Mutation-Specific CRISPR Targeting with SaCas9 and AsCas12a Restores Therapeutic Sensitivity in Treatment-Resistant Melanoma.

Background: Melanoma remains one of the most challenging cancers to treat effectively with drug resistant remaining a constant concern, primarily with activating BRAF mutations. Mutations in the BRAF gene appear in approximately 50% of patients, 90% of which are V600E. Two frontline BRAF inhibitors (BRAFi), vemurafenib and dabrafenib, are frequently used to treat unresectable or metastatic BRAF V600E melanoma. Initial response rates are high, but soon thereafter, 70-80% of patients develop resistance to treatment within a year. A major mechanism of resistance is the generation of a secondary Q61K mutation in the NRAS gene. Methods: We have developed an approach in which a CRISPR-Cas complex can be designed to distinguish between mutant genes enabling resistance to standard care in tumor cells and normal genomes of healthy cells. For the first time, we demonstrated the utility of two CRISPR-directed mutation-specific editing approaches to restore BRAFi sensitivity in BRAF^V600E/NRAS^Q61K resistant A375 cells. Results: We utilize an AsCas12a protospacer adjacent motif site created by the NRAS Q61K mutation and the Q61K mutation in the critical seed region of an SaCas9 sgRNA for Q61K-selective targeting. We show here that both approaches allow for effective NRAS targeting of only mutated-Q61K and after CRISPR-directed Q61K-targeting, previously resistant A375 cells are re-sensitized to BRAFi treatment. Conclusion: Our data support the feasibility of the development of CRISPR-Cas therapeutic approaches to the treatment of melanoma. Successful therapeutic CRISPR-directed gene editing would enable both specific and efficient editing of a mutation-specific targeting approach eliminate concern for on- and off-target damage to the genomes of healthy cells.