The genetic mechanism that makes syphilis so difficult to eradicate

Hyperaxion Apr 25, 2020

The bacteria that cause the disease have a gene that helps it avoid attacks from the immune system, scientists point out.

One of the most common sexually transmitted diseases, syphilis can remain hidden in the body for years. But a new study from the University of Washington in Seattle, USA, may explain why this happens.

The genetic mechanism that makes syphilis so difficult to eradicate
(Credit: jarun011/iStock/Getty Images).

Led by neurology professor Christina Marra, the research points out that the bacterium responsible for the disease has a gene that “deceives” the human immune system, preventing its detection. The discovery came after Marra’s team compared the genomes of syphilis bacteria collected from a man who had been infected four times.

The blood samples analyzed by the scientists were collected during two infections that occurred over an interval of six years. In addition to these infections, the study volunteer was infected and treated two more times. The researchers sought to understand if there were differences in the genomes of the bacteria between the first and the last infection, which could explain how the genes of the microorganism had changed and how this would allow new infections in a person whose immune system had already dealt with the disease.

Treponema pallidum, the bacteria that causes syphilis.
Treponema pallidum, the bacteria that causes syphilis. (Credit: Public domain).

Then, the researchers were surprised. “Across the about 1.1 million bases that make up the bacteria’s genome there were about 20 changes total. That’s very low,” noted Alex Greninger, assistant professor at the University of Washington’s Laboratory of Medicine, in a statement. “But on this one gene, we saw hundreds of changes.”

The gene called Treponema pallidum repeat gene K (tprK) is responsible for providing the necessary instructions for the synthesis of a protein on the surface of the bacteria. Because they are more easily detectable, proteins on the surface of bacteria are major targets of the immune system. In this way, tprK uses gene conversion to replace parts of the bacterial genome – and thus change its surface appearance, which prevents a response from our immune system.

Syphilis is one of the most common sexually transmitted diseases. It is curable, and can be prevented by using condoms during sexual intercourse
Syphilis is one of the most common sexually transmitted diseases. It is curable, and can be prevented by using condoms during sexual intercourse. (Credit: Centers for Disease Control and Prevention / Public domain).

Like viruses and parasites, many bacteria have a single protein on their surfaces that stands out and draws more attention from the immune system. Known as immunodominant, this type of protein can protect the bacteria from attack. “The protein acts like a distraction that draws the immune system away from proteins that might be the bacterium’s Achilles heel. More work will be required to determine if this is the case in TprK,” explains Greninger.

The researchers believe it is because of this “disguise” caused by tprK that some cases of syphilis are more persistent and reach more advanced stages. In addition, the discovery also helps to understand why some people may manifest the disease more than once, as if they had never developed any immunity to it.

Related topics:

Syphilis

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