鄭 潔，郭海英，潘思京，張鵬飛

(1.南京中醫(yī)藥大學(xué) 第二臨床醫(yī)學(xué)院，江蘇 南京 210023； 2.陜西中醫(yī)學(xué)院 針灸推拿系，陜西 咸陽 712046)

短篇綜述

表觀遺傳調(diào)控與骨質(zhì)疏松癥的研究進(jìn)展

鄭 潔1，2，郭海英1*，潘思京1，張鵬飛1

(1.南京中醫(yī)藥大學(xué) 第二臨床醫(yī)學(xué)院，江蘇 南京 210023； 2.陜西中醫(yī)學(xué)院 針灸推拿系，陜西 咸陽 712046)

表觀遺傳指所有不通過 DNA序列改變就能影響基因表達(dá)的、可遺傳的調(diào)控方式，包括 DNA甲基化、組蛋白修飾、染色質(zhì)重塑和微小RNA(miRNA)等。骨質(zhì)疏松的發(fā)生與環(huán)境和遺傳因素的相互作用密不可分，其中涉及多種表觀遺傳調(diào)控機(jī)制。

表觀遺傳; 骨質(zhì)疏松癥; DNA甲基化; 組蛋白修飾; 微小RNA

骨不僅具有支持軀體、保護(hù)內(nèi)臟器官的作用，還兼具多種代謝功能，尤其在維持機(jī)體礦物質(zhì)平衡中發(fā)揮著重要作用。骨組織自發(fā)育階段就始終處于骨吸收與骨形成不斷循環(huán)的動(dòng)態(tài)平衡，這一過程被稱為骨重塑。當(dāng)骨吸收大于骨形成時(shí)，就會(huì)引起骨量減少，進(jìn)而導(dǎo)致骨質(zhì)疏松癥的發(fā)生。表觀遺傳(epigenetics)指所有不通過 DNA序列改變就能影響基因表達(dá)的、可遺傳的調(diào)控方式，包括 DNA甲基化、組蛋白修飾、染色質(zhì)重塑和微小RNA(miRNA)等[1]。近年的研究發(fā)現(xiàn)，表觀遺傳機(jī)制在包括骨質(zhì)疏松癥在內(nèi)的多種疾病的發(fā)生中起到了不可忽視的作用。本文將近年來表觀遺傳機(jī)制在骨代謝及骨質(zhì)疏松癥發(fā)病中的研究進(jìn)展作一綜述。

1 表觀遺傳調(diào)控與骨形成

骨形成是間充質(zhì)干細(xì)胞(mesenchymal stem cells, MSCs) 通過遷移、增殖、分化，在骨吸收部位形成成骨細(xì)胞，并介導(dǎo)新骨組織形成的過程，其中MSCs 的成骨分化是骨形成的核心過程[2]。研究發(fā)現(xiàn)，MSCs 的成骨分化涉及多種復(fù)雜的表觀遺傳調(diào)控機(jī)制，包括DNA甲基化、miRNA和組蛋白修飾。

1.1 DNA甲基化與成骨分化

DNA甲基化主要發(fā)生在CpG雙核酸位點(diǎn)。通常DNA甲基化會(huì)抑制所調(diào)控基因的表達(dá)而在組織特異性基因沉默、基因組印記及染色體穩(wěn)定性維持等過程中發(fā)揮重要功能。堿性磷酸酶(alkaline phosphatase,ALP)是由成骨細(xì)胞分泌并催化骨礦化的酶，其活性在成骨細(xì)胞保持較高水平而在骨細(xì)胞逐漸降低。研究發(fā)現(xiàn)，人成骨細(xì)胞和骨細(xì)胞的ALP啟動(dòng)子區(qū)具有相反的DNA甲基化譜，前者ALP啟動(dòng)子區(qū)呈低甲基化，后者ALP啟動(dòng)子區(qū)呈高甲基化，表明DNA甲基化途徑可抑制ALP在骨細(xì)胞的表達(dá)[3]。成骨細(xì)胞骨轉(zhuǎn)移中，硬化蛋白編碼基因(SOST)在成骨細(xì)胞的低表達(dá)與SOST啟動(dòng)子區(qū)高甲基化密切相關(guān)[4]。骨髓MSCs中核心結(jié)合因子(Runx2)、骨鈣蛋白(BGLAP)及骨橋蛋白的高表達(dá)常伴隨過渡性CpG島低甲基化，表明CpG島的低甲基化參與了成骨分化的調(diào)控[5]。

