Transcriptional regulation of virulence gene families in "Plasmodium falciparum"

To date, malaria caused by Plasmodium falciparum is still a major health threat. It contributes to illness and severe disease and is responsible for up to one million deaths per year. The intra-erythrocytic asexual life cycle stage is responsible for the pathology associated with malaria. The major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1) is exposed at the surface of infected red blood cells (iRBC) and mediates binding to endothelial cells. This leads to sequestration of iRBC in the microvasculature and consequently to evasion of removal in the spleen. PfEMP1 is encoded by the 60-member var gene family, which undergoes antigenic variation by in-situ switching. Importantly, var genes are expressed in a mutually exclusive way, such that only one member is expressed whereas all other copies remain silenced. var genes as well as other gene families such as rif, stevor, phist and pfmc-2tm are located in subtelomeric heterochromatic regions. The function of these additional families is largely unknown, but they are thought to be implicated in host-parasite interactions and to contribute to antigenic variation.
With this work, I provide deeper insights into the transcriptional regulation of virulence gene families in P. falciparum by using transfection-based approaches. We functionally identified autonomous cis-acting var promoter elements including an upstream activating sequence that is essential for promoter activation. Notably, an element downstream of the transcriptional start site determines mutually exclusive locus recognition. Further, I used comparative transcriptional profiling to show that mutually exclusive expression is restricted to the var gene family and is not used in the transcription of other subtelomeric gene families. I show for the first time that knock-down of endogenous var gene transcription is also conferred by promoters of a var gene subfamily that is implicated in severe malaria. Taken together, this work provides important insight into the mechanisms involved in the regulation of virulence gene families and antigenic variation in P. falciparum. Moreover, the findings presented here are consistent with a novel mechanism of mutually exclusive gene choice in eukaryotes.