Fucci-G1 insertion partially disrupts Shh expression

The Fucci mouse lines are valuable tools for tracing cell cycle stages during development and in postnatal tissues; however, the insertion loci for the transgenes remain unidentified. Here, we report the genomic insertion sites of both the Fucci-G1 and -S/G2/M transgenes, identified through whole genome sequencing. Interestingly, the Fucci-G1 transgene is located proximal to the Shh lung gut enhancer (SLGE) on chromosome 5. Homozygous Fucci-G1 embryos survive until embryonic day 18.5 (E18.5), exhibiting craniofacial and foregut-related defects, but are perinatal lethal. Expression of Shh and canonical targets are down-regulated in the pharyngeal arches of E11.5 homozygous Fucci-G1 embryos. Furthermore, in situ hybridization analysis showed diminished Shh expression in the pharyngeal ectoderm and foregut endoderm, consistent with disruptions to SLGE activity. Our results indicate that heterozygous Fucci-G1 mice are suitable for cell cycle studies; however, we caution potential genetic interference when investigating SHH-related pathways.