Abstract 6711: Investigating competitive advantage of mismatch repair loss in colonic organoids

Abstract Lynch syndrome (LS) is an inherited predisposition to developing colon, endometrial, ovarian, and other cancers, affecting about one in 300 people. It is caused by inheriting a pathogenic variant in one copy of a DNA mismatch repair (MMR) gene. Cancers that arise in LS patients are MMR deficient, meaning the cancer cells have lost the remaining, functional MMR allele and thereby lost MMR function. It is known that losing MMR function leads to a drastically increased mutation rate. Additionally, cells in culture that lose MMR function become unresponsive to normally lethal DNA damage caused by alkylating agents. However, the question remains whether this immediate, aberrant DNA damage response contributes to cancer development in LS patients. We hypothesize that loss of this MMR-dependent damage response function provides a competitive advantage to colonic progenitor cells. Importantly, we predict that the strength of this advantage depends on the amount and type of DNA-damaging agents in the environment, such as oxidative stress, that promote expansion of MMR-deficient cells. If this hypothesis is true, it would provide insight into how this selective pressure could be mitigated using chemoprevention, and thereby decrease the incidence of colon cancer in this population. To test this, we are using stem cell-derived human colonic organoids and CRISPR gene editing to examine differences in survival and proliferation between otherwise syngeneic MMR-deficient (dMMR) and MMR-proficient (pMMR) colonic organoids. We observe that in atmospheric oxygen, dMMR colonic organoids exhibit enhanced budding compared to pMMR colonic organoids, consistent with our hypothesis of a competitive advantage of dMMR colonic cells. Addition of the antioxidant vitamin C decreases the difference in budding between pMMR and dMMR organoids suggesting this difference is mediated by oxidative stress. Additionally, mixed dMMR and pMMR colonic organoid cultures in ambient oxygen become increasingly dMMR overtime, suggesting dMMR colonic progenitor cells have a competitive advantage over pMMR colonic progenitor cells. To explore the mechanism behind this finding, we are analyzing differences in gene expression between pMMR and dMMR colonic organoids. Preliminarily, we have observed increased expression of the senescent marker gene CDKN2A (p16) and decreased expression of the colonic stem cell marker LGR5 in pMMR organoids compared to dMMR organoids. These results suggest that the MMR pathway may promote senescence and/or terminal differentiation in colonic organoid cultures grown in ambient oxygen. Taken together, these results are consistent with our hypothesis that loss of MMR function imparts a competitive advantage in colonic cells in non-transformed colonic organoids that could contribute to cancer formation in LS patients. Citation Format: Caroline Guild, Abhijit Rath, Christopher Heinen. Investigating competitive advantage of mismatch repair loss in colonic organoids [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6711.