吳 丹,葛莉萍
廣西壯族自治區(qū)南寧市紅十字會醫(yī)院產(chǎn)科,南寧 530012
妊娠期糖尿?。╣estational diabetes mellitus,GDM)為患者妊娠前糖代謝正常、妊娠期出現(xiàn)的糖耐量異常;依據(jù)診斷標(biāo)準(zhǔn)及人群不同,其發(fā)病率為10%~15%[1]。對于母體而言,GDM患者孕期易并發(fā)子癇前期,且遠(yuǎn)期罹患2型糖尿?。╰ype 2 diabetes mellitus,T2DM)及代謝性疾病的風(fēng)險大大增加;對于子代而言,其出現(xiàn)巨大兒、早產(chǎn)、胎兒生長受限、圍產(chǎn)期死亡及新生兒低血糖等并發(fā)癥的概率明顯增加,且對于胎兒的代謝影響可延續(xù)至成年甚至終生[2],這也是此前易被忽視的GDM危害性。近年來隨著對GDM研究的不斷深入,“胎兒編程”和“胎源性成人疾病”的概念愈來愈受到重視。研究[3]發(fā)現(xiàn),胎兒早期暴露于GDM母體宮內(nèi)高血糖環(huán)境后,機(jī)體代謝模式發(fā)生重編程,其影響可延續(xù)至子代成年[3]。表觀遺傳修飾在“胎兒編程”理論中被認(rèn)為具有重要的作用[4],而DNA甲基化作為研究最多的表觀遺傳機(jī)制,近年來在GDM中的報道逐步增多。
DNA甲基化指在DNA甲基轉(zhuǎn)移酶的作用下,基因組胞嘧啶和鳥嘧啶組成的串聯(lián)重復(fù)序列(CpG)中的胞嘧啶5'碳位共價鍵結(jié)合一個甲基基團(tuán);其通常發(fā)生在基因啟動子區(qū),可在不改變DNA序列的情況下,調(diào)控基因表達(dá)水平。研究[5]發(fā)現(xiàn),DNA甲基化參與癌癥、心血管疾病、糖尿病等多種疾病的發(fā)生,干預(yù)其對特定基因的遺傳修飾可成為疾病的治療策略,且對外周血中循環(huán)DNA的甲基化水平進(jìn)行檢測可望為疾病的預(yù)測與診斷提供幫助。GDM患者胎盤、外周血、臍血及脂肪組織等標(biāo)本中均出現(xiàn)部分基因甲基化水平的改變,同時GDM患者子代機(jī)體組織標(biāo)本中亦存在某些基因甲基化水平的差異。這些差異引起的基因表達(dá)水平與遺傳信息的改變,可能對GDM的發(fā)病、并發(fā)癥的發(fā)生及遠(yuǎn)期代謝性疾病罹患風(fēng)險的增加具有重要的影響。本文對目前GDM中相關(guān)基因的DNA甲基化研究進(jìn)展進(jìn)行綜述,以期為臨床上GDM新型診斷和治療手段的開發(fā)提供啟示。
胎盤是孕期母體供應(yīng)胎兒營養(yǎng)物質(zhì)、母胎交流的重要樞紐,可分泌多種激素,對維持胎兒的正常生長非常重要。GDM患者宮內(nèi)高血糖環(huán)境可促使胎盤組織形態(tài)、結(jié)構(gòu)及功能發(fā)生異常改變,對胎兒的生長與代謝模式造成直接影響,而DNA甲基化的遺傳修飾方式在其中的作用不容忽視。研究表明,GDM患者胎盤組織DNA甲基化水平發(fā)生改變的基因功能多與葡萄糖代謝、生長調(diào)控、脂質(zhì)代謝等相關(guān)。如GDM胎盤中非典型notch配體1(δ likenon-canonical notch ligand 1,DLK1)甲基化水平異常與母體葡萄糖耐量試驗2 h血糖水平和新生兒出生體質(zhì)量具有相關(guān)性[6];過氧化物酶體增殖物激活受體γ輔助激活因子1A(peroxisome proliferator activated receptorγ coactivator 1α,PPARGC1A)與胰十二指腸同源異型基因1(pancreatic and duodenal homeobox 1,PDX1)甲基化水平與胎兒葡萄糖代謝均具有一定關(guān)聯(lián)[7];胰島素樣生長因子2(insulin-like growth factor 2,IGF2)的甲基化水平改變可影響胎兒發(fā)育和出生體質(zhì)量[8];PPARGC1A啟動子甲基化水平改變與母體餐后2 h血糖水平及新生兒瘦素水平相關(guān)[9-10];胎盤脂蛋白酯酶(lipoprotein lipase,LPL)基因甲基化水平可影響母體和胎兒的脂質(zhì)譜[11]及子代5歲時的身體成分[12]。