Epigenetic repression of gene expression controls the initial phases of gametocytogenesis in the malaria parasite, Plasmodium falciparum

Abstract

The renewed focus on global malaria eradication necessitates the discovery and development of novel strategies to block the transmission of the Plasmodium parasites that remain a burden to the health and socio-economic advancement of most developing nations to this day. A comprehensive understanding of the processes that drive the development of the transmissible gametocyte stages of the malaria parasite and the regulation thereof, is foundational for the effective development of these strategies. The tight modulation of gene expression that directs asexual parasite and gametocyte stage transitions throughout the P. falciparum life cycle has been extensively documented. However, the vast majority of the mechanisms responsible for this regulation, particularly for the gametocyte stage transitions, remain unknown. The contribution of epigenetic mechanisms to transcriptional regulation in asexual parasites has also been well studied. While unique histone post-translational modification landscapes have been associated with each asexual and sexual stage, the functional relevance of histone modifications for gametocyte stage transitions remained to elucidated. In this study, the roles of H3K27me2&3 and H3K36me2&3 during early and intermediate gametocyte development were examined. As H3K27me2&3 and H3K36me2&3 peak in abundance and are present almost exclusively in the stage II gametocytes, we hypothesised that these histone modifications are crucially involved in the transcriptional reprogramming associated with the transition from early gametocyte differentiation (stage I) to intermediate development (stage III). To test this hypothesis, we employed chromatin immunoprecipitation followed by high throughput sequencing, histone methylation profiling, transcriptional fingerprint analysis of epigenetic inhibitor perturbations and molecular docking strategies. Ultimately, the data generated from these approaches and conclusions drawn make substantial contributions to the current body of knowledge regarding the epigenetic regulation in malaria parasites. More specifically, this study advances the understanding and provides novel insights into the regulatory role of histone methylation in gametocyte differentiation and development. Firstly, we provide comprehensive genome-wide maps of H3K27me2&3 and H3K36me2&3 during early and intermediate gametocyte development. Next, we show that these histone modifications are independently involved in stage II gametocyte-specific transcriptional regulation with key repressive roles deconvoluted for each. The independent enrichment and repression of asexual stage-specific genes, regulators of commitment and chromatin modifying enzymes demonstrates that as in other eukaryotes, H3K27me2&3 and H3K36me2&3 not only silence irrelevant genes directly but also reinforce this repression by modulating higher order epigenetic and transcriptional regulators. Lastly, the transcriptional fingerprints of histone methyltransferase and demethylase inhibition generated in this study illustrate the importance of epigenetic mechanisms for gametocyte development, link the activity and potency of these inhibitors with the disruption of normal histone modification patterns and identify candidate enzymes that may be responsible for the stage II gametocyte-specific abundance of H3K27me2&3 and H3K36me2&3. Ultimately, this thesis presents the first description of the roles of histone modifications and importance of epigenetic regulation as a whole for the differentiation and development of the transmissible stages of the P. falciparum parasite.