细胞核HBV cccDNA难以清除是慢性乙型肝炎难治愈的主要原因。近年发现细胞核APOBEC3A（A3A）可能通过与HBc结合，使cccDNA 的dC脱氨基为dU，进而尿嘧啶DNA糖基化酶2（UNG2）启动对dU切除修复，最终引起cccDNA特异性降解。而碱基切除修复如何引起cccDNA特异性降解的机制还不十分清楚。为了利用此效应建立清除cccDNA效应更强的抗病毒方法并阐明特异性cccDNA降解机制，我们拟在前期HBc研究基础上，重构HBc并构建核cccDNA靶向的HBc-A3A/UNG2抗病毒融合蛋白。用细胞和小鼠模型、qPCR、3D-PCR、ChIP和基因测序等技术，研究靶向蛋白引发特异性cccDNA降解的作用和机制，探索建立靶向双因子清除cccDNA的新策略，进一步阐明A3A、UNG2引发特异性cccDNA降解的机制，为研究建立系统的靶向清除乙肝病毒的治疗方案奠定理论和实验基础。

Inability to clear hepatitis B virus covalently closed circular DNA (cccDNA) from the nuclei of infected cells is primarily responsible for the failure to cure chronic HBV infection. It was recently reported that IFN-α can upregulate hepatic nuclear APOBEC3A (A3A) expression, which deaminates dC base of the cccDNA through binding to HBV core protein and converts it to dU. The resulting dU becomes a target for the excision repair by uracil DNA glycosylase (UNG2) and it ultimately leads to degradation of cccDNA. The mechanisms underlying the degradation of cccDNA by the base excision repair process remain unknown. To illustrate the mechanisms for cccDNA degradation mediated by both A3A and UNG2 and to establish a strong and targeted antiviral strategy for clearing cccDNA, we propose to modify HBc and to construct and express a HBc-A3A / UNG2 antiviral fusion protein that directly targets cccDNA, on the platform of our previous study of HBV core protein. We will investigate specific functions of this fusion protein in degrading cccDNA, explore a new strategy through studying both factors targeting cccDNA clearance, and dissect the specific mechanisms for cccDNA degradation. We will use HBV infected cell and mouse model as well as molecular and cell biology techniques of qPCR, 3D-PCR, ChIP and DNA sequencing for accomplishing proposed experiments. The results by completing this proposal are expected to provide theoretical and experimental basis for a new antiviral therapy, which systematically targets cccDNA clearance.