程香榮，胡興琳，姜琦，黃星衛(wèi)，王楠，雷蕾

?

核糖體DNA轉(zhuǎn)錄的表觀調(diào)控與腫瘤發(fā)生

程香榮，胡興琳，姜琦，黃星衛(wèi)，王楠，雷蕾

哈爾濱醫(yī)科大學(xué)組織學(xué)與胚胎學(xué)教研室，哈爾濱 150081

近年來(lái)，表觀遺傳機(jī)制的研究結(jié)果提示核糖體DNA (rDNA)表觀調(diào)控機(jī)制的缺陷可能誘導(dǎo)腫瘤發(fā)生。ATRX/DAXX復(fù)合物通過(guò)介導(dǎo)H3.3的H3K9me3修飾，建立和維持rDNA轉(zhuǎn)錄沉默。/基因在部分腫瘤中經(jīng)常發(fā)生突變，可能刺激rDNA轉(zhuǎn)錄而促進(jìn)腫瘤發(fā)生發(fā)展。本文主要闡述rDNA轉(zhuǎn)錄表達(dá)異常對(duì)腫瘤發(fā)生的促進(jìn)作用，介紹rDNA基因轉(zhuǎn)錄的表觀遺傳調(diào)控機(jī)制，以期為針對(duì)rDNA轉(zhuǎn)錄調(diào)控機(jī)制的藥物研發(fā)提供新的理論支持。

rDNA；表觀調(diào)控；H3.3；ATRX/DAXX；腫瘤發(fā)生

早在19世紀(jì)晚期科學(xué)家們就發(fā)現(xiàn)癌細(xì)胞中核仁數(shù)量增多和比例增大，20世紀(jì)初已將這種大核仁特征列為惡性腫瘤的確診依據(jù)。人非瘤病灶細(xì)胞中出現(xiàn)巨大核仁的表現(xiàn)與致瘤風(fēng)險(xiǎn)高度相關(guān)。核糖體蛋白S19 (ribosomal protein S19, RPS19)突變的遺傳性“核糖體病人”，其早期易患先天性純紅細(xì)胞再生障礙性貧血(Diamond-Blackfan anemia, DBA)，后期可發(fā)展為細(xì)胞高度增殖性疾病(癌癥)，提示核糖體缺陷可能與腫瘤發(fā)生有關(guān)[1]。因而人們提出核糖體生物合成過(guò)程中質(zhì)和量的改變可能致瘤的假說(shuō)[2]。目前，核糖體異常如何致瘤是生命科學(xué)研究領(lǐng)域的熱點(diǎn)方向之一[3]。

核糖體DNA (ribosomal DNA, rDNA)轉(zhuǎn)錄為rRNA的過(guò)程是核糖體生物合成的限速步驟[4]。基因組中rDNA基因有數(shù)百個(gè)拷貝，具有重復(fù)序列的特征，其轉(zhuǎn)錄水平的調(diào)控主要有2種方式[5]：(1)通過(guò)影響細(xì)胞的生長(zhǎng)增殖信號(hào)通路調(diào)節(jié)rDNA特異的RNA聚合酶Ⅰ(RNA polymeraseⅠ, PolⅠ)的轉(zhuǎn)錄效率；(2)通過(guò)表觀分子作用機(jī)制調(diào)節(jié)活躍態(tài)rDNA拷貝數(shù)量的所占比例[6]。前者屬于短期調(diào)節(jié)方式，細(xì)胞的營(yíng)養(yǎng)、生長(zhǎng)因子、致癌因素等會(huì)上調(diào)PolⅠ的轉(zhuǎn)錄效率，而基因毒性、代謝壓力、饑餓、病毒感染、腫瘤抑制因素等會(huì)下調(diào)PolⅠ的轉(zhuǎn)錄效率；后者屬于長(zhǎng)期穩(wěn)定的調(diào)節(jié)方式，通過(guò)建立新的表觀遺傳修飾狀態(tài)來(lái)調(diào)控rDNA的轉(zhuǎn)錄潛能，在細(xì)胞的生長(zhǎng)分化、轉(zhuǎn)化過(guò)程中至關(guān)重要[6]。本文通過(guò)闡述核糖體生物合成異常與癌癥發(fā)生發(fā)展的聯(lián)系機(jī)制，討論了rDNA的表觀調(diào)控機(jī)制的缺陷可能對(duì)腫瘤發(fā)生的誘導(dǎo)或促進(jìn)作用，以期為針對(duì)rDNA轉(zhuǎn)錄調(diào)控機(jī)制的藥物研發(fā)提供新的理論支持。

