2016, Vol. 51
人DNA拓扑异构酶,包括DNA拓扑异构酶1 (topoisomerase 1,Top1) 和拓扑异构酶2 (topoisomerase 2,Top2),广泛存在于人体细胞中,参与许多重要的细胞过程,例如DNA的复制、转录和染色质重塑等[1]。部分化疗药物和放射过程可造成拓扑异构酶介导的DNA损伤,从而杀死肿瘤细胞[4],成为肿瘤治疗的重要手段。近年来发现,酪氨酰-DNA磷酸二酯酶1 (tyrosyl-DNA phosphodiesterase 1,TDP1) 和酪氨酰- DNA磷酸二酯酶2 (tyrosyl-DNA phosphodiesterase 2,TDP2) 可以分别特异性识别和修复由Top1或Top2介导的DNA损伤,是潜在的肿瘤治疗靶点,其抑制剂可增强化疗药物或放疗措施的功效。本文从酪氨 酰-DNA磷酸二酯酶1/2的发现、结构、催化机制和生物学功能出发,讨论了其作为肿瘤治疗靶点的理论依据,并综述了其小分子抑制剂的最新研究进展。
1 酪氨酰-DNA磷酸二酯酶11996年,Nash等[7]在Saccharomyces cerevisiae酵母菌中发现了一种DNA修复酶,它可以特异性地水解DNA的3'-磷酸与酪氨酸酚羟基之间的酪氨酰磷酸二酯键 (图 1A)。由于Top1是迄今发现的唯一可以形成DNA 3'-酪氨酰磷酸二酯键的酶,因此这个新发现的酶具有修复Top1介导的DNA损伤 (Top1-mediated DNA damage,Top1-DD) 的活性,被命名为酪氨酰- DNA磷酸二酯酶1[8, 9] (图 1B,C)。
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图 1 (A) 酪氨酰-DNA磷酸二酯酶1 (TDP1) 的催化水解反应。 (B) 人TDP1的结构[9]。 (C) 人TDP1的单晶结构 (PDB code: 1NOP),催化中心 (HKN) 以蓝色棒状显示,DNA-钒酸盐-多肽复合物以红绿蓝色棒状显示 |
人TDP1属于磷脂酶D家族,是一条68 kDa的多肽[10],共有17个外显子,其中2个非编码外显子和15个编码外显子。人TDP1由2条α-β-α结构域组成[11],分别为由氨基酸残基1～350组成的氮端域,以及由氨基酸残基351～608组成的碳端域 (图 1B)。
人TDP1的氮端域含有2条核定位基序 (nuclear localization sequence,NLS)。氮端域由两部分组成[12]: 氨基酸残基为1～148的基序和149～350的基序。基序1～148不影响酶的活性,而是通过各种修饰过程,起着调控蛋白-蛋白相互作用的重要功能[13]。这些修饰过程包括: 在氮端域的聚ADP糖基化 (poly ADP glycosylation)、Ser81的磷酸化以及Lys111的小分子类泛素化[14, 15]。基序149～350中含有一段参与构成TDP1活性中心的HKN (组氨酸-赖氨酸-天冬酰胺) 催化基序[16]。在HKN基序中,组氨酸及赖氨酸对于酶的活性非常重要[10, 13, 17, 18]。TDP1共含有2条HKN催化基序,分别位于氮端域和碳端域。
人TDP1的晶体结构 (图 1C,PDB code: 1NOP) 显示[19],TDP1的每个HKN基序的氨基酸残基都面 向活性中心口袋。在口袋中,氮端域的His263与碳端域的Ser514及Glu538靠近,而碳端域的His493则与氮端域的Thr281及Gln294靠近,共同构成一个键合环道[11, 19]。在该晶体结构中,TDP1与底物钒酸共价结合,形成TDP1-DNA-钒酸盐-多肽四元复合物。单链DNA结合在一条狭长的、不对称分布着正电荷的沟槽内。沟槽表面的正电荷有助于TDP1与DNA的磷酸骨架通过静电作用相互结合。此外,狭缝内Ser400、Ser403、Lys469、Ser518、Lys519及Ala520与共价复合物中DNA的3'-酪氨酰磷酸二酯键上游3个核苷酸的磷酸基团共同构成一个极性网络,通过极性相互作用使DNA被稳定在结合沟槽内; 而Phe259、Pro461和Trp590则通过疏水作用稳定DNA[20]。另一方面,多肽则结合在与之方向相反的一条较宽的、碗口状的沟槽内。
