邴 浩，沈劍華，李異玲

中國醫(yī)科大學(xué)附屬第一醫(yī)院消化內(nèi)科，遼寧 沈陽 110001

TM6SF2 rs58542926 E167K 單核苷酸多態(tài)性與肝硬化、肝細(xì)胞癌易感性的關(guān)系

邴 浩，沈劍華，李異玲

中國醫(yī)科大學(xué)附屬第一醫(yī)院消化內(nèi)科，遼寧 沈陽 110001

跨膜蛋白6超家族成員2(transmembrane 6 superfamily member 2，TM6SF2)位于第19號染色體上，主要表達(dá)于小腸及肝臟，參與脂質(zhì)的調(diào)控。rs58542926處基因突變致其編碼的蛋白表達(dá)降低。研究顯示，TM6SF2 rs58542926 E167K單核苷酸多態(tài)性與肝內(nèi)脂質(zhì)含量、血清肝酶含量及肝纖維化均有相關(guān)性，是非酒精性脂肪性肝病(non-alcoholic fatty liver disease，NAFLD)的獨立危險因素。最近研究發(fā)現(xiàn)，TM6SF2 rs58542926 E167K單核苷酸多態(tài)性與肝硬化及肝細(xì)胞癌也具有一定相關(guān)性。

肝硬化；肝細(xì)胞癌；TM6SF2 rs58542926 E167K；易感性

肝細(xì)胞癌(hepatocellular carcinoma，HCC)在全世界癌癥發(fā)病率居第6位，占癌癥死亡原因的第3位，是最常見的消化道腫瘤，也是肝硬化患者的重要死因[1]。中國有世界上約55%的HCC患者，其5年總體生存率(overall survival，OS)只有7%[2]。HCC主要病因為慢性病毒性(HBV或HCV)肝炎[3]，非酒精性脂肪性肝病(non-alcoholic fatty liver disease,NAFLD)及非酒精性脂肪性肝炎(non-alcoholic steatohepatitis,NASH)相關(guān)性肝癌的發(fā)病率也在不斷增加。

隨著精準(zhǔn)醫(yī)學(xué)及全基因組關(guān)聯(lián)分析(genome-wide association studies，GWAS)的發(fā)展，遺傳因素不斷得到重視。PNPLA3已經(jīng)被證明與NAFLD、肝硬化及HCC的發(fā)展具有相關(guān)性[4-6]。近年來發(fā)現(xiàn)TM6SF2 rs58542926 E167K也是NAFLD發(fā)生的易感因素，本文擬就TM6SF2基因與肝硬化及HCC的關(guān)系作一概述。

1 TM6SF2的一般特性

1.1 TM6SF2 rs58542926 E167K的單核苷酸多態(tài)性 跨膜蛋白6超家族2(TM6SF2)位于第19號染色體上(19p12)，編碼一個由351個氨基酸構(gòu)成的蛋白，該蛋白為多次跨膜結(jié)構(gòu)，主要表達(dá)于肝臟、小腸及腎臟，在小腸中表達(dá)最多。TM6SF2 rs58542926 E167K突變(EK/KK)使其第499號核苷酸由胞嘧啶(C)錯義突變?yōu)樾叵汆奏?T)，導(dǎo)致其編碼的蛋白第167殘基由谷氨酸(E)突變?yōu)橘嚢彼?K)[7-8]，突變大致發(fā)生在蛋白第5個跨膜結(jié)構(gòu)[9]，使該蛋白功能喪失。

1.2 TM6SF2 rs58542926 E167K與脂質(zhì)、肝酶變化的關(guān)系 GWAS及小鼠體內(nèi)的研究[9-12]表明TM6SF2 rs58542926 E167K單核苷酸多樣性通過影響肝內(nèi)富甘油三酯脂蛋白(TG-rich lipoproteins，TRLs)及脂質(zhì)小滴含量而影響肝脂肪代謝，使肝內(nèi)甘油三酯含量(hepatic triglyceride content，HTGC)升高，而血脂(TG、LDL)含量降低；此外，抑制TM6SF2表達(dá)還會減少一些在TG合成起重要作用的基因(如PNPLA3、ACSS2、DGAT1及DGAT2)的表達(dá)，增大肝內(nèi)脂滴的范圍[12]。這使TM6SF2與各種肝病及心血管疾病[13-14]的發(fā)生具有一定的相關(guān)性。

