TET2 Loss Conspires with MYC to Foster Lymphomagenesis

Abstract

Programmed DNA methylation patterning serves the generation and maintenance of cell identity, and represents a major barrier against transformation. TET dioxygenases (TET1-3) facilitate DNA demethylation at enhancers, promoting chromatin accessibility and cell type-specific gene expression patterns. Work from our and other labs had shown that TET enzymes settle B lymphocyte identity and warrant antibody-mediated immunity. Due to the prevalence of TET2 mutations in hematological and immune cell tumors, often identified as loss-of-function (LOF) versions, TET2 LOF is considered a cancer driver. Intriguingly, although showing a predisposition for blood cancer, healthy humans carrying TET2 mutations in hematopoietic stem cells can remain cancer-free for decades despite increasing clonal dominance of the affected cells and their offspring, a condition termed clonal hematopoiesis. Furthermore, mice deficient for TET2 only rarely develop tumors. This scenario suggests roles for TET2 LOF as cancer facilitator, rather than driver. In B lymphocytes, the MYC oncoprotein is a potent driver of malignancy. However, therapeutic targeting of MYC is far from reaching the clinic. Thus, establishing functional links between MYC and dependencies imposed by TET2 LOF should provide information on the molecular mechanisms of predisposition, and guide clinical practice and targeted therapy in the future. To address whether TET2 LOF conspires with MYC, we employed the prototypic EμMYC transgenic mouse model, where enforced MYC overexpression early in the B lymphocyte lineage drives lymphomas, but additional hits are required for full transformation. Confirming a tumor suppressor role for TET2, partial or complete loss of the protein accelerated lymphomagenesis, and facilitated specifically the outgrowth of IgM+ lymphomas. The premalignant IgM+ population contained a higher fraction of cells with double strand breaks (EμMYC 2,5% vs. EμMYCTet2+/- or EμMYCTet2-/- 6%) and in the S/G2/M cell cycle phases (EμMYC 15% vs. EμMYCTet2+/- or EμMYCTet2-/- 25%), along with higher spontaneous apoptosis rates (EμMYC 4% vs. EμMYCTet2+/- or EμMYCTet2-/- 7%). Mechanistically, in the context of MYC overexpression, TET2 LOF associates with enhanced JAK/STAT signaling and selective upregulation of the pro-survival BCL2 family protein BCLX. To recapitulate alternative scenarios of TET2 LOF in diffuse large B cell lymphoma (DLBCL), a tumor that originates from germinal center B lymphocytes, we established two preclinical mouse models, (1) either with TET2 LOF in the germline to mimic the scenario of clonal hematopoiesis along with ectopic MYC expression restricted to germinal center B lymphocytes, or (2) using a fully conditional approach where we synchronize ectopic MYC expression and TET2 LOF specifically in germinal center B lymphocytes. In line with the results from the EμMYC model, germline heterozygous Tet2 loss (scenario (1)) sufficed to boost MYC-driven lymphomagenesis, a phenotype that is strongly enhanced by the absence of both Tet2 alleles. Germinal center-specific elimination of Tet2 in scenario (2) also produced lymphomas, however, heterozygous Tet2 loss in germinal center B lymphocytes was not sufficient to unleash MYC-driven lymphomagenesis. Our data from three powerful MYC-driven mouse models suggest (i) that TET2 loss in B lymphocytes nudges MYC-mediated transformation, (ii) that TET2 loss, early on in hematopoietic stem cells, potentiates the transformative potential later on in B lymphocytes, and (iii) that TET2 loss dampens apoptosis during transformation and in established lymphomas, potentially via JAK/STAT signaling and BCLX. Altogether, we provide novel in vivo evidence that TET2 prevents MYC-mediated transformation in B lymphocytes, and our results produced testable dependencies. Beyond MYC-driven blood cancer, TET2 LOF in hematopoietic stem cells predisposes to all-cause mortality in humans. Thus, identifying targetable vulnerabilities of TET2 LOF will serve healthy aging.