Abstract PR006: Impact of heterogeneity for mismatch repair activity on colon tumor development and therapeutic response
Authors: Snow Santina , Albrecht Dawn , Huebner Shane , Baus Maggie , Clark Paul , Weichert Jamey , Morris Zachary , Halberg Richard 
Published: December 2022

Abstract
Four Consensus Molecular Subtypes (CMS) of colorectal cancer (CRC) were recently recognized that distinguish CRC by mutational profile and gene expression signatures. One of the subtypes, CMS1, is enriched for cancers that are deficient for DNA mismatch repair (dMMR). The genetic instability leads to high tumor mutational burden, creating numerous neoantigens on the surface of the tumor that can elicit an immune response. CMS1 cancers with dMMR are the best candidates for immunotherapy as compared to cancers with proficient DNA mismatch repair (pMMR). Early clinical trials revealed that only 30-55% metastatic dMMR CRC responded to immunotherapy. Moreover, many responses were incomplete. If a tumor consists of a mixture of dMMR and pMMR tumor cells, then only some tumor cells may be sensitive to immunotherapy, leading to a partial response or acquired resistance. Conventional preclinical mouse models that are typically used to test treatment regimens fail to accurately predict efficacy for many reasons but foremost they lack intratumoral heterogeneity that is a common feature of human cancers. To better understand how a heterogeneous tumor microenvironment for dMMR impacts tumor development and response to immunotherapy, we have developed a new mouse model that develops tumors with a spectrum of MMR activity. Homotypic dMMR or immunologically ‘hot’ tumors that express green fluorescent protein are predicted to have the best response to immunotherapy. Homotypic pMMR or immunologically ‘cold’ tumors that express red fluorescent protein are predicted to not respond to immunotherapy. Heterotypic tumors with a mixture of dMMR and pMMR cells or immunologically ‘lukewarm’ tumors are predicted to have a partial response to immunotherapy. Surveillance bright field and fluorescent colonoscopies allow us to assess tumor size and proportion of dMMR (green) and pMMR (red) tumor cells in real-time before, during and after treatment and prior to necropsy, ex vivo imaging, and histological analysis. Mixed MMR tumors ranged from as few as 5% dMMR (green) cells to as many as 95%. The tumor response to systemic immunotherapy alone, aPD-L1, was dependent on the tumor composition. Large dMMR tumors following aPD-L1 therapy appeared to have large areas of necrosis in wholemount, whereas pMMR tumors following treatment were indistinguishable from pMMR tumors in controls. Mixed MMR tumor response varied depending on percentage of dMMR cells; tumors with a high percentage of dMMR cells remained small after aPD-L1 whereas tumors with a low percentage of dMMR had an outgrowth of pMMR cells after aPD-L1. Future analysis of histological biomarkers may assist in effectively translating the observed preclinical therapeutic responses into survival benefits for patients with CRC and other tumor types especially those heterogenous for MMR activity.
Citation Format: Santina Snow, Dawn Albrecht, Shane Huebner, Maggie Baus, Paul Clark, Jamey Weichert, Zachary Morris, Richard Halberg. Impact of heterogeneity for mismatch repair activity on colon tumor development and therapeutic response [abstract]. In: Proceedings of the AACR Special Conference on Colorectal Cancer; 2022 Oct 1-4; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_1):Abstract nr PR006.