此外,另有研究[13]發(fā)現(xiàn),中胚層特異性轉(zhuǎn)錄子(mesoderm specific transcript,MEST)的異常表觀遺傳修飾可能會導(dǎo)致GDM患者的子代終生肥胖。6-磷酸葡萄糖脫氫酶(glucose-6-phosphate dehydrogenase,G6PD)基因的異常甲基化與高血糖癥、氧化應(yīng)激相關(guān),而胰島素樣生長因子軸相關(guān)分子的甲基化改變則直接增加巨大兒的發(fā)生風(fēng)險[14]。此外,GDM環(huán)境還可以通過DNA甲基化和基因表達(dá)水平變化使胎兒胎盤內(nèi)皮細(xì)胞形態(tài)與屏障功能發(fā)生損傷[15]。研究者[16]基于生物信息學(xué)工具,發(fā)現(xiàn)GDM患者胎盤組織中低密度脂蛋白受體相關(guān)蛋白1b(LDL receptor related protein 1b,LRP1B)等基因可能為胎兒發(fā)生代謝模式重編程的候選基因??梢?,GDM患者胎盤組織中DNA甲基化水平的變化參與多種病理過程。
除胎盤外,臍帶對胎兒的正常生長亦發(fā)揮著不可替代的作用。臍血標(biāo)本信息可直接反映胎兒部分相關(guān)指標(biāo)的變化。GDM患者臍血中部分基因啟動子區(qū)域甲基化水平改變引起的基因表達(dá)異常,與機(jī)體組織多種生物學(xué)功能的損害相關(guān)。臍血是臨床上預(yù)測胎兒并發(fā)癥的發(fā)生、胎兒遠(yuǎn)期機(jī)體代謝模式的重要媒介。如臍血瘦素甲基化水平與巨大兒的發(fā)生相關(guān)[17];IGF2的甲基化水平改變與胎兒出生體質(zhì)量具有相關(guān)性[8];巨大兒臍血中Rho蛋白鳥苷酸交換因子11(Rho guanine nucleotide exchange factor 11,ARHGEF11)基因的甲基化水平明顯降低,與新生兒血糖水平和出生體質(zhì)量呈負(fù)相關(guān)[18]。研究[19]還發(fā)現(xiàn),GDM可引起1型糖尿病、主要組織相容性復(fù)合體和神經(jīng)元發(fā)育相關(guān)途徑基因發(fā)生表觀遺傳修飾,這些基因通過DNA甲基化方式參與胎兒代謝編程,對胎兒的生長和發(fā)育產(chǎn)生影響。研究[20]發(fā)現(xiàn),人缺氧誘導(dǎo)因子α(hypoxia inducible factor 3 subunitα,HIF3A)等基因可能在GDM效應(yīng)傳遞給子代的過程中發(fā)揮重要作用。此外,臍血中基因DNA甲基水平的改變還可以作為GDM患者診斷的間接指標(biāo)。如孕產(chǎn)婦GDM的發(fā)生與臍血自閉癥相關(guān)嗅覺受體家族2亞家族L成員13(olfactory receptor family 2 subfamily L member 13,OR2L13)基因和細(xì)胞色素P450同工酶2E1(cytochrome P450 family 2 subfamily E member 1,CYP2E1)基因的甲基化水平降低有關(guān)[21]。研究[22]還發(fā)現(xiàn),信號轉(zhuǎn)導(dǎo)蛋白亞基8(COP9 signalosome subunit 8,COPS8)等基因在母體發(fā)生GDM前已出現(xiàn)甲基化水平的變化,顯示DNA甲基化作為臨床生物標(biāo)志物的巨大潛力。與此同時,DNA甲基化方式參與調(diào)節(jié)臍血淋巴細(xì)胞內(nèi)丙二酰-輔酶A-?;d體蛋白轉(zhuǎn)氨酶網(wǎng)絡(luò),影響細(xì)胞脂肪酸合成及胰島素抵抗[23];在胎兒臍血中分離出的淋巴細(xì)胞中,G蛋白α亞基(GNAS complex locus,GNAS)基因甲基化水平較高,其可能與GDM母體與子代代謝疾病風(fēng)險增加相關(guān)[24]。
