基于双向孟德尔随机化的不同BMI分型非酒精性脂肪性肝病与2型糖尿病的遗传关联分析
DOI: 10.12449/JCH241011
The genetic association between nonalcoholic fatty liver disease and type 2 diabetes mellitus in different body mass index categories: A bidirectional Mendelian randomization study
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摘要:
目的 运用双向双样本孟德尔随机化(MR)评估非酒精性脂肪性肝病(NAFLD)与2型糖尿病(T2DM)的遗传关联,并进一步探讨不同BMI的NAFLD人群与T2DM的因果关系。 方法 数据来源于以欧洲人群为研究对象的全基因组关联研究,其中NAFLD的样本量为32 941例,T2DM为312 646例,BMI为681 275例。运用单变量、多变量MR方法评估NAFLD总人群及各BMI亚型与T2DM之间的双向因果关系。采用逆方差加权法、MR-Egger回归、约束最大似然与模型平均法、加权中位数法进行MR分析,采用MR多效性残差和与离群值、径向MR、MR-Egger截距法、Cochran Q检验进行敏感性分析。 结果 单变量MR分析显示NAFLD总人群与T2DM之间存在双向因果关系(正向OR=9.75,95%CI:2.57~37.00,P<0.001;反向OR=1.01,95%CI:1.00~1.01,P<0.01)。多变量MR分析显示经BMI校正后,NAFLD总人群与T2DM的因果关系仍然保持显著(OR=33.12,95%CI:7.57~144.95,P<0.000 1)。亚组分析显示,NAFLD各亚组均与T2DM存在因果关系(瘦型OR=12.19,95%CI:3.35~44.40,P<0.001;超重型OR=4.30,95%CI:1.69~10.92,P<0.01;肥胖型OR=1.67,95%CI:1.14~2.44,P<0.01)。 结论 本研究从遗传学层面揭示了NAFLD总人群及各BMI亚型与T2DM之间的因果关系。 Abstract:Objective To investigate the genetic association between nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM) using bidirectional two-sample Mendelian randomization (MR), as well as the causal relationship between NAFLD and T2DM across different body mass index (BMI) categories. Methods The data were derived from genome-wide association studies conducted in European populations, with a sample size of 32 941 cases for NAFLD, 312 646 cases for T2DM, and 681 275 cases for BMI. The univariate and multivariate MR methods were used to assess the bidirectional causal relationship between NAFLD and T2DM in the general population and across different BMI subtypes. The methods of inverse-variance weighting, MR-Egger regression, constrained maximum likelihood and model averaging, and weighted median were used to conduct the MR analysis, and MR-Pleiotropy Residual Sum and Outlier, radial MR, the MR-Egger intercept method, and