湯曉麗 鄧連瑞 張鵬霞 林加日 劉 揚 徐勁松 鄧立彬 (南昌大學醫學院,江西 南昌 00)
2型糖尿病(T2DM)是一種常見的以胰島素抵抗(IR)和β細胞分泌缺陷為特征的復雜性疾病,占糖尿病群體的大多數(95%)。因遺傳因素在T2DM的發生發展過程中起著重要的作用〔1〕,易感基因的發現一直是T2DM研究的熱點。近年來,隨著全基因組關聯研究(GWAS)在復雜性狀/疾病研究中的成功應用〔2〕,T2DM的遺傳學研究也取得了一系列成果。本文從T2DM發病機制及相關性狀兩方面對這些易感基因/位點進行總結,以期了解遺傳因素在糖尿發病過程中的作用機制。
在GWAS策略出現之前,連鎖分析和候選基因關聯研究是篩查疾病(或表型)有關易感基因/位點的主要方案。在連鎖分析方面,T2DM的基因組掃描已在全球多個群體中進行,鑒定了一些與T2DM有關的數量性狀基因座(QTLs)區域〔3〕。雖然后繼的定位克隆的確發現了可靠的易感基因(如CAPN10〔4〕),但由于該方法存在不能精細定位等缺點限制了其在復雜疾病研究中的廣泛應用。候選基因關聯研究是篩查疾病易感基因的另一傳統方法,前期T2DM候選基因多態性的研究為理解其發病機制提供了有益的線索(如 PPARG〔5〕、HNF1B〔6〕、WFS1〔7〕、TCF7L2〔8〕和 KCNJIl〔9〕),但候選基因的選擇帶有一定的盲目性。因此T2DM的遺傳學篩查亟需一個分辨力高且不存在生物假設的全基因組篩查策略。近年來,得益于組學數據的積累及芯片技術的發展,GWAS已被廣泛應用到T2DM的遺傳學研究中,取得了一系列成果。從2007年至今,全球已發表31項針對T2DM的GWAS篩查〔10~40〕,共鑒定出97個 T2DM易感基因/位點,其中包括了多個傳統方法確定的T2DM易感基因(如PPARG、KCNJ11、HNF1B、WFS1 和 TCF7L2等)。
同時,針對T2DM相關性狀的GWAS研究也的確表明,T2DM易感基因的多態性可以影響正常人的餐后2 h血糖和糖化血紅蛋白水平。其中,TCF7L2/rs12243326和腺苷酸環化酶5(ADCY5)/rs2877716既能增加空腹血糖,也能增加餐后2 h血糖水平〔41〕。而糖化血紅蛋白水平是一項反映慢性血糖水平的穩定指標,多篇研究提示錨定蛋白1基因(ANK1)〔42〕、周期素依賴性激酶5調節蛋白1樣蛋白1(CDKAL1)〔43〕、溶質載體家族 30(鋅轉運體)成員 5(SLC30A8)〔44〕和 TCF7L2〔45〕與糖化血紅蛋白水平相關。這些比較不但印證了T2DM-GWAS篩查結果的可靠性,也進一步為易感基因多態性導致疾病的分子機制研究提供線索。
目前,雖然GWAS為T2DM易感基因的發現帶來了突破性的進展,但眾多易感基因/位點的功能及其導致T2DM的分子機制仍需深入探討。
2.1 與β細胞胰島素分泌相關的T2DM易感基因 β細胞的功能下降(胰島素分泌受損)是T2DM的主要病理特征之一。在97個GWAS發現的T2DM易感基因中,目前認為與β細胞的胰島素分泌功能有關的有20個。其中,12個基因的功能被一些零散的研究所證實(KCNJ11、造血表達同源異形盒(HHEX)、HNF1B、CDKAL1、WFS1、CDKN2A/B、鋅指并列基因1(JAZF1)、電壓門控性鉀離子通道 KQT樣家族成員1(KCNQ1)、細胞分裂周期蛋白/鈣/鈣調蛋白依賴蛋白激酶ID(CDC123/CAMK1D)、甲狀腺腺瘤相關基因(THADA)、金屬肽酶含血小板反應蛋白9(ADAMTS9)、四旋蛋白8/富含亮氨酸重復單位的G蛋白耦聯受體5(TSPAN8/LGR5)〔46〕。而針對胰島素分泌相關性狀的 GWAS研究發現,GLIS家族鋅指3(GLIS3)、C2 鈣依 賴 域-含 蛋 白 4B(C2CD4B)、ADCY5、SLC30A8、TCF7L2、胰島素樣生長因子 2結合蛋白 2(IGF2BP2)、細胞周期蛋白依賴性激酶抑制劑2B(CDKN2B)和C2鈣依賴域-含蛋白4B(C2CD4A)等8個T2D易感基因的多態性可影響胰島素的分泌〔6~9,41~50〕。
