Brown C J, Carrel L, Willard H F
Department of Genetics, Case Western Reserve University School of Medicine, Cleveland, OH 44106-4955, USA.
Am J Hum Genet. 1997 Jun;60(6):1333-43. doi: 10.1086/515488.
X-chromosome inactivation results in the cis-limited inactivation of many, but not all, of the genes on one of the pair of X chromosomes in mammalian females. In addition to the genes from the pseudoautosomal region, which have long been anticipated to escape inactivation, genes from several other regions of the human X chromosome have now been shown to escape inactivation and to be expressed from both the active and inactive X chromosomes. The growing number of genes escaping inactivation emphasizes the need for a reliable system for assessing the inactivation status of X-linked genes. Since many features of the active or inactive X chromosome, including transcriptional activity, are maintained in rodent/human somatic-cell hybrids, such hybrids have been used to study the inactivation process and to determine the inactivation status of human X-linked genes. In order to assess the fidelity of inactivation status in such hybrids, we have examined the expression of 33 X-linked genes in eight mouse/human somatic-cell hybrids that contain either the human active (three hybrids) or inactive X (five hybrids) chromosome. Inactivation of nine of these genes had previously been demonstrated biochemically in human cells, and the expression of these genes only in hybrids retaining an active X, but not in those retaining an inactive X, confirms that expression in hybrids reflects expression in human cells. Although the majority of genes tested showed consistent patterns of expression among the active X hybrids or inactive X hybrids, surprisingly, 5 of the 33 genes showed heterogeneous expression among the hybrids, demonstrating a significantly higher rate of variability than previously reported for other genes in either human somatic cells or mouse/human somatic-cell hybrids. These data suggest that at least some X-linked genes may be under additional levels of epigenetic regulation not previously recognized and that somatic-cell hybrids may provide a useful approach for studying these chromosomal phenomena.
在哺乳动物雌性个体中，X染色体失活导致一对X染色体中的一条上的许多（但并非全部）基因发生顺式受限失活。除了长期以来预期会逃避失活的拟常染色体区域的基因外，现在已证明人类X染色体其他几个区域的基因也能逃避失活，并能从活性和非活性X染色体上都表达。越来越多逃避失活的基因凸显了建立一个可靠系统来评估X连锁基因失活状态的必要性。由于活性或非活性X染色体的许多特征，包括转录活性，在啮齿动物/人类体细胞杂种中得以保留，因此此类杂种已被用于研究失活过程并确定人类X连锁基因的失活状态。为了评估此类杂种中失活状态的保真度，我们检测了8个小鼠/人类体细胞杂种中33个X连锁基因的表达情况，这些杂种分别含有人类活性X染色体（3个杂种）或非活性X染色体（5个杂种）。其中9个基因的失活先前已在人类细胞中通过生化方法得到证实，这些基因仅在保留活性X染色体的杂种中表达，而在保留非活性X染色体的杂种中不表达，这证实了杂种中的表达反映了人类细胞中的表达。尽管大多数检测的基因在活性X杂种或非活性X杂种中表现出一致的表达模式，但令人惊讶的是，33个基因中有5个在杂种中表现出异质性表达，其变异性显著高于先前报道的人类体细胞或小鼠/人类体细胞杂种中的其他基因。这些数据表明，至少一些X连锁基因可能受到以前未被认识到的额外表观遗传调控水平的影响，并且体细胞杂种可能为研究这些染色体现象提供一种有用的方法。
