Gene Screen for Colon Cancer Holds Promise
Noninvasive test seeks mutation, could catch disease early
WEDNESDAY, Jan. 30, 2002 (HealthDayNews) -- A sophisticated genetic test holds promise as a screening tool to detect cancer of the colon and rectum in the early, curable stages, Johns Hopkins researchers say.
The test looks for mutations in a gene designated APC, known to be intimately involved in the process by which a growth in the intestine called a polyp changes from harmless to cancerous. In a trial with 74 individuals, the test identified all of those known to be cancer-free and detected mutations in 61 percent of patients with known cancer and half of those with pre-cancerous growths, says a report in Thursday's issue of the New England Journal of Medicine.
However, much more work is needed to make the test usable for screening, says Dr. Bert Vogelstein, lead author and a professor of oncology at the Johns Hopkins Kimmel Cancer Center. His group first identified the APC gene, and is trying to refine the procedure.
In scientific terms, the "specificity" of the test -- the ability to say who's not in danger -- is fine, Vogelstein says. However, its "sensitivity" -- the ability to say who is in danger -- needs refining.
"Our goal is to be able to detect about two-thirds of cases," he says. "We think we can get that high by technical improvements. That would be sensitivity comparable to other tests, such as mammography and Pap smears."
A simple screening test is badly needed because existing tests to detect colorectal cancer are not doing the job, "in part because of their invasive nature, in part because of lack of sensitivity," says Vogelstein.
For example, colonoscopy, a visual inspection of the intestinal tract regarded as the gold standard for detection, requires a two-day preparation by the patient and often is done under sedation. A simpler screening method, the fecal occult blood test, is much easier to do but does not have the desired sensitivity.
That test looks for signs of bleeding in a sample of feces. The newest test takes a comparable sample, and uses a technique called digital protein truncation to examine the genetic material in the sample and identify cancer-related mutations.
The next step will be a study involving about 1,000 people, some with cancer, that will take three to five years, Vogelstein says. As part of that study, "we will address issues of cost," he says. "The test now costs $500 to do in our laboratory. We will try to develop something that could be applied in a mass way."
The APC test "up to this point was not ready for prime-time usage," agrees Dr. Mark Pochapin, chief of endoscopy at the New York Weill Cornell Medical Center. "The same holds after release of this paper."
However, Pochapin calls the concept "really exciting. They are zeroing in on the genetic defect we know is active when a growth moves from being a polyp to becoming cancerous."
Like other cancer specialists, Pochapin is distraught at the thought that while colon cancer can be cured when caught early, most cases are not detected until they have progressed too far.
"What is so exciting is that if we could figure out who is at high risk for a significant neoplasia [cancer], we could skew patients to see who needs more invasive tests, such as colonoscopy," Pochapin says. "Hopefully we would be able to identify high-risk groups through this. I could see combining this with fecal occult blood testing. Maybe the two together could be more sensitive."
What To Do
"Any screening is better than no screening," Pochapin says. "This disease is the number two cause of cancer deaths, and it is highly curable if we catch it early."
For information about colorectal cancer, its detection and treatment, go to the American Cancer Society. For an overview of colonoscopy and what a patient must do to prepare for it, try the American Gastroenterological Association.