DNA Methylation in Drosophila

基因组印记是一组染色体上的可逆和可区分的标记，由传递亲本的性别决定。这种印记可能会极大地影响染色体行为或基因表达，而适当印记的破坏可能会导致染色体丢失、癌症、出生缺陷或遗传病。一个个体既有母亲的印记，也有父亲的印记，这反映了他的基因组的一半是从父母双方继承而来的。这两套印记在生物体的整个生命中都稳定地保持着。在一些单元中(例如， 人类精子干细胞)，这种维持可能会在一个世纪的过程中持续数千次的细胞分裂。当每条染色体传给下一代时，所有父母的印记都会被擦除，以支持一组新的印记，这套印记适合传播者的性别。然而，由于印记是通过未知的机制维持、建立和解释的，因此尚不清楚印记丢失是如何导致疾病的。我们的长期目标是了解男性和女性如何建立特定性别的印记， 并由后代维持，导致特定于父母的遗传行为。我们的具体假设是DNA甲基化是基因组印迹的一个重要但短暂的特征。我们基于对果蝇唯一已知DNA甲基转移酶MT2突变的基因组印迹的分析，提出了这一假设。我们的初步数据表明，DNA甲基化是母系建立父系印记所必需的，这表明父系印记受母系基因控制。基于这些观察，我们建议将我们的研究重点放在印迹建立上，以追求三个特定的目标：1.鉴定MT2甲基化的靶DNA序列。我们将通过引入一个在整个发育和有丝分裂后成年期都活跃的可印记的转基因来识别哪些序列甲基化，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.确定父系来源的染色体MT2活性的特异性模式。基因组印迹的关键特征是区分父系和母系染色体，而不受染色体序列的影响。我们将重点测试染色体歧视的可能模型，包括局部基因活性和母本和父本基因组染色质结构的差异。3.确定建立基因组印迹的遗传因素。我们已经确定了一个包含建立母体印记所需基因的遗传区间。我们将确定哪个基因与母体印记的建立有关，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.