In 1927, American gynecologist John Sampson published his theory of the etiology of endometriosis, postulating that retrograde flow of endometrial debris flows backward through the fallopian tubes during menses into the peritoneal cavity. Dr. Sampson’s notion remains the main paradigm today, mentioned still in recent articles on the topic, but it has a flaw: Although the theory may account for how endometrial tissue escapes the uterus, a 1984 study revealed that this phenomenon occurs in 90% of women. Why, then, do only 10% of women suffer from endometriosis?
Endometriosis describes a condition in which endometrial tissue lining the uterus is found outside the uterus. The disease can be painful, even crippling. As many as 30% of women in their reproductive years who have endometriosis are infertile as a consequence. The hallmarks of the condition are superficial peritoneal lesions of varying color, cysts in the ovaries, deeper nodules accompanied by scarring and adhesion, primarily in the pelvis but sometimes appearing outside the pelvis. The syndrome can be challenging to identify, requiring laparoscopy for definitive diagnosis.
John Sampson aside, scientists have struggled for the past century to identify the cause, or causes, of endometriosis. Hormones clearly play a role in its development, and women with endometriosis have an elevated risk of clear-cell and endometrioid ovarian cancer and autoimmune diseases. Immunodeficiency also could be to blame, if a faulty immune system fails to find and remove endometrial tissue outside of the uterus. A class of chemicals known as endocrine disruptors have been linked to endometriosis, but not definitively. Twin studies have demonstrated that as many as 50% of cases have a genetic basis, while mice with surgically induced endometriosis have been found to have a higher ratio of harmful to beneficial bacteria in their gut.
Several studies published this year point to new insights into the old mystery – with possible implications for ways to treat the disorder.
Perhaps the most surprising came out earlier this year in Science Translational Medicine, as a team of researchers in Japan reported that invasive infection by bacteria of the genus Fusobacterium may cause at least some cases of endometriosis.
Is Fusobacterium the new Helicobacter pylori?
The researchers, from Nagoya University, are the first to suggest that not only might a single bacterial genus cause endometriosis, but that antibiotic treatment could prevent progression of the disease. Using endometrial tissue obtained from 79 women undergoing hysterectomy for endometriosis and 76 women undergoing hysterectomy for other reasons (such as cervical cancer), the team started with gene expression profiling to explore differences between the two sets of samples.
They uncovered an interesting chain of cellular events: macrophages found in endometriotic lesions were secreting transforming growth factor-beta (TGF-beta). TGF-beta in turn stimulated high levels of expression of a gene called TAGLN in fibroblast cells from women with endometriosis but not in fibroblasts from women without endometriosis.
Turning on TAGLN transformed these previously inactive cells into active myofibroblasts, leading to increased proliferation, mobility, and attachment to mesothelial cells, the layer of cells that line body cavities and internal organs. In short, they identified some key players in an environment that seemed very favorable to the development of endometriosis.
“So, the question is: Why are macrophages activated?” said Yutaka Kondo, MD, PhD, the senior author of the study and a professor in the division of cancer biology at the Nagoya (Japan) University Graduate School of Medicine. “We think that there are always bacteria in the endometrium.”
After reviewing data from a previously published study, they used quantitative polymerase chain reaction to rule out one candidate, Erysipelothrix, but scored on their next attempt, identifying Fusobacterium species in endometrial tissue from 64% of the women with endometriosis, compared with fewer than 10% of the controls.
To confirm that the bacteria could cause disease and were not simply bystanders, Dr. Kondo’s team turned to a mouse model for endometriosis, in which endometrial cells are surgically removed from the uteri of mice and injected into the peritoneum of recipient mice, leading to the formation of endometriotic lesions. When mice received further injections of uterine tissue from mice that were infected with F. nucleatum, their lesions were more numerous when compared with mice that received injections of uninfected uterine tissue. Furthermore, antibiotic treatment with metronidazole or chloramphenicol immediately after surgery largely prevented progression to endometriosis, Dr. Kondo and his colleagues reported.
Dr. Kondo likened this relationship between Fusobacterium and endometriosis to that of the link between Helicobacter pylori and peptic ulcers but acknowledged that he doesn’t have all the answers.
“We need more clinical trials, and also we have to know what kind of treatment might be the most effective for the treatment of endometriosis,” Dr. Kondo said, pointing out that other therapies should still be pursued in addition to antibiotics, as not all the samples from women with endometriosis harbored Fusobacterium. “It might be possible that other mechanisms are also involved.”