表觀遺傳學(xué)
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表觀遺傳學(xué)又稱“擬遺傳學(xué)”、“表遺傳學(xué)”、“外遺傳學(xué)”以及“后遺傳學(xué)”(英文epigenetics),在生物學(xué)和特定的遺傳學(xué)領(lǐng)域,其研究的是在不改變DNA序列的前提下,通過某些機(jī)制引起可遺傳的基因表達(dá)或細(xì)胞表現(xiàn)型的變化[1]。
表觀遺傳學(xué)是20世紀(jì)80年代逐漸興起的一門學(xué)科,是在研究與經(jīng)典的孟德爾遺傳學(xué)遺傳法則不相符的許多生命現(xiàn)象過程中逐步發(fā)展起來的。
表觀遺傳現(xiàn)象包括DNA甲基化、RNA干擾、組蛋白修飾等。與經(jīng)典遺傳學(xué)以研究基因序列影響生物學(xué)功能為核心相比,表觀遺傳學(xué)主要研究這些“表觀遺傳現(xiàn)象”建立和維持的機(jī)制。其研究?jī)?nèi)容主要包括兩類,一類為基因選擇性轉(zhuǎn)錄表達(dá)的調(diào)控,有DNA甲基化、基因印記、組蛋白共價(jià)修飾和染色質(zhì)重塑;另一類為基因轉(zhuǎn)錄后的調(diào)控,包括基因組中非編碼RNA、微小RNA、反義RNA、內(nèi)含子及核糖開關(guān)等。
表觀遺傳學(xué)指基因組相關(guān)功能改變而不涉及核苷酸序列變化。例如DNA甲基化和組蛋白修飾,兩者均能在不改變DNA序列的前提下調(diào)節(jié)基因的表達(dá);阻遏蛋白通過結(jié)合沉默基因從而控制基因的表達(dá)。這些變化可能可以通過細(xì)胞分裂而得以保留,并且可能持續(xù)幾代。這些變化都僅是非基因因素導(dǎo)致的生物體基因表現(xiàn)(或“自我表達(dá)”)的不同[2],由于目前尚不清楚組蛋白的化學(xué)修飾是否可遺傳,有人對(duì)于用此術(shù)語(yǔ)描述組蛋白化學(xué)修飾提出了異議[3][4]。
表觀遺傳學(xué)在真核生物中主要表現(xiàn)在細(xì)胞分化過程。在胚胎形態(tài)形成過程中,全能干細(xì)胞將分化成完全不同的細(xì)胞,也就是說,一個(gè)受精卵細(xì)胞分化出各種不同類型的細(xì)胞,包括神經(jīng)細(xì)胞、肌肉細(xì)胞、上皮細(xì)胞、血管內(nèi)皮細(xì)胞等,并通過抑制其他細(xì)胞和激活相關(guān)基因而進(jìn)行持續(xù)的細(xì)胞分裂[5]。
2011年的相關(guān)研究已證實(shí),mRNA甲基化對(duì)人體內(nèi)能量平衡發(fā)揮著至關(guān)重要的作用,對(duì)RNA上的N6-甲基腺苷進(jìn)行脫甲基治療可控制FTO基因相關(guān)肥胖癥,并因此而開創(chuàng)了RNA表觀遺傳學(xué)的相關(guān)領(lǐng)域[6][7]。
目錄 |
詞源和定義
由于表觀遺傳學(xué)定義有多種,導(dǎo)致了在表觀遺傳學(xué)代表什么這一問題上出現(xiàn)了分歧。表觀遺傳學(xué)由C. H. Waddington于1942年作為后生論和遺傳學(xué)的合詞而提出[8]。
后生論是一個(gè)很古老的概念[9],現(xiàn)在更多的用于描述胚胎發(fā)育過程中的細(xì)胞分化源自干細(xì)胞的全能狀態(tài)。當(dāng)Waddington提出這一詞語(yǔ)時(shí),人們對(duì)基因的物理性質(zhì)及其在遺傳中的作用還不清楚,使用該詞語(yǔ)是表示,基因可能與環(huán)境相互作用,并產(chǎn)生表現(xiàn)型的概念。Robin Holliday將表觀遺傳學(xué)定義為“在復(fù)雜有機(jī)體的發(fā)育過程中,對(duì)基因活性在時(shí)間和空間中調(diào)控機(jī)制的研究”[10]。因此,后生論也可用于描述任何影響有機(jī)體發(fā)育的因素,而不僅僅是DNA序列。
前在科學(xué)界對(duì)表觀遺傳學(xué)有了更嚴(yán)格的定義。Arthur Riggs及其同事將其定義為,有關(guān)引起可遺傳的基因功能改變的有絲分裂和/或減數(shù)分裂的研究,這些變化以DNA序列改變無法解釋[11]。表觀遺傳學(xué)的希臘語(yǔ)前綴epi-意味著“在…之上”或“除…之外”,因此表觀遺傳學(xué)的特征是傳統(tǒng)的分子水平遺傳之上或之外的遺傳。
“表觀遺傳學(xué)”也被用于描述還未證實(shí)的組蛋白修飾的遺傳過程,因此可嘗試用更廣義的術(shù)語(yǔ)來重新定義。
例如,Adrian Bird將表觀遺傳學(xué)定義為,染色體的構(gòu)造適應(yīng),以便啟始、發(fā)出信號(hào)或保持變構(gòu)的活性狀態(tài)[12]。這個(gè)定義既包括涉及DNA修復(fù)或細(xì)胞周期的瞬態(tài)改變,也包括多代細(xì)胞的穩(wěn)態(tài)改變,但是不包含細(xì)胞膜結(jié)構(gòu)和朊病毒,除非其影響到染色體功能。但這樣的定義并不被普遍接受并仍然受到爭(zhēng)議[13]
2008年的冷泉港會(huì)議達(dá)成了關(guān)于表觀遺傳學(xué)的共識(shí),即“由染色體改變所引起的穩(wěn)定的可遺傳的表現(xiàn)型,而非DNA序列的改變[14]。
與“遺傳學(xué)”相似的詞衍生出很多平行的用法?!氨碛^基因組”是“基因組”的平行詞,指的是一個(gè)細(xì)胞的整體表觀遺傳狀態(tài)。“遺傳密碼”與“表觀遺傳密碼”對(duì)應(yīng),用于描述不同細(xì)胞產(chǎn)生不同表現(xiàn)型的一系列表觀遺傳特征。“表觀遺傳密碼”可代表細(xì)胞的總體狀態(tài),按每個(gè)分子在表觀遺傳地圖上所占的位置,可得出DNA甲基化和組蛋白修飾狀態(tài)的特定基因組區(qū)域的基因表達(dá)圖表。更典型的是,這個(gè)詞用于提及和評(píng)估特定的系統(tǒng)性措施,如組蛋白編碼或DNA甲基化模型相關(guān)的表觀遺傳學(xué)形式。 心理學(xué)家Erik Erikson在其著作中提到“后生論”,認(rèn)為后生規(guī)則是“任何生長(zhǎng)的事物都有一個(gè)平面圖,在這個(gè)圖之外各個(gè)部分先后出現(xiàn),而每個(gè)部分都有其特定的優(yōu)勢(shì)期,直至所有的部分出現(xiàn)從而形成一個(gè)功能整體。”[15]個(gè)用法有一定的歷史價(jià)值[16]。
表觀遺傳學(xué)的分子基礎(chǔ)
