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

Abstract

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 (MinApcΔ716/+) enterotoxigenic Bacteroides fragilis (ETBF) murine model is characterized by IL17-dependent, distal colon adenomas. Herein, we report that the addition of the BRAFV600E mutation to this model results in the emergence of a distinct locus of midcolon tumors. In ETBF-colonized BRAFV600ELgr5CreMin (BLM) mice, tumors have similarities to human BRAFV600E 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. 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 BRAFV600ELgr5CreMin mice, wherein tumors phenocopy aspects of human BRAF-mutated tumors and have a distinct IFNγ-dominant immune microenvironment uniquely responsive to immune checkpoint blockade. This article is highlighted in the In This Issue feature, p. 1601

Microbiota and Colorectal Cancer: From Gut to Bedside

“…Notably, this signature was associated with a recruitment of PD-L1 expressing myeloidderived suppressor cells (MDSC) and CD8 + tumour infiltrating T cells. In line with this, treatment of tumourbearing mice with PD-L1 blockade therapy led to reduced colon tumour numbers in this model (DeStefano Shields et al, 2021). Additionally, loss of membrane-associated E-cadherin, in vitro was reported after treatment of a colon cancer cell line with BFT (Wu et al, 2003).…”

“…More recently, introduction of a BRAF V600E mutation into the pre-existing Min ApcΔ716/+ ETBF mouse model ( Wu et al, 2009 ) caused a shift in the spectrum of colon tumours towards a more proximal location within the colon ( DeStefano Shields et al, 2021 ). In addition, these tumours displayed a serrated-like histopathology and were characterized by a strong IFNλ-driven immune signature.…”

Comparative Analysis of Colon Cancer-Derived Fusobacterium nucleatum Subspecies: Inflammation and Colon Tumorigenesis in Murine Models

“…vincentii failed to exhibit weight loss or changes in colon length typically seen with inflammation/colitis ( 25 ) (data not shown). Because data suggest that host-gene microbe interactions affect pathogenesis ( 26 ), we further tested CRC- F. nucleatum subsp. vincentii in SPF Apc Min/+ mice.…”

Differential pre-malignant programs and microenvironment chart distinct paths to malignancy in human colorectal polyps

“…Increased methylation has been found to be dependent on extrinsic factors such as aging ( Tao et al, 2019 ), consistent with the preponderance of BRAF V600E mutations in MSI-H CRCs in older individuals ( Lieu et al, 2019 ). Shown more recently, microbial dysbiosis can also be an environmental trigger for hypermethylation ( DeStefano Shields et al, 2021 ). Antibiotic suppression of the microbiota reduces colonic tumorigenesis in a Braf mutant model ( Leach et al, 2021 ), whereas in another study, enterotoxigenic Bacteroides fragilis (ETBF) infection is a required trigger for tumorigenesis in the proximal mid-colon in a Braf mutant mouse model ( DeStefano Shields et al, 2021 ).…”

“…Shown more recently, microbial dysbiosis can also be an environmental trigger for hypermethylation ( DeStefano Shields et al, 2021 ). Antibiotic suppression of the microbiota reduces colonic tumorigenesis in a Braf mutant model ( Leach et al, 2021 ), whereas in another study, enterotoxigenic Bacteroides fragilis (ETBF) infection is a required trigger for tumorigenesis in the proximal mid-colon in a Braf mutant mouse model ( DeStefano Shields et al, 2021 ). In the latter report, the earliest events of the ETBF response in epithelial cells prior to tumor formation occur at the colonic mucosal surface, where colonic epithelial cells and luminal contents interact.…”