EZH2 inhibition stimulates repetitive element expression and viral mimicry in resting splenic B cells

Mammalian cells repress expression of repetitive genomic sequences by forming heterochromatin. However, the consequences of ectopic repeat expression remain unclear. Here we demonstrate that inhibitors of EZH2, the catalytic subunit of the Polycomb repressive complex 2 (PRC2), stimulate repeat misexpression and cell death in resting splenic B cells. B cells are uniquely sensitive to these agents because they exhibit high levels of histone H3 lysine 27 trimethylation (H3K27me3) and correspondingly low DNA methylation at repeat elements. We generated a pattern recognition receptor loss-of-function mouse model, called RIC, with mutations in Rigi (encoding for RIG-I), Ifih1 (MDA5), and Cgas. In both wildtype and RIC mutant B cells, EZH2 inhibition caused loss of H3K27me3 at repetitive elements and upregulated their expression. However, NF-B dependent expression of inflammatory chemokines and subsequent cell death was suppressed by the RIC mutations. We further show that inhibition of EZH2 in cancer cells requires the same pattern recognition receptors to activate an interferon response. Together, the results reveal chemokine expression induced by EZH2 inhibitors in B cells as a novel inflammatory response to genomic repeat expression. Given the overlap of genes induced by EZH2 inhibitors and Epstein-Barr virus infection, this response can be described as a form of viral mimicry.

哺乳动物细胞通过形成异染色质（heterochromatin）抑制基因组重复序列的表达。然而，异位重复序列表达所产生的具体后果仍未明确。本研究证实，多梳抑制复合体2（Polycomb repressive complex 2, PRC2）的催化亚基EZH2抑制剂，可在静息脾脏B细胞中诱导重复序列异常表达并引发细胞死亡。B细胞对这类抑制剂具有独特的敏感性，原因在于其在重复序列区域存在高水平的组蛋白H3赖氨酸27三甲基化（H3K27me3），同时DNA甲基化水平相应较低。我们构建了一种模式识别受体（pattern recognition receptor）功能缺失型小鼠模型，命名为RIC，该模型携带Rigi（编码RIG-I）、Ifih1（MDA5）以及Cgas基因的突变。在野生型与RIC突变型B细胞中，EZH2抑制均会导致重复序列区域的H3K27me3丢失，并上调其表达水平。然而，RIC突变可抑制依赖核因子κB（NF-κB）的炎症趋化因子表达以及后续的细胞死亡过程。我们进一步证实，癌细胞中的EZH2抑制作用需要通过相同的模式识别受体来激活干扰素应答。综合上述结果，本研究揭示B细胞中EZH2抑制剂诱导的趋化因子表达，是一种针对基因组重复序列表达的新型炎症应答。鉴于EZH2抑制剂诱导的基因与EB病毒（Epstein-Barr virus, EBV）感染所诱导的基因存在重叠，该应答可被归类为一种病毒模拟现象。