How stable is repression of disallowed genes in pancreatic islets in response to metabolic stress?

The specific phenotype of mature differentiated beta cells not only depends on the specific presence of genes that allow beta cell function but also on the selective absence of housekeeping genes (“disallowed genes”) that would interfere with this function. Recent studies have shown that both histone modifications and DNA methylation via the de novo methyltransferase DNMT3A are involved in repression of disallowed genes in neonatal beta cells when these cells acquire their mature phenotype. It is unknown, however, if the environmental influence of advanced age, pregnancy and the metabolic stress of high fat diet or diabetes could alter the repression of disallowed genes in beta cells. In the present study, we show that islet disallowed genes—which are also deeply repressed in FACS-purified beta cells—remain deeply repressed in animals of advanced age and in pregnant females. Moreover, the stability of this repression was correlated with strong and stable histone repression marks that persisted in islets isolated from 2 year old mice and with overall high expression of Dnmt3a in islets. Furthermore, repression of disallowed genes was unaffected by the metabolic stress of high fat diet. However, repression of about half of the disallowed genes was weakened in 16 week-old diabetic db/db mice. In conclusion, we show that the disallowed status of islet genes is stable under physiological challenging conditions (advanced age, pregnancy, high fat diet) but partially lost in islets from diabetic animals.

成熟分化的胰岛β细胞的特定表型，不仅依赖于赋予其功能的基因的特异性表达，还依赖于会干扰该功能的管家基因（housekeeping genes）的选择性沉默——这类基因也被称为“禁忌基因（disallowed genes）”。近年来的研究表明，在新生β细胞获得成熟表型的过程中，组蛋白修饰与从头DNA甲基转移酶DNMT3A（de novo methyltransferase DNMT3A）介导的DNA甲基化共同参与了禁忌基因的沉默。然而目前尚不清楚，高龄、妊娠以及高脂饮食或糖尿病所引发的代谢应激这类环境因素，是否会改变β细胞中禁忌基因的沉默状态。在本研究中，我们发现胰岛禁忌基因——这类基因在经荧光激活细胞分选（FACS）纯化的β细胞中亦处于深度沉默状态——在高龄动物与妊娠雌性小鼠体内仍保持深度沉默。此外，这种沉默的稳定性与2年龄小鼠胰岛中持续存在的强效且稳定的组蛋白抑制标记，以及胰岛中Dnmt3a的整体高表达水平呈显著相关。进一步研究显示，高脂饮食带来的代谢应激并未对禁忌基因的沉默产生影响。但在16周龄的糖尿病db/db小鼠的胰岛中，约半数禁忌基因的沉默状态被削弱。综上，本研究证实，胰岛基因的禁忌状态在高龄、妊娠、高脂饮食这类生理应激条件下仍保持稳定，但在糖尿病动物的胰岛中会部分丧失。