After more than 50 years of searching, scientists have discovered a key gene that enables certain bacteria to cause blindness and debilitating genital tract infections. Using the recently completed genetic blueprint of the bacterium Chlamydia trachomatis, researchers from the National Institute of Allergy and Infectious Diseases (NIAID) have found a gene that encodes a cell-destroying toxin. Long suspected but never identified, the toxin helps explain why only some chlamydial strains cause chronic illness. The discovery, described in the Proceedings of the National Academy of Sciences November 13 online early edition, opens potential new avenues for treating or preventing chlamydial diseases. It also highlights how DNA sequencing can help scientists identify new ways to combat disabling or deadly infections.
Unlike most bacteria, C. trachomatis lives inside cells. Chronic infections of the eyelids can scar the eyes and lead to trachoma, the most common cause of preventable blindness worldwide. In the United States, C. trachomatis infection is the most common sexually transmitted disease (STD) and can lead to pelvic inflammatory disease, tubal pregnancies and infertility in women. All of these diseases are caused by chronic inflammation at the site of infection, but not all C. trachomatis strains produce this effect. Since the late 1940s, researchers have believed a toxin might cause the inflammation, but no such toxin had ever been found.
"These bacteria cause debilitating illnesses in hundreds of millions of people throughout the world," says Harlan Caldwell, Ph.D., a leading chlamydia researcher at NIAID's Rocky Mountain Laboratories in Hamilton, Mont. "If we could find a toxin that helps the bacteria attack and destroy cells, we would have a major new target for diagnostic tests, vaccines and drugs."
Dr. Caldwell joined Robert Belland, Ph.D., and their colleagues to look for the toxin using a new tool: the genetic blueprint of C. trachomatis. The researchers compared the genomes of two C. trachomatis strains: one that is restricted to mucosal surfaces and infects the eyes and genital tract, and one that is invasive and infects cells in the lymph nodes. The genes of the two strains were almost identical except for a single region of the bacterial chromosome. When the investigators studied that region in more detail, they found a stretch of DNA that resembled a known gene for the so-called toxin B of Clostridium difficile. That bacterium causes a potentially fatal infection of the large intestine in humans and is closely related to the deadly microbes that cause tetanus, botulism and gangrene. Toxin B makes the protein scaffolding inside of cells collapse, causing the cells that line mucosal passageways to separate from one another. The C. difficile bacteria can then use the spaces between the cells to invade deeper into the intestinal wall.
To see if the candidate gene found in C. trachomatis actually encoded a protein similar to clostridial toxin B, Drs. Caldwell and Belland looked for evidence of the toxin in the bacteria. Their studies showed that C. trachomatis produces specific changes in infected cells, and those changes are indistinguishable from ones induced by the clostridial toxin. Infected cells also contained toxin-encoding RNA, a type of DNA photocopy that shuttles its genetic instructions to the cell's protein-making machinery. The researchers also showed that infected cells contain a protein resembling toxin B, suggesting the cells deciphered the RNA's instructions.
"Finding the toxin would have been nearly impossible without the genome information," says Dr. Belland. Without it, he explains, the search would have resembled the proverbial needle in a haystack. The genome gave them a good idea of what haystack the needle was in, where it was located, and what the needle might look like.
Anthony S. Fauci, M.D., director of NIAID, agrees that genome sequencing offers great promise in improving global health. "Determining the DNA sequence of the world's leading infectious microbes is a high priority within NIAID," he states. "This study is one example of how that commitment can provide researchers with a powerful tool for understanding and eventually treating or preventing infectious diseases."
The discovery of a specific toxin associated with trachoma- and STD-causing C. trachomatis strains is a boon to investigators trying to understand these diseases. Drs. Caldwell and Belland plan to continue their studies to determine how the toxin helps the bacteria cause disease and how it interacts with a person's immune system. Further investigations should reveal new ways to attack the bacteria, perhaps by using an antitoxin vaccine, as is done with tetanus and diphtheria, or by developing drugs that block the toxin's ability to destroy cells.
NIAID is a component of the National Institutes of Health (NIH). NIAID supports basic and applied research to prevent, diagnose, and treat infectious and immune-mediated illnesses, including HIV/AIDS and other sexually transmitted diseases, tuberculosis, malaria, autoimmune disorders, asthma and allergies.
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