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^ApcΔ716/+) 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^V600ELgr5^CreMin (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 IFNγ 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.

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^V600ELgr5^CreMin mice, wherein tumors phenocopy aspects of human BRAF-mutated tumors and have a distinct IFNγ-dominant immune microenvironment uniquely responsive to immune checkpoint blockade.

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