Kmt5a Controls Hepatic Metabolic Pathways by Facilitating RNA Pol II Release from Promoter-Proximal Regions

H4K20 monomethylation maintains genome integrity by regulating proper mitotic condensation, DNA damage response and replication licensing. Here we show that in non-dividing hepatic cells H4K20Me1 is specifically enriched in the gene bodies of active genes and dynamically regulated by the antagonistic action of PR-Set7 methylase and Phf8 demethylase. In liver-specific PR-Set7-deficient mice, reduced H4K20Me1 levels primarily affected RNAPII release from promoter-proximal regions. Glucose and fatty acid metabolic genes were highly sensitive to impaired RNAPII release. Downregulation of glycolytic genes resulted in an energy starvation condition partially compensated by AMPK activation and increased mitochondrial activity. This metabolic reprogramming generated a normoglycemic, but highly sensitized state, which upon different metabolic stress conditions, quickly aggravated into a senescent phenotype initiated by ROS overproduction and oxidative DNA damage. The results illustrate how defects in the general process of RNAPII transition into elongation phase can trigger specific metabolic changes, which lead to genome instability. PR-Set7 (other name Set8; Setd8) knockout mice (other name Set8; Setd8) were generated by crossing PR-SET7loxp mice carrying floxed exon 7 allele of PR-SET7 (Oda et al., 2009) with Alb-Cre mice (Yakar et al., 1999) in mixed F1 background. Mice (WT = Wild type; Set8-KO = Setd8-KO; PR-SET7-KO) were sacrificed at 45 days postnatal age. Livers from these mice were used for ChIP-seq and RNA-seq approaches. For Global Run-On sequencing newly synthesized RNA GRO-EU-seq, mice were treated with xxx mg /g bodyweight Ethydyl-Uridine (EU) before sacrifice. For the GRO-Br-seq experiments nuclei were prepared from the livers and treated with Br-UTP for 3 minutes.组蛋白H4赖氨酸20单甲基化（H4K20 monomethylation）通过调控正常有丝分裂凝聚、DNA损伤应答与复制许可过程维持基因组完整性。本研究发现，在非增殖肝细胞中，H4K20单甲基化修饰（H4K20Me1）特异性富集于活性基因的基因体区域，并受PR-Set7甲基转移酶与Phf8去甲基化酶的拮抗作用动态调控。在肝脏特异性PR-Set7敲除小鼠体内，H4K20单甲基化水平的降低主要影响RNA聚合酶II（RNAPII）从启动子近端区域的释放。葡萄糖与脂肪酸代谢基因对RNAPII释放受损表现出高度敏感性。糖酵解基因的下调会引发能量饥饿状态，该状态可通过AMP活化蛋白激酶（AMPK）激活与线粒体活性增强得到部分代偿。这种代谢重编程会产生一种血糖正常但高度敏化的状态，当该状态遭遇不同代谢应激时，会快速恶化为以活性氧（ROS）过度生成与氧化性DNA损伤为起始诱因的衰老表型。本研究结果阐明了RNA聚合酶II向延伸阶段转变的通用过程出现缺陷，如何触发特异性代谢改变并最终导致基因组不稳定。PR-Set7（别名Set8、Setd8）敲除小鼠（别名同前）的构建方式为：将携带PR-SET7基因第7外显子loxP侧翼等位基因的PR-SET7loxp小鼠（Oda等，2009）与Alb-Cre小鼠（Yakar等，1999）在混合F1背景下杂交获得。实验小鼠（野生型WT；Set8-KO=Setd8-KO；PR-SET7-KO）于出生后45天处死，取其肝脏开展染色质免疫共沉淀测序（ChIP-seq）与RNA测序（RNA-seq）实验。对于全局转录运行测序（GRO-EU-seq）实验，需在处死小鼠前按xxx mg/g体重剂量向其注射乙基尿苷（EU）；而GRO-Br-seq实验则通过分离小鼠肝脏细胞核，用溴代UTP（Br-UTP）处理3分钟完成。