Genomic Imprinting in Development and Disease

与哺乳动物中表达的大多数基因不同，印记基因仅由一个亲本等位基因表达——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) 几乎完全由父系等位基因表达，而 CDK 抑制剂 p57Kip2 由母系等位基因表达。在脊椎动物中，这种形式的基因调控是哺乳动物所独有的。它为何存在仍不清楚。许多证据表明印记基因参与调节细胞增殖和活力。雄激素胚胎的整个基因组起源于父系，在妊娠中期表现出胚胎外膜过度生长和胎儿尺寸增加。整个基因组起源于母体的孤雌胚胎表现出胚胎生长迟缓。为了促进对印记的理解并识别新的印记基因，我们建立了起源于纯雄激素或孤雌生殖的成纤维细胞系。这些细胞系显示出截然相反的生长模式，雄激素细胞具有较短的细胞周期时间，达到较高的饱和密度并形成肿瘤，而单性生殖细胞则衰老并死亡。使用缺乏印记基因（例如 Igf2、其受体之一、Igf2r 和 p57Kip2）的小鼠品系表明，Igf2 是调节这些细胞增殖和活力的主要决定因素。 除了这些生长研究之外，我们还使用这些细胞系来识别新的印记基因。使用抑制性消减筛选，我们鉴定出核受体辅因子阻遏物/激活物Zac1和epsilon肌聚糖是从父系等位基因表达的印记基因，以及在大脑中强烈表达的Est，并且是从母系等位基因转录的。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。 我们正在开发一种小鼠模型，用于治疗一种与印记缺陷相关的人类先天性疾病，称为普瑞德威利综合征。在这种情况下，新生儿会出现低渗、呼吸困难，并且常常无法成长。那些幸存下来的人会出现饮食失调并经常变得肥胖。该疾病与父本 15 号染色体的部分缺失有关。在小鼠 7 号染色体上发现了同源区域。我们最近描述了缺乏位于 Prader-Willi 区域的 Necdin 基因父本等位基因的小鼠的衍生。这些小鼠在出生后不久就会因呼吸系统问题而死亡，并且模仿了普瑞德威利综合征的某一方面。人们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，印记的分子分析应该提供对基因表达的表观遗传控制的见解，这一方面与了解癌症形成中某些肿瘤抑制基因的失活越来越相关。

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.