1.2 TDP1的催化机制及其生物学功能研究认为,TDP1的催化机制可能包含两步SN2亲核进攻过程,是由HKN基序中具有催化活性的组氨酸介导完成 的[9, 21, 22]。首先,His263的咪唑氮亲核进攻底物的3'-酪氨酰磷酸二酯键。His493作为广义酸,向苯酚负离子提供一个质子,促使酪氨酸残基离去 (图 2A)。同时,His263与DNA 3'端形成一个磷酰胺键,形成一个短暂的TDP1-DNA共价中间体 (图 2B); 第二步,His493作为广义碱,激活一个水分子亲核进攻TDP1-DNA共价中间体的3'-磷酰胺键 (图 2C),使之水解,并释放出一条含有3'-磷酸基的DNA链 (图 2D)。
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图 2 人TDP1可能的催化水解酪氨酰磷酸二酯键的机制,包括两步的亲核进攻过程[9] |
已知Top1是唯一可形成DNA 3'-酪氨酰磷酸二酯键的酶。在Top1的催化过程中,Top1活性中心的酪氨酸残基进攻DNA单链的3'末端,断开DNA单链,酪氨酸残基与DNA 3'端的磷酸以可逆的共价键——酪氨酰磷酸二酯键结合,形成Top1-DNA断裂共价复合物 (Top1 cleavage complex,Top1cc,图 3A)[23]。在正常生理过程中,Top1cc仅瞬间存在。在多种外源性因素的影响下,如使用Top1毒剂[3]、烷化剂[24]、DNA嵌入剂[27, 28]或核苷类链终止剂[29],以及辐射[30]等过程,Top1cc被捕捉,造成Top1-DD (图 3B),最终 导致细胞死亡。Top1cc的蛋白必须被 降解为寡肽后 (图 3C),TDP1才能识别并水解该复合物的酪氨酰磷酸二酯键。研究认为,TDP1是与PARP1 (poly ADP- ribose polymerase 1) 共同发挥水解作用的,并得到
DNA 3' 端为磷酸基的中间体 (图 3D)[15]。由于DNA缺口必须为3'-羟基和5'-磷酸时才能在各种酶的作用下重新连接、修复得到完整的DNA。因此,该断裂DNA中间体 (图 3D) 首先在聚核苷酸激酶3'-磷酸酶 (polynucleotide kinase 3'-phosphatase,PNKP) 的作用下,DNA缺口两端分别被修饰为3'-羟基和5'-磷酸[33]。PNKP与XRCC1 (X-ray repair cross-complementing protein 1) 相互作用,并与其他修复蛋白,包括DNA聚合酶β (polymerase β,Polβ)、DNA连接酶3 (ligase 3,Lig3) 以及PARP1等,一起形成一个多蛋白的DNA修复复合物[36],共同作用下修复DNA (图 3E)。
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图 3 Top/TDP介导的DNA损伤和修复过程[37] |
这一修复过程预示,在TDP1过度表达或者活性增加的肿瘤细胞中,Top1-DD得以修复,使得肿瘤细胞对与之相关的药物治疗和放疗措施耐受。在正常 细胞中,有两条冗长的通路可以修复Top1-DD: TDP1依赖的修复通路[37] (图 3A～E) 和细胞周期检测点 依赖的修复通路。该细胞周期检测点修复通路涉及BRCA1、53BP1和MDC1等检测点蛋白[38]。但在许多肿瘤细胞中,检测点蛋白常常缺失[39],这时,TDP1依赖的通路成为主要的修复Top1-DD的通路。
以上的理论分析表明,TDP1是一个潜在的肿瘤治疗靶点: 第一、高表达TDP1的肿瘤细胞出现耐药。TDP1可修复化疗试剂 (包括Top1毒剂、烷基化试 剂和核苷试剂等) 引起的Top1-DD,降低肿瘤细胞中
