A new <scp>CRISPR</scp>‐mediated <i>Apc</i> knockout allele leads to pyloric gland adenoma‐like gastric polyps in mice with <scp>C57BL</scp>/6;<scp>FVB</scp>/N mixed background

Adenomatous polyposis coli (APC) mutations are the most frequently identified genetic alteration in sporadic colorectal cancer (CRC) cases, and a myriad of genetically engineered Apc-mutant CRC mouse models have been developed using various genetic manipulation techniques. The advent of the CRISPR/Cas9 system has revolutionized the field of genetic engineering and facilitated the development of new genetically engineered mouse models. In this study, we aimed to develop a novel Apc knockout allele using the CRISPR/Cas9 system and evaluate the phenotypic effects of this new allele in two different mouse strains. For this purpose, exon 16 of mouse Apc gene was targeted with a single-guide RNA, and the mouse carrying an Apc frameshift mutation at codon 750 (Δ750) was chosen as the founder. The mutant FVB-Apc^Δ750 mice were backcrossed with wild-type C57BL/6 mice, and the phenotypic effects of the knockout allele were evaluated in F8-FVB-Apc^Δ750, F4-B6;FVB-Apc^Δ750, and F1-B6;FVB-Apc^Δ750 by a macroscopic and microscopic examination of the gastrointestinal system. The result showed that the mean polyp number was significantly higher in F4-BL6;FVB-Apc^Δ750 than in F8-FVB-Apc^Δ750. Intestinal polyposis was more prominent in F4-BL6;FVB-Apc^Δ750, whereas a higher number of colon polyps than intestinal polyps were observed in F8-FVB-Apc^Δ750. Additionally, F1-BL6;FVB-Apc^Δ750 mixed background mice developed gastric polyps that morphologically resembled the pyloric gland adenoma of humans. In conclusion, we developed a novel CRISPR-mediated Apc knockout allele using two mouse strains. We showed that this allele can exert a strain-specific effect on the phenotype of mice and can cause gastric polyp formation.