1.2 miRNA與成骨分化

miRNA是一種普遍存在的內(nèi)源性單鏈小分子非編碼蛋白質(zhì)的RNA，由21～25個(gè)核苷酸組成，miRNA通過與目的基因結(jié)合介導(dǎo)轉(zhuǎn)錄后基因調(diào)控。miRNA陣列分析顯示，MSCs向成骨細(xì)胞分化過程中，一些miRNA表達(dá)水平的變化會(huì)影響目的基因的轉(zhuǎn)錄，進(jìn)而調(diào)控MSCs成骨分化[6]?，F(xiàn)已證實(shí)，Runx2和Smads等關(guān)鍵成骨轉(zhuǎn)錄因子的表達(dá)均受到miRNA的調(diào)控。miR-204和其同源物miR-211，以及miR-133、miR-135b等對(duì)Runx2的表達(dá)具有抑制作用[7-8]。反過來，Runx2也調(diào)控一些參與成骨過程的miRNA的表達(dá)[9]。此外，miR-23a ～ 27a ～ 24-2復(fù)合物可通過對(duì)人特異性富含AT序列結(jié)合蛋白2(SATB2)的負(fù)向調(diào)控抑制成骨過程[9]。miR-29a和miR-29c參與調(diào)節(jié)Wnt信號(hào)通路并抑制骨結(jié)合素的表達(dá)[10]。與以上miRNA對(duì)成骨分化的負(fù)向調(diào)控作用相反，還有一些miRNA對(duì)成骨過程具有促進(jìn)作用。miR-335-5p抑制Dkk1(Wnt信號(hào)通路的抑制劑)，提高Wnt信號(hào)通路活性，促進(jìn)成骨分化[11]。

此外，組蛋白修飾及染色質(zhì)重塑在成骨分化進(jìn)程中也發(fā)揮了不可忽視的調(diào)節(jié)作用。多種成骨相關(guān)的轉(zhuǎn)錄因子可在目標(biāo)啟動(dòng)子區(qū)誘導(dǎo)染色質(zhì)重塑[12]。組蛋白上的很多氨基酸可以通過各種翻譯后的可逆的共價(jià)鍵修飾，包括甲基化、乙?；?、磷酸化和泛素化等，形成理論上數(shù)目繁多的特定的“組蛋白密碼”來形成“開放”或“關(guān)閉”的局部染色質(zhì)結(jié)構(gòu)，或是決定何種蛋白結(jié)合到特定 DNA 區(qū)域，從而調(diào)節(jié)多種 DNA 功能。研究發(fā)現(xiàn)，Runx2、AP-1、ATF4和Smads等成骨相關(guān)基因均受組蛋白修飾調(diào)控[13]。

2 表觀遺傳調(diào)控與骨吸收

2.1 DNA甲基化、組蛋白修飾與破骨細(xì)胞分化

破骨細(xì)胞是唯一的骨吸收細(xì)胞，破骨細(xì)胞激活構(gòu)成了骨重塑的重要環(huán)節(jié)。破骨細(xì)胞分化是由成骨細(xì)胞系統(tǒng)啟動(dòng)，后者通過釋放細(xì)胞因子及其他一些因子調(diào)控破骨細(xì)胞功能及其分化[14]。其中，骨保護(hù)蛋白(OPG)、NF-кB受體激活劑(RANK)和NF-кB受體激活劑配體(RANKL)構(gòu)成了成骨細(xì)胞調(diào)節(jié)破骨細(xì)胞生成及功能發(fā)揮的重要系統(tǒng)，多種細(xì)胞因子或激素最終通過RANKL-RANK-OPG系統(tǒng)完成對(duì)破骨細(xì)胞的分化、成熟的調(diào)控。研究發(fā)現(xiàn)，DNA甲基化依賴機(jī)制通過影響人骨組織中RANKL和OPG的基因轉(zhuǎn)錄參與調(diào)控破骨細(xì)胞分化[15-16]。

組蛋白去乙?；敢种苿┑捏w外實(shí)驗(yàn)證實(shí)，組蛋白乙?；矃⑴c了破骨細(xì)胞的活性的調(diào)節(jié)[17]。 RANKL與其受體RANK(位于破骨細(xì)胞前體的細(xì)胞膜上)結(jié)合后，可誘導(dǎo)激活性T細(xì)胞細(xì)胞核因子(NFATc1)的活化及核轉(zhuǎn)移，通過誘導(dǎo)多種目的基因的表達(dá)促進(jìn)破骨細(xì)胞的分化[18]，而NFATc1活性可能受到Jmjd3(一種組蛋白去甲基化酶)及miR-146a的調(diào)控[19-20]。

