Table2_Methylation is maintained specifically at imprinting control regions but not other DMRs associated with imprinted genes in mice bearing a mutation in the Dnmt1 intrinsically disordered domain.xlsx

Differential methylation of imprinting control regions in mammals is essential for distinguishing the parental alleles from each other and regulating their expression accordingly. To ensure parent of origin-specific expression of imprinted genes and thereby normal developmental progression, the differentially methylated states that are inherited at fertilization must be stably maintained by DNA methyltransferase 1 throughout subsequent somatic cell division. Further epigenetic modifications, such as the acquisition of secondary regions of differential methylation, are dependent on the methylation status of imprinting control regions and are important for achieving the monoallelic expression of imprinted genes, but little is known about how imprinting control regions direct the acquisition and maintenance of methylation at these secondary sites. Recent analysis has identified mutations that reduce DNA methyltransferase 1 fidelity at some genomic sequences but not at others, suggesting that it may function differently at different loci. We examined the impact of the mutant DNA methyltransferase 1 P allele on methylation at imprinting control regions as well as at secondary differentially methylated regions and non-imprinted sequences. We found that while the P allele results in a major reduction in DNA methylation levels across the mouse genome, methylation is specifically maintained at imprinting control regions but not at their corresponding secondary DMRs. This result suggests that DNA methyltransferase 1 may work differently at imprinting control regions or that there is an alternate mechanism for maintaining methylation at these critical regulatory regions and that maintenance of methylation at secondary DMRs is not solely dependent on the methylation status of the ICR.

哺乳动物中印记调控区（imprinting control regions，ICR）的差异甲基化，对于区分亲本等位基因并相应调控其表达至关重要。为确保印记基因的亲本起源特异性表达、维持正常发育进程，受精时继承的差异甲基化状态必须在后续体细胞分裂过程中，由DNA甲基转移酶1（DNA methyltransferase 1）稳定维持。后续的表观遗传修饰（如次级差异甲基化区域的获得）依赖于印记调控区的甲基化状态，且对实现印记基因的单等位基因表达至关重要，但目前学界对印记调控区如何指导这些次级位点的甲基化获得与维持仍知之甚少。近期研究已鉴定出可降低部分基因组序列处DNA甲基转移酶1保真度的突变，而对其他基因组序列无此影响，这提示该酶在不同基因座的功能可能存在差异。本研究探讨了突变型DNA甲基转移酶1 P等位基因对印记调控区、次级差异甲基化区域（differentially methylated regions，DMR）以及非印记序列处甲基化水平的影响。研究发现，尽管该P等位基因可使小鼠全基因组的DNA甲基化水平大幅降低，但印记调控区的甲基化得以特异性维持，而其对应的次级DMR则未获得此保护。该结果提示，DNA甲基转移酶1在印记调控区的功能可能存在特异性，抑或存在维持这些关键调控区甲基化的替代机制；同时也表明，次级DMR的甲基化维持并非完全依赖于印记调控区的甲基化状态。