白癜风易感基因单核苷酸多态性的关联性研究现状与展望:单核苷酸多态性

  白癜风是一种常见的色素脱失性皮肤病,发病率逐年升高[1]。临床表现为皮肤白斑,常累及头面部、手部等暴露部位。病理学上表现为表皮内黑素细胞的减少或缺失。其病因复杂,发病机制尚不明确,多种病因假说并存,主要包括遗传学说、免疫学说、黑素细胞自毁学说、氧化应激损害学说等[1]。近年来,随着对白癜风研究的不断深入,学界对白癜风的认识也不断提高,遗传学发病机制成为学者们关注的焦点及研究热点。越来越多的学者认为白癜风是一种多基因遗传疾病,是由多条染色体上不连锁的多位点隐性等位基因相互上位作用所致。大量的临床观察和流行病学调查已经证实白癜风具有明显的家族聚集性,可能与遗传因素密切相关。此外,越来越多的白癜风易感基因和侯选基因被发现,遗传基因的变异或多态性可能造成黑素细胞功能先天性缺陷或对外界有害因素的抵御与自我修复能力不足,进而增加个体罹患白癜风的风险性[2-3]。本文就白癜风易感基因单核苷酸多态性的关联性研究现状与展望综述如下。
  1 单核苷酸多态性概述
  单核苷酸多态性(single nucleotide polymorphism,SNP)是指在基因组水平上由单个核苷酸变异所引起的DNA序列多态性,包括单碱基的转化、颠换、插入和缺失等[4],其群体发生频率大于1%。根据所在位置不同,SNP可分为编码区SNP(coding SNP, cSNP)、内含子区SNP和调控区SNP(regulatory SNP,rSNP)。其中,cSNP根据是否改变编码的氨基酸又可分为同义cSNP(synonymous cSNP)和非同义cSNP(non-synonymous cSNP)。不同类型SNP可以通过不同机制影响基因功能。非同义cSNP 可引发蛋白质氨基酸序列发生改变,通常是导致生物性状改变的直接原因。同义cSNP 可通过改变mRNA的二级结构[5]、翻译速度[6],致使蛋白质的结构和功能发生变化。内含子区SNP 能够改变剪接位点的活性,进而影响基因的功能[7]。调控区的rSNP 则可通过影响启动子元件来调控基因的表达。作为继限制性片段长度多态性(restriction fragment length polymorphism,RFLP)和微卫星多态性(microsatellite polymorphism)后的第三代遗传标记,SNP具有以下优点:①数量巨大,分布广泛,是人类最常见的可遗传变异;②具有高度稳定性,突变率极低;③部分SNP可能会影响基因的表达水平或产物蛋白质的结构和功能;④SNP只有两种等位基因型(双等位基因标记),易于基因分型及估算等位基因频率,有助于实现SNP快速、自动化及规模化检测。
  上述遗传学和生物学特性决定了SNP更适合应用于复杂性状和复杂疾病的遗传机制研究。此类研究主要有以下三个方面:①基于遗传流行病学的关联分析,研究SNP引起个体或群体疾病易感性、抵抗力、药物反应性及其他遗传表型的差异;②基于SNP影响基因表达的分子机制研究,分析SNP对基因转录、翻译及翻译后蛋白质折叠的影响;③在生化和细胞水平进行研究,通过分析酶活性、细胞信号通路等来阐释SNP对基因功能的影响。
  2 白癜风易感基因SNP关联性的研究
  白癜风遗传发病机制研究主要集中于白癜风易感基因SNP关联性的分析和探讨。现有研究主要选择参与黑素细胞生物学过程且与白癜风病因学说相关的基因为候选基因,特别关注可能具有功能效应的SNP位点,如非同义cSNP或启动子区rSNP,通过进行大规模白癜风遗传易感性研究,获得翔实证据,并在此基础上结合白癜风氧化还原失衡、自身免疫应答等方面进行系统探讨,以期更全面、综合地研究白癜风发病机制。
  2.1 氧化应激相关的分子流行病学研究:研究证实氧化应激相关基因的变异与白癜风易感性相关,可能通过氧化应激损伤,致使表皮黑素细胞发生坏死或凋亡的风险增高。研究显示:①谷胱甘肽巯基转移酶(Glutathione S-transferases,GSTs)是重要的抗氧化酶,能够对抗氧化应激、保护细胞[8]。GSTs基因外显子区SNP与白癜风易感性相关,GSTT1基因缺失型影响基因转录表达,可能导致抗氧化酶表达量下降或功能障碍,且与GSTM1基因存在联合作用,两段基因共同缺失的个体罹患白癜风风险显著增高[9];②过氧化氢酶(Catalase,CAT)是过氧化氢酶体的标志酶。研究证实白癜风患者存在CAT酶活性的降低及过氧化氢的大量蓄积[10-11]。