表觀遺傳的改變可以導(dǎo)致特定基因的激活,而不必改變DNA序列。此外,染色質(zhì)蛋白與DNA相關(guān)聯(lián)可能被激活或沉默。這是不同的細(xì)胞在多細(xì)胞有機(jī)體中只表達(dá)其活動(dòng)必需基因的原因。當(dāng)細(xì)胞進(jìn)行分裂時(shí),表觀遺傳的變化得以保存。
大多數(shù)表觀遺傳變化只發(fā)生在生物個(gè)體的一生中,但是,如果形成受精卵的精子或卵細(xì)胞發(fā)生了基因失活,那么這種表觀遺傳變化將被傳遞給下一代[17]。由此拉馬克學(xué)說提出了一個(gè)問題:這種生物體表觀遺傳的變化是否可改變DNA的基本結(jié)構(gòu)。
特殊的表觀遺傳過程包括副突變、書簽、印跡、基因沉默、X染色體失活、位置效應(yīng)、重組、縮并、母體效應(yīng)、致癌進(jìn)程、致畸劑影響、組蛋白化學(xué)修飾的調(diào)控以及異染色體和受技術(shù)局限的單性繁殖及克隆。
DNA損傷也會(huì)導(dǎo)致表觀遺傳變化[18][19][20]。DNA損傷發(fā)生頻繁,人體平均每天會(huì)發(fā)生10000次。這些損傷大部分被修復(fù),但在DNA修復(fù)時(shí)仍然可能發(fā)生表觀遺傳變化[[21]。尤其是雙鏈DNA的斷裂可能會(huì)引起未編程的表觀遺傳基因沉默,導(dǎo)致DNA甲基化和促進(jìn)沉默蛋白質(zhì)組的修飾(染色質(zhì)重構(gòu))[22]。此外,多聚二磷酸腺苷核糖酶(Parp1酶)及其產(chǎn)物多聚二磷酸腺苷核糖(PAR)在修復(fù)過程中會(huì)積聚DNA的損傷[23]。這種累積,反過來,直接補(bǔ)充和激活染色質(zhì)重塑蛋白ALC1進(jìn)而導(dǎo)致核小體的重構(gòu)[24]。而核小體的重構(gòu)會(huì)導(dǎo)致DNA修復(fù)基因MLH1的沉默[25]。能造成DNA損傷的化學(xué)物質(zhì),如苯、對(duì)苯二酚、苯乙烯、四氯化碳和三氯乙烯,可通過激活氧化應(yīng)激通路導(dǎo)致大量的DNA低甲基化[26]。
不同飲食影響老鼠表觀遺傳變化[27]。一些食物成分可增加DNA修復(fù)酶、MGMT、MLH1[27]和p53[28]和p53 [29][30])的水平,另一些食物成分如大豆異黃酮[31][32]和花青素[33]降低DNA損傷。
表觀遺傳研究廣泛使用分子生物學(xué)技術(shù),如染色質(zhì)免疫沉淀反應(yīng)、熒光原位雜交法、甲基化敏感限制酶、DNA腺嘌呤甲基轉(zhuǎn)移酶識(shí)別、亞硫酸鹽定序等,從而幫助人們更深入地理解表觀遺傳現(xiàn)象。此外,生物信息學(xué)方法也發(fā)揮著越來越重要的作用(計(jì)算表觀遺傳學(xué))。計(jì)算機(jī)模擬和分子動(dòng)力學(xué)方法揭示了原子運(yùn)動(dòng)與組蛋白尾端變構(gòu)分子的識(shí)別有關(guān).[34]。
機(jī)制
一些類型的表觀遺傳系統(tǒng)在細(xì)胞記憶中可能扮演重要角色[35],然而需注意的是,并不是所有的表觀遺傳學(xué)例子都能被普遍接受。
DNA甲基化和染色質(zhì)重組
細(xì)胞核個(gè)體的表現(xiàn)型受到自身基因轉(zhuǎn)錄的影響,因此可遺傳的轉(zhuǎn)錄能提高表觀遺傳效應(yīng)?;虮磉_(dá)分多層調(diào)控,基因調(diào)控的一種途徑是通過染色質(zhì)重組。染色質(zhì)是DNA和組蛋白結(jié)合的復(fù)合體,DNA纏繞著組蛋白球體,若DNA纏繞組蛋白的方式發(fā)生改變,基因表達(dá)也將改變。染色質(zhì)重組通過以下兩個(gè)主要機(jī)制完成:
- 第一條途徑是組成組蛋白的氨基酸的平移修改。組蛋白由長(zhǎng)鏈氨基酸構(gòu)成,如果鏈中的氨基酸改變,組蛋白的形態(tài)將發(fā)生改變。復(fù)制期間的DNA并非完全解鏈,因此,經(jīng)過修改的組蛋白可能被用于每個(gè)新復(fù)制的DNA,這些組蛋白將作為模板,以新的方式合成新形態(tài)的組蛋白。通過改變周圍蛋白的形態(tài),這些修改的組蛋白將確保分化的細(xì)胞保持分化狀態(tài),而不是重新回到干細(xì)胞狀態(tài)。
- 第二條途徑是通過增加位于CpG島上的DNA的甲基,使胞嘧啶轉(zhuǎn)化為5-甲基胞嘧啶。5-甲基胞嘧啶同正常的胞嘧啶一樣與鳥嘌呤配對(duì),然而,基因組某些區(qū)域的甲基化較多,甲基化較高的區(qū)域通過不完全清楚的機(jī)制使得轉(zhuǎn)錄的活力減小。甲基化的胞核嘧啶也可以從父母一方的生殖細(xì)胞保留在受精卵中,標(biāo)記染色體遺傳自雙親(遺傳印記)。
細(xì)胞分化過程中DNA甲基化將導(dǎo)致組蛋白性狀的變化。某些酶(如DNMT1 )對(duì)甲基化胞嘧啶有較高的親和力。如果這種酶達(dá)到DNA的“半甲基化”部分(兩條DNA鏈中只有一個(gè)甲基胞嘧啶),這種酶將催化另一部分。
雖然組蛋白修飾發(fā)生在整個(gè)序列中,非結(jié)構(gòu)化的N-末端的蛋白(稱為組蛋白尾端)特別容易被修改。這些修改包括泛素化,磷酸化和修飾作用。乙?;沁@些修飾中研究得最多的。例如,組蛋白H3尾部的K14和K9賴氨酸被組蛋白乙酰轉(zhuǎn)移酶(HATs)乙酰化通常與轉(zhuǎn)錄能力有關(guān)。
有人認(rèn)為這種與“激活的”轉(zhuǎn)錄有關(guān)的乙?;瘍A向于是一種生物物理學(xué)改變。因?yàn)橥ǔT诮M蛋白末端有一個(gè)帶正電荷的氮,賴氨酸可以與DNA主鏈帶負(fù)電荷的磷酸鹽結(jié)合。乙?;?a href="/w/%E4%BE%A7%E9%93%BE" title="側(cè)鏈">側(cè)鏈上帶正電荷的氨基團(tuán)變成中性的酰胺鍵。正電荷的去除,使DNA從組蛋白上解開。這時(shí),SWI/SNF和其他轉(zhuǎn)錄因子復(fù)合體就可以結(jié)合到DNA上使轉(zhuǎn)錄開始。這是表觀遺傳作用的“順式”模型。就是說,組蛋白尾部改變對(duì)于DNA本身有一種直接效應(yīng)。
另一種表觀遺傳作用模型是“反式”模型。在這個(gè)模型中,組蛋白尾部改變對(duì)DNA起間接作用。例如,賴氨酸乙?;梢詾槿旧|(zhì)修飾酶(和基礎(chǔ)轉(zhuǎn)錄裝置)產(chǎn)生一個(gè)結(jié)合位點(diǎn),然后該染色質(zhì)重構(gòu)體導(dǎo)致染色質(zhì)狀態(tài)改變。實(shí)際上,布羅莫結(jié)構(gòu)域——一個(gè)特異性與乙酰-賴氨酸結(jié)合的蛋白片段(域)——發(fā)現(xiàn)其幫助很多酶激活轉(zhuǎn)錄,包括SWI/SNF復(fù)合體(在polybromo蛋白上)。乙?;赡茏饔糜诖撕椭暗耐緩蕉鴰椭D(zhuǎn)錄激活。
