INHIBITING DNA MISMATCH REPAIR IN COLORECTAL CANCER

Abstract

Colorectal cancer (CRC) remains one of the most prevalent cancers in the US with a high mortality rate. Locally advanced and metastatic CRC have poor prognosis and only a small subset (5- 15%) of patient tumors with DNA mismatch repair deficiency (dMMR) respond to immunotherapy. Low numbers of tumor infiltrating lymphocytes (TIL) remain an obstacle to effective immunotherapy for the majority of mismatch repair proficient (pMMR) CRC. dMMR CRCs have increased tumor mutational burden and show a robust inflammatory multicellular network of stromal and immune cells, including TIL, within the tumor. To investigate whether disrupting MMR in pMMR CRC could improve immune responsiveness, we designed a method of tumor-targeted gene knockdown by constructing EpCAM aptamer-linked siRNAs (called aptamer- siRNA chimeras or AsiCs) to target the MMR gene Mlh1 and convert pMMR CRC to dMMR cancers sensitive to immune checkpoint blockade. The MLH1 AsiC bound to EpCAM+ CRC cell lines and induced tumor-specific MLH1 knockdown in vitro and in vivo with no apparent toxicity. Mice bearing subcutaneous mouse pMMR CRC cell line (SL4) tumors overexpressing EpCAM treated with EpCAM AsiCs against Mlh1 showed decreased tumor growth and enhanced survival. These findings suggest that tumor-targeted knockdown of MMR genes might provide a promising strategy to expand the range of CRC tumors that respond to immunotherapy.