HBX蛋白劫持宿主细胞WDR5促进HBV cccDNA复制转录机制研究

Hepatitis B virus (HBV) is able to hijack cellular mechanisms to avail its life cycle. It has been known that HBV can promote the replication and transcription of the covalent closed circle DNA (cccDNA) by using the cellular related mechanisms to methylate the histones of cccDNA chromatin. However, how HBV hijacks the cellular mechanisms to mediate the process remains unclear. In the previous experiments, we found that HBV X protein up-regulated the WDR5 protein level of host cells, a core subunit of histone lysine(K)-specific methyltransferase 2 (KMT2s). We also found that the high WDR5 protein level in HBX over-expressed cells, and HBX directly bound WDR5 and recruited the latter to HBV cccDNAchromatin, inducing a high level of H3K4me3 modification and promoting HBV replication and transcription. Basing on the literature and what we observed in our previous study, we speculate that HBX could hijack DDB1 protein, the key subunit of the ubiquitin ligase complex, to stabilize WDR5 proteinby inhibiting its ubiquitination and subsequent degradation. HBX then recruits WDR5-MKT2s to cccDNA by direct binding with WDR5, followed by H3K4 methylation of the histones of cccDNA chromatin. In this application project, by using large scale of clinical samples, we will further confirm the relationship between the increased protein level of WDR5 and the degree of HBV infection in chronic hepatitis B (CHB) patients. Then by protein-protein interaction assay and DNA pull-down techniques, we will investigate how HBX/WDR5/DDB1 interacts with each other to inhibit WDR5 protein’s ubiquitination and explore which KMT2(s) involved in the HBX induced aberrant H3K4me3 modification of cccDNA. Finally, we will utilize a series of transgenic mouse models to verify the above clues and confirm the exact roles of HBX-WDR5-KMT2-H3K4me3 axis in regulating cccDNA activity and the consequent HBV replication and transcription. The expected results in this project will not only provide new mechanisms of HBV life cycle but also provide the potential targets for the treatment of CHBat the epigenetic level.

HBV cccDNA染色质组蛋白甲基化可促进其复制转录，但该过程的具体分子机制不清。我们前期研究发现HBX蛋白可延长细胞组蛋白甲基转移酶KMT2s复合体关键亚基WDR5半衰期，HBX过表达上调cccDNA启动子区H3K4me3水平，干扰WDR5表达显著降低HBV复制与转录。我们推测HBX蛋白通过竞争性结合WDR5，抑制后者泛素化降解并将WDR5-KMT2s复合体募集到cccDNA进行H3K4me3修饰，活化cccDNA。本课题将进一步利用临床大样本验证WDR5与慢乙肝的临床相关性，并采用蛋白互作、DNA pull-down等技术拟进一步探讨HBX抑制WDR5蛋白泛素化的机制，明确HBX募集哪个(些)KMT2，并通过系列肝脏特异基因敲除小鼠模型分析HBX-WDR5-KMT2-H3K4me3轴对cccDNA调控作用。研究结果将有助于认识cccDNA转录复制和维持机制，为慢乙肝防治提供潜在新靶标

背景和目的：癌症通常被认为是一种遗传和表观遗传疾病。尽管大量研究表明表观遗传调节相关酶异常的结构、功能或表达水平与多种肿瘤类型有关，但表观遗传机制在乙型肝炎病毒（HBV）诱导的肝细胞癌（HCC）中的确切作用尚不清楚。..结果：在本研究中，我们发现组蛋白H3赖氨酸4甲基转移酶复合物的核心亚单位WD重复结构域5蛋白（WDR5）在HBV相关的HCC中高表达，并促进HCC的进展。WDR5在小鼠HBV驱动的细胞增殖和肿瘤生长中起关键作用，WDR5-0103 WDR5活性的小分子抑制剂损害HBV和HBx驱动的肿瘤增殖。异常升高的WDR5蛋白水平通过抑制损伤特异性DNA结合蛋白1/cullin-4和WDR5之间的相互作用来稳定WDR5蛋白，从而导致WDR5蛋白泛素化减少。HBx与WDR5在全基因组染色质上共定位，并通过WDR5促进全基因组H3K4me3修饰。此外，HBx向靶基因启动子的招募依赖于其通过α-螺旋结构域与WDR5的相互作用。WDR5可通过H3K4修饰共价闭合的环状DNA微染色体促进HBV的转录，WDR5-0103可抑制HBV的转录。最后，体外和体内实验进一步证明HBx的促肿瘤作用是以WDR5依赖的方式发挥的。.结论：WDR5是HBV诱导肿瘤发生的关键表观遗传学决定因素，HBx-WDR5-H3K4me3轴可能是HBV诱导肝脏病变的潜在治疗靶点。