A var gene promoter controls allelic exclusion of virulence genes in Plasmodium falciparum malaria

Abstract

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.

Variability and Exclusion in Host and Parasite: Epigenetic Regulation of Ig and var Expression

“…Yet, the intron sequences apparently do harbor an insulator activity that antagonizes spreading of chromatin activation into neighboring loci (48). This agrees with previous results, suggesting that the upsCpromoter-intron interaction has a boundary function to protect var genes in chromosome-central clusters from activation through nearby euchromatic regions and neighboring var genes (49).…”

“…Importantly, it was shown that a single active var promoter is sufficient to silence endogenous var genes and to ensure mutually exclusive expression of a single var gene (48,50). Together, it is reasonable to suggest that interplay between the promoter and intronic cis-regulatory elements generates a stably inherited silenced chromatin environment.…”

Maurer's clefts, the enigma of Plasmodium falciparum

“…In turn, exposure on the erythrocyte surface triggers antibody-dependent immune responses in which semi-immunity of exposed individuals is rooted. In a typical "arms race" the parasite circumvents elimination by the immune system through antigenic variation, exclusively expressing one of many PfEMP1 variants at any specific time point (17). Thus, again, export and trafficking of proteins into the host cell cytosol are key for parasite survival, chronic infections, and pathology of malaria.…”

Biology of Malaria Parasites

“…Thus the var promoter is sufficient for both the silencing and the monoclinic transcription of a PfEMPI allele. [ 96 ] 33. Summary…”