BACKGROUND Aberrations in DNA methylation are widespread in colon cancer (CC). Understanding origin and progression of DNA methylation aberrations is essential to develop effective preventive and therapeutic strategies. Here, we aimed to dissect CC subtype-specific methylation instability to understand underlying mechanisms and functions. METHODS We have assessed genome-wide DNA methylation in the healthy normal colon mucosa (HNM), precursor lesions and CCs in a first comprehensive study to delineate epigenetic change along the process of colon carcinogenesis. Mechanistically, we used stable cell lines, genetically engineered mouse model of mutant BRAFV600E and molecular biology analysis to establish the role of BRAFV600E-mediated-TET inhibition in CpG-island methylator phenotype (CIMP) inititation. RESULTS We identified two distinct patterns of CpG methylation instability, determined either by age-lifestyle (CC-neutral CpGs) or genetically (CIMP-CpGs). CC-neutral-CpGs showed age-dependent hypermethylation in HNM, all precursors, and CCs, while CIMP-CpGs showed hypermethylation specifically in sessile serrated adenomas/polyps (SSA/Ps) and CIMP-CCs. BRAFV600E-mutated CCs and precursors showed a significant downregulation of TET1 and TET2 DNA demethylases. Stable expression of BRAFV600E in nonCIMP CC cells and in a genetic mouse model was sufficient to repress TET1/TET2 and initiate hypermethylation at CIMP-CpGs, reversible by BRAFV600E inhibition. BRAFV600E-driven CIMP-CpG hypermethylation occurred at genes associated with established CC pathways, effecting functional changes otherwise achieved by genetic mutation in carcinogenesis. CONCLUSIONS Hence, while age-lifestyle-driven hypermethylation occurs generally in colon carcinogenesis, BRAFV600E-driven hypermethylation is specific for the "serrated" pathway. This knowledge will advance the use of epigenetic biomarkers to assess subgroup-specific CC risk and disease progression.

中文翻译：

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BRAF 介导的 TET 沉默和生活方式暴露导致的 DNA 甲基化不稳定性划分了结肠癌通路。

背景 DNA 甲基化的异常在结肠癌 (CC) 中很普遍。了解 DNA 甲基化畸变的起源和进展对于制定有效的预防和治疗策略至关重要。在这里，我们旨在剖析 CC 亚型特异性甲基化不稳定性，以了解潜在的机制和功能。方法我们在第一项全面研究中评估了健康正常结肠黏膜 (HNM)、前体病变和 CC 中的全基因组 DNA 甲基化，以描绘结肠癌发生过程中的表观遗传变化。在机制上，我们使用稳定的细胞系、突变 BRAFV600E 的基因工程小鼠模型和分子生物学分析来确定 BRAFV600E 介导的 TET 抑制在 CpG 岛甲基化表型 (CIMP) 启动中的作用。结果我们确定了两种不同的 CpG 甲基化不稳定性模式，由年龄生活方式（CC 中性 CpG）或遗传（CIMP-CpG）决定。CC-neutral-CpGs 在 HNM、所有前体和 CCs 中显示出年龄依赖性高甲基化，而 CIMP-CpGs 在无柄锯齿状腺瘤/息肉 (SSA/Ps) 和 CIMP-CCs 中显示出高甲基化。BRAFV600E 突变的 CC 和前体显示出 TET1 和 TET2 DNA 去甲基化酶的显着下调。BRAFV600E 在非 CIMP CC 细胞和遗传小鼠模型中的稳定表达足以抑制 TET1/TET2 并启动 CIMP-CpG 的超甲基化，可通过 BRAFV600E 抑制逆转。BRAFV600E 驱动的 CIMP-CpG 高甲基化发生在与已建立的 CC 通路相关的基因上，影响功能变化，否则通过致癌作用中的基因突变实现。结论因此，虽然年龄生活方式驱动的高甲基化通常发生在结肠癌发生中，但 BRAFV600E 驱动的高甲基化对“锯齿状”途径具有特异性。这些知识将促进表观遗传生物标志物的使用，以评估亚组特定的 CC 风险和疾病进展。