修回日期: 2002-08-20
接受日期: 2002-08-27
在线出版日期: 2003-01-15
基因和表遗传性改变是恶性肿瘤发生的主要机制. 表遗传性修饰-DNA甲基化在胃肠道恶性肿瘤的发生、发展中起重要作用. 细胞周期、DNA修复、血管生成和凋亡等都涉及到相关基因的CpG岛甲基化. CpG岛甲基化导致基因失活的分子机制包括DNA甲基转移酶(DNMTs)、CpG甲基化结合蛋白(MBDs)以及组蛋白去乙酰化酶(HDACs)等相关蛋白. MBD可直接抑制转录, 并与HDAC形成复合体. DNMT催化甲基化生成, 并与HDAC协同作用导致基因转录失活. 与衰老相关的甲基化可影响众多基因CpG岛, 是老年人恶性肿瘤发生的主要危险因素; 甲基化的另一种方式CpG岛甲基化表型(CIMP)具有肿瘤独特性, P16、hMLH1、E-cadherin等重要肿瘤抑制基因失活与此有关. DNA甲基化的相关深入研究将对揭示包括消化道等恶性肿瘤的发生机制以及未来临床诊断和治疗展示了广阔前景.
引文著录: 周永宁, 徐采朴, 房殿春. CpG岛甲基化与胃肠道肿瘤. 世界华人消化杂志 2003; 11(1): 65-71
Revised: August 20, 2002
Accepted: August 27, 2002
Published online: January 15, 2003
N/A
- Citation: N/A. N/A. Shijie Huaren Xiaohua Zazhi 2003; 11(1): 65-71
- URL: https://www.wjgnet.com/1009-3079/full/v11/i1/65.htm
- DOI: https://dx.doi.org/10.11569/wcjd.v11.i1.65
恶性肿瘤的生物学特征表现为浸润性生长和转移, 其发生机制在于细胞内部基因结构和功能发生了异常改变. 现代肿瘤理论认为恶性肿瘤是由遗传基因缺陷以及基因表遗传性(epigenetic)改变而引起. 基因缺陷如突变、缺失会导致编码区结构和功能破坏. 表遗传性改变尽管不存在DNA序列改变, 但通过DNA自身化学修饰方式从转录水平可影响基因的表达, 调控DNA功能. 表遗传性修饰主要包含两个层面: DNA甲基化和组蛋白去乙酰化[1,2]. 最近[3-6]发现肿瘤抑制基因失活与其启动子区域CpG岛高甲基化状态直接关联. 更为重要的是DNA甲基化与组蛋白去乙酰化在导致基因转录抑制方面具有协同作用[7-9]. 因此CpG岛甲基化在肿瘤发生、发展过程中可能发挥着关键作用.
真核细胞染色质分为常染色质和异染色质, 前者具有转录能力. 染色质的基本单位是一系列紧密排列的核小体, 每个核小体由146 bp的DNA片段围绕8聚体组蛋白构成. 正常生理条件下基因组部分基因适当转录表达, 部分则保持"沉默"(silencing), 其调控机制与核小体组蛋白及DNA的化学修饰关联. 这种化学修饰称为表遗传性或后生调控模式. 组蛋白乙酰化状态是所有真核生物基因转录的调控方式[10], 与此相关的酶分别是组蛋白乙酰基转移酶(histone acetyltransferase, HATs)和组蛋白去乙酰基酶(histone deacetylase, HDACs). DNA甲基化是哺乳动物遗传外基因转录的重要调控方式[11], 在基因转录抑制方面可能起关键作用. 曾认为DNA甲基化是脊椎动物DNA惟一的自然化学修饰方式, 研究[12-14]证明果蝇亦具有与脊椎动物同源的DNA甲基化模式. 迄今已证实[15-17]有三种与DNA甲基化相关的酶, 并已被分别克隆, 他们是DNMT1(DNA methyltransferase DNA, 甲基转移酶1)、DNMT3a和DNMT3b. 生物学甲基化方式可分为新生甲基化(de novo methylation)和持续甲基化(maintenance methylation), 据认为[17-19], DNMT3a 和DNMT3b可以催化新生甲基化形成, DNMT1主要在DNA复制时维持其持续甲基化状态. DNMT2 也已被克隆, 但体外胚胎干细胞实验证明DNMT 2不具有甲基转移酶活性[17,20,21].
