TET deficiency promotes oncogenesis and DNA damage in B cells by increasing the levels of G-quadruplexes and R-loops [G4-seq]

Enzymes of the TET family are methylcytosine dioxygenases that undergo frequent mutational or functional inactivation in both hematological and solid cancers. Recent studies have identified recurrent loss-of-function mutations in TET proteins in human patients with Diffuse Large B-Cell Lymphoma (DLBCL). Here we investigate the role of TET proteins in the pathogenesis of DLBCL by deleting the Tet2 and Tet3 genes in mature B cells in mice. Tet deletion perturbs mature B-cell homeostasis and causes spontaneous development of Germinal Center-derived B cell lymphomas. We show that an increase in G-quadruplexes and R-loops a common feature of TET deficiency in B cells as well as other hematopoietic cells. Genome-wide analyses revealed that G-quadruplexes and R-loops accumulated primarily near transcription start sites in TET-deficient B cells, and their accumulation correlated with increased DNA double-strand breaks. Moreover, CRISPR-mediated depletion of nucleases and helicases that regulate G-quadruplexes and R-loops led to decreased viability of TET-deficient but not control primary B cells. Our studies elucidate a molecular mechanism by which TET loss-of-function might predispose to development of B cell-derived and other malignancies, and highlight novel therapeutic avenues that could be further explored. G4-sequencing in CD19 Tet2/3 DKO B cells

TET家族酶（TET family）是一类甲基胞嘧啶双加氧酶，在血液系统肿瘤与实体肿瘤中均常发生突变或功能失活。近期研究证实，弥漫大B细胞淋巴瘤（Diffuse Large B-Cell Lymphoma, DLBCL）患者体内的TET蛋白存在频发的功能丧失突变。本研究通过在小鼠成熟B细胞中敲除Tet2与Tet3基因，探究TET蛋白在DLBCL发病机制中的作用。结果显示，TET缺失会扰乱成熟B细胞稳态，并自发诱发生发中心来源的B细胞淋巴瘤。我们证实，G-四链体（G-quadruplexes）与R环（R-loops）的增多是B细胞及其他造血细胞中TET缺失的共同特征。全基因组分析显示，TET缺陷型B细胞中，G-四链体与R环主要在转录起始位点附近累积，且其累积水平与DNA双链断裂（DNA double-strand breaks）的增加呈显著相关。此外，通过CRISPR技术敲除调控G-四链体与R环的核酸酶及解旋酶，会降低TET缺陷型原代B细胞的存活率，但对对照组原代B细胞无此影响。本研究阐明了TET功能丧失可能促使B细胞源性及其他恶性肿瘤发生的分子机制，并为后续探索新型治疗策略提供了方向。CD19 Tet2/3双敲除B细胞的G4测序（G4-sequencing）