1 核仁結(jié)構(gòu)與核糖體生物合成過(guò)程

核糖體是由4種rRNA分子和約80種不同的核糖體蛋白(ribosomal proteins, RPs)組成的直徑為25~30 nm的復(fù)合體微粒，負(fù)責(zé)“中心法則”的mRNA到蛋白質(zhì)這一翻譯過(guò)程，與細(xì)胞的生長(zhǎng)增殖活動(dòng)息息相關(guān)。核仁是細(xì)胞核糖體生物合成的重要場(chǎng)所，核仁結(jié)構(gòu)的增大反映出核糖體合成速率的增加。H&E染色(hematoxylin-eosin staining)顯示核仁通常表現(xiàn)為單一或多個(gè)勻質(zhì)的球形小體，電子顯微鏡顯示核仁包含3個(gè)主要結(jié)構(gòu)：纖維中心、致密纖維組分和顆粒組分[7]。rDNA位于纖維中心，而從rDNA新合成出的rRNA分子主要集中在致密纖維組分，在顆粒組分部位繼續(xù)加工成熟后，與RPs形成核糖體亞單位，最后被轉(zhuǎn)運(yùn)至胞質(zhì)中。核仁的纖維組分包含所有rDNA轉(zhuǎn)錄過(guò)程所需的物質(zhì)，包括PolⅠ、上游結(jié)合因子(upstream binding factor, UBF)、核仁素和核仁磷酸蛋白等，可以采用銀染的方法使其選擇性顯色。銀染核仁組織區(qū)域(Ag-stained nucleolar organizing region, AgNOR)的面積大小直接反映出rDNA的轉(zhuǎn)錄速率，繼而反映出核糖體生物合成的速率[8]。大約400拷貝的rDNA基因根據(jù)表觀修飾特點(diǎn)和轉(zhuǎn)錄功能被分為活躍態(tài)、沉默態(tài)[9]和中間準(zhǔn)備態(tài)(poised state)[10]，在不同分化狀態(tài)的細(xì)胞中分配著不同的比例?；钴S態(tài)rDNA在PolⅠ和至少3種基本因子[Rrn3 (TIF-IA)、SL1/TIF-IB (selectivity factor 1, SL1)、UBF]的輔助下轉(zhuǎn)錄合成47S pre-rRNA，隨后加工形成成熟的18S、5.8S、28S rRNA。另一種rRNA分子5S rRNA在核質(zhì)中由RNA聚合酶Ⅲ合成，隨后被轉(zhuǎn)運(yùn)到核仁中。各種核糖體蛋白由RNA聚合酶Ⅱ轉(zhuǎn)錄，在胞質(zhì)中翻譯成成熟蛋白質(zhì)后被轉(zhuǎn)運(yùn)到核仁中。隨后28S、5.8S、5S rRNA和49種RPs組裝形成核糖體大亞基60S，而18S和33種核糖體蛋白組裝形成核糖體小亞基40S，最后，大小亞基都被轉(zhuǎn)運(yùn)至胞質(zhì)中構(gòu)成最終的80S核糖體微粒。在整個(gè)過(guò)程中，rDNA轉(zhuǎn)錄速率是核糖體生物合成過(guò)程中的限制性步驟[4]。

2 核糖體生物合成與腫瘤發(fā)生的關(guān)系

自從發(fā)現(xiàn)癌癥細(xì)胞存在核仁過(guò)度肥大且外形不規(guī)則的特點(diǎn)后，人們就一直在探索核仁改變與癌癥發(fā)生之間的因果關(guān)系。一個(gè)快速增殖過(guò)程中的真核細(xì)胞每分鐘就可產(chǎn)生多達(dá)2000個(gè)核糖體，而高度增殖的癌細(xì)胞更加依賴于核糖體的產(chǎn)生過(guò)程。腫瘤細(xì)胞為增加其核糖體生物合成的速率，經(jīng)常突變?nèi)笔Ф鄠€(gè)負(fù)調(diào)控rDNA轉(zhuǎn)錄過(guò)程的腫瘤抑制基因(和等)[3]。RPs除了作為分子組分和rRNA的分子伴侶參與核糖體生物合成外，還在凋亡、細(xì)胞周期停滯、細(xì)胞增殖、細(xì)胞遷移和侵襲、DNA損傷修復(fù)、維持基因組穩(wěn)定等過(guò)程中發(fā)揮著重要作用[11]。對(duì)于這些“核糖體外功能”(extra-ribosomal function)，即核糖體蛋白參與的與核糖體生物合成或總體蛋白質(zhì)翻譯過(guò)程無(wú)關(guān)的其它細(xì)胞生理過(guò)程[12])，主要是通過(guò)p53-MDM2通路介導(dǎo)和調(diào)節(jié)的，調(diào)節(jié)機(jī)制的異常時(shí)也會(huì)導(dǎo)致腫瘤發(fā)生[11,13,14]。此外，除了p53依賴的信號(hào)通路外，RPs還通過(guò)c-Myc、E2F-1、ATF4和NF-κB等途徑影響到核糖體外功能，這些途徑的異常也會(huì)誘導(dǎo)腫瘤的發(fā)生[11]。本文主要以p53依賴的途徑為主，重點(diǎn)闡述核糖體生物合成與腫瘤發(fā)生的關(guān)系。