TM6SF2 rs58542926 E167K可引起血漿丙氨酸氨基轉(zhuǎn)移酶(alanine aminotransferase,ALT)含量顯著升高(P=0.003)，突變純合子(KK)有引起血漿天冬氨酸氨基轉(zhuǎn)移酶(aspartate transaminase,AST)升高的趨勢，可引起血漿堿性磷酸酶(alkaline phosphatase, ALP)顯著下降，而與谷氨酰轉(zhuǎn)肽酶(gamma glutamyl transpeptidase,GGT)的相關(guān)性并未得到[7]。研究[15]顯示rs58542926多樣性與NAFLD患者ALT(P=3.2×10-6)及AST(P=0.007)水平明顯相關(guān)，相比于C等位基因(EE)攜帶者T等位基因(EK+KK)攜帶PNPLA3 rs738409 G者轉(zhuǎn)氨酶水平明顯升高(ALT：9.8%；AST：5%)，但明顯低于對肝酶學(xué)變化的影響(ALT：28%)，慢性肝病患者中并未觀察到這種相關(guān)性。

2 TM6SF2 rs58542926 E167K與肝脂肪化及NAFLD相關(guān)性研究

NAFLD為一種代謝綜合征在肝內(nèi)的表現(xiàn)，是一種進(jìn)行性的肝臟疾病，以HTGC升高為特征，包括肝脂肪變性、NASH及肝纖維化，一些NAFLD甚至進(jìn)展為肝硬化或HCC[16]。

早期研究[17-18]發(fā)現(xiàn)與TM6SF2連鎖不平衡的基因NCAN(rs2228603 C>T)SNPs與HTGC及NAFLD明顯相關(guān)，而Kozlitina等[7]研究表明TM6SF2 rs58542926 E167K SNPs導(dǎo)致HTGC升高而非NCAN,且首次提出TM6SF2與NAFLD有關(guān)。之后的研究表明TM6SF2 rs58542926 E167K SNPs使其表達(dá)蛋白減少，致使極低密度脂蛋白(very low-density lipoprotein,VLDL)含量降低導(dǎo)致HTGC升高，并導(dǎo)致肝臟脂肪變性(OR=1.379，95%CI：1.019～1.865，P=0.037)[16]，最終導(dǎo)致NAFLD的發(fā)生[19]。TM6SF2 rs58542926 E167K與NAFLD的相關(guān)性已被很多相關(guān)研究[19-21]證實，而兩項分別基于日本人及中國人的研究[22-23]顯示TM6SF2 rs58542926 E167K SNPs與NAFLD無相關(guān)性，考慮與樣本量小及選擇偏倚的存在等因素有關(guān)。

3 TM6SF2基因與肝纖維化及肝硬化易感性的關(guān)系

3.1 TM6SF2基因與肝纖維化的關(guān)系 Liu等[16]及Sookoian等[20]研究發(fā)現(xiàn)TM6SF2 rs58542926與NAFLD患者肝纖維化程度有關(guān)(β=0.549±0.135，95%CI：0.285～0.813，P<0.01)，是NAFLD相關(guān)性肝纖維化的獨立影響因素(β=0.357±0.079，95%CI：0.203～0.511，P<0.01)；按纖維化程度不同等級分析，發(fā)現(xiàn)TM6SF2 rs58542926 C>T次等位基因明顯增加晚期肝纖維化的風(fēng)險。Dongiovanni等[24]認(rèn)為TM6SF2致肝纖維化是由于肝內(nèi)脂肪沉積而非致纖維化作用導(dǎo)致的。

一項Meta分析[25]顯示，TM6SF2突變型個體血脂含量水平較低，考慮是由于脂質(zhì)不能被肝臟分泌沉積于肝臟的原因，這也對肝臟造成損傷，進(jìn)而發(fā)展為肝纖維化。研究顯示T的攜帶者(EK+KK的個體)發(fā)生嚴(yán)重肝纖維化的風(fēng)險大于C的純合子(EE)，TM6SF2 rs58542926基因位點多態(tài)性與肝纖維化嚴(yán)重程度相關(guān)(OR=1.491，95%CI：1.131～1.851，P<0.0001)。

