DNA Methylation in Drosophila

基因组印记是一组染色体上的可逆和差异标记，由传递父母的性别决定。这种印记可能会极大地影响染色体行为或基因表达，并且适当印记的破坏可能导致染色体丢失、癌症、出生缺陷或遗传疾病。一个个体既有母亲的印记，也有父亲的印记，这反映出其基因组的一半是从父母双方遗传的。这两组印记在生物体的整个生命过程中稳定地维持。在一些细胞中（例如， 人类精子干细胞），这种维持可能会在世纪的过程中持续数千次细胞分裂。当每一条染色体传递给下一代时，所有的父母印记都被擦除，以利于一组适合传递个体性别的新印记。然而，由于印记是通过未知的机制维持、建立和解释的，因此尚不清楚印记的丧失如何导致疾病。我们的长期目标是了解男性和女性如何建立性别特异性印记， 并由后代维持，从而导致特定于亲本的遗传行为。我们的具体假设是，DNA甲基化是一个重要的，但短暂的特征基因组印记。我们基于这一假设的基因组印记在果蝇突变的唯一已知的DNA甲基转移酶，Mt2的分析。我们的初步数据表明，DNA甲基化是建立父系印记所必需的，这表明父系印记是由母系基因型控制的。基于这些观察，我们建议集中我们的研究，通过追求三个具体目标：1。鉴定Mt2甲基化靶向的DNA序列。我们将通过引入可压印的转基因来确定哪些序列被甲基化，该转基因在整个发育和有丝分裂后的成年期都是活跃的，并监测DNA甲基化水平和染色质结构，因为它们与基因活性相关。2.确定Mt2活性对父系来源染色体的特异性模式。基因组印记的关键特征是区分父系和母系染色体，而不考虑染色体序列。我们将专注于测试可能的模型，染色体歧视，包括本地化的基因活性和染色质结构的差异，母亲和父亲的基因组。3.确定负责建立基因组印记的遗传因素。我们已经确定了一个遗传区间，其中包含一个建立母系印记所需的基因。我们将确定哪些基因参与了母体印记的建立，并开始研究其作用模式。

Genomic imprinting is a reversible and differential mark on a set of chromosomes that is determined by the sex of the transmitting parent. This imprint may dramatically affect chromosome behavior or gene expression, and disruption of a proper imprint may result in chromosome loss, cancer, birth defects, or genetic disease. An individual has both maternal and paternal imprints, reflecting that half of its genome has been inherited from each parent. Both sets of imprints are stably maintained throughout the life of an organism. In some cells (e.g., human sperm stem cells), this maintenance may last through thousands of cell divisions over the course of a century. As each chromosome is transmitted to the next generation, all parental imprints are erased in favor of a new set of imprints appropriate to the sex of the transmitting individual. However, since the imprints are maintained, established, and interpreted through unknown mechanisms, it is not known how loss of imprinting leads to disease. Our long-term goal is to understand how sex-specific imprints are established by both males and females, and maintained by the offspring, resulting in parent-of-origin-specific genetic behaviors. Our specific hypothesis is that DNA methylation is an important but transient feature of genomic imprinting. We base this hypothesis on analysis of genomic imprints in fruit flies mutant for the sole known DNA methyltransferase, Mt2. Our preliminary data suggest that DNA methylation is required maternally for establishment of paternal imprints, indicating that the paternal imprint is controlled by the maternal genotype. Based on these observations, we propose to focus our research on imprint establishment by pursuing three specific aims: 1. Identify the DNA sequence that is targeted for methylation by Mt2. We will identify which sequences become methylated by introducing an imprintable transgene, active throughout development and post-mitotic adulthood, and monitoring DNA methylation levels and chromatin structure as they correlate with gene activity. 2. Determine the mode of specificity of Mt2 activity for paternally-derived chromosomes. The key feature of genomic imprinting is the discrimination of paternal from maternal chromosomes, without regard to chromosome sequence. We will focus on testing possible models of chromosome discrimination, including localized gene activity and differences in chromatin structure of maternal and paternal genomes. 3. Identify genetic factors responsible for establishing genomic imprints. We have identified a genetic interval containing a gene required for the establishment of a maternal imprint. We will identify which gene is involved in maternal imprint establishment, and begin to investigate its mode of action.