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^emI8∆1 and Bbs7^emI8∆2) 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^emI8∆1 and Bbs7^emI8∆2 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_{(Bbs7emI8∆1)} = 4.25 × 10^–7; P_{(Bbs7emI8∆2)} = 3.17 × 10^–5). Consistent with previous findings, the recessive effect of the BFMI allele was also seen for the B6N Bbs7^emI8∆1 and Bbs7^emI8∆2 alleles. However, the effect size of the B6N Bbs7^emI8∆1 and Bbs7^emI8∆2 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.