Genomic Imprinting in Development and Disease

与哺乳动物中表达的大多数基因不同，印记基因只表达于一个亲代等位基因--该等位基因取决于特定的基因。因此，胰岛素样生长因子2(IGF2)几乎完全来自父亲的等位基因，而CDK抑制因子p57Kip2则来自母亲的等位基因。在脊椎动物中，这种形式的基因调控是哺乳动物独有的。它为什么会存在仍不清楚。许多证据表明，印记基因参与调节细胞的增殖和活性。雄核发育胚胎的整个基因组起源于父系，在怀孕中期表现出胚胎外膜的过度生长和胎儿尺寸的增加。孤雌生殖胚胎，其中整个基因组是母体起源的，表现出胚胎发育迟缓。为了促进对印记的理解和鉴定新的印记基因，我们建立了起源于雄性或孤雌生殖的成纤维细胞系。这些品系表现出截然相反的生长模式，雄核发育细胞的细胞周期时间较短，达到较高的饱和密度并形成肿瘤，而孤雌生殖细胞衰老和死亡。使用缺乏印记基因的小鼠株系，如其受体之一Igf2r和p57Kip2，揭示了Igf2是调节这些细胞的增殖和活性的主要决定因素。 除了这些生长研究，我们还使用这些品系来鉴定新的印记基因。使用抑制性消减筛选，我们鉴定了核受体辅因子抑制/激活子Zac1和epsilon肌聚糖是从父亲的等位基因表达的印迹基因，以及在大脑中强烈表达的EST，是从母亲的等位基因转录而来的。目前的研究集中在确定这些基因在发育和生长调节中的功能。 我们正在为一种名为Prader-Willi综合征的人类先天性疾病开发小鼠模型，这种疾病与印记缺陷有关。在这种情况下，新生儿会出现低张，呼吸困难，往往无法茁壮成长。那些幸存下来的人会发展成饮食失调，并经常变得肥胖。这种疾病与父系15号染色体的部分缺失有关。在小鼠7号染色体上发现了一个同源区域。我们最近描述了一种缺乏位于Prader-Willi区的Necdin基因的小鼠的起源。这些小鼠出生后不久就会死于呼吸问题，并模仿了Prader-Willi综合征的一个方面。正在对它们的呼吸生理学进行更详细的研究，以确定Necdin的功能。 总体而言，印迹的分子分析应该提供对基因表达的表观遗传控制的洞察，这一方面对于理解某些肿瘤抑制基因在癌症形成中的失活越来越重要。

Unlike the majority of genes expressed in mammals, imprinted genes are expressed from only one parental allele- which allele depends on the particular gene. Thus the Insulin like growth factor 2 (Igf2) is expressed almost exclusively from the paternal allele, whereas p57Kip2, a CDK inhibitor is expressed from the maternal allele. This form of gene regulation is, among vertebrates, unique to mammals. Why it exists is still unclear. Much evidence has suggested that imprinted genes are involved in regulating cell proliferation and viability. Androgenetic embryos, in which the entire genome is paternal in origin, exhibit overgrowth of the extraembryonic membranes and in increase in fetal size at mid gestation. Parthenogenetic embryos, where the entire genome is maternal in origin show retarded embryonic growth. To facilitate an understanding of imprinting and to identify novel imprinted genes, we established fibroblast lines which are either exclusively androgenetic or parthenogenetic in origin. The lines show diametrically opposite patterns of growth with the androgenetic cells having a shorter cell cycle time, reaching a higher saturation density and forming tumors, whereas the parthenotes senesced and died. Using mouse lines deficient for imprinted genes such as Igf2, one of its receptors, the Igf2r, and p57Kip2 revealed that Igf2 was a major determinant regulating proliferation and viability of these cells. In addition to these growth studies, we have used these lines to identify novel imprinted genes. Using a suppressive subtractive screen we identified the nuclear receptor cofactor repressor/activator Zac1 and epsilon sarcoglycan as being imprinted genes expressed from the paternal allele, as well as an Est that is strongly expressed in the brain, and is transcribed from the maternal allele. Current studies are centered on determining the function of these genes in development and growth regulation. We are developing mouse models for a human congenital disease associated with a defect in imprinting called Prader-Willi syndrome. In this condition, newborns are hypotonic, have breathing difficulties and often fail to thrive. Those that survive develop an eating disorder and frequently become obese. The disease is associated with loss of part of the paternal chromosome 15. A homologous region is found on mouse chromosome 7. We recently described the derivation of mice lacking the paternal allele of a gene Necdin located in the Prader-Willi region. Such mice die shortly after birth due to respiratory problems and have mimicked one aspect of Prader-Willi syndrome. Their respiratory physiology is being studied in greater detail to determine the function of Necdin. Overall, a molecular analysis of imprinting should provide insights into the epigenetic control of gene expression, an aspect that is of increasing relevance to understanding the inactivation of certain tumor suppressor genes in cancer formation.