DNA methyltransferase inhibition reduces inflammation-induced colon tumorigenesis

Chronic inflammation is strongly associated with an increased risk of developing colorectal cancer. DNA hypermethylation of CpG islands alters the expression of genes in cancer cells and plays an important role in carcinogenesis. Chronic inflammation is also associated with DNA methylation alterations and in a mouse model of inflammation-induced colon tumorigenesis, we previously demonstrated that inflammation-induced tumours have 203 unique regions with DNA hypermethylation compared to uninflamed epithelium. To determine if altering inflammation-induced DNA hypermethylation reduces tumorigenesis, we used the same mouse model and treated mice with the DNA methyltransferase (DNMT) inhibitor decitabine (DAC) throughout the tumorigenesis time frame. DAC treatment caused a significant reduction in colon tumorigenesis. The tumours that did form after DAC treatment had reduced inflammation-specific DNA hypermethylation and alteration of expression of associated candidate genes. When compared, inflammation-induced tumours from control (PBS-treated) mice were enriched for cell proliferation associated gene expression pathways whereas inflammation-induced tumours from DAC-treated mice were enriched for interferon gene signatures. To further understand the altered tumorigenesis, we derived tumoroids from the different tumour types. Interestingly, tumoroids derived from inflammation-induced tumours from control mice maintained many of the inflammation-induced DNA hypermethylation alterations and had higher levels of DNA hypermethylation at these regions than tumoroids from DAC-treated mice. Importantly, tumoroids derived from inflammation-induced tumours from the DAC-treated mice proliferated more slowly than those derived from the inflammation-induced tumours from control mice. These studies suggest that inhibition of inflammation-induced DNA hypermethylation may be an effective strategy to reduce inflammation-induced tumorigenesis.

慢性炎症与结直肠癌发病风险升高存在显著关联。CpG岛（CpG islands）的DNA高甲基化（DNA hypermethylation）可改变癌细胞内基因的表达模式，在癌变（carcinogenesis）过程中发挥重要作用。慢性炎症同样与DNA甲基化异常相关；此前我们在炎症诱导的结肠肿瘤发生小鼠模型中证实，相较于未发生炎症的上皮组织，炎症诱导产生的肿瘤存在203个独特的DNA高甲基化区域。为明确改变炎症诱导的DNA高甲基化是否可降低肿瘤发生风险，我们采用同款小鼠模型，在整个肿瘤发生周期内，向小鼠施以DNA甲基转移酶（DNA methyltransferase, DNMT）抑制剂地西他滨（decitabine, DAC）进行处理。结果显示，地西他滨处理显著抑制了结肠肿瘤的发生。经地西他滨处理后仍形成的肿瘤，其炎症特异性DNA高甲基化水平有所降低，相关候选基因的表达也发生了改变。对比分析可见，对照组（经磷酸盐缓冲液PBS处理）小鼠的炎症诱导肿瘤，其基因表达通路以细胞增殖相关通路富集为主；而地西他滨处理组小鼠的炎症诱导肿瘤，则以干扰素基因特征富集为主要特征。为进一步探究肿瘤发生进程的改变机制，我们从不同类型的肿瘤中构建了肿瘤类器官（tumoroid）。有趣的是，对照组小鼠炎症诱导肿瘤来源的类器官，保留了多数炎症诱导的DNA高甲基化异常特征，且在对应区域的DNA高甲基化水平显著高于地西他滨处理组小鼠的肿瘤类器官。值得注意的是，地西他滨处理组小鼠炎症诱导肿瘤来源的类器官，其增殖速度显著慢于对照组小鼠炎症诱导肿瘤来源的类器官。本研究表明，抑制炎症诱导的DNA高甲基化或可成为降低炎症相关肿瘤发生的有效策略。