腺相关病毒

病毒

腺相关病毒Adeno-associated viruses,常缩写为AAV),是一种隶属细小病毒科依赖性细小病毒属、能够感染人类以及其他部分灵长类动物的病毒。腺相关病毒直径约20纳米无被膜无法自主完成复制英语Replication-defective virus,全基因组长约4.8千碱基对(kb)[1][2]

腺相关病毒
一种用作人基因治疗载体的腺相关病毒1LP3的3D原子结构
病毒分类 编辑
(未分级) 病毒 Virus
域: 单链DNA病毒域 Monodnaviria
界: 称德病毒界 Shotokuvirae
门: 科萨特病毒门 Cossaviricota
纲: 第五病毒纲 Quintoviricetes
目: 小病毒目 Piccovirales
科: 细小病毒科 Parvoviridae
属: 依赖性细小病毒属 Dependoparvovirus
种:
腺相关病毒 Adeno-associated virus
种类
  • 腺相关病毒A
    Adeno-associated dependoparvovirus A
  • 腺相关病毒B
    Adeno-associated dependoparvovirus B

尚无已知的疾病与腺相关病毒有关。腺相关病毒一般只能引发轻度的免疫反应。加之腺相关病毒须在其他病毒同时感染宿主的前提下才能复制,使研究者认为腺相关病毒适合用于改造用作人基因治疗的载体,以及体外等基因人类疾病模型英语Isogenic human disease models的构建[3]。由腺病毒改造而成的基因治疗载体能够同时感染分裂中和分裂不活跃的细胞,并将载体DNA以一种不整合入染色体的方式导入到细胞中(不过在自然条件下,也有腺相关病毒携带的DNA在感染后插入染色体的报导)[4]。目前,一部分使用腺相关病毒的基因治疗临床试验已取得正面的结果[5]

研究历史

腺相关病毒最初被认为是腺病毒制备过程中混入的污染物之一。20世纪60年代,匹兹堡大学的鲍勃·艾奇逊(Bob Atchison)与美国国立卫生研究院华莱士·罗英语Wallace P. RoweWallace P. Rowe)实验室的工作最初确定腺相关病毒是一种依赖性细小病毒。后来的血清学研究表明,腺相关病毒不能造成任何已知的人类疾病,且只能在腺病毒、疱疹病毒等辅助病毒存在的前提下才能感染人类[6]

复制周期

 
两个腺病毒颗粒周围被许多腺相关病毒颗粒环绕

绝大部分情况下,腺相关病毒需要辅助病毒的存在才能完成完整的生命周期[7]。腺相关病毒在感染细胞后,需要辅助病毒的帮助(例如,腺相关病毒的辅助病毒疱疹病毒能为其提供DNA聚合酶解旋酶以及一些对腺相关病毒转录的早期启动必要的蛋白)才能进入裂解期英语Lysogenic cycle,进行病毒复制。在没有辅助病毒的前提下,腺相关病毒基因的表达将会受限,一部分腺相关病毒DNA会在这种情况下插入至人19号染色体q13.4区域,即AAVS1位点[8]

基因组

 
腺相关病毒的基因组示意图:腺相关病毒基因组的两端是一对反向重复序列(ITR),中间存在两个开放读框(ORF)repcap

腺相关病毒的基因组由一条长约4.8千碱基对(kb)的正义或反义单链DNA构成,其两端是序列对称的反向重复序列(ITR),对腺相关病毒的复制[9]、衣壳化[10]、以及插入宿主染色体的过程有重要作用[11][12] ,中间则有repcap两个开放读框,前者包括四个序列相互重叠的基因,编码Rep78、Rep68、Rep52与Rep40等四种参与病毒基因复制的蛋白;后者包括三个序列重叠的基因,编码VP1、VP2与VP3等三种组成病毒衣壳的蛋白[13]。腺相关病毒本身不编码DNA聚合酶,仅仰赖宿主细胞或辅助病毒的DNA聚合酶合成自己的基因组[14]

反向重复序列

腺相关病毒基因组两端的反向重复序列(ITR)序列对称,长145碱基,可各自形成一茎环结构,使病毒利用其3'活性端开始DNA复制,无需由引物酶合成引物即可复制自身的DNA[14],形成相互连结的双股DNA中间产物,再以缺口酶英语Nicking enzyme将两股DNA切开[14]。除DNA复制外,反向重复序列也参与病毒插入宿主DNA的过程[11][12],且为病毒颗粒正常组装所需[10]