the Cochrane Q test were used for sensitivity analysis. Results The univariate MR analysis revealed a bidirectional causal relationship between NAFLD and T2DM in the general population (forward analysis: odds ratio [OR]=9.75, 95% confidence interval [CI]: 2.57 — 37.00, P<0.001; reverse analysis: OR=1.01, 95%CI: 1.00 — 1.01, P<0.01). After adjustment for BMI, the multivariate MR analysis showed that the causal relationship between NAFLD and T2DM remained significant in the general population (OR=33.12, 95%CI: 7.57 — 144.95, P<0.000 1). The subgroup analysis showed a causal relationship between NAFLD and T2DM across all BMI subtypes (lean subgroup: OR=12.19, 95%CI: 3.35 — 44.40, P<0.001; overweight subgroup: OR=4.30, 95%CI: 1.69 — 10.92, P<0.01; obese subgroup: OR=1.67, 95%CI: 1.14 — 2.44, P<0.01). Conclusion This study reveals the causal relationship between NAFLD and T2DM in the general population of NAFLD and across different BMI subtypes from a genetic perspective. -
非酒精性脂肪性肝病(NAFLD)是一种与代谢相关的临床病理综合征,其表现为>5%的肝细胞存在脂肪变性,且无明显的饮酒、病毒感染及其他继发性原因[1]。近年来其患病率逐步增加[2],为全人类亟待解决的健康问题之一。传统上认为NAFLD与超重或肥胖高度相关[3],然而近年来越来越多的研究[4-6]证明瘦型NAFLD亦为一种不容忽视的临床类型,其往往有着更加严重的不良结局,因此不同BMI类型的NAFLD病理生理学原理及病程转归的差异仍待进一步挖掘。
2型糖尿病(T2DM)作为全身慢性代谢障碍的重要表现形式,其发病机制与胰岛素抵抗密切相关[7]。大量流行病学资料显示NAFLD和T2DM是两种经常并存的病理状态,多达2/3的T2DM患者罹患NAFLD[8],近1/3的NAFLD患者罹患T2DM[9],且瘦型NAFLD亦常与T2DM共存[10]。一项样本量约为12 000例患者的研究[11]表明,NAFLD、肥胖和胰岛素抵抗均与T2DM风险增加独立相关,当三个危险因素同时存在时,患T2DM的风险增加了14倍。当前已有许多研究证实NAFLD、T2DM、BMI互为彼此病程进展的重要独立预测因素,存在着显著的相关性。
由于观察性研究极易受到潜在混杂因素的影响,难以明确因果关联,因此本研究拟引入一种全新的病因推断方法——孟德尔随机化(mendelian randomization, MR),通过将遗传变异,即单核苷酸多态性(single nucleotide polymorphism,SNP)作为工具变量,从基因层面模拟随机对照试验,评估暴露与结局之间的因果关联[12],从而极大地规避观察性研究的天然缺陷,并降低反向因果的不良影响[13]。本研究旨在运用双向单变量及多变量MR方法,探究NAFLD全人群及其各BMI亚型与T2DM之间的因果效应,更加深入地探讨各亚型共病的异同及潜在机理,从而更好地指导临床疾病管理,一定程度上减轻社会医疗负担。
1. 资料与方法
1.1 研究设计
