Getting answers on tape
THE blood test most commonly used to screen men for prostate cancer isn’t reliable. Some men are given a clean bill of health when they actually do have cancer, while others undergo painful biopsies only to learn they’re cancer-free.
An experimental skin test ultimately could prove to be a more precise tool. By detecting the abnormal genetic signature of prostate cancer cells on the body’s surface, it has the potential to reduce the number of unnecessary tests and save lives.
“There’s a tremendous need for a more specific test,” says Dr. Durado Brooks, director of prostate cancer for the American Cancer Society. “This is a fascinating concept, and skin is much more easily accessible than getting tissue samples or taking blood.”
Prostate cancer strikes more than 200,000 American men annually, claiming 29,000 lives. Each year, millions of men take a prostate-specific antigen, or PSA, test to determine whether they are at elevated risk for the cancer. The test measures levels of a protein produced by prostate gland cells.
However, a July 2005 study by University of Texas researchers revealed that the PSA test results in many false positives and false negatives. “Up to 20% of men with low PSA levels can have cancer, while more than half with higher levels won’t have cancer,” says Brooks.
The new test, inspired by recent research at UC San Diego, uses skin samples to test for prostate cancer. Investigators there discovered prostate cancer cells produce many types of substances that influence the growth of other tissues and organs, including the skin, the body’s largest organ.
Because the skin’s surface is so large, scientists wondered if the skin would act like a magnifying glass and allow them a better view of tumor cells percolating through the body. They also wondered whether changes in the way genes behaved when they were affected by prostate cancer were apparent on the skin’s surface, according to Dr. William Wachsman, an oncologist who conducted the recent research.
To find out whether this is true, Wachsman has begun a $1.8-million study that will eventually encompass 450 men. One third of the volunteers will have had positive biopsies for prostate cancer, another third will have elevated PSA levels but no evidence of cancer, while the remainder will have negligible PSA levels.
Skin samples will be collected from each volunteer using a technology known as Epidermal Genetic Information Retrieval. It consists of specially designed adhesive tapes, about the size of a quarter, that “can harvest surface skin cells from which genetic matter can be extracted,” says Nicholas Benson, a scientist at DermTech International, the San Diego company that makes the tapes.
The samples will then be analyzed to identify genes on the skin’s surface.
“We’re looking for skin cells that have alterations in their gene expression due to the cancer,” says Wachsman, who plans to analyze at least 47,000 genes. “Eventually, we’ll boil those numbers down to the top 25 genes that statistically are the most predictive of cancer,” he says.
If the skin sampling test proves reliable, it could provide an easy and painless way to screen for prostate cancer, and reduce the need for biopsies and the discomfort of repetitive needle sticks for blood test. In the future, the test may also be used to predict the severity of prostate cancer -- whether it is a slow-growing tumor that requires monitoring only or an aggressive malignancy that needs immediate attention.
“This could shift the paradigm of how we detect prostate cancer,” says Wachsman. “And the test could be done in a doctor’s office or even by individuals at home.”
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Other skin tests
Skin tests have been used for years to diagnose allergies and to detect tuberculosis. But skin cells may also provide clues to other diseases.
A new test called PREVU, which can measure cholesterol levels in skin cells, has been approved by the Food and Drug Administration.
And scientists at DermTech International are in the midst of studies assessing the use of adhesive skin strips to spot recurrences of melanoma, a deadly form of skin cancer, and identify changes in the skin of diabetics.