組蛋白甲基化也證實(shí)了由相關(guān)因素導(dǎo)致的對(duì)接模塊作為一種修飾方式的推斷。組蛋白H3賴氨酸9的甲基化與組成型轉(zhuǎn)錄沉默染色質(zhì)(組成型異染色質(zhì))有關(guān)。已確定轉(zhuǎn)錄阻遏蛋白HP1的一個(gè)染色質(zhì)域(特異性結(jié)合甲基-賴氨酸的域)在HP1到K9的甲基化區(qū)域發(fā)揮作用。而一個(gè)看起來像反駁甲基化的生物物理學(xué)模型,賴氨酸4上的組蛋白H3的三甲基化與轉(zhuǎn)錄激活強(qiáng)相關(guān)(且完全需要)。三甲基化將在組蛋白尾部引進(jìn)一個(gè)固定正電荷。
已研究證明,組蛋白賴氨酸轉(zhuǎn)甲基酶(KMT)在組蛋白H3和H4模式中負(fù)責(zé)甲基化激活。該酶利用一個(gè)叫SET域(Suppressor of variegation,zeste增強(qiáng)子,Trithorax)的催化活性位點(diǎn)。SET域是一個(gè)130個(gè)氨基酸的序列,參與調(diào)控基因活化。已證實(shí)其可與組蛋白尾部結(jié)合,導(dǎo)致組蛋白甲基化。[36]不同的組蛋白修飾可能通過不同的方式起作用;一個(gè)位置的乙酰化可能比另一個(gè)位置的乙?;l(fā)揮更大的作用。另外,還可以同時(shí)發(fā)生多重修飾,這些修飾可以一起作用來改變核小體的行為。多重動(dòng)態(tài)修飾以一種系統(tǒng)的可繁殖的方式調(diào)節(jié)基因轉(zhuǎn)錄叫做組蛋白密碼。
不同的組蛋白修飾可能通過不同的方式起作用;一個(gè)位置的乙?;赡鼙攘硪粋€(gè)位置的乙?;l(fā)揮更加不同的作用。另外,同時(shí)可以發(fā)生多重修飾,這些修飾可以一起工作來改變核小體的行為。多重動(dòng)態(tài)修飾以一種系統(tǒng)的和可繁殖的方式調(diào)節(jié)基因轉(zhuǎn)錄叫做組蛋白密碼。
DNA甲基化頻繁發(fā)生于重復(fù)序列,幫助抑制表達(dá)和“轉(zhuǎn)座子”的流動(dòng)性::[37]由于5-甲基胞嘧啶可以自發(fā)脫氨基(用氧替代氮)變成胸苷,除了CpG島保持未甲基化外,CpG位點(diǎn)經(jīng)常發(fā)生變化,其在基因組中逐漸變得稀少,。因此這種類型的表觀遺傳改變具有直接增加永久的基因突變頻率的潛力。已知DNA甲基化通過至少三個(gè)獨(dú)立的DNA甲基轉(zhuǎn)移酶的復(fù)雜的相互作用而對(duì)環(huán)境因子做出反應(yīng),從而使其得以建立和修改,DNMT1,DNMT3A和DNMT3B,其中任何一個(gè)缺失對(duì)于小鼠都是致命的。DNMT1在體細(xì)胞中是最多的轉(zhuǎn)甲基酶,[38] DNMT1在體細(xì)胞中是最多的轉(zhuǎn)甲基酶,[39]局限在復(fù)制中心。[40]對(duì)于半甲基化的DAN具有10-40倍的優(yōu)先權(quán),并與增殖細(xì)胞核抗原(PCNA)發(fā)生相互作用。[41]
通過優(yōu)先修飾半甲基化的DNA,DNMT1在DNA復(fù)制后將甲基化模式轉(zhuǎn)移給一條新的合成鏈,因此經(jīng)常作為“維持”甲基轉(zhuǎn)移酶被提及。[42]DNMT1對(duì)于適當(dāng)?shù)呐咛グl(fā)育、印刻銘記和X失活是必需的。[43][44]為了強(qiáng)調(diào)這個(gè)遺傳分子機(jī)制與權(quán)威的瓦特生-克里克遺傳信息的堿基配對(duì)遺傳機(jī)制的區(qū)別,引進(jìn)了“表觀遺傳模板”這個(gè)術(shù)語(yǔ)。[45]此外,除了維持和傳送甲基化DNA狀態(tài),相同的原理也能作用于保持和傳送組蛋白修飾,甚至細(xì)胞質(zhì)(結(jié)構(gòu)上)的遺傳狀態(tài)。[46]
組蛋白H3和H4也能利用組蛋白賴氨酸脫甲基酶(KDM),通過反甲基化而調(diào)節(jié)。這個(gè)最近被確認(rèn)的酶有一個(gè)叫Jumonji域(JmjC)的催化活性位點(diǎn)。當(dāng)JmjC使用多個(gè)輔助因子使甲基團(tuán)羥基化時(shí),發(fā)生了反甲基化,由此除去甲基。JmjC能夠?qū)巍㈦p和三甲基化底物進(jìn)行脫甲基。[47]
染色體區(qū)域能夠采用穩(wěn)定的和可遺傳的二選一的狀態(tài)導(dǎo)致無DNA序列變化的雙穩(wěn)態(tài)的基因表達(dá)。表觀遺傳控制經(jīng)常與非正統(tǒng)的組蛋白共價(jià)修飾有關(guān)。[48]的染色體區(qū)域的穩(wěn)定性和遺傳性狀態(tài)經(jīng)常被認(rèn)為包含正反饋,在那里被修飾的核小體動(dòng)員酶對(duì)附近的核小體進(jìn)行類似的修飾。這一發(fā)現(xiàn)證實(shí)了表觀遺傳學(xué)的一種簡(jiǎn)化隨機(jī)模型。[49][50]
由于DNA甲基化和染色質(zhì)重塑在很多表觀遺傳類型中發(fā)揮著核心作用,“表觀遺傳”這個(gè)詞有時(shí)被用來作為這些過程的一個(gè)同義詞。然而,這可能是有誤導(dǎo)性。染色質(zhì)重塑不一定遺傳,而且不是所有的表觀遺傳都包括染色質(zhì)重塑。[51]
有人認(rèn)為組蛋白密碼能夠被小RNAs的作用所調(diào)節(jié)。最近發(fā)現(xiàn)和界定的一種大量的小的(21-到26-nt)非編碼RNAs,提示有一種RNA組分可能參與表觀基因調(diào)控。小干擾RNAs能通過靶啟動(dòng)子的表觀遺傳調(diào)節(jié)來調(diào)節(jié)轉(zhuǎn)錄基因表達(dá)。[52]
RNA轉(zhuǎn)錄及其編碼蛋白
有時(shí),一個(gè)基因被發(fā)動(dòng)后轉(zhuǎn)錄成保持該基因活性的產(chǎn)物(直接或間接)。例如,Hnf4和MyoD通過編碼蛋白的轉(zhuǎn)錄因子活性而分別加強(qiáng)很多肝臟和肌肉特異性基因的轉(zhuǎn)錄,包括它們自己的轉(zhuǎn)錄。RNA信號(hào)傳輸包括有區(qū)別的募集同層次的一般染色質(zhì)修飾復(fù)合體和在分化及發(fā)展中通過RNAs使DNA轉(zhuǎn)甲基酶到特定的位點(diǎn)。[53]其他表觀遺傳變異由RNA不同粘接形式的產(chǎn)物或雙鏈RNA(RNAi)的形成來介導(dǎo)。即使基因活化的原始刺激已經(jīng)不存在,基因被發(fā)動(dòng)的細(xì)胞的后代也將繼承這種活性。這些基因?qū)σ恍┫到y(tǒng)合胞體或縫隙連接很重要,常常被信號(hào)轉(zhuǎn)導(dǎo)打開或關(guān)閉,,RNA可以通過擴(kuò)散直接傳播到其他細(xì)胞或細(xì)胞核中。大量RNA和蛋白通過母親卵子形成過程或通過足細(xì)胞促成受精卵,導(dǎo)致母體效應(yīng)的表型。少量精子RNA來自于父親,但最近證明該表觀遺傳信息能導(dǎo)致幾代后代的明顯改變。[54]