DNA甲基化是指由DNMTs介导, 在胞嘧啶(C)的第五位碳原子上加上一甲基基团, 使之变成5-甲基胞嘧啶(5-mC)的化学修饰过程. 这种修饰反应主要发生在CpG二核苷酸的胞嘧啶. 哺乳动物基因组中的大部分(70%)CpG二核苷酸存在于重复子(repetitive elements)中, 正常情况下这些CpG二核苷酸甲基化是作为一种宿主基因的保护机制[19-21], 因为他会抑制重复子转录以及重复子同源性重组. CpG二核苷酸聚集区域称为CpG岛(CpG island). CpG岛长约0.5-2.0 Kb, 呈不连续分布. 理论上讲, CpG二核苷酸含量应有1/16, 但由于生物学上的C-G抑制现象, 实际上哺乳动物细胞基因组中C-G含量仅为1%, 但C-G抑制现象并不适用于CpG岛. 整个基因组中, CpG岛通常位于基因的启动子区域(promoter region)[22], 故亦称5'-CpG岛. 与其他CpG二核苷酸不同, 正常情况下, CpG岛是以非甲基化形式存在的. CpG岛如此丰富的胞嘧啶, 何以未被甲基化, 原因尚不清楚. 对腺嘌呤磷酸核糖基转移酶(Apart)的研究[23,24]发现, 可能存在的"反作用因子"(trans-acting factor)与DNA的Sp1位点结合, 保护Apart基因CpG岛免遭甲基化. CpG岛非甲基化状态的生物学意义非常重要, 启动子非甲基化状态是相关基因转录或待转录的先决条件[11], 并通过有丝分裂得以继承和维持, 同时也是恶性肿瘤发生的关键环节. 与大多数CpG岛非甲基化状态不同, 某些生理性的CpG岛甲基化亦具有重要的生物学意义:(1)哺乳动物胚胎发育时期, DNA甲基化又再更新, 以此调节相关基因表达[17] ;(2)X-染色体失活; (3)印迹基因(imprinted gene)转录抑制; (4)防御外来入侵的寄生DNA(parasitic DNA)[19] ;(5)维护染色体的完整性[25] ;(6)控制组织特异性基因表达[26] ;(7)调控DNA重组的某些环节[27].
正常DNA甲基化模式改变, 包括DNA过低甲基化和CpG岛高甲基化, 他们在肿瘤的发生中都具有重要的意义[25,28]. 以往对CpG岛甲基化作用的认识仅限于在细胞有丝分裂时其能抑制基因重组和/或有助于染色体分离[29]. 相反对DNA过低甲基化与肿瘤的研究较多. 基因组DNA过低甲基化可促进杂和性丢失(LOH)[30], 导致基因组有害基因转录表达, 例如激活原癌基因, 使癌基因或相关因子得以表达[31]. 胚胎干细胞[29,32]在缺乏DNMT1时基因组过低甲基化, 宿主保护机制削弱, 有利于基因组重复子同源性重组, 从而导致整个基因组不稳定性增加. 免疫缺陷性着丝粒不稳定性面部异常综合征(immunodeficiency-centromeric instability-facial anomalies, ICF)与DMNT3b基因突变[33,34]造成基因组不稳有关. 此外, 在细胞染色体中心粒周围存在高度密集甲基化区域, 如果失去致密的甲基, 可导致基因损伤和突变[35,36].
1980年代发现了结肠肿瘤降钙素基因的5'-CpG岛高甲基化, 并由此拓宽了认识肿瘤与CpG岛甲基化研究的广阔视野. 随着研究的不断深入, CpG岛甲基化对肿瘤抑制基因失活的关键作用凸现出来, 成为当今肿瘤学研究的新热点. 如果说胚胎发育时期DNA甲基化的调控能有效阻止有害基因的表达, 那么肿瘤抑制基因CpG岛非甲基化状态就能保护体细胞避免发生恶性转化.
CpG岛甲基化导致相关基因失活是不争的事实. 最近认为CpG岛甲基化导致基因失活、转录抑制的机制可能有以下几点.
