR/Cas9. Two mice containing the target deletion were obtained (B6N Bbs7 and Bbs7) and were subsequently mated to BFMI and B6N to generate two families suitable for complementation. Inherited alleles were determined and body composition was measured by quantitative magnetic resonance. Evidence for a partial complementation (13.1-15.1%) of the jObes1 allele by the CRISPR/Cas9 modified B6N Bbs7 and Bbs7 alleles was found. Mice carrying the complementation alleles had a 23-27% higher fat-to-lean ratio compared to animals which have a B6N allele (P = 4.25 × 10; P = 3.17 × 10). Consistent with previous findings, the recessive effect of the BFMI allele was also seen for the B6N Bbs7 and Bbs7 alleles. However, the effect size of the B6N Bbs7 and Bbs7 alleles was smaller than the BFMI allele, and thus showed only a partial complementation. Findings suggest additional variants near Bbs7 in addition to or interacting with the deletion in intron 8.