Sequestration and Inhibition of Daxx-Mediated Transcriptional Repression by PML

PML fuses with retinoic acid receptor α (RARα) in the t(15;17) translocation that causes acute promyelocytic leukemia (APL). In addition to localizing diffusely throughout the nucleoplasm, PML mainly resides in discrete nuclear structures known as PML oncogenic domains (PODs), which are disrupted in APL and spinocellular ataxia cells. We isolated the Fas-binding protein Daxx as a PML-interacting protein in a yeast two-hybrid screen. Biochemical and immunofluorescence analyses reveal that Daxx is a nuclear protein that interacts and colocalizes with PML in the PODs. Reporter gene assay shows that Daxx drastically represses basal transcription, likely by recruiting histone deacetylases. PML, but not its oncogenic fusion PML-RARα, inhibits the repressor function of Daxx. In addition, SUMO-1 modification of PML is required for sequestration of Daxx to the PODs and for efficient inhibition of Daxx-mediated transcriptional repression. Consistently, Daxx is found at condensed chromatin in cells that lack PML. These data suggest that Daxx is a novel nuclear protein bearing transcriptional repressor activity that may be regulated by interaction with PML.

在可导致急性早幼粒细胞白血病（acute promyelocytic leukemia, APL）的t(15;17)易位事件中，早幼粒细胞白血病蛋白（PML）会与视黄酸受体α（RARα）发生融合。除弥散分布于核质之外，PML主要定位于一类被称为PML癌结构域（PML oncogenic domains, PODs）的离散核结构中，这类结构在APL细胞与脊髓细胞共济失调细胞中会发生破坏。我们通过酵母双杂交筛选（yeast two-hybrid screen）分离得到了可与PML相互作用的Fas结合蛋白（Fas-binding protein）Daxx。生化分析与免疫荧光实验结果显示，Daxx是一种核蛋白，可与PML相互作用并共定位于PODs内。报告基因实验（reporter gene assay）表明，Daxx可显著抑制基础转录，其机制可能是通过招募组蛋白去乙酰化酶（histone deacetylases）实现的。PML可抑制Daxx的转录阻遏功能，但其致癌融合蛋白PML-RARα则无此活性。此外，PML的SUMO-1修饰（SUMO-1 modification）是将Daxx隔离至PODs，以及有效抑制Daxx介导的转录阻遏所必需的。与之相一致的是，在缺失PML的细胞中，Daxx可定位于浓缩染色质（condensed chromatin）区域。上述实验结果提示，Daxx是一类具备转录阻遏活性的新型核蛋白，其功能或可通过与PML的相互作用受到调控。