基因启动子区域(包含转录因子结合位点)高甲基化状态, 直接阻止基因转录因子与启动子结合[37,38]. 高甲基化的CpG岛通过特异的甲基化结合蛋白(methyl-cpg-binding protein, MeCP), 与HDAC协同作用, 导致基因失活. 目前认为这一间接途径是基因失活的主要机制. MeCP复合体[39,40]包含MeCP1和MeCP2. MeCP2由甲基化CpG岛结合区(methyl-CpG-bindind domain, MBD)和转录抑制区(transcriptional repression domain, TRD)构成[41]. MBDs家族[42,43]包括MBD1、MBD2、MBD3和MBD4. MBD1[39,44]为单一蛋白质, 可选择性与甲基化DNA结合而抑制转录. MBD2[45]可能具有去甲基化酶活性. MBD3和/或MBD2以及HDAC与Mi-2/NuRD(nucleosome remoleling histone deacetylase)构成复合体[46,47], Mi-2是ATP酶的SWI2/SNF2超家族成员, 通过相关蛋白RbAp48增强去乙酰化酶的活性, 从而影响组蛋白-DNA的相互作用, 使染色质聚集, 基因失活. 最近[48,49]发现HDAC 和Sin3A直接相互作用与MeCP2构成复合体, Sin3A(一种基因转录抑制因子)活性得到增强. MBD4[50]与hMLH1关联, 参与DNA修复. MBD2和HDAC也是MeCP1复合体的构成部分, MeCP1复合体具有甲基结合活性, 至少能与12个对称的甲基化CpG岛结合[45]. HDACs和HATs通过调节组蛋白乙酰化状态[10], 进而调控DNA活性. 组蛋白H3或H4的氨基末端经HATs催化而乙酰化; 在组蛋白去乙酰基酶HDACs催化下去乙酰化. 乙酰化时, 组蛋白与DNA结合能力降低, 基因转录因子容易进入启动子区域; 反之基因转录因子就不易进入启动子. CpG岛甲基化, 通过MeCP可增强HDAC去乙酰化活性, 从而导致基因转录抑制. 不难发现MeCP作为"纽带"将DNA甲基化和组蛋白去乙酰化这两种表遗传性化学修饰模式有机地结合起来, 较合理的解释了CpG岛甲基化与基因转录抑制之间的因果关系.
DNMTs除具有甲基转移酶活性外, 还有抑制基因转录作用. DNMT1的C-端为甲基转移酶催化区, 能将S-腺苷甲硫氨酸的甲基基团转移至胞嘧啶[51]. N-端为功能调节区, 能将酶靶向于复制位点, 并且对半甲基化CpG(hemi-methylated CpGs)二核苷酸有高亲和力, 约是非甲基化CpG(unmethylated CpGs)的10-40倍[52]. DNMT1可能通过他的N-端与HDAC相互作用抑制转录, 并且这种抑制作用与其本身的催化活性无关. 此外, 某些因子可能与N-端构成复合体, 使DNMT1的转录抑制和/或酶活性具有调控性和靶向性. Robertson et al[53]发现DNMT1结合pRB(retinoblastoma protein), 通过E2F/pRB复合体抑制转录, 这种转录抑制似乎也与甲基化无关, 因为在启动子未发现甲基化. Rountree et al[54]证实了一种被称为DNMT1相关蛋白(DNMT1 associated protein 1, DNMP1)的因子与DNMT1的N-端结合, 再与TSG101(Tumor Susceptibility Gene 101)结合, 后者是一种肿瘤抑制基因, 具有强大的转录抑制作用. 有关DNMT1/ DNMP1相互作用的机制及其功能尚需深入研究. 有人[54,55]认为, DNMT3a和DNMT3b亦具有转录抑制功能, 且与HDAC活性有关. 这种转录抑制及其与HDAC的相关性主要由N-端类似于ATRX的区域介导.
根据经典肿瘤发生的"二次打击理论"(two-hit hypothesis), 肿瘤抑制基因失活可因基因突变或缺失引起. 但某些恶性肿瘤其DNA序列完整, 并未有突变、缺失, "二次打击理论"无法解释肿瘤抑制基因何以失活. 错配修复系统(MMR)突变导致微卫星不稳(MSI)频繁出现在遗传性非息肉结肠癌(HNPCC), MMR突变主要涉及hMLH1和hMSH2基因. 但发现[56]10-15%的散发性结肠癌尽管无MMR突变, 但亦表现MSI. 这就表明MMR相关基因失活存在另外一种机制或途径. 基于下列事实: 散发性结肠癌特别是MSI＋结肠癌出现大量hMLH1基因CpG岛甲基化; p16-INK4a失活的惟一机制[57]在于CpG岛甲基化; 某些致癌剂影响DNA甲基化方式[58] ; 故普遍认为DNA甲基化是肿瘤发生的第三种机制, 称之为"启动子高甲基化"(promoter hypermethylation)[59]. Costello et al[60]证实, 目前已发现的肿瘤细胞基因组中有45 000个CpG岛, 各类癌细胞有近600个CpG岛异常甲基化. 重要的是, CpG岛甲基化异常不是随机事件, 而是具有基因特异性和肿瘤特异性.