微小RNAs
微小RNAs(miRNAs)是非編碼RNAs的成員,大小范圍從17到25個(gè)核苷酸。正如王等研究的,[55]微小RNAs調(diào)節(jié)植物和動(dòng)物各種各樣的生物功能。迄今為止,2013年在人類中以發(fā)現(xiàn)大約有2000種微小RNAs,都可以在在線微小RNAs數(shù)據(jù)庫(kù)中找到。[56]在細(xì)胞中表達(dá)的每一種微小RNAs可靶向約100到200種由其下調(diào)的信使RNAs。[57]多數(shù)信使RNAs的下調(diào)通過靶向信使,使RNA發(fā)生衰退,另一些下調(diào)發(fā)生在翻譯成蛋白的水平。[58]
大約60%的人類蛋白編碼基因由微小RNAs調(diào)節(jié)。[59]很多微小RNAs由表觀遺傳調(diào)控。約50%的微小RNA基因與CpG島有關(guān),[60]其可能被表觀遺傳甲基化抑制。來自甲基化的CpG島的轉(zhuǎn)錄被強(qiáng)烈抑制并可遺傳。[61]其他微小RNAs通過組蛋白修飾或通過DNA甲基化和組蛋白修飾組合來進(jìn)行表觀遺傳調(diào)節(jié)。[62]
小RNAs
小RNAs是在細(xì)菌中發(fā)現(xiàn)的小的(50-250的核苷酸),高度結(jié)構(gòu)化的,非編碼的RNA片段。小RNAs控制基因表達(dá),包括病原體毒力基因,并被認(rèn)為是與細(xì)菌耐藥性作斗爭(zhēng)的新靶點(diǎn)。[63]小RNAs在很多生物進(jìn)程中發(fā)揮重要作用,與原核生物靶向信使RNA和蛋白結(jié)合。對(duì)小RNAs的系統(tǒng)發(fā)育分析,例如通過小RNA-信使RNA靶向互動(dòng)或蛋白結(jié)合特性,可建立綜合數(shù)據(jù)庫(kù)。[64]同時(shí)也建立了與微生物基因組的目標(biāo)相關(guān)的小RNA-基因圖譜。[65]
朊病毒
朊病毒是蛋白質(zhì)有傳染性的部分。通常,蛋白質(zhì)折疊成執(zhí)行不同細(xì)胞功能的不相關(guān)的單元,但有些蛋白質(zhì)也能形成有傳染性的構(gòu)象狀態(tài),如已知的感染性蛋白質(zhì)。雖然曾經(jīng)認(rèn)為朊病毒具備將相同蛋白質(zhì)的其他原生狀態(tài)催化轉(zhuǎn)變?yōu)橐环N有傳染性構(gòu)象狀態(tài)的能力,但在以后的研究中,又認(rèn)為其是表觀遺傳的代理,具有不修飾基因組而引起表型改變的能力。[66]
真菌朊病毒被認(rèn)為具有表觀遺傳,原因是由感染性蛋白質(zhì)引起的感染性表型能夠不修飾基因組而遺傳。1965年和1971年在酵母菌中發(fā)現(xiàn)的PSI+和URE3,是這種感染性蛋白質(zhì)中研究最為充分的兩個(gè)。[67][68]阮病毒可以通過抑制表型效應(yīng)蛋白的聚集,從而降低蛋白質(zhì)的活性。在PSI +細(xì)胞,Sup35蛋白質(zhì)的損失(參與翻譯終止)導(dǎo)致核糖體終止密碼子翻譯率更高,抑制其他基因中無意義突變。Sup35形成朊病毒的能力可能一直存在。它可以賦予細(xì)胞適應(yīng)性優(yōu)勢(shì),使之能夠切換到PSI+狀態(tài),表達(dá)休眠基因,而通常,這些特性被終止密碼子突變所抑制。[69][70][71][72]
結(jié)構(gòu)遺傳系統(tǒng)
在基因完全相同的纖毛蟲中,例如四膜蟲屬和草履蟲屬,其遺傳差異顯示在細(xì)胞表面纖毛紋的方式上。這種改變可以傳給子細(xì)胞,似乎存在一種結(jié)構(gòu)起到模板的作用這種遺傳的機(jī)制還不清楚,但多細(xì)胞有機(jī)體也可利用現(xiàn)存的細(xì)胞結(jié)構(gòu)來組裝一個(gè)新的有機(jī)體的假設(shè)是有理由存在的。[73][74][75]
功能和因果關(guān)系
發(fā)展
體細(xì)胞表觀遺傳通過表觀遺傳修飾,特別是通過DNA甲基化和染色質(zhì)重塑,在多細(xì)胞真核生物的發(fā)育中非常重要?;蚪M序列不變(有一些值得注意的例外),但細(xì)胞區(qū)分為很多不同的類型,執(zhí)行不同的功能,對(duì)環(huán)境和細(xì)胞間的信號(hào)做出不同的反應(yīng)。因此,作為個(gè)體發(fā)育,成形素激活或抑制在一種表觀遺傳方式里的沉默基因,賦予細(xì)胞一個(gè)“記憶”。在哺乳動(dòng)物中,多數(shù)細(xì)胞終末分化,僅干細(xì)胞保留分化成幾種細(xì)胞類型的能力(“全能性”和“多潛能性”)。在哺乳動(dòng)物中,一些干細(xì)胞在整個(gè)生命中持續(xù)產(chǎn)生新分化的細(xì)胞,但哺乳動(dòng)物不能對(duì)一些組織的失去做出反應(yīng),例如,不能再生肢體,而其他一些動(dòng)物可以。不像動(dòng)物,植物細(xì)胞不終末分化而保持全能,具有產(chǎn)生一個(gè)新植物個(gè)體的能力。雖然植物像動(dòng)物一樣利用很多相同的表觀遺傳機(jī)制,例如染色質(zhì)重塑,已有假說認(rèn)為一些種類的植物細(xì)胞不使用或不要求“細(xì)胞記憶”,而用來自環(huán)境和周圍細(xì)胞的位置信息重新設(shè)置其基因表達(dá)方式來決定其命運(yùn)。[76]
表觀遺傳可分為預(yù)定的和基于概率的。預(yù)定的表觀遺傳是一種從DNA的結(jié)構(gòu)性發(fā)展到蛋白質(zhì)的功能成熟的單向運(yùn)動(dòng)?!邦A(yù)定”在這里指發(fā)展是照本宣科和可預(yù)見的。另一方面,基于概率的表觀遺傳是一種隨著經(jīng)歷和外部造型的發(fā)展的雙向結(jié)構(gòu)-功能發(fā)育。[77]
醫(yī)學(xué)
表觀遺傳有各種各樣的潛在的醫(yī)學(xué)上的應(yīng)用,同時(shí)它在世界上也趨向多面性。[78]先天性遺傳性疾病很好理解,表觀遺傳能夠發(fā)揮作用也很清楚,例如,Angelman綜合征和普拉德-威利綜合征。由基因缺失或基因失活導(dǎo)致的遺傳疾病并不多見,這是由于基因組印記本質(zhì)上是半合子,因此單個(gè)基因敲除足夠致病,但多數(shù)病例需要兩個(gè)拷貝都被敲除。[79]
進(jìn)化