本研究遵循孟德尔随机化研究报告规范(STROBE-MR指南)[14]。由于单样本MR研究样本重叠率较高、数据较难获取,拟运用已发表的大型全基因组关联研究(GWAS)汇总数据进行双样本MR分析,具体设计如下:首先,运用单变量MR分析探究NAFLD总人群与T2DM的因果关系,并进行反向分析;随后,采用多变量MR分析,纳入BMI新变量进行校正;最后,通过单变量MR分析进一步研究NAFLD亚组人群(瘦型:BMI<25 kg/m2,超重型:25 kg/m2≤BMI<30 kg/m2,肥胖型:BMI≥30 kg/m2)与T2DM的双向因果关联,并探讨各组间的差异。研究流程见图1。
1.2 NAFLD、T2DM、BMI的GWAS数据来源
本研究中,NAFLD数据来自一项英国生物样本库(UKB)的GWAS研究,其中NAFLD的诊断依据为全肝质子密度脂肪分数≥5%,且排除了患有酒精性肝病、病毒性肝炎、药物性肝损伤等参与者,其中病例组6 623例、对照组26 318例。此外,该GWAS研究进一步根据世界卫生组织的BMI分级标准,将该队列分为瘦型、超重型和肥胖型三组,其中病例组分别为810、3 069、2 744例,对照组分别为12 804、10 641、2 873例[15]。T2DM数据源来自于常见代谢疾病知识门户(CMDKP)网站(https://hugeamp.org/)上发表的一篇GWAS-Meta研究,其中病例组33 139例、对照组279 507例[16]。BMI数据来自人体测量学性状的遗传研究联盟(GIANT)(https://portals.broadinstitute.org/collaboration/giant/index.php/GIANT_consortium)中的一篇已知样本量最大的GWAS-Meta研究[17]。暴露与结局的详细信息见表1。
表 1 各表型GWAS数据汇总信息Table 1. Summary information of GWAS data for each phenotype表型 数据来源 PMID 样本量(例) 种族 NAFLD(总人群) UKB 37235137 32 941 欧洲 瘦型NAFLD UKB 37235137 13 614 欧洲 超重型NAFLD UKB 37235137 13 710 欧洲 肥胖型NAFLD UKB 37235137 5 617 欧洲 T2DM CMDKP 34862199 312 646 欧洲 BMI GIANT 30124842 681 275 欧洲 注:PMID,PubMed唯一标识码。
1.3 工具变量选择
为了筛选出合格的遗传工具变量,制定以下标准:(1)符合关联性假设:以P<5×10-8为过滤条件,筛选出与暴露因素显著相关的SNP位点,其中部分数据(瘦型NAFLD/肥胖型NAFLD→T2DM)因缺乏足够数量的SNP(n≤2)而无法进行MR分析,故放宽过滤标准,改取P<5×10-6;(2)去除连锁不平衡:以连锁不平衡系数r2<0.001,区域宽度kb>10 000为过滤条件,保证SNP之间的独立性;(3)满足排他性假设:在上述得到的SNP中,去除与结局数据相关的SNP,P值同第一步;(4)避免反向因果:通过Steiger检验,清除与结局相关性比暴露相关性大的SNP[18];(5)运用Harmonise协同处理:将工具变量的效应等位基因对齐,去除回文SNP,最终获得能够代表暴露因素的最佳工具变量。
1.4 MR分析
采用R 4.3.1软件,调用TwoSampleMR、MendelianRandomization等软件包进行以下分析:单变量MR及亚组分析方面,IVW[19]是评价MR因果效力最高的方法,计算OR及95%CI,评估表型间潜在的因果关联,另以MR-Egger回归[20]、cML-MA[21]、WM[22]作为MR证据补充的次要方法。当IVW方法P<0.05时,代表存在因果关系,若在此基础上MR-Egger、cML-MA、WM法与IVW的效应值方向相同,则进一步加强了该因果关联的稳定性。多变量MR分析[23]方面,使用IVW方法评估多个暴露对结局的影响,当P<0.05时表示存在因果关系。此外对所有结果进行反向分析,探知是否存在反向因果关联。
1.5 敏感性分析
水平多效性方面,首先运用MR-Egger截距法识别潜在的水平多效性,当P>0.05时提示不存在多效性,遵循MR基本核心假设[24];采用MR多效性残差和与离群值(MR-PRESSO)检验[25]剔除离群值,若MR-PRESSO检验全局P值或MR-Egger截距法P值仍然<0.05,则进一步采用径向MR方法[26]剔除离群值后再进行MR分析,以加强结果的稳健性。异质性方面,采用Cochran Q检验[27]量化评估,当P>0.05时表示不存在异质性,提示MR结论相对可靠;当存在异质性时,则运用IVW随机效应模型分析。此外,采用Leave-one-out检验[28],计算逐一剔除各个SNP后剩余SNP的效应值,以评估每个SNP对总效应值的影响。