DNA甲基化异常在肿瘤形成机制中的确切作用尚不清楚, 然而众多证据表明肿瘤抑制基因CpG岛甲基化、导致基因失活和转录抑制是肿瘤发生的重要机制之一. 肿瘤细胞DNA总体甲基化水平低于正常细胞, 但某些特定基因(如肿瘤抑制基因)CpG岛却处于高甲基化状态, 可能的机制与以下因素有关.
对胚胎细胞Apart基因的研究中发现[24], Apart基因CpG岛未被甲基化可能与"反作用因子"有关. 故推测, 可能存在如"反作用因子"的保护因素使正常细胞DNA的CpG岛非甲基化. 肿瘤细胞由于某种原因缺乏相应的保护因子, CpG岛被甲基化. 这种推测也得到另一项实验支持[61]: 在E-cadherin基因启动子甲基化的癌细胞株中, 导入外源性非甲基化启动子, 其E-cadherin的表达明显小于启动子非甲基化的癌细胞株, 表明E-cadherin的完全表达需要CpG岛保护因子.
关于DNMT高表达与CpG岛甲基化之间的关系, 目前尚有争议. Belinsky et al[4,62]认为, 在恶性肿瘤细胞中DNMT1的表达上调. 但也有报告[63]指出CpG岛甲基化与DNMT1水平无关. 在人成纤维细胞, 持续DNMT1表达可导致众多基因CpG岛呈时间依赖的高甲基化[64]. Fos过度表达可使DNMT1表达上调, 在Fos导致的细胞转化中具有某种作用[65], 如Min小鼠(结肠息肉易感小鼠)在DNMT1杂和性背景下极少发生息肉病变[66]. 研究发现癌细胞株DNMT1高表达, 并可与p21-WAF1竞争性结合PCNA, 造成p21-WAF1表达上调, 进而影响细胞周期依赖激酶(CDK)活性, 使Rb蛋白磷酸化受到影响, 结果又通过正反馈调节p21-WAF1的表达, 最终导致细胞恶性生长[67,68]. 大肠癌细胞株HDT116在Dnmt1基因缺失时, 基因组甲基化水平仅少量减低(20%)[69], 但CpG岛仍然高甲基化提示可能其他甲基化转移酶维持着这种状态. 有迹象表明DNMT3a 和DNMT3b在癌细胞中也少量表达, 由此推测[70]这三种甲基化转移酶之间的平衡与协调可能具有重要意义. 有学者[71,72]进一步指出 甲基化模式的维持是由甲基化和去甲基化与组蛋白乙酰化状态之间的动态平衡所决定.
DNMT调控障碍或靶向错误可能归咎于DNMT复合体一种或多种因子的缺乏, 如pRB、DMNP1和TSG101等. 几乎所有恶性肿瘤的Rb通路(调控细胞周期)都受到破坏, 所以有理由相信癌细胞CpG岛甲基化与DNMT1/pRB相互作用障碍有关联[55]. 此外, Dmnp1基因的甲基化修饰位点(methylation modifier locus 1, MEMO1)[73]位于1号染色体的短臂, 而许多恶性肿瘤频繁发生的LOH就位于这个区域. MEMO1位点的LOH与成神经细胞瘤细胞株的异常甲基化有关[73]. 目前尚不知DMAP1是否就是MEMO1的位点; 恶性肿瘤细胞异常甲基化是否与LOH有关联.
细胞分裂增生周期是基因组复制并且表遗传性得以继承的过程. 简言之, 细胞复制时染色体要将遗传信息正确传代, 包括基因组甲基化方式、核小体的装配和定位、组蛋白修饰状态的重建以及其他保证正常转录的相关因子. 为此, 细胞在空间和时间调控基因组的复制以保证染色体正确遗传[74]. 已证实[75]增生细胞核抗原(PCNA)对异染色质遗传具有关键作用, 推测[75,76] PCNA可能给其他相关蛋白结合提供了一个平台, 并且伴随复制再建适宜的表遗传性状态. DNMT1通过与PCNA直接作用而进入细胞复制机制[76,77], 并维持子代细胞DNA适宜的甲基化方式. 有证据表明DNMT1与相关蛋白在S期形成复合体. Rountree et al[54]证实在整个S期, DNMP1与DNMT1在复制位点形成复合体. 发现[78]在S期末期, HDAC2又与DNMT1/DMAP1构成复合体, 这一点非常重要, 因为此时正处于转录"休眠"的异染色质复制阶段, 因而也可解释DNA复制后, 新合成的核小体去乙酰化的机制. 最近[79]研究发现DNMT1在复制启动阶段发挥重要作用, 当细胞开始进入S期时, pRB和E2F定位于复制位点, 也许pRB和DNMT1在这个时期相互作用对于DNA复制启动的时机恰到好处. 已发现[79]"永生"细胞在S期早期并不存在pRB-SWI/SNF复制位点, 很可能癌细胞有同样缺陷.
