Epigenetic changes by EZH2 inhibition increase translocations in B cells with high AID activity or DNA repair deficiency

The enhancer of zeste homolog 2 (EZH2) histone methyltransferase inhibitors tazemetostat and valemetostat recently have received approval for clinical use in follicular lymphoma and adult T-cell leukemia/lymphoma, respectively. In follicular lymphoma, the expression of activation-induced cytidine deaminase (AID) is responsible for increased mutational signatures and genomic instability. Because EZH2 inhibitors induce epigenetic and transcriptional changes in normal and lymphoma B cells, we studied whether these inhibitors could alter the pattern of AID-mediated chromosomal translocations. In this study, we showed that treatment with EZH2 inhibitors did not significantly change AID expression or AID-dependent chromosomal translocation frequency when used as monotherapy in either CH12F3 mouse B cells or MEC-1 human B cells. In contrast, when combined with phosphoinositide 3-kinase δ (PI3Kδ) inhibition, which enhances AID expression, EZH2 inhibition significantly increased the frequency of chromosomal translocations when compared with either EZH2 or PI3Kδ inhibition alone both in mouse CH12F3 cells and human MEC-1 cells. EZH2 inhibition also further enhanced translocation formation in mouse B cells that were DNA ligase IV (Ligase4) deficient. Mechanistically, EZH2 inhibition in B cells depletes the repressive histone modification H3 trimethylation at lysine 27 (H3K27me3) while concurrently enhancing the active histone modification H3 acetylation at lysine 27 (H3K27ac), thereby selectively increasing transcriptional activity and facilitating chromosomal translocation formation in the presence of high AID activity or Ligase4 deficiency. These findings highlight the impact of drugs that induce epigenetic changes to influence chromosomal translocations, and demonstrate the genetic safety of EZH2 inhibitors as monotherapy while highlighting the increased risk of genomic instability when used in cells prone to translocations, such as B cells with high AID levels or DNA repair deficiency.