目前,關(guān)于GDM患者外周血中全基因組DNA甲基化水平是否存在差異尚有爭議。研究[25]表明,患或不患GDM的女性外周血全基因組DNA甲基化水平差異無統(tǒng)計學(xué)意義,且基因DNA甲基化水平與空腹血糖水平及胰島素濃度均無關(guān)。而另有研究[26]顯示,GDM組和非GDM組患者之間共有1 046個CpG位點(與939個基因相關(guān))甲基化水平具有差異性,這些基因富集于與GDM相關(guān)的途徑,例如胰島素抵抗、葡萄糖代謝和炎癥;其中前5個CpG位點的DNA甲基化在GDM組和非GDM組間顯示出不同的甲基化模式,并且與母體外周血的血糖水平相關(guān);其中一個CpG位點定位于鈣調(diào)蛋白結(jié)合轉(zhuǎn)錄激活因子1(calmodulin binding transcription activator 1,CAMTA1)基因;該基因已被證明可調(diào)節(jié)胰島素的產(chǎn)生和分泌。此外,不同GDM嚴(yán)重程度的患者孕早期外周血中DNA甲基化水平亦存在差異[27],且在GDM患者母體外周血中發(fā)現(xiàn)白介素10(interleukin 10,IL10)的甲基化水平降低,濃度升高[28]。
與T2DM相似,GDM可影響患者脂肪與肌肉組織對胰島素的敏感性。研究[29]表明,與正常糖耐量患者相比,GDM患者內(nèi)臟脂肪組織中腫瘤壞死因子(tumor necrosis factor,TNF)和細(xì)胞因子信號轉(zhuǎn)導(dǎo)抑制因子3(suppressor of cytokine signaling 3,SOCS3)的mRNA表達(dá)水平顯著增加,伴隨著各自啟動子甲基化模式的特異性改變。同時,大網(wǎng)膜脂肪組織中HIF3A啟動子CpG島的甲基化可促進(jìn)GDM的胰島素抵抗[30],且GDM患者和子代的脂肪組織中胰島素受體表達(dá)的減少亦與其DNA甲基化改變有關(guān)[31]。脂肪組織和血細(xì)胞中脂聯(lián)素(adiponectin, C1Q and collagen domain containing,ADIPOQ)基因表達(dá)和甲基化的改變可影響GDM的病程與新生兒結(jié)局[32]?;诨虮磉_(dá)譜和甲基化譜的綜合分析發(fā)現(xiàn),間皮素(mesothelin,MSLN)基因的表達(dá)與其甲基化水平之間呈現(xiàn)典型的負(fù)相關(guān),該基因可參與內(nèi)臟脂肪組織的抗原加工、提呈及GDM相關(guān)通路的調(diào)控[33]。這些研究顯示,GDM患者外周組織胰島素抵抗的出現(xiàn)很可能源于DNA甲基化的表觀遺傳修飾。
DNA甲基化異常除了在GDM母體中出現(xiàn)外,在子代中同樣存在,并影響子代各組織的功能。研究[34]發(fā)現(xiàn),GDM患者子代內(nèi)臟脂肪組織中ADIPOQ基因甲基化水平增加,基因表達(dá)水平降低;胰腺基因組中GNAS等基因的DNA甲基化亦發(fā)生改變,可能與子代成年后的糖脂代謝異常、T2DM易感性和肥胖有關(guān)[35],且GDM的胰島素治療不能完全保護(hù)子代免受飲食引起的代謝紊亂。雄性子代小鼠胰島的全基因組DNA甲基化圖譜鑒定出了幾個調(diào)節(jié)胰島素分泌基因的高甲基化區(qū)域;這些基因包括ATP結(jié)合盒亞家族C成員8(ATP-binding cassette subfamily C member 8,Abcc8)等,其表達(dá)降低與胰島素分泌受損相關(guān)[36]。研究還發(fā)現(xiàn),母體宮內(nèi)高血糖環(huán)境引起子代生殖細(xì)胞的DNA甲基化重編程,從而形成代際遺傳[37-38],亦可誘發(fā)小鼠胎盤中Dlk1基因甲基化[39]。此外,GDM患者成年子代肌肉和脂肪組織中PPARGC1A基因的表達(dá)和DNA甲基化均發(fā)生改變[40];外周血中磷酸二酯酶6A(phosphodiesterase 6A,PDE6A)基因與GDM狀態(tài)相關(guān)[38]??梢姡贕DM子代中識別出的甲基化改變可能反映疾病發(fā)生機(jī)制或被利用于開發(fā)疾病的生物標(biāo)志物。
