Research Links Stomach Cancer With Bacterium
For the first time, scientists have found strong evidence that a type of cancer may be caused by a bacterium.
Stanford researchers report today in the Journal of the National Cancer Institute that virtually all of their patients with the most common type of stomach cancer are infected with a bacterium that has previously been linked to inflammation of the stomach and ulcers--strong evidence that the infectious agent plays a role in the development of the disease.
Although stomach cancer strikes only about 20,000 Americans each year, it kills nearly 14,000 and is common elsewhere, reaching epidemic status in some areas of Asia and South America.
At least two other groups of researchers have also linked the bacterium to stomach cancer, although neither has published its findings.
“All the evidence is consistent, and it is all moving in the same direction--that (the bacterium) does cause stomach cancer,” said epidemiologist Martin J. Blaser of Vanderbilt University.
A few human cancers are known to be caused by viruses, and one or two by carcinogenic chemicals produced by molds and fungi, but to date none have been clearly linked to bacteria. The majority of cancers are thought to be caused by carcinogens in the environment, radiation and genetic susceptibility.
Linking a bacterium to the disease is critical because it should be possible to make sharp inroads into the incidence of stomach cancer by developing vaccines to prevent the bacterial infections.
“I’m optimistic because, if it is correct, this is the kind of thing one could do something about,” said oncologist John Laszlo, medical director of the American Cancer Society in Atlanta. “Many people will follow it up quickly.”
But Laszlo cautioned that previous research linking bacteria to other forms of cancer, such as Hodgkin’s disease, has subsequently been disproved. In such cases, he said, the association was found to be only coincidental. “We need confirmation with much larger numbers of patients to be convinced,” he said.
The bacterium in question is called Heliobacter pylori . The spiral-shaped microorganism was isolated and identified in 1982 by Australian researchers and has since been the subject of intensive study. It is the only microorganism that can live in the acidic environment of the stomach and is rarely found elsewhere in the body, according to Stanford epidemiologist Julie Parsonnet, who conducted the study.
Heliobacter infections are common around the world. In the United States, an estimated 30% of the population carries it. Its incidence is higher in minorities and increases to as much as 60% in older populations. In some areas of the world, such as Cali, Colombia, virtually 100% of the population is infected. Researchers do not know how it is transmitted, but the most likely route is thought to be person-to-person contact.
Most people who are infected by it show no symptoms and are not harmed by the microorganism. But scientists are now convinced that a Heliobacter infection is necessary for peptic ulcers to develop and a growing body of evidence indicates that it also contributes to gastritis, an inflammation of the stomach lining. It can be killed with powerful antibiotics, but for most people the risks of side effects from the antibiotics are greater than the risk of ulcers or the odds of developing cancer.
“There are a number of theoretical considerations that would suggest it plays a role in cancer,” Blaser said. The Stanford results “support that now with some evidence.”
Parsonnet and her colleagues studied 60 stomach cancer patients who had been treated at Stanford University Hospital and Santa Clara Valley Hospital in San Jose between 1985 and 1989. They microscopically examined specimens of stomach tissue from the patients to look for the presence of Heliobacter.
They found that the bacterium was present in 89.2% of the patients with the most common type of stomach cancer, called “intestinal-type gastric adenocarcinoma,” compared to the roughly 50% incidence that would be expected in healthy populations of the same age. Because of the characteristics of the bacterium and the difficulties of identifying it, they wrote, “it can be presumed that the actual rate of H. pylori infection in intestinal-type cancer is higher than the 90% we observed and may approach 100%.”
Similar results were reported at a recent European workshop by Blaser and Nicholas Talley of the Mayo Clinic in Rochester, Minn., and by epidemiologist David Forman of Oxford University in England.
It is possible that the association between the bacterium and the tumors is coincidental, or that the tumors simply provide a fertile breeding ground for the bacterium. To eliminate this possibility, the Stanford and Vanderbilt researchers are studying groups of individuals with Heliobacter infections to determine if they develop a higher incidence of stomach cancer over a period of time. Those studies have just started, but the preliminary data “look promising,” Blaser said.
Assuming that the role of the bacterium is confirmed, there are several mechanisms by which it could trigger the cancer. It might have enzymes, for example, that would convert specific materials in food to carcinogens. Parsonnet believes, however, that it works simply by causing inflammation. “Inflammation creates risk for cancer by producing lots of byproducts that cause mutations,” she said, and those mutations are the first step along the road to cancer.
If Heliobacter is confirmed to be a cause of stomach cancer, then it should be possible to use parts of the bacterium to make a vaccine, Parsonnet says, that could prevent perhaps 8 of 10 cases of the intestinal-type stomach cancer.