2.2 miRNA與破骨細(xì)胞分化

除了DNA甲基化和組蛋白翻譯后修飾，miRNA在破骨細(xì)胞發(fā)生中也發(fā)揮了正性或負(fù)性的調(diào)控作用。miRNA的生成必須通過Dicer酶(雙鏈RNA專一的RNA內(nèi)切酶)對(duì)初始miRNA(pri-miRNA)的切割。體外實(shí)驗(yàn)發(fā)現(xiàn)，破骨細(xì)胞特異性Dicer酶的缺失可抑制破骨細(xì)胞介導(dǎo)的骨吸收過程，提示miRNA在破骨細(xì)胞生成及骨吸收過程中發(fā)揮了正性調(diào)控作用[21]。還有研究發(fā)現(xiàn)，miR-21 和 miR-155可下調(diào)一些破骨細(xì)胞分化抑制性基因的表達(dá)而促進(jìn)破骨細(xì)胞的分化[22-23]。此外，不僅成骨細(xì)胞可調(diào)節(jié)破骨細(xì)胞的活性，破骨細(xì)胞也能影響成骨分化，其機(jī)制可能涉及miRNA調(diào)控[24]。

3 表觀遺傳調(diào)控與骨質(zhì)疏松癥

以上研究表明，表觀遺傳機(jī)制在很大程度上決定著細(xì)胞分化的方向，在骨重塑中發(fā)揮著重要作用。骨質(zhì)疏松癥的發(fā)生與遺傳因素和環(huán)境因素的相互作用密不可分，其中涉及多種表觀遺傳學(xué)變化。大鼠孕產(chǎn)期飲食限制會(huì)引起編碼糖皮質(zhì)激素受體和過氧化物酶體增殖物激活受體(PPARα)基因的甲基化水平改變，這一變化會(huì)持續(xù)到母鼠斷奶后并可遺傳給下一代[25-26]。有人報(bào)道，3種DNA多態(tài)性可以改變調(diào)控人成纖維細(xì)胞生長(zhǎng)因子2(FGF2)基因水平的特異性miRNA的結(jié)合親和力，這有助于判斷骨質(zhì)疏松癥的易感性[27]。局部黏著激酶(FAK)信號(hào)途徑是成骨分化中骨力學(xué)信號(hào)傳導(dǎo)的一個(gè)關(guān)鍵環(huán)節(jié)，miR-138可抑制FAK的表達(dá)，阻礙成骨過程，與骨質(zhì)疏松的發(fā)生密切相關(guān)[28]。

4 展望

以上研究證實(shí)，表觀遺傳機(jī)制影響著骨骼發(fā)育及骨質(zhì)疏松癥發(fā)病的風(fēng)險(xiǎn)。

但DNA甲基化、組蛋白修飾和miRNA表達(dá)參與骨質(zhì)疏松癥發(fā)病的具體機(jī)制還有待闡明。表觀遺傳微陣列、高通量測(cè)序等新技術(shù)的廣泛應(yīng)用將有助于在全基因組基礎(chǔ)上建立一套完整的關(guān)于正常骨和骨疾病的表觀遺傳譜。疾病特異性表觀遺傳標(biāo)志物的識(shí)別將有助于臨床診斷與預(yù)防。DNA甲基化/去甲基化和miRNA比RNA更加穩(wěn)定，可嘗試被應(yīng)用于骨質(zhì)疏松癥的臨床治療。同時(shí)，現(xiàn)有調(diào)控骨代謝的治療手段也同樣值得深入研究，以揭示其可能存在的表觀遺傳調(diào)節(jié)機(jī)制。

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Progress of epigenetic regulation in osteoporosis

ZHENG Jie1，2, GUO Hai-ying1*, PAN Si-jing1, ZHANG Peng-fei1

(1.the Second Institute of Clinical Medicine, Nanjing Universtiy of Chinese Medicine, Nanjing 210023;2.Dept. of Acupuncture and Moxibustion, Shanxi Universtiy of Chinese Medicine, Xianyang 712046, China)

Epigenetics is involved in heritable modifications such as DNA methylation, histone modification, chromatin remodeling, as well as miRNA, without changes in DNA sequence. It has been shown that occurence of osteoporosis is closely related to the interactions between the genome and enviroment, in which epigenetic regulation plays an important role.

epigenetics; osteoporosis; DNA methylation; histone modification; miRNA

2013-03-18

2013-05-07

*通信作者(correspondingauthor)： ghying63@126.com

1001-6325(2014)03-0406-04

R 683

A