d Graaff Wim, Tomotsune Daihachiro, Oosterveen Tony, Takihara Yoshihiro, Koseki Haruhiko, Deschamps Jacqueline
Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Uppsalalaan 8, 3485 CT Utrecht, The Netherlands.
Proc Natl Acad Sci U S A. 2003 Nov 11;100(23):13362-7. doi: 10.1073/pnas.2237046100. Epub 2003 Oct 31.
Polycomb-group (Pc-G) proteins ensure late maintenance of transcriptional repression outside the expression domain of target genes in flies and vertebrates. They act in complexes, presumably by modulating chromatin structure. In Drosophila, they have been found to be associated with transcriptionally inactive loci but seem to be present in association with actively transcribed promoters as well, a feature which is not yet understood. In the mouse, mutations in several Pc-G genes result in an often subtle, local derepression of only a subset of the Hox genes rostral to their expression domains. We report here that Hox/reporter fusion genes, either randomly integrated as transgenes or as insertions within endogenous loci, are transcriptionally silenced in two mouse Pc-G-null mutants, Mel18 and rae28. Transcriptional silencing of Hox/reporter transgenes in Pc-G mutants was accompanied by increased DNA methylation in the promoter region. Gene silencing was observed at early developmental stages, long before Pc-G and trithorax-group proteins exert their function in maintenance of the Hox patterns. Although all five Hox genes tested as Hox/reporter fusions were silenced in the Pc-G mutants, transcription of the endogenous loci was mildly decreased in a subset of these Hox genes, and Hoxb1 was the most strongly affected. We discuss the possibilities that the observed negative effect of Pc-G mutations on Hox and Hox/reporter expression may reflect a positive involvement of the Pc-G epigenetic repressors in initial Hox gene transcription and that this requirement is exacerbated by the reporter insertion.
多梳蛋白家族（Pc-G）蛋白可确保果蝇和脊椎动物中靶基因表达域外转录抑制的后期维持。它们以复合物的形式发挥作用，可能是通过调节染色质结构来实现。在果蝇中，已发现它们与转录不活跃的基因座相关，但似乎也与活跃转录的启动子相关，这一特征尚不清楚。在小鼠中，几个Pc-G基因的突变通常只会导致其表达域前方的一部分Hox基因出现细微的局部去抑制。我们在此报告，Hox/报告基因融合基因，无论是作为转基因随机整合还是作为内源性基因座内的插入片段，在两种小鼠Pc-G基因敲除突变体Mel18和rae28中都会发生转录沉默。Pc-G突变体中Hox/报告基因转基因的转录沉默伴随着启动子区域DNA甲基化的增加。在发育早期阶段就观察到了基因沉默，远早于Pc-G和三胸节蛋白家族蛋白在维持Hox模式中发挥作用的时间。尽管作为Hox/报告基因融合体测试的所有五个Hox基因在Pc-G突变体中都被沉默，但这些Hox基因中的一部分内源性基因座的转录略有下降，其中Hoxb1受影响最为严重。我们讨论了Pc-G突变对Hox和Hox/报告基因表达产生的观察到的负面影响可能反映了Pc-G表观遗传抑制因子在Hox基因初始转录中的积极参与，以及报告基因插入加剧了这种需求的可能性。