當(dāng)表觀遺傳改變可遺傳時(shí),表觀遺傳可影響進(jìn)化。一個(gè)隔離的種系或魏斯曼屏障對(duì)于動(dòng)物是特異的,表觀遺傳在植物和微生物中更為普遍。Eva Jablonka和Marion Lamb已經(jīng)爭(zhēng)論過這些作用,認(rèn)為可能需要推進(jìn)現(xiàn)代綜合進(jìn)化論標(biāo)準(zhǔn)的概念框架。[80][81]其他進(jìn)化生物學(xué)家則建議結(jié)合表觀遺傳與群體遺傳學(xué)模型[82]或表示公開懷疑。[83]
表觀遺傳有兩個(gè)重要方式,可與傳統(tǒng)遺傳相區(qū)別,對(duì)于進(jìn)化有重要的作用,這就是表突變率比一般突變率快得多[84]及表突變更容易逆轉(zhuǎn)。[85]種表觀遺傳要素,如PSI陽(yáng)性系統(tǒng)可充當(dāng)“臨時(shí)替代者”,由于短期適應(yīng)足夠好,使得此血統(tǒng)存活足夠長(zhǎng),直到突變和/或復(fù)合以遺傳同化適應(yīng)性的表型改變。[82] [86]這種存在可能增強(qiáng)一個(gè)物種的進(jìn)化力。
樣本
觀遺傳改變已被觀察到在對(duì)環(huán)境暴露產(chǎn)生反應(yīng)時(shí)發(fā)生,例如,給予膳食補(bǔ)充劑的小鼠具有影響基因表達(dá)的表觀遺傳改變,影響其毛色,體重和患癌癥的傾向。[87][88]
就人類在不同環(huán)境暴露下來說,F(xiàn)raga等研究年輕的和年老的單卵雙胞胎。發(fā)現(xiàn)盡管這些雙胞胎在早年很難從表觀遺傳上區(qū)分,但老年雙胞胎在5-甲基胞嘧啶DNA和組蛋白乙?;恼w含量及基因組分布上具有顯著差異。共度時(shí)間較短的雙胞胎和/或醫(yī)療史差異較大的雙胞胎在5甲基胞嘧啶DNA和組蛋白H3及H4乙?;讲町愐哺?。
在廣泛的有機(jī)體范圍內(nèi),包括原核生物,植物和動(dòng)物,已有超過100種的跨代的表觀遺傳現(xiàn)象被報(bào)道。[89]
最近的分析提示,胞嘧啶脫氨酶APOBEC/AID家族的成員能夠利用類似的分子機(jī)制同時(shí)調(diào)節(jié)基因的和表觀的遺傳。[90]
人類的表觀遺傳效應(yīng)
基因組印跡和相關(guān)疾病
一些人類疾病與基因組印記有關(guān),在哺乳動(dòng)物中有一種現(xiàn)象,即父親和母親在其生殖細(xì)胞中對(duì)特定的染色體組位點(diǎn)貢獻(xiàn)不同的表觀遺傳模式。[91]在人類疾病中眾所周知的印記案例是Angelman綜合征和普拉德-威利綜合征——兩者可由相同的基因突變產(chǎn)生,染色體15q部分缺失,這個(gè)特別的綜合征將依賴于突變是繼承于母親還是父親而發(fā)展。[92]原因是在這個(gè)區(qū)域里存在基因組印記。
Beckwith-Wiedemann綜合征也與基因組印記有關(guān),經(jīng)常由母體基因組印記的染色體11上的一個(gè)區(qū)域異常導(dǎo)致。
跨代表觀遺傳觀察
見主要文章 跨代表觀遺傳 在?verkalix研究中,Marcus Pembrey等[93]觀察到,在19世紀(jì),瑞典男子如在青春期前遭受營(yíng)養(yǎng)不良,則其孫子可能較少死于心血管疾病。如果這些男子的食物豐富,那其孫子的糖尿病死亡率就增加,提示這是一種跨代的表觀遺傳。[94]在女性中觀察到相反的效應(yīng)——如女子在在子宮內(nèi)經(jīng)歷過營(yíng)養(yǎng)不良(且其卵子正在形成),則其孫女的平均壽命短一些。[95]
表觀遺傳與發(fā)育異常
很多致畸劑通過表觀遺傳機(jī)制對(duì)胎兒發(fā)揮特定作用。[96][97]表觀遺傳效應(yīng)可以保持致畸劑的作用,如己烯雌酚可以影響兒童的整個(gè)生命周期,但由父親暴露引起后代出生缺陷的可能性因?yàn)槿狈碚摶A(chǔ)而不能成立。[98]然而,一系列由男性介導(dǎo)的異常已被證實(shí),如阿扎胞苷[99] ,FDA規(guī)定,當(dāng)使用5-阿扎胞苷(當(dāng)其整合進(jìn)入DNA后形成低甲基化胞苷成為不可甲基化類似物的物質(zhì))時(shí),“男性應(yīng)注意避孕”。證據(jù)是:5-阿扎胞苷處理過的雄性小鼠繁殖力下降,增加了胚胎丟失和異常胚胎發(fā)育的機(jī)會(huì)。[100]在暴露于嗎啡的雄性大鼠的后代中觀察到內(nèi)分泌差異。[101]在小鼠中,己烯雌酚的第二代效應(yīng)已被描述為是通過表觀遺傳機(jī)制而發(fā)生的。[102]
除了形成受精卵的卵子和精子的基因發(fā)生表觀遺傳變化會(huì)傳遞給下一代外,正在發(fā)育的胎兒在宮內(nèi)也會(huì)因?yàn)槟赣H暴露于某些因素而發(fā)生表觀遺傳變化。很多流行病學(xué)調(diào)查顯示,胎兒在宮內(nèi)的生長(zhǎng)發(fā)育狀況與某些成人疾病的發(fā)生存在一定的關(guān)系。如Barker著名的“成人疾病胎兒起源”假說。該假說認(rèn)為,胎兒在孕中晚期營(yíng)養(yǎng)不良,會(huì)引起生長(zhǎng)發(fā)育失調(diào),且成年后易患冠心病。與低出生體重相關(guān)的疾病還包括動(dòng)脈粥樣硬化、冠心病、2型糖尿病等。
表觀遺傳與癌癥
多種復(fù)合物被認(rèn)為是表觀遺傳致癌物——導(dǎo)致腫瘤發(fā)生率增加,但不顯示誘變活性(有毒復(fù)合物和導(dǎo)致腫瘤發(fā)生或復(fù)發(fā)的病原體應(yīng)該被排除)。實(shí)例包括己烯雌酚,亞砷酸鹽,六氯苯和鎳復(fù)合物。最近的研究已顯示,系白血病(MLL)基因通過在不同染色體中重排和接合其他基因?qū)е掳籽?,是一個(gè)在表觀遺傳控制下的過程。[103]
其他研究證實(shí),在許多基因中發(fā)生的組蛋白乙酰化改變和DNA甲基化對(duì)前列腺癌起作用。[104]前列腺癌的基因表達(dá)可被營(yíng)養(yǎng)和生活方式改變所調(diào)節(jié)。[105]
2008年國(guó)家衛(wèi)生研究院宣布,在接下來的5年中將投資1.9億美元用于表觀遺傳研究。在宣告書中,政府注意到表觀遺傳具有解釋老化機(jī)制,人類發(fā)育和癌癥起源,心臟病,精神疾病及其他的健康狀況的潛力。一些研究者,如杜克大學(xué)醫(yī)學(xué)中心博士Randy Jirtle認(rèn)為,在疾病治療方面,對(duì)于以上疾病,表觀遺傳學(xué)研究可能比遺傳學(xué)具有更大的作用。[106]
在癌癥中的DNA甲基化
