Dnmt3a associates with promoters and enhancers to protect epidermal stem cells from cancer [MeDIP-seq]

The de novo DNA methyltransferase Dnmt3a is mutated in human acute myeloid leukemia, and suppresses tumorigenesis in murine models of leukemia and lung cancer. Conversely, deregulation of the other de novo DNA methyltransferase, Dnmt3b, predominantly promotes tumorigenesis. However, the molecular mechanisms underlying the roles of Dnmt3a and Dnmt3b in cancer remain poorly understood. Using conditional knockout mice, here we show that Dnmt3a -- but not Dnmt3b -- strongly protects epidermal stem cells from carcinogen-induced tumor initiation, without affecting the progression of benign lesions to aggressive carcinomas. Only upon combined deletion of Dnmt3a and Dnmt3b, squamous cell carcinomas acquired a more aggressive fate and even became metastatic, indicating that Dnmt3b is tumor-suppressive, rather than pro-tumorigenic, in epidermal neoplasia. Mechanistically, Dnmt3a promotes the expression of epidermal differentiation genes by interacting with their enhancers, and inhibits the expression of lipid metabolism and cell proliferation genes by directly methylating their promoters. Altogether, we demonstrate that Dnmt3a, but not Dnmt3b, is critical for suppressing epidermal tumor initiation, while both enzymes prevent tumor progression. Overall design: The study was performed to analyse the differentially methylated regions between skin tumors derived from mouse epidermis of wild type and Dnmt3a-deficient animals.

从头型DNA甲基转移酶（de novo DNA methyltransferase）Dnmt3a在人类急性髓系白血病（acute myeloid leukemia）中存在突变，且在白血病与肺癌的小鼠模型中可抑制肿瘤发生。与之相反，另一款从头型DNA甲基转移酶Dnmt3b的表达失调则主要促进肿瘤发生。然而，Dnmt3a与Dnmt3b在癌症中发挥功能的分子机制仍未被充分阐明。本研究借助条件性基因敲除小鼠（conditional knockout mice）模型，证实仅Dnmt3a（而非Dnmt3b）可强效保护表皮干细胞（epidermal stem cells）免受致癌物诱导的肿瘤起始，且不影响良性病变向侵袭性癌的进展过程。仅当同时敲除Dnmt3a与Dnmt3b时，鳞状细胞癌（squamous cell carcinomas）才会呈现出更强的侵袭性，甚至发生转移，这表明在表皮肿瘤发生过程中，Dnmt3b实为抑癌因子，而非促肿瘤因子。从分子机制层面来看，Dnmt3a可通过结合表皮分化基因的增强子（enhancers）促进其表达，并通过直接甲基化脂质代谢与细胞增殖相关基因的启动子（promoters）抑制其转录。综上，本研究证实：Dnmt3a（而非Dnmt3b）是抑制表皮肿瘤起始的关键因子，而两种酶均可阻碍肿瘤进展。总体实验设计：本研究旨在分析野生型（wild type）与Dnmt3a缺陷型小鼠表皮来源的皮肤肿瘤之间的差异甲基化区域（differentially methylated regions）。