APOBEC3基因拷贝数变异与乳腺癌易感性的关联

为了探讨APOBEC3基因缺失拷贝数变异的多态性与汉族女性乳腺癌易感性的关联性,本研究应用聚合酶链式反应-限制性多态性内切酶技术检测281例乳腺癌和292例正常对照组。我们发现APOBEC3基因拷贝数变异位点的等位基因、基因型频率分布在乳腺癌和对照组之间比较有显著性差异(p<0.05),校正混杂因素后基因型Del/Del,Del/Ins,显性模型和加性模型下对乳腺癌的风险预测值分别为2.753 (95%Cl:1.363～3.560; p=0.005)、1.462(95%Cl:1.021～2.094; p=0.038)、2.282(95%Cl:1.155～3.508; p=0.018)和1.596(95%Cl:1.129～2.256; p=0.008)。研究表明APOBEC3基因的缺失变异位点多态性与乳腺癌的发病风险存在关联,等位基因del是乳腺癌发病的危险因素。本研究的结果可以为本地区群体的乳腺癌易感基因的筛选以及评估易感基因对患病的风险程度提供参考数据,为乳腺癌的防治、诊断和个体化治疗研究提供了帮助。     

APOBEC deaminases-mutases with defensive roles for immunity

In recent years, tremendous progress has been made in the elucidation of the biological roles and molecular mechanisms of the apolioprotein B mRNA-editing enzyme catalytic polypeptide (APOBEC) family of enzymes. The APOBEC family of cytidine deaminases has important functional roles within the adaptive and innate immune system. Activation induced cytidine deaminase (AID) plays a central role in the biochemical steps of somatic hypermutation and class switch recombination during antibody maturation, and the APOBEC 3 enzymes are able to inhibit the mobility of retroelements and the replication of retroviruses and DNA viruses, such as the human immunodeficiency virus type-1 and hepatitis B virus. Recent advances in structural and functional studies of the APOBEC enzymes provide new biochemical insights for how these enzymes carry out their biological roles. In this review, we provide an overview of these recent advances in the APOBEC field with a special emphasis on AID and APOBEC3G.     

Extensive mutagenesis experiments corroborate a structural model for the DNA deaminase domain of APOBEC3G

APOBEC3G is a single-strand DNA cytosine deaminase capable of blocking retrovirus and retrotransposon replication. APOBEC3G has two conserved zinc-coordinating motifs but only one is required for catalysis. Here, deletion analyses revealed that the minimal catalytic domain consists of residues 198-384. Size exclusion assays indicated that this protein is monomeric. Many (31/69) alanine substitution derivatives of APOBEC3G198-384 retained significant to full levels of activity. These data corroborated an APOBEC2-based structural model for the catalytic domain of APOBEC3G indicating that most non-essential residues are solvent accessible and most essential residues cluster within the protein core.     

Selective inhibition of Alu retrotransposition by APOBEC3G

The non-LTR retrotransposon LINE-1 (L1) comprises  17% of the human genome, and the L1-encoded proteins can function in trans to mediate the retrotransposition of non-autonomous retrotransposons (i.e., Alu and probably SVA elements) and cellular mRNAs to generate processed pseudogenes. Here, we have examined the effect of APOBEC3G and APOBEC3F, cytidine deaminases that inhibit Vif-deficient HIV-1 replication, on Alu retrotransposition and other L1-mediated retrotransposition processes. We demonstrate that APOBEC3G selectively inhibits Alu retrotransposition in an ORF1p-independent manner. An active cytidine deaminase site is not required for the inhibition of Alu retrotransposition and the resultant integration events lack G to A or C to T hypermutation. These data demonstrate a differential restriction of L1 and Alu retrotransposition by APOBEC3G, and suggest that the Alu ribonucleoprotein complex may be targeted by APOBEC3G.     

Concerted action of activation-induced cytidine deaminase and uracil-DNA glycosylase reduces covalently closed circular DNA of duck hepatitis B virus

Covalently closed circular DNA (cccDNA) forms a template for the replication of hepatitis B virus (HBV) and duck HBV (DHBV). Recent studies suggest that activation-induced cytidine deaminase (AID) functions in innate immunity, although its molecular mechanism of action remains unclear, particularly regarding HBV restriction. Here we demonstrated that overexpression of chicken AID caused hypermutation and reduction of DHBV cccDNA levels. Inhibition of uracil-DNA glycosylase (UNG) by UNG inhibitor protein (UGI) abolished AID-induced cccDNA reduction, suggesting that the AID/UNG pathway triggers the degradation of cccDNA via cytosine deamination and uracil excision.     

High expression of APOBEC3G in patients infected with hepatitis C virus

APOBEC3G (an apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G; also known as CEM15), a member of the APOBEC family, which possesses cytidine deaminase activity that causes C/G to T/A transition mutations in virus genomes such as human immunodeficiency virus 1 and hepatitis B virus, is reported to play an important role in host-defense mechanisms. However, APOBEC3G expression in patients infected with chronic hepatitis C virus (HCV), of which there are currently more than 170 million worldwide, has not yet been well studied. We investigated this issue herein, and demonstrated an increased expression of APOBEC3G in both hepatocytes and lymphocytes of chronic hepatitis patients infected with HCV. Transfection of the NS5A gene, but not any other non-structural protein genes of HCV tested, to the hepatocellular carcinoma cell line enhanced APOBEC3G expression. Incubation of the cells with interferon also resulted in the augmentation. These results may provide new insight into the pathogenesis of chronic HCV infection.     

Zinc-mediated binding of a low-molecular-weight stabilizer of the host anti-viral factor apolipoprotein B mRNA-editing enzyme,catalytic polypeptide-like 3G

Apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G (APOBEC3G, A3G), is a human anti-virus restriction protein which works deaminase-dependently and -independently. A3G is known to be ubiquitinated by HIV-1 viral infectivity factor (Vif) protein, leading to proteasomal degradation. A3G contains two zinc ions at the N-terminal domain and the C-terminal domain. Four lysine residues, K²⁹⁷, K³⁰¹, K³⁰³, and K³³⁴, are known to be required for Vif-mediated A3G ubiquitination and degradation. Previously, we reported compound SN-1, a zinc chelator that increases steady-state expression level of A3G in the presence of Vif. In this study, we prepared Biotin-SN-1, a biotinylated derivative of SN-1, to study the SN-1–A3G interaction. A pull-down assay revealed that Biotin-SN-1 bound A3G. A zinc-abstraction experiment indicated that SN-1 binds to the zinc site of A3G. We carried out a SN-1–A3G docking study using molecular operating environment. The calculations revealed that SN-1 binds to the C-terminal domain through Zn²⁺, H²¹⁶, P²⁴⁷, C²⁸⁸, and Y³¹⁵. Notably, SN-1-binding covers the H²⁵⁷, E²⁵⁹, C²⁸⁸, and C²⁹¹ residues that participate in zinc-mediated deamination, and the ubiquitination regions of A3G. The binding of SN-1 presumably perturbs the secondary structure between C²⁸⁸ and Y³¹⁵, leading to less efficient ubiquitination.