DNA羟化酶TET1在甲状腺癌发生发展中的作用及其分子机制

DNA hydroxylase TET1 is a dioxygenase that convert 5-methylcytosine (5-mC) to 5-hydroxylmethylcytosine (5-hmC), resulting in DNA demethylation. TET1-mediated epigenetic events play an important role in tumroigenesis. Recently, interesting clues on the role of TET proteins including TET1 in cancer formation are quickly emerging. It has been reported that loss of TET1 and 5-hmC in a broad spectrum of solid tumors, including breast cancer, hepatocarcinoma, melanoma, gastric cancer and colon cancer. These observations suggest that TET1 may be a potential tumor suppressor. However, the role of TET1 in thyroid carcinogenesis remains largely unknown. Our preliminary data showed that TET1 expression and 5-hmC levels were significantly down-regulated in thyroid cancer tissues as compared with matched normal thyroid tissues. Restoring TET1 expression in thyroid cancer cells significantly inhibited cell proliferation, colony formation, migration and invasion. Conversely, TET1 depletion promoted thyroid cancer cell growth and invasiveness. Taken together, these results suggest that TET1 should be a novel tumor suppressor in thyroid cancer. In addition, our data revealed that TET1 suppressed cellular generation of reactive oxygen species (ROS) and limited extracellular acidification, indicating that TET1 may play a role in tumor redox stasis and glucose metabolism. The aims of this project is to further explore biological role and molecular mechanisms of TET1 in thyroid tumorigenesis by using cell and mice models, and identify its downstream target genes using ChIP-Seq, gene expression microarray and Protein Chip approaches.

DNA羟化酶TET1将5甲基胞嘧啶（5-mC）氧化成5-羟甲基胞嘧啶（5-hmC），进而实现DNA去甲基化。其介导的表观遗传学事件参与多种肿瘤的发生与发展。有研究显示TET1在多种肿瘤中表达下调，与之相应的5-hmC水平也显著降低，提示TET1可能发挥抑癌作用。然而，该基因在甲状腺癌中的作用至今未见报道。本项目前期研究发现甲状腺癌组织中TET1的表达和5-hmC水平显著低于癌旁组织。过表达和敲除甲状腺癌细胞中TET1分别抑制和增强细胞增殖、克隆形成、迁移和侵袭能力，提示TET1可能是一个新的甲状腺癌抑制因子。同时，我们的研究还显示TET1可能在甲状腺癌细胞糖代谢及氧化还原稳态中发挥独特作用。基于此，本项目拟通过细胞及动物实验，进一步探讨甲状腺癌中TET1表达下调的分子机制，鉴定TET1抑癌机制的关键作用靶点，明确TET1参与肿瘤细胞糖代谢及氧化还原稳态的作用机制，绘制其基因/蛋白调控网络。

Tet 甲基胞嘧啶双加氧酶1（TET1）在DNA去甲基化及染色质重塑过程中发挥重要作用。但TET1在甲状腺癌中的表达和作用尚不清楚。此外，缺氧是甲状腺癌快速生长的特征之一，且与癌症预后差直接相关。在本项目中，我们试图揭示TET1在低氧性甲状腺癌中的作用及其机制。我们分析了TCGA数据库中的乳头状甲状腺癌（PTC）的数据，并通过比较了PTC和癌旁组织中的TET1表达和5-羟甲基胞嘧啶（5-hmC）含量。结果发现，在甲状腺癌组织中TET1表达下调，并伴随5-hmC含量降低，提示TET1可能发挥抑癌作用，体外功能实验进一步这一结论。进而，我们在BRAFV600E驱动的PTC小鼠模型的基础上条件性敲除了Tet1。结果发现，敲除Tet1基因使小鼠死亡率下降；相较于对照组，基因敲除小鼠体重增加，甲状腺肿块更小，并且预后更好。这一结论与TCGA数据库中PTC患者预后情况相符，即TET1低表达的患者预后更好。为了揭示TET1在体内外的功能差异，我们在缺氧条件下培养肿瘤细胞，发现TET1能促进肿瘤细胞增殖并提高细胞及小鼠PTC组织中HIF1α的表达。机制研究证实，TET1通过协助HIF1α上调CK2β的表达，而CK2激酶通过磷酸化PTEN 使其失活，导致AKT激酶活性升高；激活的AKT通过抑制GSK3β对HIF1α的磷酸化，从而阻断HIF1α分子的泛素蛋白酶体降解途径，增加其稳定性；而HIF1α反过来上调CK2的表达，进而形成正反馈环路，在缺氧环境下促进甲状腺癌的恶性进展。.在该基金的资助下，发表SCI论文19篇，申请人均作为通讯作者，影响因子总计156.2。

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