rep基因

腺相关病毒基因组中上游的rep读框可由左侧的p5p19两个启动子开始转录,产生长度不一的两种RNA,这两种RNA中皆含有一个内含子,可能在转录后被切除,依使用启动子的不同与内含子被切除与否,rep基因可转录出四种mRNA,进而产生四种蛋白质变体:Rep78、Rep68、Rep52,以及Rep40(Rep后面的数字代表这种蛋白的分子量为多少千道尔顿[15],前两者是由p5启动子转录产生,可与反向重复序列形成的茎环结合,参与病毒DNA的复制,后两者则是由p19启动子转录产生[16]。四种Rep蛋白皆可与ATP结合,且皆具解旋酶活性,它们可抑制自身的启动子(p5p19)转录,并可活化转录cap基因蛋白的p40启动子[15][17][18][19][20]

cap基因

cap基因位于rep基因的下游,p40启动子可调控其转录,产生序列重叠的VP1、VP2、VP3与AAP(组装活化蛋白)四种蛋白,VP1、VP2与VP3的大小分别为87、72与62千道尔顿[21],可以1:1:10的比例组成腺相关病毒二十面体的衣壳[22],AAP亦为衣壳组装所需的蛋白[23]cap基因转录出的RNA可能将一段较长的内含子或一段较短的内含子切除,分别形成长2.3kb或长2.6kb的mRNA,其中长内含子被切除的情况较为常见,因此2.3kb的mRNA为大宗,可转译产生VP3蛋白,但其起始密码子(AUG)上游有一个ACG密码子周边亦形成可起始转译的Kozak序列,少数核糖体会自此开始转译,产生比VP3蛋白多出一N端延伸区域的VP2蛋白;而切除短内含子的2.6kb的mRNA则会转译产生VP1蛋白,也比VP3多了一N端延伸区[24][25][26][27]

由于2.3mRNA的数量较多,且其上游ACG密码子启动转译的能力比正常的起始密码子弱,三种衣壳蛋白中VP3被转译生成的量远多于VP1与VP2,与其在衣壳中的组成比例相符[28]。有研究显示VP1蛋白的N端延伸区有磷脂酶A2活性,可能有助于病毒自细胞释出[29],因而为病毒复制周期所需,相较之下VP2可能并非病毒组装必须的蛋白[30]

分类

腺相关病毒在人类与其他灵长类中相当普遍,目前已知有11个血清型[31],其中AAV-2、AAV-3、AAV-5与AAV-6为自人类细胞发现,AAV-1、AAV-4与AAV-7至AAV-11为自其他灵长类细胞发现[32],其中对AAV-2的研究最多[33][34][35],此血清型可能使用硫酸肝素蛋白多糖英语Heparan sulfate(HSPG)、aVβ5整合素成纤维细胞生长因子受体1英语Fibroblast growth factor receptor 1(FGFR-1)为受体感染细胞,其中前者为主要受体,后两者为辅受体,可协助病毒以受质媒介内吞作用英语Receptor-mediated endocytosis进入细胞[36][37][38],但学界对此尚有争议[39]

2013年起,国际病毒分类委员会(ICTV)将腺相关病毒分为A型与B型两个物种,前者包含AAV-1、AAV-2、AAV-3与AAV-4,后者即AAV-5[40][41]

为提升在腺相关病毒在临床与研究中作为载体的效率,研究人员研发英语Recombinant AAV mediated genome engineering了许多腺相关病毒的人工变型,以提升基因表现、适应不同的目标组织并使其更易逃避免疫系统的攻击,例如AAV-2的基因组与AAV-5的衣壳蛋白基因融合组成的AAV-2/5,以及由8型AAV序列融合组成的AAV-DJ 等[42]

用于基因治疗

腺相关病毒因毒性低、不能自主复制、不致病,且较少整合到宿主基因组中,因而被认为是一种较为理想的基因治疗载体。将腺相关病毒改造为基因治疗载体的具体操作方法是腺相关病毒基因组上的repcap区域替换为目的基因[3][43]。腺相关病毒载体导入人体后,不分裂的细胞能够一直保持输入的目的基因片段,而持续分裂细胞则会逐渐随细胞分裂丢失经由腺相关病毒导入的基因片段,因此腺相关病毒疗法不太适合以持续分裂的细胞(如干细胞)作为靶标[43]

腺相关病毒作为基因治疗载体存在一些缺点。首先是容量低,最多只能容纳4.5kb的插入片段,无法容纳DMD英语Dystrophin等较长的人类基因[43]。其次,输入的腺相关病毒载体可能被体内存在的针对天然腺相关病毒的抗体中和。此外,腺相关病毒也存在整合入宿主基因组的情况[44]

过去认为将腺相关病毒用于基因治疗时,反向重复序列是病毒基因组中唯一需与转入的基因顺式in cis)组装的元件,caprep蛋白皆可反式in trans)导入,但有新研究结果显示rep基因编码区域中有一称为顺式rep依赖型元件(cis-acting Rep-dependent element,简称CARE)的序列与转入的基因顺式组装时也可促进病毒的复制与组装[45][46][47][48]

根据2019年的数据,全球范围内共有超过250件使用腺相关病毒载体技术的临床试验,占基于病毒载体的基因治疗临床试验总数的8.3%[49]

参见

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