Mutation of tet methylcytosine dioxygenase 2 (encoded by TET2) drives myeloid malignancy initiation and progression^1,2,3. TET2 deficiency is known to cause a globally opened chromatin state and activation of genes contributing to aberrant haematopoietic stem cell self-renewal^4,5. However, the open chromatin observed in TET2-deficient mouse embryonic stem cells, leukaemic cells and haematopoietic stem and progenitor cells⁵ is inconsistent with the designated role of DNA 5-methylcytosine oxidation of TET2. Here we show that chromatin-associated retrotransposon RNA 5-methylcytosine (m⁵C) can be recognized by the methyl-CpG-binding-domain protein MBD6, which guides deubiquitination of nearby monoubiquitinated Lys119 of histone H2A (H2AK119ub) to promote an open chromatin state. TET2 oxidizes m⁵C and antagonizes this MBD6-dependent H2AK119ub deubiquitination. TET2 depletion thereby leads to globally decreased H2AK119ub, more open chromatin and increased transcription in stem cells. TET2-mutant human leukaemia becomes dependent on this gene activation pathway, with MBD6 depletion selectively blocking proliferation of TET2-mutant leukaemic cells and largely reversing the haematopoiesis defects caused by Tet2 loss in mouse models. Together, our findings reveal a chromatin regulation pathway by TET2 through retrotransposon RNA m⁵C oxidation and identify the downstream MBD6 protein as a feasible target for developing therapies specific against TET2 mutant malignancies.

tet 甲基胞嘧啶双加氧酶 2（由 TET2 编码）突变驱动髓系恶性肿瘤的发生和进展^1,2,3。已知 TET2 缺陷会导致整体染色质状态开放和基因激活，从而导致造血干细胞自我更新异常^4,5。然而，在 TET2 缺陷的小鼠胚胎干细胞、白血病细胞和造血干细胞和祖细胞⁵ 中观察到的开放染色质与 TET2 的 DNA 5-甲基胞嘧啶氧化的指定作用不一致。在这里，我们表明染色质相关的反转录转座子 RNA 5-甲基胞嘧啶 （m⁵C） 可以被甲基 CpG 结合域蛋白 MBD6 识别，该蛋白指导组蛋白 H2A （H2AK119ub） 附近单泛素化 Lys119 的去泛素化以促进开放染色质状态。TET2 氧化 m⁵C 并拮抗这种 MBD6 依赖性 H2AK119ub 去泛素化。因此，TET2 耗竭导致 H2AK119ub 整体降低，染色质开放增加，干细胞转录增加。TET2 突变型人类白血病变得依赖于这种基因激活途径，MBD6 耗竭选择性地阻断了 TET2 突变型白血病细胞的增殖，并在很大程度上逆转了小鼠模型中 Tet2 缺失引起的造血缺陷。总之，我们的研究结果揭示了 TET2 通过反转录转座子 RNA m⁵C 氧化的染色质调节途径，并将下游 MBD6 蛋白确定为开发针对 TET2 突变恶性肿瘤的特异性疗法的可行靶标。