DNA甲基化是一種基因轉(zhuǎn)錄的重要的調(diào)節(jié)器,許多證據(jù)已經(jīng)證實(shí),異常的DNA甲基化與不定期的基因沉默有關(guān),若在啟動(dòng)子區(qū)域具有高水平的5-甲基胞嘧啶,將發(fā)生基因沉默。DNA甲基化在胚胎發(fā)育期間是必需的,在體細(xì)胞中,DNA甲基化的方式通常是高保真的傳給子細(xì)胞。異常的DNA甲基化模式與大量的人類惡性腫瘤有關(guān),并發(fā)現(xiàn)其與正常組織相比存在兩種不尋常的形式:超甲基化和低甲基化。超甲基化是主要的表觀遺傳修飾中的一種,其通過腫瘤抑制基因的啟動(dòng)子區(qū)抑制轉(zhuǎn)錄。超甲基化通常發(fā)生在啟動(dòng)子區(qū)的CpG島,且與基因失活有關(guān)。整體的低甲基化也通過不同機(jī)制與癌癥的發(fā)生和發(fā)展有關(guān)。[107]
在癌癥中的DNA修復(fù)表觀遺傳學(xué)
種系(家族的)突變已在34種導(dǎo)致癌癥高風(fēng)險(xiǎn)的不同的DNA修復(fù)基因中被確定,包括如BRCA1和ATM。這些被列于“DNA repair-deficiency disorder”一文中。然而,由這樣的種系突變導(dǎo)致的癌癥僅占癌癥中非常小的比例。例如,種系突變僅導(dǎo)致2%到5%的結(jié)腸癌病例。[108]
DNA修復(fù)基因表達(dá)的表觀遺傳減少,在散發(fā)性(非種系)癌癥中非常頻繁,如在下表中顯示的,在一些代表性的散發(fā)性癌癥中DNA修復(fù)基因的突變非常罕見。[109]
癌癥 | 基因 | 后生變化 | 頻率 | 文獻(xiàn). |
---|---|---|---|---|
乳房 | BRCA1 | CpG島甲基化 | 13% | 1 |
WRN | CpG島甲基化 | 17% | 2 | |
卵巢 | WRN | CpG島甲基化 | 36% | 3 |
BRCA1 | CpG島甲基化 | 5%-30% | 1,11,12 | |
FANCF | CpG島甲基化 | 21% | 11 | |
RAD51C | CpG島甲基化 | 3% | 11 | |
結(jié)腸直腸 | MGMT | CpG島甲基化 | 40%-90% | 4-8 |
WRN | CpG島甲基化 | 38% | 2 | |
MLH1 | CpG島甲基化 | 2%-65% | 2,5,9 | |
MSH2 | CpG島甲基化 | 13% | 6 | |
ERCC1 | 表觀遺傳類型未知 | 100% | 10 | |
Xpf | 表觀遺傳類型未知 | 55% | 10 | |
頭頸部 | MGMT | CpG島甲基化 | 35%-57% | 13-16 |
MLH1 | CpG島甲基化 | 27%-33% | 17,19,20 | |
NEIL1 | CpG島甲基化 | 62% | 13 | |
FANCB | CpG島甲基化 | 46% | 13 | |
MSH4 | CpG島甲基化 | 46% | 13 | |
ATM | CpG島甲基化 | 25% | 18 |
表中文獻(xiàn)如下:
1, [110]2, [111]3, [112]4, [113]5, [114]6, [115] 7, [116] 8, [117]9, [118]10, [119]11, [120]12, [121]13, [122] 14, [123]15, [124]16, [125] 17, [126]18, [127]19, [128] 20[129]
DNA修復(fù)基因表達(dá)不足導(dǎo)致突變率增加。如由于DNA修復(fù)基因PMS2, MLH1,MSH2,MSH3或MSH6缺陷或DNA修復(fù)基因BRCA2,[130][131]或DNA修復(fù)基因BRCA2, [132]錯(cuò)配,小鼠突變率增加,同時(shí)注意到在DNA修復(fù)基因BLM有缺陷時(shí),人類染色體重排和非整倍性有所增加。[133]因此,DNA修復(fù)缺陷可導(dǎo)致基因組不穩(wěn)定,且這種基因組不穩(wěn)定可能是導(dǎo)致癌癥的遺傳改變的主要潛在原因。實(shí)際上,如Nowak等指出的,通過一種數(shù)學(xué)計(jì)算,很多散發(fā)性腫瘤的首要事件是一種遺傳性改變,其影響遺傳不穩(wěn)定性,并且應(yīng)注意到DNA修復(fù)的表觀遺傳缺陷是由體細(xì)胞遺傳的。
癌癥中的組蛋白變體H2A
H2A家族的組蛋白變異體在哺乳動(dòng)物中被高度保存,其通過改變?nèi)旧|(zhì)結(jié)構(gòu)在很多調(diào)節(jié)核內(nèi)過程中發(fā)揮決定性作用。其中一種主要的H2A突變體,H2A.X,標(biāo)志著DNA損傷,需要補(bǔ)充DNA修復(fù)蛋白來促進(jìn)恢復(fù)基因組的完整性。另一種突變體,H2A.Z,在基因活化和抑制中發(fā)揮重要作用。在很多癌癥中廣泛發(fā)現(xiàn)有高水平的H2A.Z表達(dá),并且與細(xì)胞增殖和基因組不穩(wěn)定顯著相關(guān)。[134]組蛋白變異體macroH2A1在很多類型癌癥的發(fā)病機(jī)理中也很重要,例如肝癌。[135]
癌癥治療
最近的研究已顯示,表觀遺傳藥物可替代當(dāng)前公認(rèn)的治療方法,如放射治療和化學(xué)治療,或作為輔助治療提高當(dāng)前療法的效果。[136]已證明,原癌基因區(qū)的表觀遺傳控制和腫瘤抑制序列可通過組蛋白構(gòu)象變化而直接影響癌癥的形成和進(jìn)展。[137]此外,表觀遺傳具有可逆性,是其他任何一種癌癥治療法所不能提供的特性。[138]
藥物發(fā)展主要聚焦于組蛋白乙酰轉(zhuǎn)移酶(HAT)和組蛋白脫抑制劑,[139]其在口腔鱗狀細(xì)胞癌的進(jìn)展中發(fā)揮整體作用。[140]對(duì)當(dāng)前領(lǐng)跑的新藥靶點(diǎn)候選者還有組蛋白賴氨酸甲基轉(zhuǎn)移酶(KMT)和蛋白質(zhì)精氨酸甲基轉(zhuǎn)移酶(PRMT)。[141]
孿生子研究
最近涉及雙卵和單卵雙胞胎的研究也提供了一些人類表觀遺傳影響的證據(jù)。[142][143][144]
微生物中的表觀遺傳
細(xì)菌廣泛利用DNA甲基化的表觀遺傳,控制DNA-蛋白的相互作用。細(xì)菌利用DNA腺嘌呤甲基化(不是DNA胞核嘧啶甲基化)作為一種表觀遺傳信號(hào)。DNA腺嘌呤甲基化對(duì)于細(xì)菌在有機(jī)體內(nèi)的毒力很重要,如大腸桿菌,沙門氏菌屬,弧菌屬,耶爾森氏菌屬,嗜血桿菌屬和布氏桿菌屬。對(duì)于甲型變形菌,腺嘌呤甲基化可調(diào)節(jié)從細(xì)胞周期和配對(duì)基因轉(zhuǎn)錄到DNA復(fù)制。對(duì)于丙型變形菌,腺嘌呤甲基化為DNA復(fù)制,染色體分離,錯(cuò)配修復(fù),噬菌體包裝,轉(zhuǎn)座酶活性和基因表達(dá)控制提供了信號(hào)。[145][146]
