Exome sequencing in the knockin mice generated using the CRISPR/Cas system

Knockin (KI) mouse carrying a point mutation has been an invaluable tool for disease modeling and analysis. Genome editing technologies using the CRISPR/Cas system has emerged as an alternative way to create KI mice. However, if the mice carry nucleotide insertions and/or deletions (InDels) in other genes, which could have unintentionally occurred during the establishment of the KI mouse line and potentially have larger impact than a point mutation, it would confound phenotyping of the KI mice. In this study, we performed whole exome sequencing of multiple lines of F1 heterozygous Ntrk1 KI mice generated using the CRISPR/Cas system in comparison to that of a wild-type mouse used as a control. We found three InDels in four KI mice but not in a control mouse. In vitro digestion assay suggested that each InDel occurred as a de novo mutation, was carried-over from the parental mice, or was incorporated through the Cas9 nuclease mediated off-target cleavage. These results suggest that frequency of InDels found in KI mice generated by the CRISPR/Cas technology is not high, but cannot be neglected and careful assessment of these mutations is warranted.