Anderson Catherine L, Brown Carolyn J
Department of Medical Genetics, University of British Columbia, Vancouver, Canada.
Hum Genet. 2002 Mar;110(3):271-8. doi: 10.1007/s00439-002-0676-8. Epub 2002 Feb 1.
X chromosome inactivation results in dosage equivalency for X-linked gene expression between males and females. However, some X-linked genes show variable X inactivation, being expressed from the inactive X in some females but subject to inactivation in other women. The human tissue inhibitor of metalloproteinases-1 ( TIMP1) gene falls into this category. As TIMP1 and its target metalloproteinases are involved in many biological processes, women with elevated TIMP1 expression may exhibit different disease susceptibilities. To address the potential impact of variable X inactivation, we analyzed TIMP1 expression levels by using an RNase protection assay. The substantial variation of TIMP1 expression observed in cells with monoallelic TIMP1 expression precluded analysis of the contribution of the inactive X to total TIMP1 RNA levels in females, so we examined expression in rodent/human somatic cell hybrids. TIMP1 expression levels varied more widely in hybrids retaining an inactive X than in those with an active X chromosome, suggesting variable retention of the epigenetic silencing mechanisms associated with X inactivation. Therefore, we investigated the contribution of methylation at the promoter to expression level variation and found that methylation of the TIMP1 promoter correlated with instability and low level expression, whereas stable TIMP1expression from the inactive X equivalent to that seen from the active X chromosome was observed when the promoter was unmethylated. Since all female cell lines examined showed methylation of the TIMP1 promoter, the contribution of expression from the inactive X appears minimal. However, as women age, they may accumulate cells stably expressing TIMP1 from the inactive X, with a resulting increase of TIMP1, which may explain some sex differences in various late-onset disorders.
X染色体失活导致男性和女性之间X连锁基因表达的剂量等效。然而，一些X连锁基因表现出可变的X失活，在一些女性中从失活的X染色体上表达，但在其他女性中则会失活。人类金属蛋白酶组织抑制剂-1（TIMP1）基因就属于这一类。由于TIMP1及其靶标金属蛋白酶参与许多生物学过程，TIMP1表达升高的女性可能表现出不同的疾病易感性。为了探讨可变X失活的潜在影响，我们使用核糖核酸酶保护试验分析了TIMP1的表达水平。在单等位基因TIMP1表达的细胞中观察到的TIMP1表达的显著差异，使得无法分析失活的X染色体对女性总TIMP1 RNA水平的贡献，因此我们检测了啮齿动物/人类体细胞杂种中的表达情况。与保留活跃X染色体的杂种相比，保留失活X染色体的杂种中TIMP1表达水平的变化更为广泛，这表明与X失活相关的表观遗传沉默机制存在可变保留。因此，我们研究了启动子甲基化对表达水平变化的影响，发现TIMP1启动子的甲基化与不稳定性和低水平表达相关，而当启动子未甲基化时，观察到来自失活X染色体的TIMP1表达稳定，与来自活跃X染色体的表达相当。由于所有检测的女性细胞系均显示TIMP1启动子甲基化，来自失活X染色体的表达贡献似乎最小。然而，随着女性年龄的增长，她们可能会积累从失活X染色体稳定表达TIMP1的细胞，从而导致TIMP1增加，这可能解释了各种迟发性疾病中的一些性别差异。
