A small freshwater parasite burrows into the bare skin of an unsuspecting swimmer and enters their bloodstream. Once inside the blood, it grows into an adult worm, quietly feasting on its victim’s nutrients and breeding for some time, until one day its destruction becomes serious and life-threatening. While this could be the storyline of a bad horror film, it’s tragically the real tale of schistosomiasis – a deadly neglected tropical disease that kills more than 200,000 people each year in developing countries, predominantly in Africa.
In an effort to understand and control schistosomiasis, health officials around the globe are seeking strategies that target the life cycle of the parasite, ones that can block early development of the organism. Because epigenetics plays a role in host/pathogen co-evolutionary biology, researchers participating in an International study examined DNA methylation, a common epigenetic signaling tool, with the purpose of revealing key factors associated with this disease’s transmission. Their study was recently published in the May issue of PLOS: Neglected Tropical Diseases.
According the World Health Organization (WHO), schistosomiasis is one of the most devastating parasitic diseases on the planet, second only to malaria. Impacting an estimated 218 million people in 78 countries, schistosomiasis is a considered a disease of poverty. This is because the lack of clean water and sanitation in the deprived communities where it is found create the ideal environment for the disease-causing parasite, which is transmitted simply by contact with contaminated water (e.g. wading, swimming, and washing).
Often called bilharziosis or snail fever, schistosomiasis is caused by a trematode flatworm whose larval form is emitted by freshwater snails (Biomphalaria glabrata or B. glabrata). Once in the water, this microscopic organism can easily penetrate the skin of an exposed person. It then matures inside the human host’s blood vessels, where it reproduces and sheds some of its eggs via the body’s urine or feces. When an infected person urinates or defecates in freshwater areas, the eggs migrate back to the snails and the cycle starts all over again. Other eggs can become trapped in body tissues, causing immune reactions and progressive damage to organs. Without treatment, schistosomiasis can persist for years, resulting in long-term ill health and death.
Medical control of the disease relies primarily on the availability of the chemotherapeutic drug praziquantel. However, mass treatment with this drug (as promoted in sub-Saharan Africa) merely increases the possibility of the parasite’s resistance to it. Chemical and biological eradication of the host snail population has been somewhat successful; however, large-scale efforts to exterminate this species of mollusks have failed. The snail’s high reproductive rate and temperature tolerance, combined with dam and irrigation construction, have unfortunately expanded the disease’s spread to unaffected regions. And, it’s predicted that global warming will accelerate this spread more northward. Without a preventative vaccine available or access to healthcare, individuals living in and visiting these at-risk areas are susceptible, not only to initial infection, but continuous recurrence. Therefore, it is vital to find alternative methods to control the spread of schistosomiasis.
In the international study, which had strong contributions from the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University (UK), the School of Veterinary Medicine University of Wisconsin (US), and the Université Perpignan Via Domitia (France), the group of researchers focused on the parasite’s intermediate host snail stage. They chose to investigate the snail’s underlying biology and molecular processes, in particular DNA methylation.
“While much is known about how the definitive host responds to schistosome infection,” they reported, “there is comparably less information available describing the snail’s response to infection.” Their aim at studying the snail’s epigenetic machinery was to gain a better understanding the host-parasite relationship, as well as identify new targets that could be useful in finding integrated ways to combat the disease.
DNA methylation is an epigenetic mechanism used by cells to control gene expression. It plays an important role in many biological phenomena including development, genome stability and phenotypic plasticity. While most our current knowledge about DNA methylation comes from studying mammals, limited data exist on invertebrates, particularly mollusks. Yet, one study on Pacific oysters revealed that their genome displayed intragenic DNA methylation and that it contained the genes necessary for this epigenetic mechanism. Moreover, the role of DNA methylation in host-virus interactions and virulence is well documented, whereas less is known with regard to parasitism. In earlier articles, we discussed how the flu viruses use DNA methylation to evade the human immune system and how pathogens like the Toxoplasma parasite have the potential to similarly alter DNA methylation.
With a need for more data and the recent availability of B. glabrata’s DNA sequencing, the research team used a variety of diverse approaches to functionally characterize the critically important molecular process of DNA methylation in the snail species. In addition, they evaluated the mollusk’s responsiveness to schistosome soluble products, specifically from the more common Schistosoma mansoni genus. The mechanisms identified in their study included DNMT1, DNMT2 and methyl-CpG-binding domain protein (MBD2/3).
Using popular enzymatic assays from EpiGentek, the group measured both DNA methyltransferase and MBD binding activity within nuclear extracts derived from the tissues of 4 individual snail strains. They also quantified 5-mC levels in their global DNA pools using the company’s SuperSense Methylated DNA Quantification Kit. Their results showed that MBD2/3 and DNMT1 genes were transcriptionally enriched in gonadal vs. somatic tissue, and that treatment with demethylating agent, 5-azacytidine (5-AzaC), significantly inhibited egg production and embryo development. They also found higher levels or 5-mC, DNMT activity, and 5-mC binding in hybrid vs inbred snail populations. High resolution bisulfite (BS)-PCR analysis further revealed the presence of 5-mC within an exonic region of a housekeeping protein-coding gene (14-3-3), thus supporting computer simulated calculations and whole genome BS-Seq analysis previous done on this species’ genome.
Lastly, the researchers looked at how the parasite is able to overcome the snail’s immune system response. Using the B. glabrata embryonic cell line (Bge) they cultured the cells in the presence or absence of schistosome larval transformation products (LTP). “Interestingly, the Bge cells exposed to schistosome LTP significantly increased their expression of both Bgdnmt1 as well as Bgmbd2/3”, the authors wrote, “indicating that the snail’s epigenetic machinery is responsive to biotic stress and is specifically reactive to parasite products.” These results provide first evidence for a Schistosoma mansoni –provoked modulation of the snail’s DNA methylation system.
While global eradication of schistosomiasis will take time, it can happen. The WHO projects that by the year 2025 this disease should be wiped out. As it stands now, however, there is no effective vaccine, only one viable drug, and the increasing risk of the pathogen’s territory expansion. It’s clear that alternative tools to control schistosomiasis must be explored. Perhaps, disrupting the parasite during its developmental stage in snails may be the key to truly ending this epidemic.
This study is only preliminary, but the extended data it provides on B. glabrata epigenetics offers up new targets and molecular processes that should be further explored. In a parallel investigation, the same research team analyzed the snail’s genome, providing details on the biological properties that contribute to the snail’s suitability as host for the transmission of human schistosomiasis. It’s optimistic that these efforts together may lead to new approaches that help control the spread of this serious neglected tropical disease, as well as other similar parasitic diseases.
Source: Geyer KK, Niazi UH, Duval D, Cosseau C, Tomlinson C, Chalmers IW, et al. (2017) The Biomphalaria glabrata DNA methylation machinery displays spatial tissue expression, is differentially active in distinct snail populations and is modulated by interactions with Schistosoma mansoni. PLoS Neglected Tropical Disease 11(5): e0005246.