2611. Enterotoxigenic Bacteroides fragilis Alters the Genome of Colon Epithelial Cells

BACKGROUND: Individuals born in 1990 have twice the risk of developing colon cancer and four times the risk of developing rectal cancer as those born in 1950. The gut microbiome is being proposed as a potential contributor to this difference because of the surge in obesity in the United States, the link between obesity and gut dysbiosis, and the growing number of studies which have associated a dysbiotic gut microbiome with CRC. Enterotoxigenic Bacteroides fragilis (ETBF) is one of the bacteria most studied in relation to CRC development; it is found at a higher frequency in both the stool and mucosa of CRC patients, and it rapidly induces tumor formation in an Apc(min/+) mouse model of CRC. In this model, tumor formation typically occurs via loss of heterozygosity (LOH) of the Apc gene, the genetic mutation found in approximately 80% of sporadic CRC cases. ETBF produces a potent exotoxin (BFT) which induces E-cadherin cleavage, β-catenin nuclear localization and colonic epithelial cell proliferation. But we still do not understand how these downstream effects cause lasting changes in the genome of colon epithelial cells that then initiate tumor formation and growth. As cancer is ultimately a disease that arises and progresses via changes in the genome, understanding these interactions is essential. METHODS: We hypothesize that ETBF induces DNA mutations via BFT that encourage tumor formation, and enhance tumor growth. To test this hypothesis, we performed whole-exome sequencing on tumors and normal tissue isolated from Apc(min/+) mice after ETBF or sham inoculation. Additionally, we isolated colon organoids from Apc(min/+) mouse normal tissue (colonoids) and Apc(min/+) mouse tumors (tumoroids) after ETBF or sham inoculation. We performed in vitro DNA damage assays and qPCR for Apc LOH on these colon organoids. RESULTS: Our preliminary data indicate that ETBF-induced tumors have lower rates of Apc LOH and that double-stranded DNA breaks are observed as soon as 3-hours after BFT treatment of colonoids and as soon as 72-hours after ETBF inoculation. CONCLUSION: These data suggest that in vivo, ETBF may induce mutations in cancer-driver genes which cause tumor formation via pathways other than somatic recombination at the Apc locus, a result we are now testing with additional (N = 19) whole-exome tumor sequencing in-progress. [Image: see text] DISCLOSURES: All authors: No reported disclosures.