DNA and RNA sequencing of mouse 4T1 knocked out for MSH2

Mismatch repair deficiency (MMRD) leads to the inability of cells to correct errors during DNA replication, causing a high mutational burden and the accumulation of insertions/deletions (indels) at microsatellite sites. This results in high microsatellite instability (MSI-H), immune infiltration, and responsiveness to immunotherapies. The MMRD phenotype is present in over 15 types of cancers, especially colorectal (CRC) and endometrial cancers. MSI-H tumors generally have a better prognosis in early stages and are less likely to metastasize than microsatellite-stable (MSS) tumors, as seen in CRC. However, in advanced stages, MSI-H tumors lose this survival advantage, with the underlying remaining unclear. Leveraging the tumor-agnostic features of the MMRD phenotype, we established a syngeneic mouse model of MSI cancer using the highly metastatic 4T1 breast cancer cell line, knocked out for the MMR gene Msh2. This model recapitulated the genomic features common to all MSI-H cancers, showed a 17.3% reduction in metastatic incidence and fewer lesion-bearing sites in the MSI metastatic group compared to the MSS group, which mirrors findings from human MSI CRC data. Interestingly, primary MSI-H tumors showed enrichment of immune-related gene expression signatures that negatively correlated with metastatic status. Moreover, primary MSI-H tumors that develop metastasis exhibited a distinct enrichment of a hybrid epithelial-mesenchymal signature, indicating greater aggressiveness compared to MSS tumors. Furthermore, we identified a unique population of immature myeloid cells present at both primary and metastatic sites in MSI-H tumor-bearing mice, suggesting that the deficiency in MMR pathway elicits a specific immune response beyond T-cell activation.

错配修复缺陷（Mismatch Repair Deficiency, MMRD）会使细胞无法纠正DNA复制过程中产生的错误，进而引发较高的突变负荷，并在微卫星位点累积插入缺失（insertions/deletions, indels）。该异常会导致高微卫星不稳定性（Microsatellite Instability-High, MSI-H）、免疫浸润以及对免疫治疗的响应性。MMRD表型存在于超过15种癌症类型中，尤以结直肠癌（Colorectal Cancer, CRC）和子宫内膜癌最为典型。相较于微卫星稳定（Microsatellite-Stable, MSS）肿瘤（以结直肠癌为例），MSI-H肿瘤在早期通常预后更佳，且转移风险更低。但在晚期阶段，MSI-H肿瘤会丧失这一生存优势，其背后的潜在机制目前仍未阐明。本研究利用MMRD表型的泛癌特征，以高转移性4T1乳腺癌细胞系为载体，通过敲除错配修复基因Msh2，构建了MSI型癌症的同基因小鼠模型。该模型重现了所有MSI-H癌症共有的基因组特征；与MSS组相比，MSI转移组的转移发生率降低了17.3%，病灶累及位点更少，这一结果与人类MSI结直肠癌数据中的发现相一致。值得注意的是，原发MSI-H肿瘤的免疫相关基因表达特征富集程度与转移状态呈负相关。此外，发生转移的原发MSI-H肿瘤会显著富集上皮间质混合特征，提示其侵袭性较MSS肿瘤更强。进一步研究表明，在携带MSI-H肿瘤的小鼠中，原发肿瘤部位与转移部位均存在一类独特的未成熟髓系细胞群，这提示错配修复通路的缺陷会触发除T细胞激活之外的特异性免疫应答。