如上所述, 基因组复制在S期具有时空性. DNMT1及其相关复合体在S期不同阶段的调控作用是DNA复制正常的重要因素之一, 也是恶性肿瘤发生的重要环节. 由于复制时间错误, 某些基因CpG岛不被甲基化而失去转录抑制(癌基因); 而另一些基因则发生不恰当的甲基化(肿瘤抑制基因). 可以说, 肿瘤抑制基因CpG岛在S期末期甲基化导致基因失活是在错误的时间所发生的错误事件.
DNA甲基化方式改变涉及染色体结构、DNA复制以及基因表达. CpG岛甲基化与某些遗传性疾病[80](如fragile-X syndrom)、衰老[81]以及恶性肿瘤[4]有关. 近50%的人类家族性恶性肿瘤细胞发现有肿瘤抑制基因突变[4]; 但仍有部分癌细胞虽无编码区突变的证据, 肿瘤抑制基因却因CpG岛甲基化处于失活状态, 表明CpG岛甲基化是癌细胞相关基因失活的重要机制之一. 最近[82-87]发现众多恶性肿瘤不同程度的存在一个或多个肿瘤抑制基因CpG岛甲基化, 如乳腺癌、舌癌、肝癌、肺癌、胰腺癌、胃癌和结肠癌等. 这些相关的肿瘤抑制基因失活涉及多个环节, 从而导致肿瘤的发生和进展[55](表1).
| Pathway affected | Genes silenced by CpG island methylation |
| Cell cycle control | Rb, p16(INK4a), p15, p14(ARF), p73 |
| DNA damage repair | MLH1, O6MGMT, GSTp, BRCA1 |
| Inhibiting apoptosis | DAP-kinase, Caspase-8, TMS1 |
| Invasion tumor architecture | E-cadherin, VHL, APC, LKB1, TIMP3, Thrombospondin1 |
| Growth factor response | ER, RARb, Androgen Receptor, Endothelin B Receptor, RASSF1A |
CpG岛甲基化是胃肠道恶性肿瘤发生的早期事件, 在早癌及癌前病变就存在CpG岛甲基化[88]. 对食管癌检测[89]发现除食管癌外, 反流性食管炎、Barrett's 食管也出现ER(雌激素受体)、APC和p16基因CpG岛甲基化. Esteller et al[90]的研究也发现结肠腺瘤p16和p14基因CpG岛甲基化. Kang et al[91]则证实 DAP-kinase、hMLH1、THBS1(thromobospondin-1)、TIMP-3(金属蛋白酶-3)在胃癌及癌前病变组织CpG岛甲基化, 且THBS1和TIMP-3的CpG岛甲基化频率随胃炎→肠化生→腺瘤→胃癌的不同阶段显著上升. 此外也发现肝硬变时染色体16q22.1高甲基化[92].
Issa et al[58]首先报告, 在正常老年人结肠黏膜细胞ER基因CpG岛甲基化. Toyota et al[93,94]采用MCA(Methylation CpG island amplification)技术(同时进行甲基化分析和克隆基因组不同的甲基化DNA)对26个DNA序列的CpG岛甲基化与结肠癌的关系作了较为系统的研究. 根据结肠癌不同的CpG岛甲基化状况而分为两型: A型和C型. A型甲基化(19/26, 73%)在结肠癌频繁发生(30%-100%), 部分正常结肠黏膜尤其是老年人也存在, 称之为"衰老独特性甲基化"(aging-specific methylation). 由此表明[81,93,94]老年化是结肠癌等恶性肿瘤发生的主要危险因素, 在衰老进程中, 由于基因组DNA的CpG岛甲基化发生和不断积累, 可引起细胞周期、分化及凋亡调控机制障碍. C型甲基化(7/26, 27%)在结肠癌的发生频率较低(30%-50%), 但明显不同于A型, 仅出现在癌组织, 亦称"癌独特性甲基化"(cancer-specific methylation). 由于两型甲基化同时都有新生甲基化异常, 故推测[94] C型甲基化机制可能主要在于抗甲基化的保护因子如"反作用因子"的缺乏.
