Concomitant loss of TET2 and TET3 results in T cell expansion and genomic instability

TET proteins are tumor suppressors that through their catalytic activity oxidize 5-methylcytosine to 5-hydroxymethylcytosine, to promote DNA demethylation and to regulate gene expression. Notably, TET2 is the second most frequently mutated gene in hematological malignancies, including T cell lymphomas. However, murine models with deletion of TET2 do not exhibit T cell expansion, presumably due to redundancy with other members of the TET family of proteins. In order to gain insight on the TET mediated molecular events that safeguard T cells from aberrant proliferation we performed serial adoptive transfers of murine CD4 T cells that lack concomitantly TET2 and TET3 to fully immunocompetent congenic mice. Our data reveal a progressive acquisition of malignant traits upon loss of TET2 and TET3 that is characterized by loss of genomic integrity, acquisition of aneuploidy and upregulation of the protooncogene Myc. In this report, we aim to decipher the impact of TET proteins in controlling in vivo proliferation of peripheral T cells. We employ adoptive transfer of Tet2/3 DKO T cells and we discover that Tet2/3 DKO CD4 cells exhibit upregulation of cell cycle genes upon serial transplantation. This altered gene expression program endows Tet2/3 DKO CD4 cells with increased proliferative capacity and acquisition of malignant traits, such as oligoclonal expansion and chromosomal copy number (CCN) variations, resulting in aggressive lymphoproliferative disorders. In addition, we demonstrate that as the TET deficient cells acquire a hyperproliferative profile they can expand remarkably fast, within 10 days, in recipient mice. During this rapid expansion of transplanted Tet2/3 DKO T cells, by employing an unbiased, single cell transcriptomic approach, we reveal that while the immune environment of the host mice becomes rapidly activated, it fails to annihilate the expansion of the donor T cells.

TET蛋白（TET proteins）是一类肿瘤抑制因子，其通过催化活性将5-甲基胞嘧啶氧化为5-羟甲基胞嘧啶，从而促进DNA去甲基化并调控基因表达。值得注意的是，TET2是血液系统恶性肿瘤（包括T细胞淋巴瘤）中第二常见的突变基因。然而，仅敲除TET2的小鼠模型并未出现T细胞扩增现象，推测这可能与TET家族其他蛋白的功能冗余有关。为深入解析TET蛋白介导的、保护T细胞免于异常增殖的分子事件，我们对同时缺失TET2与TET3的小鼠CD4 T细胞开展连续过继转移实验，将其输注至完全免疫健全的同系受体小鼠体内。我们的研究数据显示，在同时缺失TET2与TET3后，细胞会逐步获得恶性表型，具体表现为基因组完整性丧失、非整倍体形成以及原癌基因Myc的表达上调。本研究旨在阐明TET蛋白在调控外周T细胞体内增殖过程中的作用。我们采用Tet2/3双敲除（DKO）T细胞的过继转移模型，发现经连续移植后，Tet2/3 DKO CD4 T细胞的细胞周期相关基因表达出现上调。这种异常的基因表达程序赋予Tet2/3 DKO CD4 T细胞更强的增殖能力，并使其获得恶性特征，包括寡克隆扩增与染色体拷贝数（CCN）变异，最终引发侵袭性淋巴增殖性疾病。此外，我们证实，当TET缺陷细胞获得高增殖表型后，可在受体小鼠体内于10天内快速扩增。在移植后的Tet2/3 DKO T细胞快速扩增阶段，我们通过无偏倚单细胞转录组学方法开展分析，结果显示尽管受体小鼠的免疫微环境被快速激活，但仍无法清除供体T细胞的扩增。