吳宛玲 李爽
環(huán)狀RNA是一種內(nèi)源性的RNA分子,通過(guò)反式剪接的形式由其3'端和5'端以共價(jià)鍵形成閉合環(huán)狀結(jié)構(gòu),不具有“PolyA尾”。環(huán)狀RNA序列高度保守,在生物體內(nèi)穩(wěn)定表達(dá),含量豐富,其表達(dá)具有組織特異性和發(fā)育階段特異性。有研究證明,環(huán)狀RNA有microRNA“海綿”、RNA結(jié)合蛋白及核轉(zhuǎn)錄調(diào)節(jié)等功能,對(duì)生命活動(dòng)起到重要的調(diào)控作用[1]。在目前的研究中,環(huán)狀RNA主要以miRNA“海綿”的形式調(diào)控腫瘤的發(fā)生和發(fā)展。隨著研究的不斷深入,還發(fā)現(xiàn)環(huán)狀RNA可能在腫瘤的診斷、預(yù)后和靶向治療中同樣發(fā)揮一定的作用。現(xiàn)已報(bào)道的環(huán)狀RNA在腫瘤中的功能及臨床應(yīng)用見(jiàn)表1。本文就環(huán)狀RNA在腫瘤中的作用機(jī)制及對(duì)腫瘤診斷、預(yù)后和治療的應(yīng)用進(jìn)行綜述。
大部分的環(huán)狀RNA都具有miRNA結(jié)合位點(diǎn),可以作為miRNA“海綿”分子,通過(guò)大量結(jié)合miRNA來(lái)抑制miRNA對(duì)下游靶基因的調(diào)控作用。在腫瘤疾病中,環(huán)狀RNA主要通過(guò)作為miRNA“海綿”發(fā)揮其間接調(diào)控作用。CDR1as/miR-7是較為經(jīng)典的腫瘤相關(guān)的環(huán)狀RNA-miRNA體系。CDR1as含有70余個(gè)miR-7的結(jié)合位點(diǎn),可發(fā)揮其“海綿”作用與miR-7大量地結(jié)合抑制miR-7的基因調(diào)控作用而間接達(dá)到抑瘤作用,既往已有多項(xiàng)研究表明,過(guò)表達(dá)miR-7可以顯著抑制膠質(zhì)瘤、乳腺癌、胃癌、結(jié)直腸癌等多種腫瘤細(xì)胞的增殖活性和侵襲性[1-3]。
具有促瘤作用的環(huán)狀RNA還有多種,這些環(huán)狀RNA在腫瘤中的表達(dá)是上調(diào)的。circHIPK3是Zheng等[4]發(fā)現(xiàn)的一個(gè)新的腫瘤相關(guān)的環(huán)狀RNA,circHIPK3主要存在于細(xì)胞質(zhì)中。circHIPK3通過(guò)與miR-124結(jié)合發(fā)揮促細(xì)胞增殖的功能。Xie等[5]研究發(fā)現(xiàn),hsa_circ_001569通過(guò)與miR-145結(jié)合上調(diào)受miR-145負(fù)向調(diào)控的E2F5、BAG4和FMNL2的表達(dá)來(lái)促進(jìn)結(jié)直腸癌細(xì)胞的增殖和侵襲。Zhong等[6]也在膀胱癌組織中發(fā)現(xiàn)了一個(gè)明顯上調(diào)的環(huán)狀RNA:circTCF25,并證實(shí)了circTCF25-miR-103a-3p/miR-107-CDK6通路在膀胱癌中發(fā)揮重要的調(diào)控作用,過(guò)表達(dá)circTCF25能增加癌細(xì)胞增殖遷移能力。
表1 腫瘤相關(guān)的環(huán)狀RNA功能及臨床應(yīng)用
表1 腫瘤相關(guān)的環(huán)狀RNA功能及臨床應(yīng)用(續(xù)表1)
也有部分環(huán)狀RNA發(fā)揮著完全相反的作用,這一部分環(huán)狀RNA在腫瘤中的表達(dá)下調(diào)。cir-ITCH也是一個(gè)具有miRNA“海綿”功能的環(huán)狀RNA。在食管癌的研究中顯示,circ-ITCH包含了miR-214、miR-7和miR-17的結(jié)合位點(diǎn),過(guò)表達(dá)circ-ITCH可以使miRNA的靶基因ITCH表達(dá)上調(diào),ITCH使得磷酸化的Dlv2發(fā)生泛素化而降解,從而抑制Wnt/β-catenin信號(hào)通路,癌細(xì)胞活性降低,增殖受抑[7]。而在結(jié)直腸癌中,cir-ITCH不再作為miR-214“海綿”,而是與miR-7和miR-20a結(jié)合來(lái)抑制Wnt/β-catenin通路來(lái)發(fā)揮其腫瘤抑制效應(yīng)[8]。
