New Heterochromatin Formation and Maintenance in the Malaria Parasite

Jenn Hoskins
4th June, 2025

New Heterochromatin Formation and Maintenance in the Malaria Parasite

This analysis of transgenic malaria parasites (Plasmodium falciparum) (a, b) demonstrates that specific DNA fragments from the var gene and proximal pfap2-g locus, unlike distal pfap2-g or ama1 sequences, have the intrinsic capacity to nucleate de novo heterochromatin (c, d), establishing primary DNA sequence as a key determinant of epigenetic silencing.

Image adapted from: Pérez-Cantero et al. / CC BY (Source)

Key Findings

  • In Plasmodium falciparum, researchers showed that specific DNA sequences signal where to build heterochromatin—a compact structure that turns genes off
  • They found that var gene upstream regions efficiently trigger silencing, while pfap2‐g sequences require longer or repeated segments for similar effects
  • The study also revealed that different DNA parts are needed to maintain this silenced state across cell generations
[1] Researchers from ISGlobal; Universitat de Barcelona; ICREA; Cornell University have recently shed light on how heterochromatin (HC) nucleation is directed by the DNA sequence in the malaria parasite Plasmodium falciparum, an organism responsible for the most severe form of malaria in humans. Heterochromatin is a compact form of chromatin that plays an important role in controlling gene expression. In particular, the histone mark tri-methylation of histone H3 lysine 9 (H3K9me3) is essential for HC assembly, which involves three steps: nucleation (the initial formation of HC), spreading (extension along the chromatin), and maintenance (preserving the established state). In many organisms, histone modifications such as H3K9me3 are critical for establishing regions of gene silencing and ensuring genome integrity[2]. In P. falciparum, these modifications safeguard the regulation of genes that are important for the parasite’s survival and pathogenesis, making them a potential target for new strategies to combat malaria. However, the details of how HC nucleation is initiated in this parasite have remained largely unexplored. The study addressed this gap by investigating the cis-acting DNA elements that direct the nucleation of heterochromatin. The researchers used an approach that involved integrating various fragments from heterochromatic gene regions into a euchromatic (less compact and generally active) part of the genome. By then applying H3K9me3 chromatin immunoprecipitation (ChIP) analysis—a technique that helps identify the regions of the genome associated with specific histone modifications—they were able to determine which DNA fragments are capable of initiating HC formation. A key finding from the study is that DNA fragments upstream of var genes were highly effective in nucleating heterochromatin. Var genes play a significant role in antigenic variation, which is a strategy used by the parasite to evade the host’s immune response. The efficient nucleation of HC by these upstream regions suggests that their DNA sequence contains specific signals or motifs that attract the machinery necessary for depositing H3K9me3 marks. In contrast, fragments from the upstream region of pfap2‐g, a gene that controls sexual conversion in the parasite, and from the mspdbl2 locus did not show this capability. The researchers also observed that fragments from the beginning of the coding sequence of pfap2‐g could induce HC formation, but only with low efficiency, and only when long fragments (about 2 kilobases) were used. This suggests that not only is the DNA sequence important, but that a certain length or density of information might be required for effective nucleation. This research builds on previous studies that emphasized the role of modified histones in epigenetic inheritance[2]. The "read-write" mechanism, where pre-existing H3K9me3 marks help recruit histone methyltransferases to add further modifications, is well documented. The new study expands this understanding by showing that in malaria parasites, the primary DNA sequence itself is a major determinant of where heterochromatin begins to form. In other words, it is not only the histone modifications that matter but also the underlying genetic code that instructs the cell where to establish these modifications. Other work in model organisms, such as studies in fission yeast, has shown that heterochromatin nucleation can also involve non-DNA sequence factors. For example, proteins from the ATF/CREB family, Atf1 and Pcr1, have been shown to work in parallel with RNA interference (RNAi) pathways to promote heterochromatin formation at specific regions[3]. Although P. falciparum may use different mechanisms, the underlying principle of relying on specific nucleation signals remains consistent. Furthermore, research exploring heterochromatin spreading mechanisms[4] has highlighted how enzymes that deposit histone modifications are regulated by additional factors to extend HC over large chromosomal domains, a process essential for sustained gene silencing. The current study thus fits into a larger framework of understanding how cells build and maintain regions of heterochromatin across diverse organisms. By comparing the capabilities of different genomic fragments to nucleate heterochromatin, the study provides insight not only into the initiation of HC but also into its maintenance. While the fragments from var gene regions effectively initiate HC, the study reveals that different subregions within the pfap2‐g locus are necessary for keeping HC in place over time. This distinction between nucleation and maintenance is crucial because it suggests that once HC is established, a separate set of DNA elements may be responsible for preserving the silenced state during cell divisions. Such findings have potential implications for understanding how malaria parasites regulate critical genes involved in life cycle transitions and pathogenicity. Overall, this study contributes significant new insights into the role of primary DNA sequence in directing heterochromatin formation in a major human pathogen. It ties into previous work on histone modifications and heterochromatin spreading[2][4] and may help guide future research aimed at targeting epigenetic regulation in malaria parasites. By elucidating the cis determinants of HC nucleation and maintenance, the research opens up possibilities for designing novel interventions that disrupt the parasite’s gene regulation machinery without affecting the host.

GeneticsBiochem

References

Main Study

1) Heterochromatin de novo formation and maintenance in Plasmodium falciparum

Published 2nd June, 2025

https://doi.org/10.1371/journal.ppat.1013137

Related Studies

2) The molecular basis of heterochromatin assembly and epigenetic inheritance.

https://doi.org/10.1016/j.molcel.2023.04.020

3) RNAi-independent heterochromatin nucleation by the stress-activated ATF/CREB family proteins.

Journal: Science (New York, N.Y.), Issue: Vol 304, Issue 5679, Jun 2004

4) Regulation of the heterochromatin spreading reaction by trans-acting factors.

https://doi.org/10.1098/rsob.230271


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