Maintenance of the mammalian inactive X chromosome

小鼠胚胎癌细胞系MC12携带两条X染色体，其中一条在S期后期复制，与正常失活的X染色体具有相同的特性，因此被认为是失活的。由于激活的X染色体上的次黄嘌呤磷酸核糖转移酶(HPRT)基因发生突变(HPRT^-)，MC12细胞缺乏HPRT活性。然而，在HAT培养基中对MC12细胞进行筛选后，出现了一些抗HAT的克隆(HAT^R)。HAT抗性的高频率(3.18x10^&lt；-4&gt；)表明HPRT^+在失活的X染色体上重新激活，而不是HPRT^-的逆转。与这一观点一致，细胞学分析表明，在分离的20个HAT^R克隆中，有11个发生了超过非活性X染色体长度的重新激活。其余9个克隆保留了正常的异染色质不活跃的X染色体。失活X染色体上HPRT^+的自发复活率相对较高(1.34x10^~(-6))，与Xist缺失的体细胞的复活率相当(Csankovszki等，2001)，这表明MC12细胞的失活状态很难维持。

The mouse embryonal carcinoma cell line MC12 carries two X chromosomes, one of which replicates late in S phase and shares properties with the normal inactive X chromosome, and is therefore considered to be inactivated. Since the hypoxanthine phosphoribosyl transferase (HPRT) gene on the active X chromosome is mutated (hprt^-), MC12 cells lack HPRT activity. After subjecting MC12 cells to selection in HAT medium, however, a number of HAT-resistant clones (HAT^R) appeared. The high frequency of HAT resistance (3.18x10^<-4>) suggested reactivation of hprt^+ on the inactive X chromosome rather than reversion of hprt^-. Consistent with this view, cytological analyses showed that the reactivation occurred over the length of the inactive X chromosome in 11 out of 20 HAT^R clones isolated. The remaining 9 clones retained a normal heterochromatic inactive X chromosome. The spontaneous reactivation rate of the hprt^+ on the inactive X chromosome was relatively high (1.34x10^<-6>) and comparable to that observed for Xist-deleted somatic cells (Csankovszki et al., 2001), suggesting that the inactivated state is poorly maintained in MC12 cells.