宿主デアミナーゼによるウイルス生き残り戦略

金沢大学医学系これまでB型肝炎の病態をモデル化した試験管内実験系により以下の事を明らかにしてきた。1)、B型肝炎ウイルスレプリコンをヒト肝細胞株に発現させるとウイルス複製と培地中へのウイルスの分泌が起こる。これはB型肝炎ウイルス治療薬のラミブジンで複製阻害できる。2)、HBVレプリコンとAPOBEC3Gを発現させるとAPOBEC3G依存的部分的複製抑制がおこりさらにはHBVゲノムDNAに高頻度変異が導入される。高頻度変異は主にG-to-Aが最も多い。3)、2)は内在性APOBEC3Gをインターフェロンで発現誘導させてもウイルス複製抑制と高頻度変異が起こせる。そこで本年度は、まずはラミブジン耐性株(M204V)のPCRによる高感度検出方法の条件検討を行なった。また人工的にM204V変異を導入したウイルスクローンが本研究のHBV試験管内実験系でラミブジン存在下でもウイルス複製能を維持する条件検討を行なった。点突然変異を検出するのに特化したプライマーと全HBVを検出できるプラィマーのマルチプレックスPCRの条件検討したところ、野生型100に対してM204V株1の出現比率でも薬剤耐性株を検出できる系を構築できた。また野生型HBVは、50uMのラミブジン処理では複製抑制されたがM204V変異型HBVは、複製効率はラミブジンによって影響を受けなかった。これらの結果は、ウイルスの生残り戦略の指標である薬剤耐性獲得の検出系の確率と生残り戦略を発揮できる微小環境の基本的設定を実験系に組み入れる基礎ができた事を意味する意義のある結果である。H23年度は、この成果を踏まえて宿主デアミナーゼの役割をウイルス生残り戦略の観点から正していく予定であるが、制度上本萌芽研究は辞退し、最先端次世代プロジェクトで継続していく。研究課題/領域番号:21659120, 研究期間(年度):2009 – 2011出典：研究課題「宿主デアミナーゼによるウイルス生き残り戦略」課題番号21659120（KAKEN：科学研究費助成事業データベース（国立情報学研究所）） （https://kaken.nii.ac.jp/ja/grant/KAKENHI-PROJECT-21659120/）を加工して作

B型肝炎ウイルスHypermutationと発ガン

金沢大学医薬保健研究域医学系前年度は、実験材料として樹立したB型肝炎ウイルス(HBV)を構成的に発現するヒト肝細胞株に不具合が見つかったのでやむなく本年度まで研究継続した。HBV株の樹立を再度行ない、これまでの知見を確認した。まずはDNA編集酵素APOBEC3G(およびそのファミリー)をこの細胞株に強制発現させると、ウイルスの複製が抑制された。抑制は、キャップシド内の逆転写のレベルで起こっていた。またこの際、キャップシド内に存在するHBV DNAに高頻度突然変異を見いだした。この高頻度突然変異は、ほとんどG-to-A変異であり、マイナス鎖DNAが逆転写された際にAPOBEC3GがDNA上のCのアミノ基を加水分解したことを示唆した。さらに培地中に分泌されたウイルス粒子内のHBV DNAでも同様の知見を得た。肝炎病態では、治療手段としてあるいは免疫系活性化の結果として、局所環境でインターフェロンが分泌される可能性がある。そこでこの細胞株にインターフェロンを投与し細胞を刺激すると、内在性APOBEC3Gが発現誘導され、強制発現の際に酷似するウイルス複製抑制と高頻度変異を観察した。またDNAデアミナーゼが作るC-to-U変異は塩基除去修復系酵素UNG感受性であると考えられるので、UNGの活性を阻害した環境で、ウイルス複製と高頻度変異を見たところ、ウイルス複製はUNG活性の有無でさほど違いは無かったが高頻度変異は増強した。以上の知見から慢性のB型肝炎の際にはAPOBEC3Gなどのデアミナーゼが発現しウイルス複製は若干抑制する一方でウイルスゲノム多様性は増悪し、さらに除去修復系活性が下がるとウイルス多様性は増強する事が見いだされた。この事はDNAデアミナーゼが慢性化の一つの原因であるウイルス多様性を増強し間接的に発ガンに寄与する可能性を示唆している。研究課題/領域番号:20012018, 研究期間(年度):2008 – 2009出典：研究課題「B型肝炎ウイルスHypermutationと発ガン」課題番号20012018（KAKEN：科学研究費助成事業データベース（国立情報学研究所））（https://kaken.nii.ac.jp/ja/grant/KAKENHI-PROJECT-20012018/）を加工して作

