The malaria parasite Plasmodium falciparum undergoes antigenic variation to evade host immune responses through switching expression of variant surface proteins encoded by the var gene family. We demonstrate that both a subtelomeric transgene and var genes are subject to reversible gene silencing. Var gene silencing involves the SIR complex as gene disruption of PfSIR2 results in activation of this gene family. We also demonstrate that perinuclear gene activation involves chromatin alterations and repositioning into a location that may be permissive for transcription. Together, this implies that locus repositioning and heterochromatic silencing play important roles in the epigenetic regulation of virulence genes in P. falciparum.

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.

No abstract