Hansen R S, Canfield T K, Fjeld A D, Mumm S, Laird C D, Gartler S M
Department of Medicine, University of Washington, Seattle, WA 98195, USA.
Proc Natl Acad Sci U S A. 1997 Apr 29;94(9):4587-92. doi: 10.1073/pnas.94.9.4587.
The timing of DNA replication in the Xq27 portion of the human X chromosome was studied in cells derived from normal and fragile X males to further characterize the replication delay on fragile X chromosomes. By examining a number of sequence-tagged sites (STSs) that span several megabases of Xq27, we found this portion of the normal active X chromosome to be composed of two large zones with different replication times in fibroblasts, lymphocytes, and lymphoblastoid cells. The centromere-proximal zone replicates very late in S, whereas the distal zone normally replicates somewhat earlier and contains FMR1, the gene responsible for fragile X syndrome when mutated. Our analysis of the region of delayed replication in fragile X cells indicates that it extends at least 400 kb 5' of FMR1 and appears to merge with the normal zone of very late replication in proximal Xq27. The distal border of delayed replication varies among different fragile X males, thereby defining three replicon-sized domains that can be affected in fragile X syndrome. The distal boundary of the largest region of delayed replication is located between 350 and 600 kb 3' of FMR1. This example of variable spreading of late replication into multiple replicons in fragile X provides a model for the spread of inactivation associated with position-effect variegation or X chromosome inactivation.
为了进一步明确脆性X染色体上复制延迟的特征，我们研究了来自正常男性和脆性X男性的细胞中人类X染色体Xq27区域的DNA复制时间。通过检测一系列跨越Xq27几个兆碱基的序列标签位点（STS），我们发现正常活性X染色体的这一部分在成纤维细胞、淋巴细胞和淋巴母细胞中由两个具有不同复制时间的大区域组成。着丝粒近端区域在S期很晚才复制，而远端区域通常复制得稍早一些，并且包含FMR1基因，该基因在发生突变时会导致脆性X综合征。我们对脆性X细胞中复制延迟区域的分析表明，它在FMR1基因5'端至少延伸400 kb，并且似乎与Xq27近端非常晚复制的正常区域融合。不同脆性X男性中复制延迟的远端边界各不相同，从而定义了三个在脆性X综合征中可能受到影响的复制子大小的结构域。复制延迟最大区域的远端边界位于FMR1基因3'端350至600 kb之间。脆性X中晚期复制可变地扩展到多个复制子的这个例子为与位置效应斑驳或X染色体失活相关的失活扩展提供了一个模型。