盡管大部分的報(bào)道均證實(shí)了環(huán)狀RNA作為miR?NA“海綿”的作用,但仍有部分研究表明,環(huán)狀RNAs不能起到miRNA“海綿”的作用。如目前研究較多的circ-Foxo3,一方面主要通過(guò)與蛋白CDK2和P21結(jié)合調(diào)控細(xì)胞周期進(jìn)程[9];另一方面則與MDM2結(jié)合,促進(jìn)其介導(dǎo)的p53的泛素化和降解,抑制FOXO3的泛素化和降解,從而抑制細(xì)胞增殖,誘導(dǎo)細(xì)胞凋亡[10]。但環(huán)狀RNA這種非miRNA“海綿”的作用機(jī)制在具體的腫瘤疾病中少有報(bào)道,這還有待于進(jìn)一步的研究。
環(huán)狀RNA近幾年來(lái)已被證實(shí)可在外周血中檢測(cè)到[11],尤其是在血清外泌體中,環(huán)狀RNA大量且穩(wěn)定地存在,且這個(gè)過(guò)程是受調(diào)控的,腫瘤外泌體中特定環(huán)狀RNA的表達(dá)明顯高于正常組織的外泌體[12],其富集程度與腫瘤的大小呈正相關(guān)。值得注意的是,環(huán)狀RNA 在腫瘤中也存在差異表達(dá)[1,8,13],但相反的是,環(huán)狀RNA在癌組織中的表達(dá)水平明顯低于正常組織,原因可能與癌細(xì)胞具有更強(qiáng)的增殖能力有關(guān)。Bachmayrheyda等[13]研究發(fā)現(xiàn),環(huán)狀RNA在腦和心臟等細(xì)胞增殖弱的組織細(xì)胞中表達(dá)水平高,而在乳腺、結(jié)腸和卵巢細(xì)胞增殖強(qiáng)的組織細(xì)胞中表達(dá)偏低。
環(huán)狀RNA的差異性表達(dá)是其能夠用于生物標(biāo)記物的基礎(chǔ),尤其是在血清中的高表達(dá)和較為明顯的表達(dá)差異,更是讓環(huán)狀RNA的檢測(cè)變得更為簡(jiǎn)便。近些年有多項(xiàng)環(huán)狀RNA擬作腫瘤早期診斷生物標(biāo)記物可能性的研究迅速開展,均得到了肯定的答案。有研究發(fā)現(xiàn),胃癌組織中的hsa-circ-0000190與原有的兩種經(jīng)典的胃癌生物標(biāo)記物CEA和CA-199相比,具有更好地靈敏度和特異度[14];肝癌組織中發(fā)現(xiàn)的hsa_circ_005075[15],結(jié) 直 腸 癌 組 織 中 的 hsa_circ_001988[16],作為腫瘤診斷的生物標(biāo)記物均具有較高的診斷真實(shí)度。
近年來(lái),關(guān)于環(huán)狀RNA用于腫瘤預(yù)后評(píng)估的研究也逐漸增加。大多數(shù)在腫瘤疾病中呈現(xiàn)表達(dá)差異的環(huán)狀RNA均不同程度的與部分預(yù)后相關(guān)指標(biāo),如腫瘤大小、分期、遠(yuǎn)處及淋巴結(jié)轉(zhuǎn)移等具有相關(guān)性,提示其預(yù)后意義。如在喉鱗狀上皮癌中,hsa_cir?cRNA_100855在喉癌組織中較正常組織高表達(dá),尤其是在T3~T4期腫瘤、淋巴結(jié)轉(zhuǎn)移的腫瘤、聲門上型腫瘤和臨床晚期腫瘤中表達(dá)水平更高,而hsa_cir?cRNA_104912在喉癌組織中低表達(dá),在T3~T4期腫瘤、淋巴結(jié)轉(zhuǎn)移的腫瘤、低分化腫瘤及臨床晚期的腫瘤中表達(dá)水平更低[17]。
部分研究為了明確環(huán)狀RNA的預(yù)后意義開展了進(jìn)一步的生存分析,結(jié)果同樣肯定了環(huán)狀RNA的預(yù)后價(jià)值。如在一項(xiàng)關(guān)于非小細(xì)胞肺癌的研究中,首先發(fā)現(xiàn)了circRNA_100876和腫瘤分期及淋巴轉(zhuǎn)移相關(guān),進(jìn)一步的生存分析顯示,circRNA_100876高表達(dá)的患者的總生存時(shí)間明顯低于circRNA_100876低表達(dá)的患者[18];在肝細(xì)胞癌的研究中也發(fā)現(xiàn)了同樣的結(jié)果,hsa_circRNA_100338能提高腫瘤的侵襲和轉(zhuǎn)移能力,hsa_circRNA_100338高表達(dá)的肝癌患者累積生存率明顯降低[19];在胃癌的研究中也肯定了環(huán)狀RNA的預(yù)后價(jià)值,circPVT1高表達(dá)的胃癌患者生存率較高,并且circPVT1結(jié)合其末基因PVT1的表達(dá)量是更好的胃癌預(yù)后指標(biāo),circPVT1高表達(dá)而PVT1低表達(dá)的患者生存期明顯延長(zhǎng)[20]。
