'Passenger gene' problem in transgenic C57BL/6 mice used in hearing research

Despite recent advances in genome engineering technologies, traditional transgenic mice generated on a mixed genetic background of C57BL/6 and 129/Sv mice remain widely used in age-related hearing loss (AHL) research, since C57BL/6 mice exhibit early onset and progression of AHL due to a mutation in cadherin 23-encoding gene (Cdh23753G>A). In these transgenic mice, backcrossing for more than 10 generations results in replacement of the donor background (129/Sv) with that of the recipient (C57BL/6), so that approximately 99.9% of genes are C57BL/6-derived and are considered congenic. However, the regions flanking the target gene may still be of 129/Sv origin, creating a so-called "passenger gene problem" where the normal 129/Sv-derived Cdh23753G allele can travel with the target gene. In this study, we investigated the role of fatty acid-binding protein 7 (Fabp7), which is important for cellular uptake and intracellular trafficking of fatty acids in the cochlea, using traditional Fabp7 knockout (KO) mice on the C57BL/6 background. We found that Fabp7 KO mice showed delayed AHL progression and milder cochlear degeneration. However, the genotype of the Cdh23 region flanking Fabp7 was still that of 129/Sv origin (Cdh23753GG). Our findings reveal the potential risk of contamination for traditional transgenic mice generated on the C57BL/6 background.