3.2 TM6SF2基因與肝硬化易感性的關(guān)系 Falleti等[26]研究將受試對象分為病毒性肝硬化、酒精性肝硬化及對照組，結(jié)果顯示病毒性肝硬化組與對照組TM6SF2 rs58542926次等位基因頻率無明顯差異，而酒精性肝硬化組與對照組基因頻率具有明顯差異。說明TM6SF2 rs58542926與酒精性肝硬化可能存在一定的相關(guān)性，而具體關(guān)系及機制還需TM6SF2 rs58542926(T)更多研究證實。

4 TM6SF2基因與HCC易感性的關(guān)系

4.1 TM6SF2基因與NAFLD相關(guān)性HCC易感性的關(guān)系 Liu等[16]研究顯示TM6SF2 rs58542926次等位基因與NAFLD相關(guān)性HCC發(fā)病相關(guān)(OR=1.922，95%CI：1.31～2.81，P<0.01)。但當(dāng)與其他危險因素一起分析時，差異無統(tǒng)計學(xué)意義(P=0.42)，考慮可能與樣本量小、基因頻率低等有關(guān)。

4.2 TM6SF2基因與肝硬化惡變HCC的關(guān)系 Falleti等[26]研究了PNPLA3及TM6SF2與HCC的關(guān)系，研究納入了511例肝硬化的患者做回顧性調(diào)查。研究顯示與HCC的發(fā)生相關(guān)(OR=1.66，95%CI：1.01～2.74，P=0.043)，而與病毒相關(guān)性HCC的發(fā)生差異無統(tǒng)計學(xué)意義。兩種基因在HCC的作用中沒有相互作用。TM6SF2 rs58542926 T等位基因被稱為HCC的高風(fēng)險基因型(high risk genotypes，HGR)，C等位基因被稱為低風(fēng)險基因型(low risk genotypes，LGR)，TM6SF2 rs58542926 T/*基因型在HCC患者中的比例高于非HCC患者(OR=2.57，P=0.035)，HGR在乙肝、丙肝及酒精性肝硬化導(dǎo)致的HCC患者中頻率依次升高，且與酒精性肝硬化相關(guān)HCC的發(fā)生具有明顯相關(guān)性(P=0.0007)，而與病毒相關(guān)HCC的數(shù)據(jù)分析差異卻無統(tǒng)計學(xué)意義，考慮與基因?qū)χ|(zhì)的調(diào)節(jié)作用有關(guān)。

總之，TM6SF2 rs58542926 E167K的作用至今仍未完全明確，逐漸有研究證明其與肝硬化、HCC的相關(guān)性。但TM6SF2 rs58542926 E167K如何介導(dǎo)HCC、肝硬化的具體機制仍需研究，與不同類型的肝硬化、HCC發(fā)生的關(guān)系還需更多研究證實。而TM6SF2 rs58542926次等位基因T純合子基因頻率低(<1%)，尚需更多的研究充分證實基因的影響。

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(責(zé)任編輯：王全楚)

The relationship between TM6SF2 rs58542926 E167K single nucleotide polymorphisms and the susceptibility of liver cirrhosis and hepatocellular carcinoma

BING Hao, SHEN Jianhua, LI Yiling

Department of Gastroenterology, the First Affiliated Hospital of China Medical University, Shenyang 110001, China

The transmembrane 6 superfamily member 2 (TM6SF2) is located on chromosome 19, mainly expressed in the intestine and liver, involved in lipid metabolism. The genic mutation of rs58542926 decreases the protein expression. Studies show that TM6SF2 rs58542926 E167K single nucleotide polymorphisms are associated with levels of liver fat, plasma liver enzyme levels and fibrosis severity, which are independent risk factors for nonalcoholic fatty liver disease (NAFLD). Recent studies find that TM6SF2 rs58542926 E167K single nucleotide polymorphisms are also associated with liver cirrhosis and hepatocellular carcinoma.

Liver cirrhosis; Hepatocellular carcinoma; TM6SF2 rs58542926 E167K; Susceptibility

國家自然科學(xué)基金(81570519)

邴浩，碩士研究生,研究方向：非酒精性脂肪性肝病的相關(guān)基因?qū)W研究。E-mail：2247725858@qq.com

李異玲，博士，主任醫(yī)師，教授，研究方向：非酒精性脂肪性肝病的相關(guān)基因?qū)W研究。E-mail：lyl-72@163.com

10.3969/j.issn.1006-5709.2017.04.003

R735.7；R575.2

A

1006-5709(2017)04-0369-03

2016-09-12