2. 结果
2.1 NAFLD总人群与T2DM的单变量MR分析
IVW分析结果显示,NAFLD与T2DM存在正向因果关联(OR=9.75,95%CI:2.57~37.00,P<0.001)。除MR-Egger回归以外,cML-MA、WM法的结果亦具有统计学意义,且各补充方法的效应值方向均与IVW分析结果保持一致。反向分析方面,IVW分析提示T2DM与NAFLD亦存在因果关联(OR=1.01,95%CI:1.00~1.01,P<0.01)(图2)。
2.2 经BMI调整的多变量MR分析
IVW分析结果显示,经BMI调整后,NAFLD与T2DM的因果关系仍然保持显著(OR=33.12,95%CI:7.57~144.95,P<0.000 1)(图3)。
2.3 以BMI分层的亚组分析(图4)
2.3.1 瘦型NAFLD
IVW分析结果显示,瘦型NAFLD与T2DM存在双向因果关联(正向OR=12.19,95%CI: 3.35~44.40,P<0.001;反向OR=1.01,95%CI:1.00~1.01,P<0.01)。
2.3.2 超重型NAFLD
IVW分析结果显示,超重型NAFLD与T2DM存在双向因果关联(正向OR=4.30,95%CI:1.69~10.92,P<0.01;反向OR=1.01,95%CI:1.00~1.02,P<0.01)。
2.3.3 肥胖型NAFLD
IVW分析结果显示,肥胖型NAFLD与T2DM存在正向因果关联(OR=1.67,95%CI:1.14~2.44,P<0.01)。反向IVW分析显示,T2DM与肥胖型NAFLD未观察到因果关联(OR=1.01,95%CI:0.99~1.02,P=0.54)。
2.4 敏感性分析
水平多效性方面,亚组分析中的瘦型NAFLD与T2DM的MR-Egger截距法P<0.05,故而采用径向MR方法进一步剔除离群值,调整后MR-PRESSO检验P值>0.05,不存在水平多效性;经BMI调整的多变量组MR-Egger截距法计算P=0.71,不存在水平多效性;其余各组结果亦提示不存在水平多效性。异质性方面,Cochran Q检验显示除NAFLD总人群对T2DM组(P=0.02)、经BMI调整的多变量组(P<0.000 1)、超重型NAFLD对T2DM组(P=0.04)结果存在异质性,采用随机效应模型,其余各组P值均>0.05,提示不存在异质性(表2)。Leave-one-out检验森林图可见每组各SNP剔除后的效应值均位于0的同侧,提示各组结果较为稳健(图5)。
表 2 NAFLD各组与T2DM的敏感性分析Table 2. Sensitivity analysis of NAFLD subgroups and T2DM暴露 结局 正向 反向 异质性(P值) 水平多效性 异质性(P值) 水平多效性 MR-Egger截距法P值 MR-PRESSO P值 MR-PRESSO 剔除SNP MR-Egger截距法P值 MR-PRESSO P值 MR-PRESSO 剔除SNP NAFLD总人群 T2DM 0.02 0.69 0.32 无 0.41 0.91 0.38 无 瘦型NAFLD T2DM 0.98 0.05 0.71 rs17547923,
rs584898061)
0.99 0.30 0.99 无 超重型NAFLD T2DM 0.04 0.39 0.21 无 0.61 0.40 0.58 无 肥胖型NAFLD T2DM 0.74 0.32 0.60 无 1.00 0.87 1.00 无 注:1)第一次MR-Egger截距法P=0.002,进一步采用径向MR剔除的SNP离群值。
3. 讨论
NAFLD是一种与胰岛素抵抗和遗传易感密切相关的代谢应激性肝病[29],长期临床观察发现NAFLD与T2DM常常共病,因此两疾病间潜在的因果效应历来为研究者的热点话题。近年虽然已有两疾病相关MR研究发表[30-31],但鲜有探讨NAFLD各BMI亚型与T2DM之间的关系。本研究基于欧洲人群GWAS数据来源,运用MR分析,一方面明确了NAFLD总人群与T2DM之间的双向因果关系,并发现以NAFLD指向T2DM较为显著;另一方面通过亚组分析量化了瘦型、超重型、肥胖型NAFLD与T2DM之间的因果效应,发现OR效应值与BMI呈潜在的负向线性关系,即BMI越小,NAFLD与T2DM的因果关联越强。
