Phenotypic divergence in two lines of L-Fabp^-/- mice reflects substrain differences and environmental modifiers

Phenotypic divergence in diet-induced obesity (DIO) and hepatic steatosis has been reported in two independently generated lines of L-Fabp^-/- mice [New Jersey (NJ) L-Fabp^-/- vs. Washington University (WU) L-Fabp^-/- mice]. We performed side-by-side studies to examine differences between the lines and investigate the role of genetic background, intestinal microbiota, sex, and diet in the divergen tphenotypes. Fasting-induced steatosis was attenuated in both L-Fabp^-/- lines compared with C57BL/6J controls, with restoration of hepatic triglyceride levels following adenoviral L-Fabp rescue. Both lines were protected against DIO after high-saturated-fat diet feeding. Hepatic steatosis was attenuated in WU but not NJ LFabp ^-/- mice, although this difference between the lines disappeared upon antibiotic treatment and cohousing. In contrast, there was phenotypicdivergence in L-Fabp^-/- mice fed a high cocoa butter fat diet, with WU L-Fabp^-/- mice, but not NJ L-Fabp^-/- mice, showing protection against both DIO and hepatic steatosis, with some sexdependent (female > male) differences. Dense mapping revealed no evidence of unintended targeting, duplications, or deletions surrounding the Fabp1 locus in either line and only minor differences in mRNA expression of genes located near the targeted allele. However, a C57BL/6 substrain screen showed that the NJ L-Fabp^-/- line contains ~40% C57BL/6N genomic DNA, despite reports that these mice were backcrossed six generations. Overall, these findings suggest that some of the phenotypic divergence between the two L-Fabp^-/- lines may reflect unanticipated differences in genetic background, underscoring the importance of genetic background in phenotypic characterization.