Roles of TET2 Loss in B Cell Transformation

TET enzymes (TET1-3) catalyze the oxidation of 5-methylcytosine (5mC) in DNA. These oxidized 5mC variants associate with chromatin accessibility and transcription, maintaining gene regulatory elements in an open confirmation. Using mouse models, we and others have shown that, in B cells, TET2 and TET3 are recruited to the enhancers of genes such as IRF4, PU.1 or AICDA, allowing developmental progression and proper antibody generation. Loss-of-function (LOF) mutations in the TET2 gene are commonly found in leukemias and lymphomas. Furthermore, functional depletion of TET2 can be exerted by transcriptional repression via miRNAs, oncometabolites or enhanced caspase-dependent TET2 degradation. Being intensely studied in myeloid leukemias, TET2 mutations are also found in up to 30% of diffuse large B cell lymphomas (DLBCL), and recent data suggest that TET2 mutated DLBCLs display a distinct gene expression profile. In DLBCL patients, TET2 mutations may arise in the B cell type of origin, the germinal center B cell, or be inherited from hematopoietic stem cells. However, TET2 LOF on its own is only weakly tumorigenic in mouse models, and although showing a predisposition for blood cancer, humans carrying such mutations in hematopoietic stem cells can remain cancer-free for decades despite increasing clonal dominance of the affected cells, a condition termed clonal hematopoiesis. This scenario suggests roles for TET2 LOF as cancer facilitator, conspiring with prototypic oncogenes such as MYC to drive cancer. It is, however, unclear how TET2 LOF impacts the transformation process and outcome. To address whether TET2 LOF serves MYC-mediated transformation, we employ different murine MYC-driven B cell lymphoma models, where enforced MYC expression drives lymphomas, but additional hits are required for full transformation. Using a pre-clinical model with enforced MYC expression exclusively in germinal center B cells, we can show that additional TET2 LOF in hematopoietic stem cells generally enhances the transformation of germinal center B cells. In contrast, enforced MYC expression along with TET2 LOF specifically in germinal center B cells enhances lymphomagenesis only in approx. 30% of animals. Independent both these model systems, TET2 LOF results in lymphomas with plasmacytic differentiation. Thus, our data suggests that the establishment of TET2 LOF hematopoietic stem cell clones boosts oncogene-driven transformation of germinal center B cells. Mechanistic studies in premalignant B cells show that TET2 LOF reduces the expression of genes involved in ensuring a faithful passage through mitosis, such as PLK1, AURKB, or BUB1b, a phenomenon that is dependent on B cell receptor expression. The resulting cell death vulnerabilities may be overcome by BCL2 network remodeling during transformation, as the ultimately transformed TET2 LOF B cell lymphomas show robust expression of the pro-survival BCL2 family protein BCLX. Altogether, our studies provide evidence that TET2 LOF promotes genomic instability along with BCL2 network rewiring, fueling B cell transformation.