Association of the CpG Methylation Pattern of the Proximal Insulin Gene Promoter with Type 1 Diabetes

The insulin (INS) region is the second most important locus associated with Type 1 Diabetes (T1D). The study of the DNA methylation pattern of the 7 CpGs proximal to the TSS in the INS gene promoter revealed that T1D patients have a lower level of methylation of CpG -19, -135 and -234 (p = 2.10−16) and a higher methylation of CpG -180 than controls, while methylation was comparable for CpG -69, -102, -206. The magnitude of the hypomethylation relative to a control population was 8–15% of the corresponding levels in controls and was correlated in CpGs -19 and -135 (r = 0.77) and CpG -135 and -234 (r = 0.65). 70/485 (14%) of T1D patients had a simultaneous decrease in methylation of CpG -19, -135, -234 versus none in 317 controls. CpG methylation did not correlate with glycated hemoglobin or with T1D duration. The methylation of CpG -69, -102, -180, -206, but not CpG -19, -135, -234 was strongly influenced by the cis-genotype at rs689, a SNP known to show a strong association with T1D. We hypothesize that part of this genetic association could in fact be mediated at the statistical and functional level by the underlying changes in neighboring CpG methylation. Our observation of a CpG-specific, locus-specific methylation pattern, although it can provide an epigenetic biomarker of a multifactorial disease, does not indicate whether the reported epigenetic pattern preexists or follows the establishment of T1D. To explore the effect of chronic hyperglycemia on CpG methylation, we studied non obese patients with type 2 diabetes (T2D) who were found to have decreased CpG-19 methylation versus age-matched controls, similar to T1D (p = 2.10−6) but increased CpG-234 methylation (p = 5.10−8), the opposite of T1D. The causality and natural history of the different epigenetic changes associated with T1D or T2D remain to be determined.

胰岛素（INS）基因区域是与1型糖尿病（Type 1 Diabetes, T1D）相关的第二大重要位点。本研究针对胰岛素基因启动子区域转录起始位点（Transcription Start Site, TSS）近端的7个CpG位点的DNA甲基化模式展开分析，结果显示，1型糖尿病患者的CpG-19、CpG-135及CpG-234位点甲基化水平显著低于健康对照组（p=2×10^-16），而CpG-180位点甲基化水平则高于对照组；其余CpG-69、CpG-102、CpG-206位点的甲基化水平在两组间无显著差异。与对照组人群相比，患者的低甲基化幅度为对照组对应水平的8%~15%，且CpG-19与CpG-135（r=0.77）、CpG-135与CpG-234（r=0.65）的甲基化水平呈显著正相关。485名1型糖尿病患者中，70例（占比14%）同时出现CpG-19、-135、-234位点的甲基化水平降低，而317名健康对照组中未发现此类病例。CpG位点的甲基化水平与糖化血红蛋白水平及1型糖尿病病程均无相关性。CpG-69、-102、-180、-206位点的甲基化水平受rs689位点的顺式基因型显著影响；rs689是一种已被证实与1型糖尿病存在强关联的单核苷酸多态性（Single Nucleotide Polymorphism, SNP），而CpG-19、-135、-234位点的甲基化则不受该基因型调控。我们推测，该遗传关联效应的部分机制，可能通过邻近CpG位点甲基化的潜在改变，在统计学与功能层面实现介导。尽管我们观察到的CpG特异性、位点特异性甲基化模式可作为多因素疾病的表观遗传生物标志物（epigenetic biomarker），但该结果无法明确所报道的表观遗传模式是先于1型糖尿病发生，还是在疾病发生后才出现。为探索慢性高血糖对CpG甲基化的影响，本研究纳入非肥胖的2型糖尿病（Type 2 Diabetes, T2D）患者进行分析，结果显示其CpG-19位点甲基化水平较年龄匹配的对照组显著降低（p=2×10^-6），这一点与1型糖尿病患者的表现一致；但CpG-234位点甲基化水平则显著升高（p=5×10^-8），与1型糖尿病患者的表现恰好相反。与1型或2型糖尿病相关的不同表观遗传改变的因果关系及自然病程，仍有待进一步明确。