DNA损伤水平[12,26,40],使得肿瘤细胞对这些化疗药物耐受[12,41,42,42,43]。研究发现,TDP1在多种肿瘤细胞或组织中高表达,如肺癌[42, 45]、横纹肌肉瘤[44]、甲状腺癌、乳腺癌、肝癌、子宫内膜癌和卵巢癌等[46]。在高表达TDP1的非小细胞肺癌组织中,TDP1活性与肿瘤的体积呈正相关[45]。TDP1成为肿瘤耐药的主要原因。第二、TDP1抑制剂可逆转肿瘤耐药,与某些化疗药物 (包括Top1毒剂、烷化剂、DNA嵌入剂和核苷类药物等) 以及放疗措施具有协同作用。研究发现,Tdp1基因突变[47, 48]或耗尽TDP1的细胞对Top1毒剂、烷基化试剂和核苷类链终止剂高度敏感[26, 40, 44, 49, 50];
TDP1抑制剂可提高乳腺癌肿瘤细胞对喜树碱 (CPT) 的敏感性[46]。第三、PARP1抑制剂的成功上市[51],证明“合成致死”(synthetic lethality) 理论的可行 性[52, 53]。鉴于TDP1功能与PARP1功能的相似性,其抑制剂可能具有“合成致死”的功能。值得注意 的是,由于正常细胞存在两条平行的修复通路,抑制TDP1对正常细胞的影响可能不大,预示着TDP1抑制剂的毒副作用可能较低[46]。
1.3 TDP1小分子抑制剂尽管类生物大分子钒酸盐和钨酸盐可以抑制TDP1活性[22],但是该类抑制剂不具有成药性。自从人TDP1-DNA-多肽-钒酸盐复合物晶体被发现后[19],TDP1小分子抑制剂成为研究热点。理论上,TDP1小分子抑制剂可以作用在催化过程的每个阶段。例如,抑制剂可以阻止His263或His493的亲核进攻,分别抑制酪氨酰磷酸二酯键 (图 2A) 或磷酰胺键 (图 2C) 的水解; 能够稳定TDP1-DNA中间体 (图 2B),使TDP1被耗尽。其中,稳定TDP1- DNA中间体的方式类似于稳定Top1cc[4, 54],似乎更具可能性[55]。
美国国立卫生研究院的Pommier课题组[56]在TDP1小分子抑制剂的研究中做了大量的工作。首先发现了氨基糖苷类小分子抑制剂——新霉素B和巴龙霉素I (neomycin B,paromomycin I,图 4)。遗憾的是,该类抑制剂的活性较低。新霉素B和巴龙霉素I的IC50值分别为8 和21 mmol×L-1。动力学研究发现,新霉素B是一个可逆的TDP1抑制剂,它可能是直接作用于TDP1或TDP1-DNA复合物而发挥抑制活性的。
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图 4 TDP1小分子抑制剂的结构 |
最近,Tian等[57]从澳大利亚热带雨林植物Macropteranthes leichhardtii的皮中分离得到了2个吡喃葡萄糖苷类化合物 (图 4,化合物1和2),具有良好的TDP1抑制活性,其IC50值均为1 μmol·L-1。
Pommier课题组建立了2种TDP1高通量筛选方法,并先后发现了呋喃二脒类 (NSC 305831) 和methyl-3,4-dephostatin小分子抑制剂 (图 4)。
NSC 305831的抑制活性在微摩尔浓度水平,其IC50值约为90 μmol·L-1[58]。呋喃二脒类化合物是DNA沟槽结合试剂。在生理条件下,NSC 305831质子化后可与DNA的磷酸骨架发生静电相互作用。研究发现,NSC 305831与单链DNA的解离平衡常数 (Kd) 约为70 μmol·L-1,其TDP1抑制活性可能与其DNA的结合能力 相关。
Methyl-3,4-dephostatin的TDP1抑制活性在亚 微摩尔浓度 (submicromolar) 水平,其IC50值为0.36 μmol·L-1。Pommier等[59]认为,methyl-3,4-dephostatin 作用于TDP1催化水解酪氨酰磷酸二酯键阶段,抑制TDP1的水解功能。Methyl-3,4-dephostatin的化学结构稳定,成为工具药或先导药物的可能性较大。
甾体类化合物 (NSC 88915,图 4) 是该课题组发现的另一类TDP1抑制剂,其IC50值为7.7 μmol·L-1[33]。但是,它对细胞粗提液 的TDP1 (whole cell extract,WCE TDP1) 抑制活性显著下降,表明它在细胞环境下有脱靶效应 (off-target effects)[60]。进一步的机制研究表明,NSC 88915是竞争性抑制剂。作为底物类似物,NSC 88915首先作用于酶,阻止TDP1与底物的结合。
