Epigenetic changes by EZH2 inhibition increase translocations in B cells with high AID activity or DNA repair deficiency [Cut&Tag for H3K27me3]

The EZH2 histone methyltransferase inhibitors tazemetostat and valemetostat have recently received approval for clinical use in follicular lymphoma and adult T-cell leukemia/lymphoma, respectively. In follicular lymphoma, the expression of 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. Here we show 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 PI3Kδ inhibition, which enhances AID expression, EZH2 inhibition significantly increased the frequency of chromosomal translocations compared to 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 Ligase4 deficient. Mechanistically, EZH2 inhibition in B cells depletes the repressive histone modification H3K27me3 while concurrently enhancing the active histone modification 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.