空腹血糖水平(FBG)可反映β細胞的基礎分泌能力。2010年,MAGIC(Meta-Analyses of Glucose and Insulin-related traits Consortium)進行了一項針對FBG的GWAS研究,發現了16個 FBG關聯的基因/位點,其中5個(GLIS3、C2CD4B、ADCY5、SLC30A8和TCF7L2)為已知的T2DM易感基因〔48〕。后續的meta分析,不但印證了MAGIC的發現;還報道了20個新的與FBG關聯的基因,其中包含2個已知的T2DM易感基因(IGF2BP2、CDKN2B)〔49〕。這些研究表明已知的 T2DM 易感基因中有 7個與 FBG關聯,其中 4個基因 GLIS3、TCF7L2、SLC30A8和ADCY5得到了重復驗證。另一方面,這些研究也提示多數的FBG關聯基因可能僅與“生理”狀態下血糖的變化相關,并不影響“病理”狀態下血糖水平,但這一假設還需更深入的研究證實。
胰島素原是胰島素的前體,其水解過程是胰島素產生和分泌的關鍵步驟。2011年,Strawbridge等〔47〕的 GWAS分析發現了8個與血Proinsulin水平關聯的基因,其中有3個是T2DM的易感基因(TCF7L2、SLC30A8和C2CD4A/B)。有趣的是,這三個基因均被證實同空腹血糖性狀關聯,提示研究胰島素的加工成熟過程可為理解T2DM病理生理機制提供新的見解。而后續的的功能學研究進一步證實 TCF7L2〔51〕和 SLC30A8〔52〕的確與胰島素原轉化為胰島素有關;其中TCF7L2-rs7903146的TT基因型與胰島素原轉化能力降低及胰島素分泌下降相關〔53〕,但具體機制尚不明確。
2.2 與胰島素敏感性相關的T2DM易感基因 IR是T2DM的另一個病理特征,是指體內周圍組織對胰島素的敏感性降低。將前期針對IR衡量指標(HOMA-IR)及其他IR有關性狀(空腹胰島素水平、高胰島素血癥)的GWAS研究綜合共發現11個T2DM的易感基因(WFS1、胰島素受體底物1(IRS1)、TCF7L2、SLC30A8、鋅指AN1型域3(ZFAND3)、萌芽同源物2(果蠅)(SPRY2)、肽酶D(PEPD)、神經突觸素2(SYN2)、生長因子受體結合蛋白14(GRB14)、PPARG和CDKAL1)可影響胰島素敏感性〔49,54〕。此外,非GWAS研究提示,其他一些基因與外周胰島素敏感性或胰島素抵抗相關,如ENPP1、ADIPOQ、AHSG、ADAMTS9、CAPN10、SREBF1、PPARGC1A 和 SHBG 等。其中 ADAMTS9已被GWAS研究證實是T2DM的易感基因〔48〕。
HOMA-IR是數值化衡量IR的常用指標〔55〕,在97個T2DM易感基因中已確定 6 個基因(WFS1、IRS1、TCF7L2、CDKAL1、SLC30A8和ZFAND3)與其相關〔54〕??崭挂葝u素水平是反映IR的另一常用指標,在血糖水平正?;蛏叩娜巳褐?,空腹胰島素水平增高表明IR情況的存在。而另一些研究則提示,空腹胰島素水平可識別糖尿病前期〔56〕。在97個GWAS確定的T2DM 的易感基因中,7 個基因 SPRY2、PEPD、IRS1、SYN2、GRB14、PPARG和TCF7L2與空腹胰島素水平相關〔49〕。高胰島素血癥和IR緊密聯系,被認為是IR的又一標志,是引發糖尿病患者大血管并發癥(如心肌梗死、腦卒中、高血壓、血脂紊亂、糖尿病腎病等糖尿病并發癥)的主要原因。在GWAS確定的T2DM 的易感 基 因中,WFS1、IRS1、TCF7L2、CDKAL1 和SLC30A8等與高胰島素血癥相關〔54〕,這幾個基因也與HOMAIR指標關聯。
