TET deficiency perturbs mature B cell homeostasis and promotes oncogenesis associated with accumulation of G-quadruplex and R-loop structures

Enzymes of the TET family are methylcytosine dioxygenases that undergo frequent mutational or functional inactivation in human cancers. Recurrent loss-of-function mutations in TET proteins are frequent in human diffuse large B cell lymphoma (DLBCL). Here, we investigate the role of TET proteins in B cell homeostasis and development of B cell lymphomas with features of DLBCL. We show that deletion of Tet2 and Tet3 genes in mature B cells in mice perturbs B cell homeostasis and results in spontaneous development of germinal center (GC)-derived B cell lymphomas with increased G-quadruplexes and R-loops. At a genome-wide level, G-quadruplexes and R-loops were associated with increased DNA double-strand breaks (DSBs) at immunoglobulin switch regions. Deletion of the DNA methyltransferase DNMT1 in TET-deficient B cells prevented expansion of GC B cells, diminished the accumulation of G-quadruplexes and R-loops and delayed B lymphoma development, consistent with the opposing functions of DNMT and TET enzymes in DNA methylation and demethylation. Clustered regularly interspaced short palindromic repeats (CRISPR)-mediated depletion of nucleases and helicases that regulate G-quadruplexes and R-loops decreased the viability of TET-deficient B cells. Our studies suggest a molecular mechanism by which TET loss of function might predispose to the development of B cell malignancies.