另外,他们与美国普渡大学的Cushman课题组合作,首次发现了一类茚并异喹啉类TDP1/Top1双效抑制剂[60]。该类化合物直接作用于酶,属于竞争性抑制剂,而且对于TDP1和WCE TDP1的抑制活性相近。其代表化合物3 (图 4) 对TDP1的IC50值在1～12 μmol·L-1之间[61],对Top1的抑制活性与CPT相 似,具有良好的细胞毒活性。他们测试了化合物3抑制NCI60的活性 (美国肿瘤研究所建立的一种抗肿瘤药物筛选方法,测试药物对60株临床肿瘤细胞的细胞毒活性[64]),其平均GI50为0.027 μmol·L-1[65]。TDP1抑制活性最高的是一个双茚并异喹啉化合物4(图 4),IC50值为1.52 μmol·L-1[60]; 同样具有良好Top1抑制活性和细胞毒活性,Top1抑制活性与CPT相近,对NCI60的平均GI50为0.394 μmol·L-1[66]。
Sirivolu等[67]发现,thioxothiazolidinone类抑制 剂具有较高的TDP1抑制活性 (图 4)。其代表化合 物5的IC50值为0.87 μmol·L-1,但对WCE TDP1 的抑制活性降低了约40倍 (IC50 35 μmol·L-1),表明该化合物具有严重的脱靶效应。另外,化合物5也 是TDP2抑制剂,其IC50值为17 μmol·L-1。表面等 离子共振实验 (surface plasmon resonance) 表明,化合物5与核酸底物没有作用,而是作用于TDP1[67]。Thioxothiazolidinone类抑制剂不仅有脱靶效应,其化学结构也不稳定[68],大大降低了其成药性。
Dean等[46]发现了thioxopyrimidinedione类TDP1抑制剂CD00509 (图 4),其IC50值为0.71 μmol·L-1。CD00509可以增加CPT引起的DNA损伤,提高乳腺癌肿瘤细胞MCF-7对CPT的敏感性,二者联用使得MCF-7的增殖降低了25%。另外,单独使用CD00509 (10 μmol·L-1) 不影响人乳腺上皮细胞的生长,表明它的细胞毒性很低。
Takagi等[69]从变形真菌Anamorphicfungus次级代谢产物中分离得到了JBIR-21 (图 4),其抑制TDP1的IC50为18 μmol·L-1。JBIR-21具有较好的细胞毒活性,对4株肿瘤细胞的IC50值在3.5～13 μmol·L -1之间; 并可有效抑制小鼠结肠癌 (HT-29) 的生长,用药21天后,肿瘤体积降低了49%,而且没有明显的毒副作用。
最近,Zakharenko等[70]报道了一类苯并五硫平 类化合物,具有良好的TDP1抑制活性。化合物6 (图 4) 为迄今报道的抑制活性最佳的化合物,其IC50为0.22 μmol·L-1。该类化合物可引起MCF-7和肝癌细胞HepG2的凋亡。
在以上报道的TDP1小分子抑制剂中,大多未说明这些抑制剂的选择性 (如是否抑制TDP2)。本课题组发现了一类异喹啉类抑制剂,其抑制活性在低微摩尔浓度水平。尤其值得注意的是,该类抑制剂在111 μmol·L-1时对TDP2没有抑制活性,属于TDP1选择性抑制剂。
2 酪氨酰-DNA磷酸二酯酶2相比于TDP1,酪氨酰-DNA磷酸二酯酶2的发现较晚。2000年,Pype等[71]在研究CD40信号转导通路时发现了一种蛋白,作为细胞内肿瘤坏死因子受体结合蛋白,被命名为TTRAP (TRAF and TNF receptor- associated protein),但此时并未发现它的5'-酪氨酰磷酸二酯酶活性。2003年,Pei等[72]在酵母中也发现了此种蛋白,主要功能为转录因子ETS1 (E-twenty six 1 protein) 和AP1 (APETALA 1) 的负转录调控因子,并命名为EAP II (ETS1-associated protein 2)。2009年,Cortes Ledesma等[73]在人的细胞中再次发现了这种蛋白,并发现其可特异性地水解DNA的5'-磷酸与酪氨酸酚羟基形成的酪氨酰磷酸二酯键。如图 5A所示,TDP2水解DNA 5'-端的酪氨酰磷酸二酯键,清除酪氨酸残基,使5'-磷酸基裸露,以便与其他DNA片段连接,恢复DNA的完整性。为了与TDP1名称相对应,将其命名为TDP2。