2.1 核糖體生物合成與細(xì)胞周期

當(dāng)細(xì)胞開始增殖時(shí)，蛋白合成的需求就會(huì)迅猛增加，以滿足細(xì)胞分裂時(shí)所需的大量結(jié)構(gòu)和功能組分。蛋白質(zhì)合成速率的增加是通過(guò)上調(diào)核糖體合成速率來(lái)實(shí)現(xiàn)的[15]，只有G1期達(dá)到足夠多的核糖體儲(chǔ)備時(shí)才會(huì)允許細(xì)胞經(jīng)過(guò)G1-S期限制點(diǎn)[16]。事實(shí)上，有絲分裂原和生長(zhǎng)因子主要通過(guò)以下3種途徑刺激細(xì)胞增殖過(guò)程(圖1)：(1)通過(guò)MAPK/ERK信號(hào)通路，激活PolⅠ和RNA聚合酶Ⅲ的轉(zhuǎn)錄[6]；(2)激活基因，Myc蛋白正向調(diào)節(jié)核糖體生物合成過(guò)程[17]；(3)激活mTOR信號(hào)通路，誘導(dǎo)促進(jìn)PolⅠ和RNA聚合酶Ⅲ的轉(zhuǎn)錄過(guò)程[18]。細(xì)胞增殖信號(hào)可以激活核糖體的生物合成，同時(shí)核糖體生物合成的增加也能促進(jìn)細(xì)胞增殖。在細(xì)胞靜息狀態(tài)下，較高水平的p53阻礙pRb的磷酸化過(guò)程，使pRb保持低磷酸化水平。低磷酸化的pRb與E2Fs結(jié)合，阻止其激活E2F靶基因。E2F的靶基因產(chǎn)物是進(jìn)入S期所必須的，因此腫瘤抑制蛋白pRb與p53在細(xì)胞周期的調(diào)控中起了重要作用[19]。在細(xì)胞正常生理狀態(tài)下，核糖體蛋白與rRNA分子結(jié)合形成核糖體，留下少量游離的RPs。其中，游離的大亞基核糖體蛋白R(shí)PL5、RPL11與5S rRNA形成復(fù)合物，通過(guò)結(jié)合MDM2 (mouse double minute 2, MDM2)而抑制其E3泛素連接酶的功能，剩余未被結(jié)合的MDM2降解p53而使其保持穩(wěn)定水平[20,21]。當(dāng)抑制或擾亂rDNA轉(zhuǎn)錄或成熟過(guò)程，如DNA損傷、RP基因突變、藥物作用、饑餓狀態(tài)或致癌基因激活造成核仁壓力(核糖體壓力)時(shí)，多余游離的RPs將更多地結(jié)合上MDM2，阻礙P53的蛋白酶降解過(guò)程，導(dǎo)致p53水平增高，細(xì)胞周期停滯[22,23]。

圖1 核糖體蛋白-MDM2-p53通路

rDNA轉(zhuǎn)錄是細(xì)胞活動(dòng)的中心。mTOR、MYC與MAPK/ERK通路對(duì)聚合酶Ⅰ(PolⅠ)的轉(zhuǎn)錄具有促進(jìn)作用，促進(jìn)核糖體生物合成。核糖體蛋白(RPs)參與核糖體生物合成過(guò)程，因而導(dǎo)致游離的核糖體蛋白R(shí)PL5、RPL11與5S RNA組成的復(fù)合物減少，從而釋放出更多的MDM2。游離的MDM2導(dǎo)致p53的降解增多，p53蛋白水平的下降將促進(jìn)細(xì)胞周期進(jìn)程，從而促進(jìn)腫瘤發(fā)生。本課題組發(fā)現(xiàn)ATRX/DAXX復(fù)合物與沉默rDNA相關(guān)，經(jīng)常突變基因的腫瘤細(xì)胞中，可能是通過(guò)開放更多活躍rDNA，從而促進(jìn)腫瘤發(fā)生的重要途徑之一。

2.2 核糖體生物合成與腫瘤發(fā)生

腫瘤細(xì)胞會(huì)利用各種方式來(lái)提高核糖體的生物合成水平。例如，p53基因的失活是腫瘤細(xì)胞中最常見的基因突變方式。p53蛋白負(fù)性調(diào)節(jié)rDNA轉(zhuǎn)錄，所以在p53突變?nèi)笔У哪[瘤中rDNA的轉(zhuǎn)錄水平更高[7]。除了p53，腫瘤抑制基因(如和)與致癌基因(如)分別對(duì) PolⅠ轉(zhuǎn)錄起著抑制和促進(jìn)的調(diào)控作用[6]。如果抑癌基因和致癌基因發(fā)生突變失衡，或在癌癥細(xì)胞中給予胰島素或IL-6處理，增強(qiáng)rRNA的轉(zhuǎn)錄，就會(huì)消耗更多的RPs用于核糖體生物合成，從而減少了與MDM2結(jié)合抑制的RPs，導(dǎo)致p53蛋白降解增多，從而游離的p53蛋白水平下降。最終，腫瘤細(xì)胞將得益于核糖體合成水平的增高和細(xì)胞周期限制的解除，更加快速地增殖和侵襲[20,24]。