通过单变量MR分析,本研究发现NAFLD总人群与T2DM互相存在因果关系(正向OR=9.75,反向OR=1.01),且正向效应更为显著。Mantovani等[32]在一项涉及296 439例人群的Meta分析中发现,NAFLD患者发生T2DM的风险比无NAFLD的患者高约2倍。他们在随后的研究[33]中还指出,脂肪变性和肝纤维化越严重,罹患T2DM的风险越高,并且与年龄、性别和肥胖无关。NAFLD和T2DM之间存在很强的双向关系,有大量流行病学数据[34]表明NAFLD多发生于T2DM之前,但也有少部分资料表明T2DM优先发生。从病理机制角度来看,一方面NAFLD通过脂肪沉积、炎症、内质网应激等途径加剧肝脏胰岛素抵抗,促进血糖升高,推动T2DM进展;另一方面,T2DM还可通过胰岛素抵抗、炎症、氧化应激等加剧糖脂代谢紊乱,引发NAFLD,加速肝纤维化、肝硬化乃至肝癌的进程[35]。二者互相促进疾病的发展,大大增加不良结局的风险[36]。此外,众所周知NAFLD的发生发展是遗传因素、环境因素综合作用的结果,其中patatin样磷脂酶结构域蛋白3(PNPLA3)rs738409、跨膜蛋白6超家族成员2(TM6SF2)rs58542926基因多态性均是较强的遗传预测因子[37]。在一项涉及30万余人的大型GWAS研究[38]中,PNPLA3 I148M与TM6SF2 E167K的遗传变异不仅与脂肪变性、非酒精性脂肪性肝炎、肝硬化和肝细胞癌相关,还与较高的T2DM风险密切相关。一项动物实验研究[30]显示,与对照组小鼠相比,PNPLA3 I148M小鼠不仅表现出明显的NAFLD遗传易感性,还在糖耐量试验中表现出了高血糖状态和葡萄糖清除率延迟。因此,遗传变异层面也可印证NAFLD与T2DM之间的相关性。
随后,本研究加入BMI表型进行了多变量MR分析。众所周知,超重/肥胖、T2DM是代谢综合征的重要特征,NAFLD也常被认为是代谢综合征的肝脏表现[39]。然而,尽管NAFLD、T2DM患病率的升高趋势与超重/肥胖保持一致[3,40],但瘦型患者中NAFLD的流行现象提示超重/肥胖并非NAFLD的唯一驱动因子,不应将超重/肥胖作为NAFLD筛查的唯一标准[41]。多变量MR分析结果表明,在校正遗传预测的BMI后NAFLD和T2DM的因果关系仍然成立(OR=33.12),这一定程度上验证了NAFLD对T2DM的风险并不全与超重/肥胖相关,这一结果对解释瘦型NAFLD的存在具有重要意义。
对NAFLD进行BMI分层的亚组分析,结果显示NAFLD各BMI亚型与T2DM均存在正向因果关系(瘦型OR=12.19,超重型OR=4.30,肥胖型OR=1.67),且OR效应值随BMI的升高而降低。这意味着瘦型NAFLD与T2DM的因果关系更强烈,超重型NAFLD次之,肥胖型NAFLD最弱。这也预示着瘦型NAFLD与超重/肥胖型NAFLD在病理机制方面可能存在差异。首先,PNPLA3 rs738409基因多态性是瘦型NAFLD的主要危险因素之一,它通过降低甘油三酯水解酶活性、提高溶血磷脂酸酰基转移酶活性,促使脂质分解下降而合成增加,从而增加肝脏脂肪含量[42-43]。研究[5]表明,与超重/肥胖型患者相比,瘦型NAFLD患者携带PNPLA3 rs738409 G等位基因的比例较高,且它是与瘦型NAFLD患者发生非酒精性脂肪性肝炎和显著纤维化(分级≥2级)相关的唯一独立变量。因此,该基因的遗传易感性使得一部分瘦型人群患病风险大大增加。同时,在胰岛素抵抗方面,超重/肥胖型NAFLD患者由于肝脏和肝外胰岛素输送和提取的显著增加不足以补偿胰岛素敏感性的降低,从而导致葡萄糖稳态受损[44]。而瘦型NAFLD患者不仅存在隐形的内脏肥胖,其骨骼肌质量也比超重/肥胖NAFLD患者更低[45],而骨骼肌质量的减少不仅会导致更严重的脂肪变性和肝纤维化[46],还会促使葡萄糖摄取和代谢减少,加重胰岛素抵抗[47],进而更容易导致T2DM的发展。综上所述,瘦型NAFLD以胰岛素抵抗和内脏肥胖为核心,受遗传易感性等因素的影响更强,因此可能会导致更高的T2DM风险。