綜上所述,多項(xiàng)證據(jù)均提示環(huán)狀RNA的表達(dá)水平可用來(lái)鑒定腫瘤分期和評(píng)估預(yù)后。
miRNA的“海綿”技術(shù)是一種新型的RNA治療方法,能實(shí)現(xiàn)對(duì)miRNA的長(zhǎng)效抑制[21]。已有多項(xiàng)針對(duì)癌基因的線性“海綿”被發(fā)現(xiàn),當(dāng)將其轉(zhuǎn)入人腫瘤細(xì)胞時(shí)能夠有效地逆轉(zhuǎn)癌細(xì)胞表型[12]。近些年來(lái),環(huán)狀RNA的miRNA“海綿”功能使得其在這一技術(shù)上的應(yīng)用備受期待。
Ivanov等[22]研究發(fā)現(xiàn),除了內(nèi)含子上ALU重復(fù)序列介導(dǎo)外顯子環(huán)化外,人環(huán)狀RNA的側(cè)翼內(nèi)含子區(qū)富集著反向互補(bǔ)配對(duì)序列(reverse complementary matches,RCMs)。進(jìn)一步研究發(fā)現(xiàn)上述內(nèi)含子RCMs對(duì)環(huán)狀RNA生成非常關(guān)鍵,雙鏈RNA編輯酶ADAR1參與該過(guò)程,敲低ADAR1后可顯著特異性地上調(diào)環(huán)狀RNAs的表達(dá),并認(rèn)為內(nèi)含子間的RCMs通過(guò)競(jìng)爭(zhēng)性結(jié)合方式介導(dǎo)外顯子環(huán)化。如果通過(guò)某種方式使得ADAR在生理狀態(tài)下瞬時(shí)降低后,則可能明顯上調(diào)環(huán)狀RNAs的功能。既往有研究顯示,如果胚胎干細(xì)胞中ADAR基因表達(dá)降低,干細(xì)胞可以分化成神經(jīng)元細(xì)胞[23]。環(huán)狀RNA是否參與干細(xì)胞分化過(guò)程值得進(jìn)一步深入研究。
隨著RNA測(cè)序、芯片等技術(shù)的發(fā)展,越來(lái)越多的研究表明環(huán)狀RNA和許多疾病,尤其是腫瘤,存在著較為重要的聯(lián)系。環(huán)狀RNA被發(fā)現(xiàn)具有miRNA“海綿”、蛋白調(diào)控、剪切及轉(zhuǎn)錄功能,以及近期越來(lái)越多的研究發(fā)現(xiàn)蛋白翻譯功能,環(huán)狀RNA的功能研究不斷出現(xiàn)新的突破,在腫瘤疾病的發(fā)生發(fā)展中發(fā)揮著較為重要的調(diào)控作用。對(duì)腫瘤中環(huán)狀RNA功能的研究,不僅可以促進(jìn)對(duì)疾病進(jìn)展的理解還幫助臨床對(duì)疾病更為掌握,是將其應(yīng)用于臨床的前提。就臨床應(yīng)用而言,環(huán)狀RNA已經(jīng)體現(xiàn)出了其研究?jī)r(jià)值。環(huán)狀RNA可以作為腫瘤早期診斷的生物標(biāo)記物,也可作為評(píng)估療效和預(yù)后的敏感指標(biāo)。目前,臨床應(yīng)用的部分生物標(biāo)記物,如CEA,在多種消化道腫瘤,如胰腺癌、結(jié)腸癌中均有升高,因此在應(yīng)用于診斷時(shí)難免缺乏特異性。而環(huán)狀RNA具有組織特異性,能夠更好地協(xié)助診斷。通過(guò)對(duì)環(huán)狀RNA調(diào)控機(jī)制的研究,明確環(huán)狀RNA在腫瘤疾病中具體的調(diào)控網(wǎng)絡(luò),則能以環(huán)狀RNA為抗癌靶點(diǎn)作為腫瘤治療的新方向,腫瘤化療藥物耐藥相關(guān)的環(huán)狀RNA也將成為新的研究熱點(diǎn)。
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