Lack of Support for the Association between <em>GAD2</em> Polymorphisms and Severe Human Obesity

The demonstration of association between common genetic variants and chronic human diseases such as obesity could have profound implications for the prediction, prevention, and treatment of these conditions. Unequivocal proof of such an association, however, requires independent replication of initial positive findings. Recently, three (−243 A>G, +61450 C>A, and +83897 T>A) single nucleotide polymorphisms (SNPs) within glutamate decarboxylase 2 (GAD2) were found to be associated with class III obesity (body mass index > 40 kg/m2). The association was observed among 188 families (612 individuals) segregating the condition, and a case-control study of 575 cases and 646 lean controls. Functional data supporting a pathophysiological role for one of the SNPs (−243 A>G) were also presented. The gene GAD2 encodes the 65-kDa subunit of glutamic acid decarboxylase—GAD65. In the present study, we attempted to replicate this association in larger groups of individuals, and to extend the functional studies of the −243 A>G SNP. Among 2,359 individuals comprising 693 German nuclear families with severe, early-onset obesity, we found no evidence for a relationship between the three GAD2 SNPs and obesity, whether SNPs were studied individually or as haplotypes. In two independent case-control studies (a total of 680 class III obesity cases and 1,186 lean controls), there was no significant relationship between the −243 A>G SNP and obesity (OR = 0.99, 95% CI 0.83–1.18, p = 0.89) in the pooled sample. These negative findings were recapitulated in a meta-analysis, incorporating all published data for the association between the −243G allele and class III obesity, which yielded an OR of 1.11 (95% CI 0.90–1.36, p = 0.28) in a total sample of 1,252 class III obese cases and 1,800 lean controls. Moreover, analysis of common haplotypes encompassing the GAD2 locus revealed no association with severe obesity in families with the condition. We also obtained functional data for the −243 A>G SNP that does not support a pathophysiological role for this variant in obesity. Potential confounding variables in association studies involving common variants and complex diseases (low power to detect modest genetic effects, overinterpretation of marginal data, population stratification, and biological plausibility) are also discussed in the context of GAD2 and severe obesity.常见遗传变异与肥胖等慢性人类疾病之间关联的阐明，对这类疾病的预测、预防与治疗具有深远意义。然而，要明确证实这类关联，需对最初的阳性发现进行独立重复验证。近期，研究人员在谷氨酸脱羧酶2（glutamate decarboxylase 2, GAD2）基因内发现3个单核苷酸多态性（single nucleotide polymorphism, SNP）位点——−243 A>G、+61450 C>A与+83897 T>A，它们与III级肥胖（体质量指数>40 kg/m²）存在关联。该关联在188个携带有该疾病的核心家系（共612名个体）以及一项纳入575名肥胖病例与646名瘦体型对照的病例-对照研究中均被观察到。同时，研究人员还提供了支持其中一个SNP位点（−243 A>G）存在病理生理学（pathophysiology）作用的功能实验数据。 GAD2基因编码谷氨酸脱羧酶的65 kDa亚基——GAD65。本研究旨在在更大规模的人群中重复验证该关联，并拓展对−243 A>G位点的功能研究。在纳入693个德国核心家系（共2359名个体，均患有重度早发性肥胖）的队列中，无论对3个GAD2基因的SNP位点进行单独分析还是单倍型（haplotype）分析，均未发现其与肥胖存在关联。 在两项独立的病例-对照研究中（共纳入680名III级肥胖病例与1186名瘦体型对照），合并样本分析显示−243 A>G位点与肥胖无显著关联（比值比=0.99，95%置信区间CI：0.83~1.18，p=0.89）。针对−243G等位基因与III级肥胖关联的所有已发表数据进行荟萃分析（meta-analysis）后，同样得到了阴性结果：在总计1252名III级肥胖病例与1800名瘦体型对照的总样本中，比值比为1.11（95%CI：0.90~1.36，p=0.28）。此外，对覆盖GAD2基因座（locus）的常见单倍型进行分析，同样未发现其与该家系人群的重度肥胖存在关联。 本研究还获得了−243 A>G位点的功能实验数据，结果不支持该变异在肥胖发生中发挥病理生理学作用。本文还结合GAD2基因与重度肥胖的关联研究，讨论了常见变异与复杂疾病关联研究中潜在的混杂因素，包括检测微弱遗传效应的效力不足、对边缘显著性数据的过度解读、人群分层以及生物学合理性等问题。