Bacterial-Driven Inflammation and Mutant BRAF Expression Combine to Promote Murine Colon Tumorigenesis That Is Sensitive to Immune Checkpoint Therapy

Colorectal cancer is multifaceted, with subtypes defined by genetic, histologic, and immunologic features that are potentially influenced by inflammation, mutagens, and/or microbiota. Colorectal cancers with activating mutations in BRAF are associated with distinct clinical characteristics, although the pathogenesis is not well understood. The Wnt-driven multiple intestinal neoplasia (Min(Apc Delta 716/+)) enterotoxigenic Bacteroides fragilis (ETBF) murine model is characterized by IL17-dependent, distal colon adenomas. Herein, we report that the addition of the BRAF(V600E) mutation to this model results in the emergence of a distinct locus of midcolon tumors. In ETBF-colonized BRAF(V600E)Lgr5(Cre)Min (BLM) mice, tumors have similarities to human BRAF(V600E) tumors, including histology, CpG island DNA hypermethylation, and immune signatures. In comparison to Min ETBF tumors, BLM ETBF tumors are infiltrated by CD8(+) T cells, express IFN gamma signatures, and are sensitive to anti-PD-L1 treatment. These results provide direct evidence for critical roles of host genetic and microbiota interactions in colorectal cancer pathogenesis and sensitivity to immunotherapy. SIGNIFICANCE: Colorectal cancers with BRAF mutations have distinct characteristics. We present evidence of specific colorectal cancer gene-microbial interactions in which colonization with toxigenic bacteria drives tumorigenesis in BRAF(V600E)Lgr5CreMin mice, wherein tumors phenocopy aspects of human BRAF-mutated tumors and have a distinct IFN gamma-dominant immune microenvironment uniquely responsive to immune checkpoint blockade.