相關性狀的研究發現IRS1和TCF7L2與HOMA-IR、空腹胰島素及高胰島素血癥等三種性狀均相關。其中IRS1為胰島素受體底物1,可以與胰島素受體結合并參與胰島素介導的信號轉導;其作為T2DM的易感基因,無論是從基因及蛋白的表達水平,還是變異導致氨基酸的替換方面都已被廣泛的研究。如在脂肪細胞中IRS1基因及蛋白的低表達可預測胰島素抵抗和 T2DM〔57〕。Almind 等〔58〕提出 IRS 的 Gly971Arg替換可能損害胰島素刺激的信號傳導,導致胰島素抵抗。而Clausen等〔59〕報道,971Arg等位基因可聯合肥胖因素,使胰島素敏感性降低50%。而對于TCF7L2基因,目前多數研究證實其可以通過降低胰島素分泌來增加T2DM的易感性;但另也有研究提示它可通過和胰島素分泌不足兩個環節共同起作用,具體機制尚待深入研究。
2.3 肥胖及相關體質人類學性狀 肥胖是導致繼發性IR的最重要的因素。在97個 T2D易感基因/位點中,4個基因SPRY2、IRS1、脂肪量和肥胖相關基因(FTO)和包含WW域的氧化還原酶(WWOX)與肥胖相關〔60〕。研究表明,FTO是通過肥胖增加 T2DM的易感性〔61〕。此外,5個 T2DM易感基因,GRB14、血管內皮生長因子(VEGFA)、ADAMTS9、FTO和 CDKAL1與肥胖相關的數量性狀相關。如GRB14、VEGFA和ADAMTS9 與腰臀比相關〔62〕;FTO 與臀圍、腰圍和體重相關〔63,64〕;CDKAL1和FTO與身體質量指數(BMI)相關〔65〕。另外,FTO還與兒童早期極端肥胖〔33〕及皮下脂肪組織相關,ADAMTS9和IRS1與內臟脂肪組織/皮下脂肪組織的比例相關〔66〕。雖然肥胖與T2DM之間的關系已相對明確,但相當一部分重要的肥胖關聯基因,僅與T2DM存在弱關聯甚至不關聯〔61〕,因此肥胖和糖尿病之間的相互關系還有待深入研究。
同時,在T2DM的易感基因中,還有部分基因與其他一些性狀關聯:如FTO和TCF7L2與代謝綜合征相關〔67〕;ADCY5與出生體重相關〔68〕;GRB14 與血壓相關〔69〕;IRS1、CMIP、PEPD 和VEGFA與脂聯素水平相關〔70〕等。其中T2DM易感基因與脂代謝紊亂的關聯最具臨床意義,因為相當一部分糖尿病人都伴有高脂血癥。在97個T2DM的易感基因中,Kruppel樣因子14(KLF14)、c-Maf誘導蛋白(CMIP)、IRS1和肝細胞核因子4α(HNF4A)與高密度膽固醇相關;肝細胞核因子1α(HNF1A)與低密度膽固醇水平相關;HNF4A和HNF1A與總膽固醇量相關;IRS1與甘油三酯相關〔71〕。
綜上所述,近年發現的T2DM易感基因/位點極大擴展了我們對T2DM遺傳因素的認識,但GWAS的結果與傳統研究尚存在一些出入,如CAPN10、SREBF1、SHBG和PPARGC1A等基因在GWAS篩查中并未得到驗證。而在作用機制方面,雖然先前的一些研究認為IR是T2DM發病的重要分子機制;但篩查發現的多數易感基因與胰島素分泌關聯,提示β細胞的功能在T2DM中也發揮著重要作用。因此T2D的遺傳學研究不但需要后續進一步基于大樣本篩查易感基因,還需要對T2DM易感基因/位點的致病機制進行深入的功能學研究。
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