DNA mismatch repair (MMR) genes (e.g., MLH1, MSH2, MSH6, PMS2, and EPCAM) play an important role in maintaining genomic stability during DNA replication and recombination. Deficiency in MMR resulting from mutations in these genes leads to mutations in microsatellite regions throughout the genome (microsatellite instability; MSI) and in cancer driver oncogenes or tumor suppressor genes, which accumulate over time and eventually lead to cancer formation. Monoallelic germline mutation in MMR genes causes Lynch syndrome (LS). Among LS-related cancer types, the lifetime risk for colorectal cancer (CRC) is the highest (~80%). Frameshift mutations (FSMs) in coding microsatellites produce neoantigens, which have been shown to elicit immune responses. It was thus postulated that they can serve as vaccine targets. To develop a prophylactic vaccine and prevention strategy for this high-risk population, we characterized a LS mouse model (Msh2LoxP/LoxP;Villin-Cre) to determine whether these mice recapitulate the human LS oncogenic process. We found that tumor development was already notable at 7-8 months of age and median survival was 11.5 months. Histopathological analysis showed that tumors were adenoma or adenocarcinoma mixed with mucinous features. Using a targeted sequencing approach, a panel of FSMs in mononucleotide regions were identified in both tumors and histologically normal mucosa, suggesting that Msh2 deletion and FSMs were not sufficient for tumor development. In addition, Apc, Ctnnb, and Trp53 mutations were also observed with low frequency in organoids derived from these tumors, indicating that other driver mutations may be required for tumor initiation and progression, and most FSMs detected in tumors and mucosa were probably passenger mutations. To determine if fecal samples can be used to monitor the FSM load, fecal DNA from different time points was sequenced. We found that FSMs can be detected at 1month of age although the number of FSMs was relatively low compared to that from older mice, indicating that FSMs accumulate over time. MSI detection via fragment analysis confirmed that these tumors were MSI-H. Interestingly, mucosa and fecal samples from a time course study showed progressive increase in microsatellite instability, suggesting the possibility of using MSI score for disease monitoring. Our preliminary data indicates that combined fecal FSM status and MSI score can be potentially used as a biomarker to monitor the tumor development and disease progression for LS colorectal cancer.

Funded by the National Cancer Institute, National Institutes of Health, under Contract No. HHSN261201500003I

Citation Format: Yurong Song, Shaneen Baxter, Lisheng Dai, Chelsea Sanders, Holli Loomans-Kropp, Brandon Somerville, Ryan N. Baugher, Stephanie D. Mellott, Todd B. Young, Heidi E. Lawhorn, Teri M. Plona, Bingfang Xu, Lei Wei, Qiang Hu, Song Liu, Alan Hutson, Baktiar Karim, Simone Difilippantonio, Ligia Pinto, Matthias Kloor, Steven M. Lipkin, Shizuko Sei, Robert H. Shoemaker. Time course genomic characterization reveals progressive accumulation of mutations during tumor development in a Lynch syndrome mouse model. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6518.