CAT基因启动子区-89 A>T与白癜风易感性显著相关,携带CAT-89AA的个体血清CAT酶活性较高,其对机体的保护作用呈现剂量相关效应[12];③ 环氧合酶-2(Cyclooxygenase2,COX2)是具有环氧合酶和过氧化物酶功能的双重酶。此外,COX2能够调控表皮角质形成细胞产生前列腺素E2 (Prostaglandin E2,PGE2)[13-14],进而调节表皮黑素细胞增殖和黑素合成过程[15-16]。COX2启动子区-1195A>G与白癜风易感性密切相关,G等位基因能够通过降低COX2 mRNA水平,进而降低PGE2表达量,是白癜风的危险性因素[17];④儿茶酚邻位甲基转移酶(Catechol-O-Methylt-ransferase,COMT)是降解儿茶酚胺的主要代谢酶。COMT基因启动子区-158 G>A位点能够增加寻常型白癜风的患病风险,这种影响在早发型白癜风、具有白癜风家族史以及女性患者中更为显著[18];⑤ 核因子E2相关因子2(Nuclear factor erythroid-2- related factor 2,Nrf2)是氧化应激相关的转录因子,Nrf2基因启动子区-650C>A与白癜风易感性相关[19]。
  2.2 自身免疫相关的分子流行病学研究:大量研究表明白癜风与自身免疫密切相关,异常自身免疫应答介导表皮黑素细胞破坏。研究发现多个免疫相关基因变异与白癜风遗传易感性关系密切,主要包括:① 人类白细胞抗原(Human leukocyte antigen,HLA)多个SNP位点与白癜风相关,此外补体(Complement,C)分子C4的编码基因C4B 异常及其HLA的关系可能是本病的危险因素[23];② 抗原处理相关基因:TOLL样受体家族(Toll like receptor,TLR)中TLR2和TLR4[34]以及抗原处理蛋白(Transporter associated with antigen processing,TAP)-1 基因多态性与白癜风存在关联性[26];③ 免疫活化相关基因:细胞毒性T 淋巴细胞抗原4(Cytotoxic T lymphocyte-associated antigen-4,CTLA-4)基因[24]和编码CD4的CD4*A4等位基因[28]及甘露(聚)糖结合凝集素( Mannan-binding Lectin,MBL2)[30]可能在白癜风的遗传病因学上起到一定作用;④ 免疫调节相关基因:自身免疫调节因子(autoimmune regulator,AIRE)[29]、白细胞介素(Interleukin,IL)-4(IL-4)[31]、IL-10[32]、IL-19[33] 白介素1 受体拮抗剂(Interleukin 1 receptor antagonist,IL1-RN)[24]等基因变异能够增加个体罹患白癜风的风险性;⑤ 诱导型一氧化氮合酶(Inducible nitric oxide synthase,iNOS)能够催化受损机体生成一氧化氮,后者参与多种自身免疫性疾病的发病过程[20]。研究表明一氧化氮可能通过参与黑素细胞破坏和调节自身免疫在白癜风发病中发挥作用[21]。iNOS -954 G>C与白癜风有明显的相关性,携带C等位基因的个体血清iNOS活性较高[22]。此外,血管紧张素转化酶(Angiotensin converting enzyme,ACE)基因的I/D 多态性[25]以及X-盒结合蛋白1(X-box binding protein 1,XBP1)基因 4804 G>C位点与HLA-DRB1*07间的上位效应[27]与白癜风的发病具有明显的相关性。   2.3 黑素细胞凋亡相关的分子流行病学研究:Fas蛋白及其配体FasLG是重要的细胞凋亡相关膜表面分子,参与细胞毒性T淋巴细胞的杀伤机制,介导淋巴细胞凋亡,其基因变异或功能异常能够引发多种疾病的产生[35-38]。研究证实白癜风患者皮损、外周血单个核细胞上和循环中有Fas/FasL及sFas/sFasL的异常表达[39]。此外,白癜风自身免疫机制研究发现阻断Fas与FasL结合及相互作用能够在一定程度上抑制 CD4+T淋巴细胞介导的黑素细胞破坏[40]。我们前期工作发现FAS基因启动子区-1377G>A可能通过改变该基因的转录活性,影响细胞免疫功能,导致黑素细胞损伤,进而增加白癜风罹患风险[41]。