然而,在真实世界的观察性研究中不难发现,瘦型NAFLD人群中患有T2DM的比例往往比超重/肥胖型NAFLD人群低,如一项Meta分析[6]显示,仅有19.56%的瘦型NAFLD受试者患有T2DM,而45.70%的肥胖型NAFLD受试者患有T2DM。这可能与研究只采用了单一的BMI作为衡量肥胖的指标有关。有研究[48]发现,在正常腰围组(男性<90 cm,女性<80 cm)中,超重型NAFLD是T2DM的独立危险因素,而在高腰围组(男性≥90 cm, 女性≥80 cm)中,瘦型NAFLD是T2DM的独立危险因素,并且发生T2DM的风险略高于超重/肥胖型NAFLD(HR:3.88 vs 3.30)。同样,另一项观察性研究[49]也表明在高腰围组(男性≥102 cm,女性≥88 cm)中,瘦型NAFLD患者发生T2DM的风险显著高于超重/肥胖型NAFLD患者(OR:13.0 vs 5.3)。这提示在未来的研究中,宜在以BMI分层的基础上,加入更多中心性肥胖指标(如腰围、腰臀比等)进一步探讨二者共病的风险。
本研究首次运用MR探讨以BMI分层视角下NAFLD与T2DM的关系,优势在于采用MR研究规避了传统观察性研究存在的混杂、偏倚相关难题,结合多变量分析、亚组分析探讨了NAFLD、T2DM和BMI之间的复杂关系,增强了因果推断结论的可靠性。此外,本研究也存在一定的局限性:(1)本文中部分反向OR结果显示接近于不显著,虽然通过MR补充方法、敏感性分析确认了结果具有较好的稳健性,但仍待在未来开展更多的GWAS研究进一步验证结果的准确性及可重复性;(2)本文观察到部分组别存在异质性,这可能源于不同研究人群、环境因素,导致效应估计的不一致,尽管已经引入了IVW随机效应模型进行评估,但其结果可能与临床存在部分差异;(3)本文中部分工具变量数量较少,可能一定程度上会限制研究的统计效能和结果的可靠性,此外,在肥胖型NAFLD与T2DM的反向MR分析中,未观察到有统计学意义的因果关联,可能与样本量较少有关,以上问题均提示还需待基因检测等技术进步后确定更多的工具变量。
NAFLD与T2DM的共病作为当前的研究热点,其因果关系、病理基础、关联机制等一直受到众多研究者的关注。本文采用MR,从基因层面揭示了二者的双向因果关系,并对以BMI分层的NAFLD亚型进行深入探究,为两病互治互防的临床策略提供一定参考。
致谢:感谢GWAS Catalog、代谢疾病知识门户网站、人体测量学性状的遗传研究联盟、英国生物样本库等公开数据集及有关工作人员的努力。
数据可用性声明:本研究所用数据均来自公共数据库,可从文中提到的PMID或数据链接进行下载。
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表 1 各表型GWAS数据汇总信息
Table 1. Summary information of GWAS data for each phenotype
表型 数据来源 PMID 样本量(例) 种族 NAFLD(总人群) UKB 37235137 32 941 欧洲 瘦型NAFLD UKB 37235137 13 614 欧洲 超重型NAFLD UKB 37235137 13 710 欧洲 肥胖型NAFLD UKB 37235137 5 617 欧洲 T2DM CMDKP 34862199 312 646 欧洲 BMI GIANT 30124842 681 275 欧洲 注:PMID,PubMed唯一标识码。
表 2 NAFLD各组与T2DM的敏感性分析
Table 2. Sensitivity analysis of NAFLD subgroups and T2DM
暴露 结局 正向 反向 异质性(P值) 水平多效性 异质性(P值) 水平多效性 MR-Egger截距法P值 MR-PRESSO P值 MR-PRESSO 剔除SNP MR-Egger截距法P值 MR-PRESSO P值 MR-PRESSO 剔除SNP NAFLD总人群 T2DM 0.02 0.69 0.32 无 0.41 0.91 0.38 无 瘦型NAFLD T2DM 0.98 0.05 0.71 rs17547923,
rs584898061)
0.99 0.30 0.99 无 超重型NAFLD T2DM 0.04 0.39 0.21 无 0.61 0.40 0.58 无 肥胖型NAFLD T2DM 0.74 0.32 0.60 无 1.00 0.87 1.00 无 注:1)第一次MR-Egger截距法P=0.002,进一步采用径向MR剔除的SNP离群值。
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