Isolation and characterization of candidate genes for Down syndrome

为了分离可能参与唐氏综合征发病机制的基因，我们使用来自染色体21q22.1-q22.2上的“唐氏综合征关键区（DSCR）”的一系列粘粒克隆进行了外显子捕获实验。到目前为止，我们已经分离出155个不同的外显子。这些外显子包括来自已知人类基因（ERG、HCS、CAF-1）的外显子和与人类cDNA片段或来自其他物种的cDNA同源的外显子（果蝇专一（sim）、果蝇迷你脑（mnb）和小鼠Girk 2基因）。在鉴定出人类同源物的三种基因中，果蝇sim基因作为主发育调节因子发挥作用，其指导神经外胚层细胞的特定亚群发育成中枢神经系统中线细胞，果蝇mnb基因编码神经发生期间成神经细胞增殖所必需的丝氨酸/苏氨酸激酶，小鼠Girk 2基因编码G蛋白偶联的内向整流钾通道，据报道该通道是一种韦弗突变体的原因。因此，从DSCR中鉴定这些人类同源物（SIM 2、MNB和GIRK 2）是非常有趣的。我们分离了编码SIM 2和MNB蛋白的cDNA克隆。我们进一步证明了小鼠胚胎早期间脑特异性表达SIM 2 mRNA。MNB mRNA在包括胎儿和成人脑在内的各种组织中表达。利用4-Mb粘粒/PAC重叠群构建DSCR基因的EcoRI限制性内切酶图谱，对SIM 2、HCS、MNB、GIRK 2、ERG基因以及新近报道的TPRD和Kir1.3（内向整流钾通道）基因的95个外显子进行定位。这些研究将为唐氏综合征的发病机制提供更多的候选基因。

To isolate genes potentially involved in the pathogenesis of Down syndrome, we have performed exon trapping experiments using a series of cosmid clones derived from the "Down syndrome critical region (DSCR)" on chromosome 21q22.1-q22.2. To date, we have isolated 155 different exons. These include exons derived from known human genes (ERG,HCS,CAF-1) and exons that were homologous to human cDNA fragments or cDNAs from other species (Drosophila single-minded (sim), Drosophila minibrain (mnb), and mouse Girk2 genes). Among the three genes for which human homologs were identified, the Drosophila sim gene functions as a master developmental regulator which instructs a specific subset of neuroectodermal cells to develop into central nervous system midline cells, the Drosophila mnb gene encodes a serine/threonine kinase that is essential for the neuroblast proliferation during neurogenesis, and the mouse Girk2 gene encodes a G-protein coupled inward rectifier potassium channel for which the cause of a weaver mutant was reported. Thus the identification of these human homologs (SIM2, MNB,and GIRK2) from the DSCR is quite intriguing. We isolated the cDNA clones encoding SIM2 and MNB proteins. We further demonstrated diencephalon-specific expression of SIM2 mRNA in early stages of mouse embryos. The MNB mRNA is expressed in various tissues including fetal and adult brains. Ninety-five exons including those of SIM2, HCS,MNB,GIRK2, ERG genes as well as newly reported genes for TPRD and Kir1.3 (inward rectifier potassium channel) have been mapped on an EcoRI restricition map of the DSCR which was constructed from 4-Mb cosmid/PAC contigs. These studies should yield additional candidate genes for the pathogenesis of Down syndrome.