核糖體生物合成的“量變”或者“質(zhì)變”都有可能升高患癌風(fēng)險(xiǎn)。對(duì)人類疾病的研究數(shù)據(jù)表明，患有慢性炎癥、二型糖尿病、肥胖的人群中細(xì)胞惡性轉(zhuǎn)化的出現(xiàn)頻率更高[25~27]，這些人具有更高的IL-6或血漿胰島素水平，IL-6或胰島素將刺激rDNA的轉(zhuǎn)錄過(guò)程[20,24]。另外，大量實(shí)驗(yàn)研究數(shù)據(jù)證明了核糖體生物合成在誘導(dǎo)腫瘤發(fā)生過(guò)程中的重要性。敲除TIP5 (rDNA相關(guān)沉默復(fù)合物NoRC的大亞基)將導(dǎo)致rDNA的轉(zhuǎn)錄水平升高，誘導(dǎo)正常NIH3T3細(xì)胞出現(xiàn)轉(zhuǎn)化表型[28]。解除MTG16a蛋白對(duì)核糖體DNA的抑制作用，將誘導(dǎo)人正常乳腺上皮細(xì)胞出現(xiàn)乳腺癌發(fā)生時(shí)的形態(tài)表型和分子特征[29]。IL-6通過(guò)刺激核糖體生物合成過(guò)程，誘導(dǎo)人正常結(jié)腸黏膜上皮細(xì)胞系NCM460出現(xiàn)上皮間質(zhì)轉(zhuǎn)化、侵襲的特征[24]。因此，核糖體生物合成速率增高的細(xì)胞將會(huì)有更高的腫瘤發(fā)生風(fēng)險(xiǎn)。此外，核糖體生物合成“質(zhì)變”的“核糖體病人”(如DBA和5q-綜合征患者)因其具有核糖體生物合成缺陷而也會(huì)具有更高的患癌風(fēng)險(xiǎn)[30,31]。RPs缺陷通常導(dǎo)致細(xì)胞低增殖性的表型：如貧血。低增殖特點(diǎn)給細(xì)胞帶來(lái)選擇壓力，驅(qū)使細(xì)胞發(fā)生二次突變，從而獲得高增殖的特性，導(dǎo)致細(xì)胞異常克隆并增殖成癌[1,3]。

3 調(diào)控rDNA轉(zhuǎn)錄的表觀遺傳異常可能參與腫瘤發(fā)生

已分化細(xì)胞中大約一半的rDNA基因處于異染色質(zhì)化的沉默狀態(tài)[32]。最新研究提示，影響rDNA轉(zhuǎn)錄的表觀調(diào)控機(jī)制異?？赡苁前┌Y發(fā)生的驅(qū)動(dòng)因素，rDNA的表觀遺傳調(diào)控基因突變也可觸發(fā)腫瘤發(fā)生[28,33]。敲減H3K4me3/H3K36me2的去甲基化酶JHDM1B(JmjC domain-containing histone demethylase 1B, JHDM1B)能夠誘導(dǎo)rDNA的表觀遺傳修飾發(fā)生重塑，導(dǎo)致轉(zhuǎn)化與未轉(zhuǎn)化的乳腺上皮細(xì)胞更具侵襲性[33]。NoRC復(fù)合物負(fù)責(zé)建立和維持rDNA基因的沉默態(tài)，敲除其亞基成分將導(dǎo)致沉默rDNA及大小衛(wèi)星序列的不穩(wěn)定，rDNA的轉(zhuǎn)錄水平增高，從而誘導(dǎo)正常NIH3T3細(xì)胞出現(xiàn)轉(zhuǎn)化表型[28]。

外顯子測(cè)序結(jié)果顯示，超過(guò)60%的胰腺神經(jīng)內(nèi)分泌瘤(pancreatic neuroendocrine tumor, PNETs)突變?nèi)笔Я酥辽僖韵?種基因之一：(multiple endocrine neoplasia type 1)、(death domain- associated protein)、(alpha- thalassemia/mental retardation X-linked syndrome protein)[34]。免疫熒光原位雜交實(shí)驗(yàn)證實(shí)，這些突變與腫瘤的ALT表型(alternative lengthening of telomeres，腫瘤細(xì)胞為達(dá)到永生而采取的端粒延長(zhǎng)機(jī)制)相關(guān)[34,35]。與胰腺神經(jīng)內(nèi)分泌瘤具有相似突變特點(diǎn)的是膠質(zhì)瘤。例如在兒童多形性膠質(zhì)母細(xì)胞瘤(glioblastoma multiforme, GBM)中，、與(編碼組蛋白變體H3.3的基因之一)經(jīng)常突變，突變頻率可高達(dá)45%，且與GBM細(xì)胞的ALT表型有很強(qiáng)相關(guān)性[36]。又如在低級(jí)別膠質(zhì)瘤(low-grade gliomas, LGGs)和繼發(fā)性膠質(zhì)母細(xì)胞瘤中，突變還經(jīng)常與(Isocitrate dehydro-genase 1/NADP+)、突變伴隨發(fā)生[37]。的蛋白產(chǎn)物Menin是組蛋白甲基轉(zhuǎn)移酶復(fù)合物的組成成分，參與組蛋白甲基轉(zhuǎn)移酶MLL (mixed lineage leukemia)、PRMT5 (protein arginine methyl-ransferase 5)、SUV39H1 (suppressor of variegation 3-9 homolog protein 1)的功能[38]；ATRX是SWI/SNF家族成員的染色質(zhì)重塑ATP酶，與H3.3特異分子伴侶DAXX組成復(fù)合物，將H3.3沉積于異染色質(zhì)區(qū)，建立H3K9me3修飾，從而維持染色質(zhì)結(jié)構(gòu)穩(wěn)定[39~41]；突變將會(huì)促使全基因組范圍的DNA和組蛋白的高甲基化狀態(tài)[42]?？紤]到和等基因都具有表觀遺傳調(diào)控的作用，因此該類基因的突變可能重塑了某些腫瘤相關(guān)基因的表觀遺傳修飾狀態(tài)，有利于腫瘤的發(fā)生發(fā)展[43]。

