Diffuse large B-cell lymphomas (DLBCLs) are aggressive tumors derived from germinal center (GC) B cells. Despite progress in the treatment of DLBCL, approximately 40% of patients relapse or are refractory to the treatment, which usually leads to fatality. DLBCLs are characterized by aberrant DNA methylation and this feature correlates with poor clinical outcome. We and others have shown that deregulated epigenetic mechanisms contribute to lymphoma formation. Particularly, we have demonstrated that TET2, an enzyme that converts methylcytosine (mC) into hydroxymethylcytosine (hmC) and is mutated in ~10% of DLBCLs, is a B-cell tumor suppressor. GC-specific deletion of TET2 (Cg1Cre/Tet2-/-) resulted in accelerated lymphomagenesis in DLBCL mouse models driven by BCL6 overexpression, with 100% Cg1Cre/Tet2-/-;ImBcl6 mice developing lymphoma at 7 months compared to only 50% in ImBcl6 control mice. In addition, TET2 deletion in hematopoietic stem cells (VavCre/Tet2-/-) induced GC B-cell hyperplasia (B220+GL7+CD95+ cells; 10% VavCre/Tet2-/- vs 5% VavCre/Tet2+/+), promoting malignant transformation. Further analysis of the GC reaction revealed that TET2-deficient GC B cells displayed anomalous patterns of DNA methylation. GC B cells from VavCre/Tet2-/- mice presented 1) focal loss of hmC—using hMeDIPseq—with 25,000 differentially hydroxymethylated regions (DHMR) lost compared to VavCre/Tet2+/+ GC B cells and 2) increased mC—using RRBS—with almost 11,000 differentially methylated cytosines (DMCs), 84% hypermethylated, compared to VavCre/Tet2+/+ GC B cells. TET2-mediated reduction of hmC and hypermethylation affected enhancers and promoters, respectively, of genes mediating GC exit and terminal differentiation of GC B cells, especially those regulated at enhancers by the opposing functions of CREBBP and HDAC3. We are currently investigating the potential cooperative role between TET2-mediated hmC and CREBBP-mediated H3K27Ac, supported by reduced H3K27Ac at enhancers activated by CREBBP in VavCre/Tet2-/- GC B cells and mutual exclusion between TET2-mutant and CREBBP-mutant primary DLBCL. RNA sequencing analysis revealed that the genes epigenetically regulated by TET2 were aberrantly repressed in VavCre/Tet2+/+ GC B cells, explaining the observed GC hyperplasia in TET2-deficient GC B cells since these genes control the differentiation of GC B cells into plasma cells. Importantly, TET2-mutant DLBCL primary samples display a similar repressive transcriptional signature associated with GC B-cell terminal differentiation. Our data show how TET2-induced epigenetic changes contribute to lymphoma development and highlight the multilayered nature of the epigenome, which can be therapeutically exploited. We are evaluating the therapeutic potential in TET2-mutant DLBCL of a combinatorial therapy consisting of DNA methylation inhibitors (DNMTi), to revert hypermethylation at promoters, plus specific HDAC3 inhibitors, to compensate for the loss of hmC at enhancers.
Citation Format: Pilar M. Dominguez, Wojciech Rosikiewicz, Xiaowen Chen, Hussein Ghamlouch, Said Aoufouchi, Olivier A. Bernard, Ari M. Melnick, Sheng Li, Ricky W. Johnstone. TET2 deficiency alters the epigenome of germinal center B cells, contributing to lymphoma formation [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-05.