Activation-induced Deaminase (AID)-directed Hypermutation in the Immunoglobulin Sμ Region: Implication of AID Involvement in a Common Step of Class Switch Recombination and Somatic Hypermutation

Somatic hypermutation (SHM) and class switch recombination (CSR) cause distinct genetic alterations at different regions of immunoglobulin genes in B lymphocytes: point mutations in variable regions and large deletions in S regions, respectively. Yet both depend on activation-induced deaminase (AID), the function of which in the two reactions has been an enigma. Here we report that B cell stimulation which induces CSR but not SHM, leads to AID-dependent accumulation of SHM-like point mutations in the switch μ region, uncoupled with CSR. These findings strongly suggest that AID itself or a single molecule generated by RNA editing function of AID may mediate a common step of SHM and CSR, which is likely to be involved in DNA cleavage

Unmutated Immunoglobulin M Can Protect Mice from Death by Influenza Virus Infection

To elucidate the role of class switch recombination (CSR) and somatic hypermutation (SHM) in virus infection, we have investigated the influence of the primary and secondary infections of influenza virus on mice deficient of activation-induced cytidine deaminase (AID), which is absolutely required for CSR and SHM. In the primary infection, AID deficiency caused no significant difference in mortality but did cause difference in morbidity. In the secondary infection with a lethal dose of influenza virus, both AID−/− and AID+/− mice survived completely. However, AID−/− mice could not completely block replication of the virus and their body weights decreased severely whereas AID+/− mice showed almost complete prevention from the reinfection. Depletion of CD8+ T cells by administration of an anti-CD8 monoclonal antibody caused slightly severer body weight loss but did not alter the survival rate of AID−/− mice in secondary infection. These results indicate that unmutated immunoglobulin (Ig)M alone is capable of protecting mice from death upon primary and secondary infections. Because the titers of virus-neutralizing antibodies were comparable between AID−/− and AID+/− mice at the time of the secondary infection, a defect of AID−/− mice in protection of morbidity might be due to the absence of either other Ig classes such as IgG, high affinity antibodies with SHM, or both

DNA Double-Strand Breaks: Prior to but not Sufficient in Targeting Hypermutation

The activation-induced cytidine deaminase (AID) is required for somatic hypermutation (SHM) and class-switch recombination (CSR) of immunoglobulin (Ig) genes, both of which are associated with DNA double-strand breaks (DSBs). As AID is capable of deaminating deoxy-cytidine (dC) to deoxy-uracil (dU), it might induce nicks (single strand DNA breaks) and also DNA DSBs via a U-DNA glycosylase-mediated base excision repair pathway (‘DNA-substrate model’). Alternatively, AID functions like its closest homologue Apobec1 as a catalytic subunit of a RNA editing holoenzyme (‘RNA-substrate model’). Although rearranged Vλ genes are preferred targets of SHM we found that germinal center (GC) B cells of AID-proficient and -deficient Vλ1-expressing GC B cells display a similar frequency, distribution, and sequence preference of DSBs in rearranged and also in germline Vλ1 genes. The possible roles of DSBs in relation to AID function and SHM are discussed

Constitutive Expression of AID Leads to Tumorigenesis

Genome stability is regulated by the balance between efficiencies of the repair machinery and genetic alterations such as mutations and chromosomal rearrangements. It has been postulated that deregulation of class switch recombination (CSR) and somatic hypermutation (SHM), which modify the immunoglobulin (Ig) genes in activated B cells, may be responsible for aberrant chromosomal translocations and mutations of non-Ig genes that lead to lymphocyte malignancy. However, the molecular basis for these genetic instabilities is not clearly understood. Activation-induced cytidine deaminase (AID) is shown to be essential and sufficient to induce both CSR and SHM in artificial substrates in fibroblasts as well as B cells. Here we show that constitutive and ubiquitous expression of AID in transgenic mice caused both T cell lymphomas and dysgenetic lesions of epithelium of respiratory bronchioles (micro-adenomas) in all individual mice. Point mutations, but not translocations, were massively introduced in expressed T cell receptor (TCR) and c-myc genes in T lymphoma cells. The results indicate that AID can mutate non-Ig genes including oncogenes, implying that aberrant AID expression could be a cause of human malignancy

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. © 2013 Federation of European Biochemical Societies

Uracil DNA Glycosylase Counteracts APOBEC3G-Induced Hypermutation of Hepatitis B Viral Genomes: Excision Repair of Covalently Closed Circular DNA