雖然突變可以通過(guò)誘導(dǎo)重復(fù)序列的異常DNA重組而導(dǎo)致ALT表型的出現(xiàn)，使腫瘤細(xì)胞永生化[44,45]，但ALT并不是腫瘤發(fā)生的機(jī)制。突變?nèi)笔Т龠M(jìn)腫瘤發(fā)生的機(jī)制可能是通過(guò)誘導(dǎo)基因轉(zhuǎn)錄水平改變、DNA修復(fù)機(jī)制異常來(lái)實(shí)現(xiàn)的[39]。ATRX綜合征患者顯示亞端粒區(qū)和rDNA基因的DNA甲基化水平降低，特別是rDNA的CpG島區(qū)，這說(shuō)明ATRX的染色質(zhì)重塑活性與rDNA的甲基化修飾相關(guān)，突變可能對(duì)rDNA的表達(dá)造成影響[46]。本實(shí)驗(yàn)室利用NCBI-GEO數(shù)據(jù)庫(kù)中小鼠胚胎干細(xì)胞(mouse embryonic stem cell, mESC)的H3.3與H3K9me3的reChIP-seq (連續(xù)染色質(zhì)免疫共沉淀與測(cè)序技術(shù))數(shù)據(jù)[GSE59189][47]，采取最近生物信息學(xué)分析方法[48]重新分析單個(gè)rDNA序列單元上的富集信息。通過(guò)分析H3K9甲基轉(zhuǎn)移酶野生型和敲除時(shí)H3.3與H3K9me3共富集的情況，結(jié)果發(fā)現(xiàn)H3.3與H3K9me3主要共富集于rDNA的啟動(dòng)子和編碼區(qū)，這種共富集狀態(tài)明顯受敲除的影響。本實(shí)驗(yàn)室還證明及能夠在rDNA的啟動(dòng)子和編碼區(qū)特異性富集，且隨細(xì)胞分化過(guò)程中沉默rDNA的比例增加而增多。本實(shí)驗(yàn)室證明了沉默態(tài)rDNA的建立和維持需要ATRX/DAXX復(fù)合物與H3.3介導(dǎo)的H3K9me3修飾，敲除將導(dǎo)致rDNA啟動(dòng)子區(qū)的甲基化水平降低，UBF結(jié)合的活躍rDNA拷貝數(shù)量增加等現(xiàn)象(待發(fā)表)。在其他報(bào)道中，A缺失明顯促進(jìn)膠質(zhì)母細(xì)胞瘤的生長(zhǎng)、縮短小鼠的生存期[49]。在胰腺神經(jīng)內(nèi)分泌瘤的臨床樣本中，低表達(dá)與更高的(增殖相關(guān)的基因)、更高的WHO分級(jí)明顯相關(guān)[50]。以上結(jié)果共同提示，或缺失會(huì)誘導(dǎo)rDNA轉(zhuǎn)錄活性增加，可能是腫瘤發(fā)生發(fā)展的新機(jī)制(圖1)。

4 結(jié)語(yǔ)與展望

核糖體的快速合成是癌細(xì)胞增殖侵襲的基礎(chǔ)。核糖體蛋白除了被認(rèn)為參與核糖體的生物合成外，還具有多種涉及腫瘤發(fā)生的核糖體外功能，因此核糖體生物合成過(guò)程直接或間接的與腫瘤發(fā)生相關(guān)。多種腫瘤細(xì)胞通過(guò)基因突變、代謝方式改變來(lái)促進(jìn)核糖體的生物合成過(guò)程，滿足細(xì)胞代謝需求。ATRX/ DAXX復(fù)合物與rDNA的沉默相關(guān)，然而其對(duì)rDNA轉(zhuǎn)錄的表觀調(diào)控作用是否與ATRX或DAXX經(jīng)常突變的腫瘤發(fā)生直接相關(guān)，還需更多的探索。目前，抑制核糖體生物合成已成為抗癌藥物研發(fā)的重要路徑。抑制核糖體生物合成將有利于大量游離的RPs結(jié)合MDM2，激活抑癌基因并抑制致癌基因；另外核糖體生物合成不足將阻礙腫瘤細(xì)胞的蛋白質(zhì)合成過(guò)程，通過(guò)影響細(xì)胞周期、細(xì)胞凋亡等其他途徑，綜合抑制腫瘤細(xì)胞的生長(zhǎng)[51]。抑制核糖體生物合成的藥物比起通過(guò)引發(fā)DNA損傷來(lái)激活、誘導(dǎo)細(xì)胞凋亡的傳統(tǒng)方式，其基因毒性的副作用更小[52]。目前抑制核糖體生物合成的治療藥物主要靶向抑制PolⅠ、翻譯起始因子EIF4A、EIF4e、EIF2S1和信號(hào)通路mTOR/PI3K，從而抑制核糖體的產(chǎn)生和蛋白翻譯的起始階段[53]，未來(lái)還可根據(jù)rDNA轉(zhuǎn)錄的表觀遺傳調(diào)控環(huán)節(jié)的異常來(lái)研發(fā)新藥。

[1] De Keersmaecker K, Sulima SO, Dinman JD. Ribosom-opathies and the paradox of cellular hypo- to hyperproli-feration., 2015, 125(9): 1377–1382.

