Targeting genomic rearrangements in tumor cells through Cas9-mediated insertion of a suicide gene

Insertion of the sequence encoding herpes simplex virus 1 thymidine kinase at the breakpoints of fusion genes in cancer cells causes cell death and regression of mouse xenograft tumors. Specifically targeting genomic rearrangements and mutations in tumor cells remains an elusive goal in cancer therapy. Here, we used Cas9-based genome editing to introduce the gene encoding the prodrug-converting enzyme herpes simplex virus type 1 thymidine kinase (HSV1-tk) into the genomes of cancer cells carrying unique sequences resulting from genome rearrangements. Specifically, we targeted the breakpoints of TMEM135–CCDC67 and MAN2A1–FER fusions in human prostate cancer or hepatocellular carcinoma cells in vitro and in mouse xenografts. We designed one adenovirus to deliver the nickase Cas9 D10A and guide RNAs targeting the breakpoint sequences, and another to deliver an EGFP-HSV1-tk construct flanked by sequences homologous to those surrounding the breakpoint. Infection with both viruses resulted in breakpoint-dependent expression of EGFP-tk and ganciclovir-mediated apoptosis. When mouse xenografts were treated with adenoviruses and ganciclovir, all animals showed decreased tumor burden and no mortality during the study. Thus, Cas9-mediated suicide-gene insertion may be a viable genotype-specific cancer therapy.