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图 5 (A) 酪氨酰-DNA磷酸二酯酶2 (TDP2) 的催化水解反应。(B) 小鼠TDP2的结构[76]。(C) 小鼠TDP2的单晶结构 (PDB code: 4GZ1),催化中心以蓝色棒状显示,Mg2+离子以绿色球状显示,DNA底物以红绿蓝色棒状显示。(D) 人TDP2催化水解酪氨酰磷酸二酯键可能的机制[75] |
TDP2属于二价金属离子 (Mg2+/ Mn2+) 依赖的磷酸二酯酶[73]。2012年,Aihara和Williams课题组[37, 76]同时报道了鼠、线虫和斑马鱼的TDP2晶体结构。如图 5B所示,鼠TDP2是一种双结构域DNA修复蛋白,其氮端为泛素相关 (ubiquitin- associated,UBA) 结构域,可能与TDP2的细胞信号转导和应激反应功能有关; 其碳端为核酸外切酶-核酸内切酶-磷酸酶 (exonuclease-endonuclease-phosphatase) 催化结构域,是TDP2发挥其磷酸二酯酶活性的关键区域[76]。如图 5C所示,TDP2 (PDB code: 4GZ1) 有一个深而窄的DNA底物结合沟槽。保守的疏水性残基Trp307、Leu315、Ile317和Phe325形成一个高度精密的结合平面,与DNA的碱基发生堆积
作用,并与脱氧核糖相互作用,“捕捉”单链DNA的5'端,使其5'-酪氨酸的芳香环结合在疏水口袋中,限制了非专一性的核酸内切或外切过程。氨基酸残基Glu162、Arg216、His236、Ser239、Asp272、Asp326、His359和一个Mg2+离子共同组成了TDP2的活性中心[76]。
目前,人TDP2的单晶结构尚未报道。序列分析发现,人TDP2和APE1 (apurinic endonuclease 1) 具有30% 的序列相似性以及14% 的序列一致性。Gao等[75]以人APE1为基础,通过在线蛋白预测程序I-Tasser模拟了人TDP2结构。预测的结构显示,作为TDP2的催化中心,其碳端的核心区域由2个α螺旋夹着3个β折叠组成,这很可能就是DNA的结合沟槽。在这个区域的中心,4个关键残基: Asn120、Glu152、Asp262和His351组成了TDP2的催化口袋。
2.2 TDP2的催化机制及其生物学功能尽管小鼠和斑马鱼的TDP2单晶结构均显示,活性中心包含一个Mg2+ 离子[37, 76],但Gao等[75]通过生物化学方法证实,人的TDP2需要两个Mg2+参与。他们认为,人TDP2催化水解DNA 5'-酪氨酰磷酸二酯键是一步完成的。如图 5D所示,其中一个Mg2+离子与Asp262、His351、Asn264以及一个OH-离子络合,OH-离子亲核进攻P原子; 另一个Mg2+离子与Asp122和Glu152络合,辅助酪氨酰磷酸酯键断裂; Asn120通过桥键间接连接两个Mg2+金属离子。在协同作用下水解酪氨酰磷酸二酯键,去除酪氨酸残基,获得末端为5'-磷酸的缺口DNA[75]。
含有5'-酪氨酰磷酸二酯键的缺口DNA主要是由Top2引起的。与Top1不同,Top2同时断开DNA的两条链,并分别生成一个5'-酪氨酰磷酸二酯键,形成Top2-DNA断裂共价复合物Top2cc (Top2 cleavage complex)。在外源性或内源性因素刺激下,Top2cc被“捕捉”(图 3G),造成Top2介导的DNA损伤 (Top2- mediated DNA damage,Top2-DD),最终导致细胞死亡。造成外源性损伤的因素包括使用Top2毒剂[5]、DNA嵌入剂[77]和AraC等药物治疗[78],以及辐射[79]等 (图 3F)。类似于TDP1,TDP2必须在Top2cc的蛋白被水解代谢后才能识别Top2来源的多肽-DNA复合物 (图 3H)[80],并水解该底物的5'-酪氨酰磷酸二酯键,获得含有3'-OH和5'-磷酸末端的缺口DNA (图 3I)。然后在XRCC4/Lig4、Ku70/Ku80/DNA PK等修复酶的参与下,通过非同源末端连接 (nonhomologous end joining,NHEJ) 通路,进行双链断裂DNA的无错修复 (error-free repair)[81],获得没有碱基缺失的、完整的DNA (图 3J)。
