CYP3A4、CYP3A5多态性与肿瘤的关系

许菊青 蒋峰

CYP3A4、CYP3A5多态性与肿瘤的关系

许菊青 蒋峰

CYP3A家族(cytochrome P450, subfamily ⅢA)是细胞色素酶CYP450(cytochrome P450 enzyme system)重要成员,其含量丰富,底物覆盖广泛,主要功能是参与许多内、外源性物质的代谢,是药物代谢反应中最主要的限速酶[1-2]。CYP3A亚家族包含4个成员:CYP3A4、CYP3A5、CYP3A7和CYP3A43。CYP3A4主要分布在肝脏和小肠,是肝内最常见最丰富的细胞色素亚家族;CYP3A5主要分布于一些肝外组织如肠壁、肾和前列腺等,是CYP3A亚家族最重要的肝外分布形式。

CYP3A4和CYP3A5基因位于人类第7号染色体q21.1-22.1,基因全长分别为27.6 kb和31.8 kb,它们都有13个外显子,编码CYP3A4和CYP3A5的氨基酸有84%的相似性[3-4]。CYP3A蛋白受CYP3A基因编码调控。CYP3A基因多态性决定了CYP3A酶表达含量的多寡及功能的强弱。

在已经发现的CYP3A4基因单核苷酸多态性(single nucleotide polymorphisms，SNPs)中,CYP3A4*1B基因变异最常见,CYP3A4*1B基因变异(rs2740574)位于第5′端侧翼区的A-392G转换,可下调CYP3A4酶的表达及活性[5]。

CYP3A5基因型呈高度多态性,具有广泛的个体及种族差异。Kuehl等[6]发现携带至少1个CYP3A5*1等位基因的人,肝脏和小肠CYP3A5酶的含量显著增加,可达CYP3A酶总量的50%以上,提示携带CYP3A5*1等位基因可产生高水平的CYP3A5 mRNA和蛋白。CYP3A5*1基因的种族差异性分布显示:非裔美国人占45%,高加索人占15%,中国人占35%,日本人占15%。CYP3A5另一常见基因型是CYP3A5*3,其第3内含子(A6986G)突变可引起可变剪接,产生不稳定蛋白从而导致部分人组织中CYP3A5表达缺失。在中国人群中,CYP3A5*3的发生频率高达71%～76%[7]。

目前,CYP3A4、CYP3A5在肿瘤方面的研究主要集中在流行病学方面,研究表明CYP3A SNPs与部分肿瘤发生风险及化疗药物代谢具有相关性。本文对CYP3A4、CYP3A5基因多态性与常见肿瘤发生风险之间的关联做一详细阐述。

1 前列腺癌

CYP3A4主要表达于成人肝脏,在肝外组织表达含量较低,在前列腺中表达仅占0～14%,目前没有发现CYP3A4促进或抑制前列腺癌发生发展的直接证据;但一系列研究表明,细胞色素酶P450家族中CYP17、CYP19和CYP11A1酶可影响肾上腺及性腺中类固醇的生物合成,与激素依赖性肿瘤的癌症易感性相关。同样,作为P450家族的重要成员之一,CYP3A4酶可参与内源性睾酮的氧化反应,使其代谢成无活性的2β,6β,15β-羟睾酮[8];另外,作为CYP3A4基因最常见的SNPs,CYP3A4*1B可下调CYP3A4酶的表达及酶活性,导致CYP3A4表达下降或缺失。关于CYP3A4*1B与前列腺癌风险关联的研究发现,前列腺癌高发的欧美地区,CYP3A4*1B突变率明显增加,且CYP3A4*1B基因表达与前列腺癌发生风险显著相关[9];回顾性研究表明，前列腺癌病人CYP3A4*1B等位基因突变率明显高于健康人群[10-11];相反,在前列腺癌发生率较低的亚洲人中并没有发现CYP3A4*1B基因突变[12]。综上,CYP3A4*1B基因突变与前列腺癌发生发展密切相关,具有促癌作用,其可能的机制是CYP3A4*1B突变下调CYP3A4蛋白表达,导致内源性雄激素灭活减少，促进了激素依赖性前列腺癌的发生。

CYP3A5在正常前列腺组织及肿瘤组织中均有表达,目前研究普遍认为CYP3A5 SNP与肿瘤的发生发展密切相关,但存在较大争议。Plummer等[11]发现表达CYP3A4*1B/CYP3A5*3单体型基因人群前列腺癌发生率明显增加,而存在CYP3A5*1基因突变的人群,前列腺癌的发生率则显著降低。Vaarala等[13]研究也发现CYP3A5*3(6986A>G)基因型的表达与前列腺癌临床病理参数呈正相关;已知的CYP3A5基因多态性中,CYP3A5*3基因突变可使CYP3A5低表达或表达缺失,而CYP3A5*1基因则可产生高水平CYP3A5 mRNA和蛋白。早期研究也证明CYP3A5活性酶能催化6β-羟基睾酮,诱导睾酮失活,从而抑制激素依赖性的前列腺细胞的恶性增殖[14]。以上研究表明,CYP3A5*1基因型可作为CYP3A5功能性酶的活化基因，可阻止前列腺癌的发生。但也有研究提出了相反的结论:CYP3A5基因多态性与前列腺癌发生风险及临床病理特征之间无显著关联[15-16]。因此,CYP3A5基因多态性与激素依赖性肿瘤的相关性仍需大量研究证实。

