表观遗传学

一项新研究发现DNA甲基化是控制MMP13酶基因的开关，而MMP13酶具有重要的软骨破坏作用。文章发表在FASEB Journal杂志的网站上。
骨关节炎是一种退行性关节疾病，该疾病的典型症状是患者关节软骨持续不可逆的损失，这主要是由于基质金属蛋白酶MMP13破坏基质中的II型胶原引起。该研究是一项突破性的表观遗传学应用实例，英国科学家发现人骨关节炎组织样本具有标志性的表观遗传学修饰（DNA甲基化），是控制MMP13这种破坏性酶基因的开关。而这种会提高MMP13蛋白水平的表观遗传学修饰，将和MMP13酶一同成为骨关节炎药物研发的主要靶点。该研究除了为骨关节炎治疗提供表观遗传学新途径，也展示了在癌症以外的疾病中，表观遗传学修饰同样也扮演了重要角色。
“随着人口的老龄化，骨关节炎日趋成为一种社会和经济问题，”该研究的研究者之一，英国泰晤士河畔的纽卡斯尔大学细胞医学研究所肌肉骨骼研究组的David A. Young博士说。“我们的研究能帮助人们更好的了解骨关节炎造成的软骨破坏事件，这项研究成果将有助于研发相应的治疗药物，改善许多骨关节炎患者的生活水平和活动能力。”
Young及其同事将骨关节炎患者和年龄相近的健康人的软骨组织进行了比较，研究了二者之间的DNA甲基化差异。研究人员发现，骨关节炎患者MMP13酶基因上的一个小位点的DNA甲基化水平较低。随后他们证实这一基因甲基化程度的降低，会增加破坏性酶MMP13的表达水平。
研究人员发现与健康组织样品相比，骨关节炎组织样品中MMP13启动子中CpG岛特异位点−104 CpG存在去甲基化现象，而这种现象会提高MMP13的表达水平。用10μMDNA甲基转移酶抑制剂5-aza-2′-deoxycytidine处理人关节软骨细胞，启动子的−104 CpG位点被持续去甲基化，也会导致MMP13的表达水平升高。研究人员通过CpG-free荧光素酶报告系统发现，cAMP 应答元件结合蛋白CREB在−104 CpG去甲基化的情况下，作为调控因子特异性结合MMP13启动子，并在随后的染色质免疫沉淀ChIP实验中证实了这一点。
文章总结道，骨关节炎组织中MMP13启动子的−104 CpG位点去甲基化，招募cAMP 应答元件结合蛋白CREB结合，导致MMP13表达水平升高并破坏软骨。
“我们已经看到表观遗传学研究对癌症治疗很有帮助，而现在我们发现它在其他疾病中也有着重要的影响，”FASEB Journal杂志总编Gerald Weissmann博士说，“这项研究不仅为骨关节炎疾病研究打下了基础，也说明从前关节炎研究界' either/or' nature v. nurture的争论已经过时：表观遗传学告诉我们nature（关节的日常磨损）会调控nurture（软骨基因）引发关节炎。”

原文摘要：
cAMP response element-binding (CREB) recruitment following a specific CpG demethylation leads to the elevated expression of the matrix metalloproteinase 13 in human articular chondrocytes and osteoarthritis
Osteoarthritis is a degenerative joint disease characterized by a progressive and irreversible loss of the articular cartilage, due in main part to the cleavage of type II collagen within the matrix by the enzyme matrix metalloproteinase (MMP)13. Here, we examined the methylation status of MMP13 promoter and report the demethylation of specific CpG dinucleotides within its promoter in osteoarthritic compared to normal cartilage, which correlates with increased MMP13 expression. Of the promoter CpG sites examined, the −104 CpG was consistently demethylated following treatment of human articular chondrocytes with 10μM DNA-methyltransferase inhibitor 5-aza-2′-deoxycytidine, again correlating with increased MMP13 expression. Methylation of the −104 CpG site resulted in reduced promoter activity in the chondrosarcoma cell line SW1353 as shown by CpG-free luciferase reporter. Using electrophoretic mobility shift assays, we identified CREB as the regulating factor able to only bind to the MMP13 promoter when the −104 CpG is demethylated, and confirmed this binding by chromatin immunoprecipitation. Finally, we demonstrated that CREB induces MMP13 expression only following treatment of SW1353 with 0.5 μM Ca2+ ionophore A23187. In summary, the −104 CpG is demethylated in osteoarthritic cartilage, correlating with the elevated MMP13 expression and cartilage destruction, providing a highly novel link between epigenetic status and arthritic disease.—Bui, C., Barter, M. J., Scott, J. L., Xu, Y., Galler, M., Reynard, L. N., Rowan, A. D., Young, D. A. cAMP response element-binding (CREB) recruitment following a specific CpG demethylation leads to the elevated expression of the matrix metalloproteinase 13 in human articular chondrocytes and osteoarthritis.