絲狀真菌粗糙鏈孢霉有助于理解胞核嘧啶甲基化在一個(gè)突觸的模型系統(tǒng)中的控制和功能。在這個(gè)有機(jī)體內(nèi),DNA甲基化抑制轉(zhuǎn)錄延伸,與RIP(重復(fù)誘導(dǎo)點(diǎn)突變)的基因組防御系統(tǒng)的殘余物和沉默基因表達(dá)有關(guān)。[147]
酵母菌感染性蛋白(PSI)由一種翻譯終止因子的某一構(gòu)象改變而產(chǎn)生,其子細(xì)胞可繼承這種改變,并在不利條件下提供一種生存優(yōu)勢(shì)。這是表觀遺傳調(diào)節(jié)使單細(xì)胞有機(jī)體能夠快速對(duì)環(huán)境應(yīng)激產(chǎn)生反應(yīng)的一個(gè)范例。朊病毒可被視為能夠誘導(dǎo)表型改變而不修飾基因組的表觀遺傳中介。[148]
用單分子實(shí)時(shí)排序方法可以在微生物中直接檢查表觀遺傳標(biāo)志,聚合酶的敏感性允許在測(cè)序時(shí)測(cè)量一個(gè)DNA分子的甲基化和其他修飾。[149]幾項(xiàng)研究已經(jīng)證實(shí),該方法具備在細(xì)菌中收集整組基因表觀遺傳資料的能力。[150][151][152][153]
方式
- DNA甲基化(DNA methylation)
- 基因組印記(genomic imprinting)
- 母體效應(yīng)(maternal effects)
- 基因沉默(gene silencing)
- 核仁顯性
- 休眠轉(zhuǎn)座子激活
- RNA編輯(RNA editing)
參見
- B chromosome
- Baldwin effect
- Behavioral epigenetics
- Centromere
- Computational epigenetics
- DNA demethylation
- Dutch famine of 1944(Legacy)
- Ecotype
- Emergenesis
- Epigenetic landscape
- Epigenetic theory
- Epigenetics in psychology
- Epigenomics
- Evolutionary capacitance
- Evolutionary developmental psychology
- Extranuclear inheritance
- Histone code
- Hologenome theory of evolution
- Human genome
- Molecular biology
- Nutriepigenomics
- Position-effect variegation
- Preformationism
- Somatic epitype
- Synthetic genetic array
- Weismann barrier
- 時(shí)代精神運(yùn)動(dòng)
文獻(xiàn)
- ↑ Spector, Tim (2012). Identically Different: Why You Can Change Your Genes. London: Weidenfeld & Nicolson. p. 8. "Just over ten years ago researchers found that the diets of pregnant mothers could alter the behaviour of genes in their children and that these changes could last a lifetime and then be passed on in turn to their children. The genes were literally being switched on or off by a new mechanism we call epigenetics – meaning in Greek 'around the gene'. Contrary to traditional genetic dogma, these changes could be transferred to the next generation. In this case the mothers just happened to be rats, but recent similar findings in humans have created a revolution in our thinking."
- ↑ Bird A (May 2007). "Perceptions of epigenetics". Nature 447 (7143): 396–8. doi:10.1038/nature05913. PMID 17522671.
- ↑ "Special report: 'What genes remember' by Philip Hunter | Prospect Magazine May 2008 issue 146". Web.archive.org. 2008-05-01. Retrieved 2012-07-26.
- ↑ Ledford H. (2008). "Disputed definitions". Nature 455 (7216): 1023–8. PMID 18948925
- ↑ Reik W (May 2007). "Stability and flexibility of epigenetic gene regulation in mammalian development". Nature 447 (7143): 425–32. doi:10.1038/nature05918. PMID 17522676
- ↑ Jia, Guifang; Fu, Ye, Zhao, Xu, Dai, Qing, Zheng, Guanqun, Yang, Ying, Yi, Chengqi, Lindahl, Tomas, Pan, Tao, Yang, Yun-Gui, He, Chuan (16 October 2011). "N6-Methyladenosine in nuclear RNA is a major substrate of the obesity-associated FTO". Nature Chemical Biology 7 (12): 885–887. doi:10.1038/nchembio.687. PMC 3218240. PMID 22002720.
- ↑ "New research links common RNA modification to obesity". Physorg.com. Retrieved 2012-07-26.