2 乳腺癌

在乳腺癌的研究中发现,肿瘤中CYP3A4表达水平与乳腺癌临床病理特征及不良预后相关[17-18],CYP3A4是雌激素16α-羟化代谢反应中的重要酶,CYP3A4可催化雌激素16α-羟化反应产生16α-羟雌酮,后者与雌激素受体结合,转换细胞核DNA合成,可诱导乳腺异常细胞增殖、有潜在的致癌作用[19],提示CYP3A4通过影响雌激素水平间接促进了乳腺癌的发生发展。另外,Mitra等[17]通过体外实验发现CYP3A4促进激素受体阳性乳腺癌细胞恶性增殖的信号通路,CYP3A4促进环氧二十碳三烯酸(EET)生物合成,抑制了激活因子3(STAT-3)磷酸化作用和核转运,从而导致恶性细胞的无限增殖。

流行病学研究已经提供了雌激素水平与乳腺癌风险之间关联的直接和间接证据。女性体内性激素的差异受遗传因素影响,CYP3A位点SNPs与乳腺癌罹患风险关系的研究证明常见遗传变异CYP3A4*1B与激素水平或乳腺癌风险之间无明显相关性[20-21]。虽然CYP3A4*1B与乳腺癌风险无直接关联,但研究显示CYP3A4*1B变异与性早熟和他莫昔芬诱发的子宫内膜癌相关[22-23],而早熟是乳腺癌的高危因素之一。美国放射技师协会进行雌激素代谢相关基因的常见异常的巢式病例对照研究报告显示,罕见的非同义CYP3A4的SNP(rs4986910,M445T)可降低乳腺癌风险[24],然而这一结果并没有在Johnson等[25]研究中得到复制,但该研究发现了距离3′CYP3A5约50 kb碱基的标签SNP(rs10273424)与绝经前女性尿中较低的雌酮葡萄苷酸(E1G)水平存在显著关联,后者作为雌激素降解产物反映了体内雌二醇(E2)的水平;这一发现表明携带罕见基因CYP3A5(rs10273424)可降低乳腺癌风险。

3 肝癌

研究表明,CYP3A酶参与了黄曲霉素B1代谢活化，生成具有致癌作用的AFB1-外-8,9-环氧化物(AFBO)，诱导肝细胞肝癌发生[26]。一项关于CYP3A亚家族与肝癌侵袭转移关联的研究发现,CYP3A4可能具有促进肿瘤细胞增殖的“癌基因”生物学功能;机制可能是通过造成肝癌细胞低氧环境来诱导低氧反应元件(HRE)启动子活性的增强,促进促红细胞生成素(EPO)和血管内皮生长因子(VEGF)mRNA表达含量的增加,继而诱导磷酸肌醇3激酶/蛋白激酶B(PI3K/Akt)信号通路的活化[27],PI3K/Akt参与调节肿瘤细胞的增殖迁移。而Jiang等[28]发现CYP3A5可作为一个保护性因子抑制肝细胞肝癌侵袭转移,其抑制肝癌细胞侵袭转移的分子机制是CYP3A5通过细胞内活性氧簇(ROS)单纯抑制雷帕霉素靶点白复合物2(mTORC2)的活性,进而选择性抑制AKT(Ser473)活化,引起AKT信号通路下调,从而抑制肝细胞肝癌侵袭转移。提示CYP3A5具有区别于传统代谢功能的“抑癌基因”功能。

4 肺癌

研究发现,CYP1A1、CYP2C9等可参与烟草中前致癌物质多环芳烃(PAH)的代谢活化,合成DNA加合物,从而诱导肺癌的发生[29]。研究发现吸烟病人PAH-DNA加合物的水平与肺泡巨噬细胞CYP3A5的含量呈正相关[30],提示CYP3A5可能通过参与PAH-DNA合成代谢诱导了肺癌的发生。而对CYP3A SNP研究显示,CYP3A5*3纯合子基因型使肺癌的发生风险增加了4.3倍[31],另一项包含了801例肺癌的病例对照研究显示，携带CYP3A4*1B基因型的病人小细胞肺癌发生风险明显增加,亚组分析显示,携带CYP3A4*1B基因型的人群中吸烟>20包/年肺癌的发病风险将进一步增加,可能与CYP3A4*1B基因型增加烟草中致癌物质的代谢活化从而诱导癌症发生有关[32]。但Timofeeva等[33]研究却并未发现CYP3A4或CYP3A5基因多态性与肺癌发生风险之间存在显著关联。CYP3A4、CYP3A5在肺癌中的作用机制仍需大量研究。

5 其他肿瘤

研究显示骨肉瘤[34]、尤文肉瘤[35]CYP3A4或CYP3A5高表达的病人较易出现远处转移,提示CYP3A4或CYP3A5可能参与了肿瘤细胞的增殖迁徙过程。Hyland等[36]研究发现中国人群CYP3A5基因多态性与食管癌风险相关,其中携带至少1个CYP3A5*1等位基因的人食管癌发生风险明显升高。

综上所述,CYP3A4、CYP3A5在体内的差异性表达受遗传因素影响,CYP3A SNP是造成个体、种族差异的基础,并影响了癌症的易感性。目前,CYP3A SNP影响癌症发生的具体机制尚未明确,有待进一步研究。寻找可以预测癌症风险的候选等位基因,并探索它们与肿瘤风险或药物代谢的关系对预防癌症发生及指导治疗具有重要意义。

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210029江苏省南京市，南京医科大学第四临床医学院(许菊青)；210009江苏省南京市，南京医科大学附属肿瘤医院胸外科(蒋峰)

蒋峰，Email：jiangfeng174@sohu.com

R 730.2

A

10.3969/j.issn.1003-9198.2017.01.023

2016-11-23)