In female mammalian cells, inactivation of one of the X chromosomes compensates the increased dosage of X-linked genes as compared with their male counterparts. This process is initiated by the X-inactive specific transcripts of the xist/XIST gene in cis, resulting in methylation of specific sites of genes to be silenced. However, in male germ cells, X inactivation is established by xist/XIST expression only. We investigated the X inactivation pattern in human testicular tumors of different histogenesis by analysis of XIST expression and methylation of the androgen receptor gene. XIST was expressed only in tumors derived from the germ cell lineage with supernumerical X chromosomesseminomas, nonseminomas, and spermatocytic seminomas. Although low expression was present in testicular parenchyma with spermatogenesis, XIST was expressed at a higher level in parenchyma with carcinoma in situ, the precursor lesion of seminomas and nonseminomas. Despite the consistent expression of XIST in germ-cell-derived tumors with gain of X chromosomes, methylation of the androgen receptor gene was present in all differentiated but only in a proportion of the undifferentiated nonseminomas. This differential pattern of methylation was also found in a number of representative cell lines. Our data indicate that the counting mechanism resulting in X inactivation is functional in testicular cancers of different histogenesis. Moreover, the differentiation-dependent pattern of X inactivation as reported during normal development in the case of multiple X chromosomes by methylation is retained in these tumors. We conclude therefore that X inactivation allows the excessive gain of X chromosomes found in germ-cell-derived tumors of the adult testis. In addition, this offers an interesting model to study the fundamental mechanisms of these processes.

译文

在雌性哺乳动物细胞中,与雄性对应物相比,X染色体之一的失活补偿了X连锁基因剂量的增加。此过程由cis中xist/XIST基因的X失活特异性转录本启动,导致待沉默基因的特定位点甲基化。然而,在雄性生殖细胞中,X失活仅通过xist/XIST表达建立。我们通过分析XIST表达和雄激素受体基因的甲基化,研究了不同组织发生的人睾丸肿瘤中的X失活模式。XIST仅在源自生殖细胞谱系的肿瘤中表达,这些肿瘤具有超数字X染色体精原细胞瘤,非精原细胞瘤和精细胞精原细胞瘤。尽管在精子发生的睾丸实质中存在低表达,但XIST在原位癌,精原细胞瘤和非精原细胞瘤的前体病变的实质中表达较高。尽管XIST在获得X染色体的生殖细胞衍生的肿瘤中一致表达,但在所有分化的非精原细胞瘤中,雄激素受体基因的甲基化都存在,但仅在未分化的非精原细胞瘤中。在许多代表性细胞系中也发现了这种甲基化的差异模式。我们的数据表明,导致X失活的计数机制在不同组织发生的睾丸癌中起作用。此外,在这些肿瘤中保留了在正常发育过程中通过甲基化引起的多个X染色体的X失活的分化依赖性模式。因此,我们得出结论,X失活允许在成年睾丸的生殖细胞衍生肿瘤中发现的X染色体的过度增加。此外,这提供了一个有趣的模型来研究这些过程的基本机制。

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