Novel insights into the epigenetic regulation of Plasmodium falciparum virulence genes

Plasmodium falciparum malaria still places a major health burden upon the developing world. The high virulence of this parasite is linked to the variegated expression of single P. falciparum erythrocyte membrane protein 1 (PfEMP1) variants that are encoded by the var gene family. Most vars and other virulence genes implicated in host-parasite interactions are found within transcriptionally inert chromatin domains at the nuclear periphery and are associated with heterochromatin protein 1 (PfHP1). Mutually exclusive gene activation relies on the escape of a single var locus from this repressive environment into a transcriptionally competent perinuclear zone and non-coding elements play an essential role in this process. The exact mechanisms involved in singular gene choice, however, remain elusive.
Using a transfection-based approach we identified cis-acting regulatory promoter elements, namely an upstream activating sequence and a mutual exclusion element (MEE), which are essential for var gene induction and singular locus recognition, respectively. Interestingly, in absence of the MEE, active var promoters are no longer introduced into the mutual exclusion programme. A yet unknown nuclear factor binds specifically to the MEE and I consider this interaction to be a prime candidate for the regulation of singular var activity. In addition, the MEE regulates PfEMP1 expression post-transcriptionally. On the mRNA level, this element is able to drastically reduce translation in an autonomous manner.
In an independent project, I generated a conditional PfHP1 loss-of-function mutant to functionally characterise this epigenetic regulator. After inducing PfHP1 depletion during early intra-erythrocytic development, parasites progress normally through schizogony but do not enter mitosis in the subsequent generation. Detailed analysis of these PfHP1-deprived parasites revealed four striking phenotypes: (1) they show a massive de-repression of PfHP1-controlled virulence genes, including the entire var repertoire; (2) over 50% of the cells in the parasite population represent viable early stage gametocytes that complete sexual development in absence of PfHP1; (3) this high conversion rate is linked to the induction of an apiap2 TF family member during schizogony of the previous cell cycle; (4) the non-gametocyte cells represent asexual parasites that are reversibly arrested prior to enter S-phase and mitosis. We could thus identify essential roles for PfHP1 in gene silencing, cell cycle progression and showed for the first time that the mechanism of sexual conversion involves a strong epigenetic component.