[2] Ruggero D, Pandolfi PP. Does the ribosome translate cancer?, 2003, 3(3): 179–192.

[3] Sulima SO, Hofman IJF, De Keersmaecker K, Dinman JD. How ribosomes translate cancer., 2017, 7(10): 1069–1087.

[4] Kopp K, Gasiorowski JZ, Chen D, Gilmore R, Norton JT, Wang C, Leary DJ, Chan EK, Dean DA, Huang S. Pol I transcription and pre-rRNA processing are coordinated in a transcription-dependent manner in mammalian cells., 2007, 18(2): 394–403.

[5] Cheng XR, Wei YJ, Huang XW, Wang N, Jiang Q, Liu H, Lei L. Research progress on epigenetic regulation of rDNA transcription., 2018,45(5): 485–493.程香榮, 魏艷軍, 黃星衛(wèi), 王楠, 姜琦, 劉惠, 雷蕾. 表觀調(diào)控rDNA轉(zhuǎn)錄的研究進(jìn)展. 生物化學(xué)與生物物理進(jìn)展, 2018,45(5): 485–493.

[6] Grummt I. Wisely chosen paths--regulation of rRNA synthesis: delivered on 30 june 2010 at the 35th FEBS congress in gothenburg, sweden., 2010, 277(22): 4626–4639.

[7] Derenzini M, Montanaro L, Trerè D. Ribosome biogenesis and cancer., 2017, 119(3): 190–197.

[8] Derenzini M. The AgNORs., 2000, 31, 117–120.

[9] McStay B, Grummt I. The epigenetics of rRNA genes: from molecular to chromosome biology., 2008, 24, 131–157.

[10] Xie W, Ling T, Zhou Y, Feng W, Zhu Q, Stunnenberg HG, Grummt I, Tao W. The chromatin remodeling complex NuRD establishes the poised state of rRNA genes charac-terized by bivalent histone modifications and altered nucleosome positions., 2012, 109(21): 8161–8166.

[11] Xu X, Xiong X, Sun Y. The role of ribosomal proteins in the regulation of cell proliferation, tumorigenesis, and genomic integrity., 2016, 59(7): 656– 672.

[12] Warner JR, McIntosh KB. How common are extraribo-somal functions of ribosomal proteins?, 2009, 34(1): 3–11.

[13] Zhou X, Liao JM, Liao WJ, Lu H. Scission of the p53-MDM2 Loop by ribosomal proteins., 2012, 3(3–4): 298–310.

[14] de Las Heras-Rubio A, Perucho L, Paciucci R, Vilardell J, LLeonart ME. Ribosomal proteins as novel players in tumorigenesis., 2014, 33(1): 115– 141.

[15] Thomas G. An encore for ribosome biogenesis in the control of cell proliferation., 2000, 2(5): E71–72.

[16] Derenzini M, Montanaro L, Chillà A, Tosti E, Vici M, Barbieri S, Govoni M, Mazzini G, Treré D. Key role of the achievement of an appropriate ribosomal RNA comple-ment for G1-S phase transition in H4-II-E-C3 rat hepatoma cells., 2005, 202(2): 483–491.

[17] Zhu J, Blenis J, Yuan J. Activation of PI3K/Akt and MAPK pathways regulates Myc-mediated transcription by phosphorylating and promoting the degradation of Mad1., 2008, 105(18), 6584–6589.

[18] Mayer C, Grummt I. Ribosome biogenesis and cell growth: mTOR coordinates transcription by all three classes of nuclear RNA polymerases., 2006, 25(48): 6384–6391.

[19] Levine AJ. p53, the cellular gatekeeper for growth and division., 1997, 88(3): 323–331.

[20] Donati G, Bertoni S, Brighenti E, Vici M, Treré D, Volarevic S, Montanaro L, Derenzini M. The balance between rRNA and ribosomal protein synthesis up- and downregulates the tumour suppressor p53 in mammalian cells., 2011, 30(29): 3274–3288.

[21] Donati G, Peddigari S, Mercer CA, Thomas G. 5S ribosomal RNA is an essential component of a nascent ribosomal precursor complex that regulates the Hdm2-p53 checkpoint., 2013, 4(1): 87–98.

[22] Boulon S, Westman BJ, Hutten S, Boisvert FM, Lamond AI. The nucleolus under stress., 2010, 40(2): 216–227.

[23] Russo A, Russo G. Ribosomal proteins control or bypass p53 during nucleolar stress., 2017, 18(1).

[24] Brighenti E, Calabrese C, Liguori G, Giannone FA, Trerè D, Montanaro L, Derenzini M. Interleukin 6 downregu-lates p53 expression and activity by stimulating ribosome biogenesis: a new pathway connecting inflammation to cancer., 2014, 33(35): 4396–4406.

[25] Roberts DL, Dive C, Renehan AG. Biological mechanisms linking obesity and cancer risk: new perspectives., 2010, 61, 301–316.

[26] Giovannucci E, Harlan DM, Archer MC, Bergenstal RM, Gapstur SM, Habel LA, Pollak M, Regensteiner JG, Yee D. Diabetes and cancer: a consensus report., 2010, 33(7): 1674–1685.

