類3號DNA甲基化酶監管表觀基因體抵抗細胞衰老

Abstract

細胞增生與衰老之間的平衡，與器官生成、老化、再生醫學及癌症發生有密切關係。隨著細胞衰老，細胞核發生結構重整以及表觀基因體重塑，而衰老細胞中異染色質比例下降，會漸漸失去對反轉錄跳躍子的抑制。活化的反轉錄跳躍子將威脅基因體完整性。為維持細胞分裂複製基因模板的正確性，無法修復的細胞將進入「不分裂程序」，我們稱這樣的細胞作「殭屍細胞 (senescence cell)」。這些不分裂的殭屍細胞並未被修復，在被清除前仍為不穩定之存在。本研究中意外發現，表現外源類三號DNA甲基化因子(DNA methylatransferase 3-like,DNMT3L) 可以在分裂代數較晚的小鼠胚胎纖維母細胞 (mouse embryonic fibrobalsts, MEFs) 中，透過細胞核結構重整以及表觀基因修飾重塑，召集表觀基因調控蛋白以靜默跳躍子，顯著延緩老化的細胞進入不分裂程序。類三號DNA甲基化因子相對顯著表現於生殖細胞與幹細胞中，作用為協助抑制跳躍子及建立親緣特異性之基因組印痕標記。完整的DNMT3L蛋白在體細胞內難以測得存在。本研究發現僅需「暫時性」給予老化的小鼠纖維母細胞外源性DNMT3L即可修復大半老化細胞中監管異常的表觀基因，使整體基因表現趨勢相對受控。反觀現有的細胞抗老化研究模式：多強行使衰老細胞重新進入細胞周期，而未修復可能具癌化風險的老化表觀基因體，本研究利用表觀基因組及轉錄基因組的分析，解構DNMT3L造成抗老化現象之分子機轉，並鑑定參與抗老化的因子，提供一新穎的角度開發相對安全的抗老化因子─利用DNMT3L短暫表現模式，緩解基因體不穩定的危險因子，開啟抗老化相關研究的新方向並提供降低癌症的風險的線索，希冀在未來應用於開發老化相關疾病的預防及治療。

Loosening epigenetic control during cellular aging increases chromatin instability. To mitigate damage, cells with irreparable damage of all kinds would enter senescence. However, senescence only blocks cell proliferation for these damaged cells without fixing their aberrant chromatin signatures, which remain unstable and could be cancer-prone. As a serendipitous finding via studying retroviral silencing activities, we discovered that the transient ectopic expression of DNA methyltransferase 3-like (DNMT3L) was sufficient to drive late-passage mouse embryonic fibroblasts (MEFs) to halt senescence progression. DNMT3L promotes repression of some endogenous transposable elements and de-repressed coding genes in old MEFs. While DNMT3L repressed endogenous retroviruses by attracting H3K9me3 modification, the coding genes re-repressed in old MEFs by DNMT3L were mostly Polycomb Repressive Complex 2 (PRC2) targets in young MEFs. The up-regulation of those genes in old MEFs was associated with loss of the PRC2 mediated repressive mark, H3K27me3. After transient ectopic DNMT3L expression, a panel of PRC2 modulated genes regained repressive chromatin features. We demonstrated the interaction between DNMT3L and PRC2 in our system. Our data suggest that ectopic DNMT3L may guide PRC2 to redress certain loosened chromatin regions in aging cells. This study opens perspectives in the development of an epigenetic reinforcement strategy to overcome aging-associated epimutation and senescence.