从上述通路可知,TDP2主要在修复Top2-DD的起始阶段发挥作用,而泛素化则能避免将正常复制转录的DNA误认为可裂解底物[82]。这种准确、精细的修复功能使得细胞基因组在面对内源性或外源性的威胁时,依然能保持DNA的完整性和稳定性[83]。但是对于肿瘤细胞来说,这样的修复功能使得它们对上述外源性治疗方法不再敏感[37]。研究证实,在肺癌细胞和组织中高表达TDP2可促进肿瘤细胞增殖以及肿瘤组织生长[84],并使肺癌细胞对Top2毒剂——依托泊苷耐药[85]。
在正常细胞中,同样存在两条通路修复Top2-DD: TDP2依赖的修复通路 (如图 3F～J) 和细胞周期检测点依赖的修复通路。如前所述,在许多肿瘤细胞 中,检测点蛋白常常缺失[39],TDP2依赖的通路成为主要的修复通路。理论上分析,在相关检测点蛋白缺失的肿瘤细胞中,抑制TDP2活性将对这些肿瘤细胞增殖产生很大的影响,其抑制剂可以增强Top2毒剂、DNA嵌入剂或AraC类药物的疗效,甚至提高放疗效果。研究证实,删除Tdp2基因的小鼠对Top2-DD敏感[81]; 如果抑制TDP2的活性或敲除TDP2,可增加细胞对依托泊苷和阿霉素的敏感性[85, 86]。
通过以上的理论分析和研究发现可知,TDP2是一个潜在的肿瘤治疗靶点。其抑制剂与Top2毒剂、DNA嵌入剂或核苷类链终止剂联用,具有协同治疗肿瘤的功效。
2.3 TDP2小分子抑制剂由于TDP2的发现时间尚短,其抑制剂的报道较少。2013年,Raoof等[87]发现了两类TDP2选择性抑制剂 —— toxoflavin类化合物和deazaflavin类化合物。如图 6所示,代表化合物7和8的TDP2抑制活性IC50值分别为1.51和0.59 μmol·L-1。然而,toxoflavin类化合物具有很强的氧化还原活性,而deazaflavin类化合物则细胞膜透过性不佳。这些性质限制了它们成为药物的可能性。
|
图 6 TDP2小分子抑制剂的结构 |
本课题组发现了一类呋喃并喹啉二酮类化合物,具有良好的TDP2选择性抑制活性。目前,该研究正在进行中。总之,TDP2抑制剂的研究尚处于起始阶段,有待更多的研究发现。
3 问题与展望作为近年来才发现的DNA修复酶,TDP1和TDP2具有特殊的生物学功能,是潜在的肿瘤治疗靶点。但是,目前对其研究和认识尚处于初级阶段,需要进行深入研究: 如它们是如何专一性识别底物的; 水解酪氨酰磷酸二酯键的详细过程; 当前发现的抑制剂种类较少,尤其是TDP2抑制剂,仅报道了两类。对于药物化学家来说,发现新的抑制剂,尤其是高活性的、选择性抑制剂将为解决以上问题提供有力帮助,甚至发现以其为靶点的创新药物,证实其作为肿瘤治疗靶点的可行性。
TDP1和TDP2除了作为肿瘤治疗靶点外,还具有更加广泛的功能。如研究发现: ① 低剂量的伊立替康和TDP1抑制剂呋喃二脒NSC 305831联用,可显著改善狼疮肾炎小鼠的生存率,预示着Top1/TDP1可能是系统性红斑狼疮的联合治疗靶点[88]; ② TDP1和TDP2可调控人类乳头瘤病毒 (human papillomavirus) 游离基因的稳定性[89]; ③ TDP2还是小核糖 核酸病毒 (picornaviruses) 的断裂酶 (unlinkase)[90]。一旦感染小核糖核酸病毒,宿主的TDP2会将其RNA 5' 端的一段小的病毒蛋白移除,病毒才能开始翻译 自己的蛋白[90, 91]。另外,宿主TDP2在乙肝病毒的感染中也扮演着重要角色,可能与乙肝病毒cccDNA (covalently closed circular DNA) 的形成有关[92]。因 此,TDP1和TDP2不仅是潜在的肿瘤治疗靶点,还可能在感染性疾病的治疗中扮演重要角色。
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