Voss Till S, Healer Julie, Marty Allison J, Duffy Michael F, Thompson Jennifer K, Beeson James G, Reeder John C, Crabb Brendan S, Cowman Alan F
The Walter and Eliza Hall Institute of Medical Research, Parkville 3050, Australia.
Nature. 2006 Feb 23;439(7079):1004-8. doi: 10.1038/nature04407. Epub 2005 Dec 28.
Mono-allelic expression of gene families is used by many organisms to mediate phenotypic variation of surface proteins. In the apicomplexan parasite Plasmodium falciparum, responsible for the severe form of malaria in humans, this is exemplified by antigenic variation of the highly polymorphic P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1, encoded by the 60-member var gene family, represents a major virulence factor due to its central role in immune evasion and intravascular parasite sequestration. Mutually exclusive expression of PfEMP1 is controlled by epigenetic mechanisms involving chromatin modification and perinuclear var locus repositioning. Here we show that a var promoter mediates the nucleation and spreading of stably inherited silenced chromatin. Transcriptional activation of this promoter occurs at the nuclear periphery in association with chromosome-end clusters. Additionally, the var promoter sequence is sufficient to infiltrate a transgene into the allelic exclusion programme of var gene expression, as transcriptional activation of this transgene results in silencing of endogenous var gene transcription. These results show that a var promoter is sufficient for epigenetic silencing and mono-allelic transcription of this virulence gene family, and are fundamental for our understanding of antigenic variation in P. falciparum. Furthermore, the PfEMP1 knockdown parasites obtained in this study will be important tools to increase our understanding of P. falciparum-mediated virulence and immune evasion.
许多生物体利用基因家族的单等位基因表达来介导表面蛋白的表型变异。在导致人类严重疟疾的顶复门寄生虫恶性疟原虫中,高度多态的恶性疟原虫红细胞膜蛋白1(PfEMP1)的抗原变异就是一个例证。PfEMP1由60个成员组成的var基因家族编码,由于其在免疫逃避和血管内寄生虫滞留中的核心作用,它是一种主要的毒力因子。PfEMP1的互斥表达受涉及染色质修饰和核周var基因座重新定位的表观遗传机制控制。在这里,我们表明一个var启动子介导了稳定遗传的沉默染色质的成核和扩散。该启动子的转录激活发生在核周与染色体末端簇相关的位置。此外,var启动子序列足以将一个转基因纳入var基因表达的等位基因排斥程序,因为该转基因的转录激活会导致内源性var基因转录的沉默。这些结果表明,一个var启动子足以实现这个毒力基因家族的表观遗传沉默和单等位基因转录,对于我们理解恶性疟原虫的抗原变异至关重要。此外,本研究中获得的PfEMP1敲低寄生虫将是增进我们对恶性疟原虫介导的毒力和免疫逃避理解的重要工具。