Common parasite genome sequenced

Researchers from the National Institute of Allergy and Infectious Diseases, part of the US National Institutes of Health (NIH), have decoded the genetic makeup of the parasite that causes trichomoniasis, one of the most common sexually transmitted infections (STIs).

The genome sequencing project is detailed in the January 12 issue of the journal Science. One of the more interesting aspects of the research was the discovery of a very large amount of repetitive genes, giving clues to the evolution of the parasite.

The Trichomonas vaginalis parasite is pear-shaped with thread-like flagella that propel its movement. Once it attaches to cells lining the host's urinary or genital tract, it flattens out and begins to ingest the cells, as well as white and red blood cells, causing direct damage to the urinary and vaginal tissues and resulting in inflammation.

T. vaginalis also consumes bacteria that may be present in the urinary and genital areas, including the bacteria necessary for maintaining a normal healthy environment in the vagina.

In generating the genetic blueprint for the parasite, researchers were surprised to find such a large and highly repetitive genome comprising nearly 26,000 predicted genes as determined by computer models and previously sequenced parasitic genomes. Repetitive genes accounted for roughly 65 per cent of the genome.

"Parasites generally have smaller amounts of DNA than non-parasitic organisms, but in this case, there was ten times as much DNA than we originally thought there would be," lead author Dr Jane Carlton, who led the project while at The Institute for Genomic Research (TIGR), said. Carlton is now an associate professor in the Department of Medical Parasitology at New York University's School of Medicine.

Although it is not entirely clear why the genome is so large and repetitive, researchers theorise that the parasite evolved over time, previously inhabiting the intestine and later moving to the urogenital tract, which resulted in increased cell size and, subsequently, a considerably expanded genome.

The researchers also discovered more than 150 instances where bacterial genes may have transferred into the parasite's genome, suggesting that bacteria may have influenced the development of the parasite's metabolism.

The decoded genome also revealed 800 genes for surface proteins that likely enable T. vaginalis to adhere to cells in the urinary and genital tracts and cause infection. Additionally, the researchers were able to analyse proteins thought to be linked to the parasite's hydrogenosome - its energy source and the target of the two drugs approved to treat trichomoniasis - and identified possible ways the parasite may become resistant to these medications.

Source: US National Institutes of Health (NIH)