GDM除影響子代胰島素相關(guān)組織與器官外,還可通過DNA甲基化的方式影響其他類型組織的功能。研究顯示,GDM中脫嘌呤/脫嘧啶核酸內(nèi)切酶1(apurinic/apyrimidinic endodeoxyribonuclease 1,APEX1)基因甲基化水平的改變能夠調(diào)節(jié)子代上皮干細(xì)胞的增殖和自我更新[41],亦可引起子代神經(jīng)系統(tǒng)疾病的發(fā)生[42],且子代DNA甲基化模式的改變還與隨后的T2DM風(fēng)險相關(guān)[43]。與此同時,GDM患者所育新生兒的內(nèi)皮集落形成細(xì)胞功能受損,其原因在于暴露于GDM宮內(nèi)環(huán)境后內(nèi)皮集落形成細(xì)胞中胎盤特異蛋白8(placenta associated 8,PLAC8)基因表達(dá)增加,其表達(dá)水平與PLAC8中17個CpG位點的甲基化狀態(tài)呈負(fù)相關(guān)[44]。在心血管系統(tǒng)中,GDM子代沉默信息調(diào)節(jié)因子1(silent information regulator 1,Sirt1)的甲基化水平降低可促進(jìn)心肌缺血敏感性表型的胎兒編程[45]。更多研究發(fā)現(xiàn),GDM子代原始生殖細(xì)胞及睪丸中出現(xiàn)了胰島素抵抗和脂肪蓄積相關(guān)基因酪氨酸激酶(Fyn proto-oncogene,Src family tyrosine kinase,F(xiàn)yn)基因的低甲基化[37],且子代生長相關(guān)并發(fā)癥的發(fā)生可能與脂肪細(xì)胞因子如ADIPOQ的甲基化水平改變相關(guān)[32]。GDM患者及子代不同組織中基因DNA甲基化水平異常改變對母體及子代的影響見表1。
表1 GDM相關(guān)組織樣本中DNA甲基化水平的改變Tab 1 Changes of DNA methylation levels in GDM-related specimens
DNA甲基化作為一種常見的表觀遺傳修飾方式,發(fā)生機(jī)制與局部微環(huán)境關(guān)系密切,但其確切的調(diào)控機(jī)制尚不清楚。研究[46]發(fā)現(xiàn),人類GDM胎盤組織中miR-98的上調(diào)可靶向抑制甲基CpG結(jié)合蛋白2(methyl-CpG binding protein 2,MECP2)基因表達(dá),而MECP2表達(dá)水平的改變影響全基因組的甲基化狀態(tài),表明微小RNA的作用可能是DNA甲基化的一種調(diào)控模式。盡管目前DNA甲基化發(fā)生的原因尚不明確,但是其在GDM的診斷與治療中仍具有重要的應(yīng)用前景。一方面,DNA甲基化的改變廣泛存在于外周血中,并與機(jī)體多種指標(biāo)狀態(tài)相關(guān),如外周血中循環(huán)DNA的甲基化差異可用于監(jiān)測胰島β細(xì)胞的死亡狀況[47],顯示了其作為一種新型生物標(biāo)志物的巨大潛力;另一方面,DNA甲基化水平改變直接影響基因表達(dá)水平,進(jìn)而促進(jìn)基因相關(guān)功能發(fā)生改變,可見該表觀遺傳修飾方式亦可作為疾病治療的干預(yù)靶點。由于GDM宮內(nèi)高血糖環(huán)境可對胎兒代謝進(jìn)行重編程,以遺傳的方式產(chǎn)生代際間傳遞,對子代的健康影響嚴(yán)重且持久,DNA甲基化的調(diào)控可能成為一種預(yù)防GDM母胎危害的策略。隨著相關(guān)研究不斷深入,DNA甲基化有望為臨床上GDM的診斷與治療帶來新的啟示。
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