[27] Candido J, Hagemann T. Cancer-related inflammation., 2013, 33, S79–S84.

[28] Guetg C, Lienemann P, Sirri V, Grummt I, Hernandez- Verdun D, Hottiger MO, Fussenegger M, Santoro R. The NoRC complex mediates the heterochromatin formation and stability of silent rRNA genes and centromeric repeats., 2010, 29(13): 2135–2146.

[29] Rossetti S, Hoogeveen AT, Esposito J, Sacchi N. Loss of MTG16a (CBFA2T3), a novel rDNA repressor, leads to increased ribogenesis and disruption of breast acinar morphogenesis., 2010, 14(8), 2186–2186.

[30] Barlow JL, Drynan LF, Trim NL, Erber WN, Warren AJ, McKenzie AN. New insights into 5q- syndrome as a ribosomopathy., 2010, 9(21): 4286–4293.

[31] Doherty L, Sheen MR, Vlachos A, Choesmel V, O'Donohue MF, Clinton C, Schneider HE, Sieff CA, Newburger PE, Ball SE,Niewiadomska E, Matysiak M, Glader B, Arceci RJ, Farrar JE, Atsidaftos E, Lipton JM, Gleizes PE, Gazda HTRibosomal protein genes RPS10 and RPS26 are commonly mutated in Diamond-Blackfan anemia., 2010, 86(2): 222–228.

[32] Grummt I. Different epigenetic layers engage in complex crosstalk to define the epigenetic state of mammalian rRNA genes., 2007, 16 Spec No 1, R21–27.

[33] Galbiati A, Penzo M, Bacalini MG, Onofrillo C, Guerrieri AN, Garagnani P, Franceschi C, Trere D, Montanaro L. Epigenetic up-regulation of ribosome biogenesis and more aggressive phenotype triggered by the lack of the histone demethylase JHDM1B in mammary epithelial cells., 2017, 8(23): 37091–37103.

[34] Jiao Y, Shi C, Edil BH, de Wilde RF, Klimstra DS, Maitra A, Schulick RD, Tang LH, Wolfgang CL, Choti MA,Velculescu VE, Diaz LA Jr, Vogelstein B, Kinzler KW, Hruban RH, Papadopoulos NDAXX/ATRX, MEN1, and mTOR pathway genes are frequently altered in pancreatic neuroendocrine tumors., 2011, 331(6021): 1199– 1203.

[35] Heaphy CM, de Wilde RF, Jiao Y, Klein AP, Edil BH, Shi C, Bettegowda C, Rodriguez FJ, Eberhart CG, Hebbar S, Offerhaus GJ, McLendon R, Rasheed BA, He Y, Yan H, Bigner DD, Oba-Shinjo SM, Marie SK, Riggins GJ, Kinzler KW, Vogelstein B, Hruban RH, Maitra A, Papadopoulos N, Meeker AKAltered telomeres in tumors with ATRX and DAXX mutations., 2011, 333 (6041): 425.

[36] Schwartzentruber J, Korshunov A, Liu XY, Jones DT, Pfaff E, Jacob K, Sturm D, Fontebasso AM, Quang DA, T?njes M, Hovestadt V, Albrecht S, Kool M, Nantel A, Konermann C, Lindroth A, J?ger N, Rausch T, Ryzhova M, Korbel JO, Hielscher T, Hauser P, Garami M, Klekner A, Bognar L, Ebinger M, Schuhmann MU, Scheurlen W, Pekrun A, Frühwald MC, Roggendorf W, Kramm C, Dürken M, Atkinson J, Lepage P, Montpetit A, Zakrzewska M, Zakrzewski K, Liberski PP, Dong Z, Siegel P, Kulozik AE, Zapatka M, Guha A, Malkin D, Felsberg J, Reifenberger G, von Deimling A, Ichimura K, Collins VP, Witt H, Milde T, Witt O, Zhang C, Castelo-Branco P, Lichter P, Faury D, Tabori U, Plass C, Majewski J, Pfister SM, Jabado NDriver mutations in histone H3.3 and chromatin remode-lling genes in paediatric glioblastoma., 2012, 482(7384): 226–231.

[37] Kannan K, Inagaki A, Silber J, Gorovets D, Zhang J, Kastenhuber ER, Heguy A, Petrini JH, Chan TA, Huse JT. Whole-exome sequencing identifies ATRX mutation as a key molecular determinant in lower-grade glioma., 2012, 3(10): 1194–1203.

[38] Feng Z, Ma J, Hua X. Epigenetic regulation by the menin pathway., 2017, 24(10): T147– T159.

[39] Dyer MA, Qadeer ZA, Valle-Garcia D, Bernstein E. ATRX and DAXX: Mechanisms and Mutations.,2017, 7(3):DOI: 10.1101/cshperspect.a026567.

[40] Goldberg AD, Banaszynski LA, Noh KM, Lewis PW, Elsaesser SJ, Stadler S, Dewell S, Law M, Guo X, Li X, Wen D, Chapgier A, DeKelver RC, Miller JC, Lee YL, Boydston EA, Holmes MC, Gregory PD, Greally JM, Rafii S, Yang C, Scambler PJ, Garrick D, Gibbons RJ, Higgs DR, Cristea IM, Urnov FD, Zheng D, Allis CDDistinct factors control histone variant H3.3 localization at specific genomic regions., 2010, 140(5): 678–691.

