De Novo Methylation and Cancer

我们的目标是了解在正常细胞和发育过程中调节从头DNA甲基化的因素，并确定这些控制途径如何在肿瘤细胞中出错并有助于肿瘤发生。DNA甲基化，或在CpG二核苷酸的背景下将甲基共价添加到胞嘧啶的5位，是哺乳动物发育所需的，对染色质结构具有深远的影响，并且在DNA修复和基因组稳定性中具有作用。已经清楚的是，虽然对发育至关重要，但DNA甲基化模式在癌症中变得失调，具有普遍的全基因组低甲基化（丢失）与区域特异性高甲基化（获得）的组合，主要在CpG岛。此外，DNA超甲基化已被证明可以有效地和遗传性地使基因失活。当这种情况发生在生长调节基因的启动子区域时，它可以使细胞获得生长优势，并最终导致癌症。 在正常细胞中建立或靶向甲基化模式的机制几乎完全未知，因此我们将研究重点放在细胞酶促甲基化机制DNA甲基转移酶（DNMT）的调节上。甲基化显然可以被靶向，因为某些基因组区域总是高度甲基化的，如近着丝粒异染色质（邻近着丝粒的DNA区域），而许多必需基因的调控区域总是未甲基化的。我们已经采取了一种生物化学方法来确定与DNMT相互作用的因素，这些因素可能会改变它们的核定位，并增强或抑制它们对特定DNA底物的酶活性。我们的研究表明，最丰富的甲基转移酶，DNMT1，与视网膜母细胞瘤基因产物（Rb）相互作用。Rb是细胞分裂的关键调节因子，帮助细胞决定是分裂还是进入半永久性静止状态。Rb或Rb途径的其他组分在几乎所有肿瘤细胞中都发生突变。我们已经确定Rb的癌症相关突变消除了Rb与DNMT1相互作用的能力。因此，我们可能已经确定了一种机制，即甲基化模式在肿瘤中由于Rb不能与DNMT1相互作用而被破坏。识别与正常细胞相互作用并指导甲基化的因素为理解肿瘤发展开辟了令人兴奋的新可能性，并可能最终导致旨在恢复正常甲基化模式和生长控制的新疗法。

Our objective is to understand the factors regulating de novo DNA methylation in normal cells and during development and determine how these control pathways go awry in tumor cells and contribute to tumorigenesis. DNA methylation, or the covalent addition of a methyl group to the 5-position of cytosine in the context of the CpG dinucleotide, is required for mammalian development, has profound effects on chromatin structure, and has roles in both DNA repair and genome stability. It has become clear that while essential for development, DNA methylation patterns become dysregulated in cancer, with a generalized genome-wide hypomethylation (loss) in combination with region-specific hypermethylation (gain), primarily at CpG islands. In addition, DNA hypermethylation has been shown to efficiently and heritably inactivate genes. When this occurs in the promoter region of a growth-regulatory gene it can give that cell a growth advantage and ultimately lead to cancer. Mechanisms for the establishment or targeting of methylation patterns in normal cells are almost completely unknown and thus we have focused our studies on the regulation of the cellular enzymatic methylation machinery, the DNA methyltransferases (DNMTs).. Methylation clearly can be targeted since certain genomic regions are always heavily methylated, like pericentromeric heterochromatin (regions of DNA adjacent to the centromere) while regulatory regions for many essential genes are always unmethylated. We have taken a biochemical approach to identify factors that interact with the DNMTs that may alter their nuclear localization and enhance or inhibit their enzymatic activites on particular DNA substrates. Our studies reveal that the most abundant methyltransferase, DNMT1, interacts with the retinoblastoma gene product (Rb). Rb is a critical regulator of cell division and helps a cell decide whether to divide or to go into a semi-permanent resting state. Rb, or other components of the Rb pathway, are mutated in almost all tumor cells. We have determined that cancer-associated mutations in Rb abolish the ability of Rb to interact with DNMT1. Thus we may have identified one mechanism whereby methylation patterns become disrupted in tumors due to an inability of Rb to interact with DNMT1. The identification of factors that interact with and direct methylation in normal cells opens up exciting new possibilities for understanding tumor development and may ultimately lead to novel therapies designed to restore normal methylation patterns and growth control.