  2.4 黑素细胞相关基因的分子流行病学研究:芳香烃受体(Aryl hydrocarbon receptor,AHR)是一种胞浆转录因子,能够结合于黑素细胞相关基因的启动子区,通过调控其转录活性参与调节黑素细胞增殖分化及黑素合成过程[42-45]。AHR基因多态性或功能障碍,可能通过影响黑素细胞相关基因的转录表达,调控皮肤着色过程,进而增加个体对白癜风的易感性。前期研究证实,AHR基因启动子区-129 C>T与白癜风易感性显著相关,其中T等位基因具有保护作用,能够降低个体罹患白癜风的风险性[46]。此外,酪氨酸酶基因(TYR)[47]、酪氨酸酶相关蛋白1基因(TRP1)[48-49]、雌激素受体基因(ESR1)[50]、黑素皮质素受体基因(MC1R)[51]及黑素细胞增殖基因(MYG)[52]等基因多态性与白癜风相关。
  2.5 其他:研究发现一些与神经机制、黑素细胞粘附、细胞自噬、微环境等方面相关的基因多态性可能增加个体对白癜风易感性,如:细胞周期蛋白依赖性激酶基因(CDK5RAP1)、盘状结构域受体1基因(DDR1)[53]、紫外线照射耐药相关基因(UVRAG)[54]、干细胞因子基因(SCF)[55]、干细胞生长因子基因(SCGF)[55]和内皮素-1基因(EDN1)[56]等。
  3 讨论
  白癜风易感基因多态性的研究进行了十多年,在国内外多个研究课题组的努力下,此领域取得了显著成果。目前,已经明确了白癜风与数十种易感基因或候选基因SNP位点的关系,也通过功能学研究进一步探讨了部分基因多态性参与白癜风黑素细胞损伤的生物学机制,极大地促进了白癜风遗传机制研究的发展,对于全面深入了解白癜风复杂的发病机制意义重大。在白癜风遗传学机制取得长足进步的同时,我们更应该关注现有研究尚存在以下问题:①对于同一基因或同一SNP位点的关联分析,结论有时大相径庭,甚至相反。主要发生在不同种族人群的研究间,在同种族人群研究中也存在这种结论相悖的情况;②白癜风是复杂的多基因疾病,多个基因联合作用构成遗传背景,同时在环境因素的制约下参与黑素细胞损伤过程。现有研究多为单个基因多位点或单个位点与白癜风相关性的分析,缺乏多基因多态性的联合分析。此外,如果对每一个SNP都进行独立研究,那么对几百万SNP的研究就会导致成千上万次的假关联,结果会掩盖真实的关联性;③基于遗传流行病学的关联分析方法,有助于建立序列变异与白癜风罹患风险之间的关系,且为基因在整体水平的功能研究提供重要线索,但部分研究未深入开展相关基因的功能学检测,难以在生物化学方面和细胞水平阐明基因参与白癜风的生物学机制;④部分研究涉及到基因多态性的功能学研究,主要集中于对非同义cSNP和启动子区rSNP的生物学机制研究。目前,对于rSNP的功能学研究仅局限在其对启动子区转录活性影响的检测,而未能深入了解并明确rSNP的具体分子调控机制。此外,对于内含子SNP及同义cSNP的功能学研究甚少,且缺乏较为有效、可靠的研究手段,无法在基因转录水平、转录后水平、翻译水平、翻译后蛋白折叠及蛋白的细胞定位水平全面分析SNP对基因功能的影响;⑤通过SNP功能学研究得到的结论在一定程度上反映了SNP对基因功能影响的机制,但对于SNP是否参与白癜风发病、如何参与以及作用重要程度等机理的解释还不够深刻。
  综合分析目前研究现状,可为今后研究提出以下建议:①增加样本量,提高样本均质性和代表性,有益于客观反映实际情况。选择人群起源相同且各项性状特征对应良好的患者和对照入组,以排除非处理因素的干扰,真实反映SNP与疾病的关联效应;②在单基因多态性与白癜风关联研究的基础上,加强多基因联合分析;③应用致病等位基因与其周围SNP的连锁不平衡性提高关联分析的效果。与疾病相关联的阳性标记未必可确定其周围的致病序列,且采用任何一个或少数标记得到的阴性关联分析结果并不能排除其周围序列对疾病的风险,针对上述情况可应用一系列随机SNP扫描标记周围的DNA序列以期发现能指示起病效应的关联信号;④从多水平、多角度加强白癜风易感基因和候选基因SNP位点的功能学研究,将微观研究与宏观研究有机结合,有助于阐明白癜风的发病机制,也为研发高效且特异性强的治疗手段提供新思路。
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  [收稿日期]2012-06-04 [修回日期]2012-08-01
  编辑/李阳利

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