[41] Huang XW, Cheng XR, Wang N, Zhang YW, Liao C, Jin LH, Lei L. Histone variant H3.3 and its functions in reprogramming., 2018, 40(3): 186–196.黃星衛(wèi), 程香榮, 王楠, 張雨薇, 廖辰, 金連弘, 雷蕾.組蛋白H3變體H3.3及其在細(xì)胞重編程中的作用. 遺傳, 2018, 40(3): 186–196.

[42] Liu X, Ling ZQ. Role of isocitrate dehydrogenase 1/2 (IDH 1/2) gene mutations in human tumors., 2015, 30(10): 1155–1160.

[43] Elsasser SJ, Allis CD, Lewis PW. Cancer. New epigenetic drivers of cancers., 2011, 331(6021): 1145–1146.

[44] Clynes D, Jelinska C, Xella B, Ayyub H, Scott C, Mitson M, Taylor S, Higgs DR, Gibbons RJ. Suppression of the alternative lengthening of telomere pathway by the chromatin remodelling factor ATRX., 2015, 6, 7538.

[45] He J, Mansouri A, Das S. Alpha thalassemia/mental retardation syndrome X-linked, the alternative lengthening of telomere phenotype, and gliomagenesis: current understandings and future potential., 2017, 7, 322.

[46] Gibbons RJ, McDowell TL, Raman S, O'Rourke DM, Garrick D, Ayyub H, Higgs DR. Mutations in ATRX, encoding a SWI/SNF-like protein, cause diverse changes in the pattern of DNA methylation., 2000, 24(4): 368–371.

[47] Els?sser SJ, Noh KM, Diaz N, Allis CD, Banaszynski LA. Histone H3.3 is required for endogenous retroviral element silencing in embryonic stem cells., 2015, 522(7555): 240–244.

[48] Mars JC, Sabourin-Felix M, Tremblay MG, Moss T. A deconvolution protocol for ChIP-Seq reveals analogous enhancer structures on the mouse and human ribosomal RNA genes., 2018, 8(1): 303–314.

[49] Koschmann C, Calinescu AA, Nunez FJ, Mackay A, Fazal-Salom J, Thomas D, Mendez F, Kamran N, Dzaman M, Mulpuri L, Krasinkiewicz J, Doherty R, Lemons R, Brosnan-Cashman JA, Li Y, Roh S, Zhao L, Appelman H, Ferguson D, Gorbunova V, Meeker A, Jones C, Lowenstein PR, Castro MGATRX loss promotes tumor growth and impairs nonhomologous end joining DNA repair in glioma., 2016, 8(328): 328ra28.

[50] Ueda H, Akiyama Y, Shimada S, Mogushi K, Serizawa M, Matsumura S, Mitsunori Y, Aihara A, Ban D, Ochiai T, Kudo A, Tanabe M, Tanaka STumor suppressor functions of DAXX through histone H3.3/H3K9me3 pathway in pancreatic NETs., 2018, 25(6): 619– 631.

[51] Zhou X, Hao Q, Liao J, Zhang Q, Lu H. Ribosomal protein S14 unties the MDM2-p53 loop upon ribosomal stress., 2013, 32(3): 388–396.

[52] Woods SJ, Hannan KM, Pearson RB, Hannan RD. The nucleolus as a fundamental regulator of the p53 response and a new target for cancer therapy., 2014, 1849(7): 821–829.

[53] Pelletier J, Thomas G, Volarevi? S. Ribosome biogenesis in cancer: new players and therapeutic avenues., 2018, 18(1): 51–63.

The epigenetic regulation of ribosomal DNA and tumorigenesis

Xiangrong Cheng, Xinglin Hu, Qi Jiang, Xingwei Huang, Nan Wang, Lei Lei

Recent research in epigenetics suggests that defects in epigenetic regulation of ribosomal DNA (rDNA) transcription may contribute to tumorigenesis. ATRX/DAXX complex is involved in the establishment and maintenance of the silence of the rDNA gene through H3K9me3 modification at histone variant H3.3. The ATRX/DAXX-related genes are frequently mutated in some types of tumors, which may increase rDNA transcription and promote cancer development and progression. In this review, we focus on the mechanism that abnormal transcription of rDNA potentially influences tumorigenesis. We also summarize the epigenetic regulatory mechanism of rDNA transcription, which may provide new theoretical support for drug development based on rDNA transcriptional regulation.

rDNA; epigenetic regulation; H3.3; ATRX/DAXX; tumorigenesis

2018-10-15;

2019-01-15

國(guó)家自然科學(xué)基金項(xiàng)目(編號(hào)：31671545) 資助[Supported by the National Natural Science Foundation of China (No. 31671545)]

程香榮，在讀碩士研究生，專業(yè)方向：基礎(chǔ)醫(yī)學(xué)。E-mail: xiangrongcheng@foxmail.com

雷蕾，博士，教授，研究方向：生物發(fā)育學(xué)。E-mail: leiys2002@yahoo.com

10.16288/j.yczz.18-244

2019/2/25 15:21:42

URI: http://kns.cnki.net/kcms/detail/11.1913.R.20190225.1521.002.html

(責(zé)任編委: 宋旭)