- ↑ Waddington CH (1942). "The epigenotype". Endeavour 1: 18–20.
- ↑ According to the Oxford English Dictionary: The word is used by W. Harvey, Exercitationes 1651, p. 148, and in the English Anatomical Exercitations 1653, p. 272. It is explained to mean‘partium super-exorientium additamentum’,‘the additament of parts budding one out of another’. It is also worth quoting this adumbration of the definition given there (viz., "The formation of an organic germ as a new product"): theory of epigenesis: the theory that the germ is brought into existence (by successive accretions), and not merely developed, in the process of reproduction. [...] The opposite theory was formerly known as the‘theory of evolution’; to avoid the ambiguity of this name, it is now spoken of chiefly as the‘theory of preformation’, sometimes as that of‘encasement’or‘embo?tement’.
- ↑ Holliday R (November 1990). "Mechanisms for the control of gene activity during development". Biol Rev Camb Philos Soc 65 (4): 431–71. PMID 2265224.
- ↑ Riggs AD, Russo VEA, Martienssen RA (1996). Epigenetic mechanisms of gene regulation. Plainview, N.Y: Cold Spring Harbor Laboratory Press. ISBN 0-87969-490-4.
- ↑ Bird A (May 2007). "Perceptions of epigenetics". Nature 447 (7143): 396–8. doi:10.1038/nature05913. PMID 17522671.
- ↑ Ledford H. (2008). "Disputed definitions". Nature 455 (7216): 1023–8. PMID 18948925.
- ↑ Ledford H. (2008). "Disputed definitions". Nature 455 (7216): 1023–8. PMID 18948925.
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- ↑ Fraga MF, Ballestar E, Paz MF, Ropero S, Setien F, Ballestar ML, Heine-Su?er D, Cigudosa JC, Urioste M, Benitez J, Boix-Chornet M, Sanchez-Aguilera A, Ling C, Carlsson E, Poulsen P, Vaag A, Stephan Z, Spector TD, Wu YZ, Plass C, Esteller M (July 2005). "Epigenetic differences arise during the lifetime of monozygotic twins". Proc. Natl. Acad. Sci. U.S.A. 102 (30): 10604–9. doi:10.1073/pnas.0500398102. PMC 1174919. PMID 16009939.
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- ↑ Jorg Tost (2008). Epigenetics. Norfolk, England: Caister Academic Press. ISBN 1-904455-23-9.
- ↑ Lewis ZA, Honda S, Khlafallah TK, Jeffress JK, Freitag M, Mohn F, Schübeler D, Selker EU (March 2009). "Relics of repeat-induced point mutation direct heterochromatin formation in Neurospora crassa". Genome Res. 19 (3): 427–37. doi:10.1101/gr.086231.108. PMC 2661801. PMID 19092133.
- ↑ Jorg Tost (2008). Epigenetics. Norfolk, England: Caister Academic Press. ISBN 1-904455-23-9.
- ↑ http://genome.cshlp.org/content/early/2012/10/23/gr.136739.111.full.pdf+html
- ↑ http://www.sciencedirect.com/science/article/pii/S1369527413000155
- ↑ http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1003191
- ↑ http://nar.oxfordjournals.org/content/early/2012/10/02/nar.gks891.abstract?keytype=ref&ijkey=5wszQKtd4ssMjGh
- ↑ http://www.nature.com/nbt/journal/v30/n12/abs/nbt.2432.html
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