The covalently closed circular DNA (cccDNA) of the hepatitis B virus (HBV) plays an essential role in chronic hepatitis. The cellular repair system is proposed to convert cytoplasmic nucleocapsid (NC) DNA (partially double-stranded DNA) into cccDNA in the nucleus. Recently, antiviral cytidine deaminases, AID/APOBEC proteins, were shown to generate uracil residues in the NC-DNA through deamination, resulting in cytidine-to-uracil (C-to-U) hypermutation of the viral genome. We investigated whether uracil residues in hepadnavirus DNA were excised by uracil-DNA glycosylase (UNG), a host factor for base excision repair (BER). When UNG activity was inhibited by the expression of the UNG inhibitory protein (UGI), hypermutation of NC-DNA induced by either APOBEC3G or interferon treatment was enhanced in a human hepatocyte cell line. To assess the effect of UNG on the cccDNA viral intermediate, we used the duck HBV (DHBV) replication model. Sequence analyses of DHBV DNAs showed that cccDNA accumulated G-to-A or C-to-T mutations in APOBEC3G-expressing cells, and this was extensively enhanced by UNG inhibition. The cccDNA hypermutation generated many premature stop codons in the P gene. UNG inhibition also enhanced the APOBEC3G-mediated suppression of viral replication, including reduction of NC-DNA, pre-C mRNA, and secreted viral particle-associated DNA in prolonged culture. Enhancement of APOBEC3G-mediated suppression by UNG inhibition was not observed when the catalytic site of APOBEC3G was mutated. Transfection experiments of recloned cccDNAs revealed that the combination of UNG inhibition and APOBEC3G expression reduced the replication ability of cccDNA. Taken together, these data indicate that UNG excises uracil residues from the viral genome during or after cccDNA formation in the nucleus and imply that BER pathway activities decrease the antiviral effect of APOBEC3-mediated hypermutation. © 2013 Kitamura et al

RNA editing of hepatitis B virus transcripts by activation-induced cytidine deaminase.

Activation-induced cytidine deaminase (AID) is essential for the somatic hypermutation (SHM) and class-switch recombination (CSR) of Ig genes. The mechanism by which AID triggers SHM and CSR has been explained by two distinct models. In the DNA deamination model, AID converts cytidine bases in DNA into uridine. The uridine is recognized by the DNA repair system, which produces DNA strand breakages and point mutations. In the alternative model, RNA edited by AID is responsible for triggering CSR and SHM. However, RNA deamination by AID has not been demonstrated. Here we found that C-to-T and G-to-A mutations accumulated in hepatitis B virus (HBV) nucleocapsid DNA when AID was expressed in HBV-replicating hepatic cell lines. AID expression caused C-to-T mutations in the nucleocapsid DNA of RNase H-defective HBV, which does not produce plus-strand viral DNA. Furthermore, the RT-PCR products of nucleocapsid viral RNA from AID-expressing cells exhibited significant C-to-T mutations, whereas viral RNAs outside the nucleocapsid did not accumulate C-to-U mutations. Moreover, AID was packaged within the nucleocapsid by forming a ribonucleoprotein complex with HBV RNA and the HBV polymerase protein. The encapsidation of the AID protein with viral RNA and DNA provides an efficient environment for evaluating AID's RNA and DNA deamination activities. A bona fide RNA-editing enzyme, apolipoprotein B mRNA editing catalytic polypeptide 1, induced a similar level of C-to-U mutations in nucleocapsid RNA as AID. Taken together, the results indicate that AID can deaminate the nucleocapsid RNA of HBV

Interferon-alpha responsible EPN3 regulates hepatitis B virus replication

Hepatitis B virus (HBV) infection remains a major health problem worldwide, and the current antiviral therapy, including nucleoside analogs, cannot achieve life-long cure, and clarification of antiviral host immunity is necessary for eradication. Here, we found that a clathrin-binding membrane protein epsin3 (EPN3) negatively regulates the expression of HBV RNA. EPN3 expression was induced by transfection of an HBV replicon plasmid, and reduced HBV-RNA level in hepatic cell lines and murine livers hydrodynamically injected with the HBV replicon plasmid. Viral RNA reduction by EPN3 was dependent on transcription, and independent from epsilon structure of viral RNA. Viral RNA reduction by overexpression of p53 or IFN-α treatment, was attenuated by knockdown of EPN3, suggesting its role downstream of IFN-α and p53. Taken together, this study demonstrates the anti-HBV role of EPN3. The mechanism how it decreases HBV transcription is discussed