进一步对50例结肠癌研究[94]发现, CpG岛甲基化状况在两组间明显不同: 第一组C型极少见, 平均每个标本0.3个位点, 癌肿主要发生在远端; 第二组C型甲基化高频率, 平均5.1个位点, 且癌肿主要在近端; 两组之间A型甲基化无明显差异. 第二组结肠癌因以C型甲基化为特征而称为CpG岛甲基化表型(CpG island methylator phenotype, CIMP). 推测CIMP＋和CIMP-结肠癌的发生机制可能不同. 对于多数的CIMP-结肠癌而言, 伴随着年龄进展, 结肠黏膜细胞基因组众多基因甲基化, 并不断积累, 影响相关基因表达. 由于细胞周期、分化及凋亡调控机制障碍, 导致细胞处于高增生状态, 最终可能细胞恶性转化. 在此过程中, 环境因素如炎症、病毒感染等也可参与其中. CIMP＋的发生则与细胞转化过程中肿瘤抑制基因失活密切相关, 如p16-INK4a[95]、THBS1[91]、E-cadherin[96] 、hMLH1[97,98]及Cox-2[99]等. 已报告胰腺癌[100]、胃癌[101]及乳腺癌[102]等亦存在CIMP.
晚近[59,103]认为, 在肿瘤演进过程中, 常伴随基因和表遗传性分子复杂交错的多样性改变, 故称"表遗传性-突变表型"(epi-mutator phenotype)较CIMP更为贴切.
Grady et al[104]发现家族性弥漫型胃癌生殖细胞突变的E-cadherin基因存在启动子CpG岛甲基化. Machado et al[105]随后对23例散发性胃癌的研究中证实, 16例弥漫型胃癌中有9例存在E-cadherin基因突变, 同时免疫组化检测均表达异常. 9例中仅1例LOH, 即符合经典的"二次打击理论"模式; 但6例CpG岛甲基化的事实表明在E-cadherin基因突变的基础上, 其CpG岛甲基化是散发性弥漫型胃癌的"第二次打击". 这项研究展示出CpG岛甲基化在恶性肿瘤发病机制中另一个重要途径或模式.
恶性肿瘤DNA甲基化异常具有鲜明特征[60,106] : 肿瘤特异性, 基因和组织特异性以及可逆性. 因此DNA甲基化检测作为一种新的分子生物学手段将会广泛应用于临床诊断、病情监测以及疗效评价. 甲基化检测技术的高敏感性和特异性, 使其优于如微卫星不稳定性及LOH等检测方法. PCR技术更适用于检测标本量较少的组织切片、痰及尿液等, 并能定量分析以此进行临床随访.
DNA甲基化的可逆性特征为临床抗肿瘤治疗提供了一种新途径. 5-氮-2-脱氧胞苷(DAC或5aza-dc)是有效的甲基转移酶抑制剂, 大剂量使用时因其细胞毒性而具有抗肿瘤作用, 小剂量DAC导致细胞分化可能与去甲基化和调控分化的特异性基因再表达有关[107,108]. 通过对ICF的治疗, 证明DAC可增加染色体转录[36]. 此外, DAC与组蛋白去乙酰化抑制剂TSA协同作用值得关注. Cameron et al[109]证实去甲基化药物协同HDAC抑制剂可诱导失活的基因再表达; Zhu et al[110]亦发现去甲基化药物协同HDAC抑制剂可导致肺癌细胞大量凋亡, 提示与凋亡关联的甲基化基因在去甲基化后, 该基因被激活. 最近Kim et al[111]报告对胃MALT淋巴瘤抗幽门螺杆菌(H. pylori)治疗后平均随访28mo, 发现3例(3/7)p16的CpG岛甲基化消失, 提示H. pylori感染与DNA甲基化的关系如何？DNA甲基化检测能否作为胃MALT淋巴瘤H. pylori根除治疗的分子生物学指标？
DNA甲基化是当今肿瘤研究的热点, 其中许多内在机制尚待深入研究, 如甲基化与细胞复制周期、恶性肿瘤DNA甲基化和染色质异常改变方式等. 相信对甲基化机制的不断研究将会为认识肿瘤发病机制以及未来临床诊断和治疗展示广阔前景.
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