To screen for prostate cancer, physicians have long relied on a blood test that searches for abnormally high levels of a protein known as prostate specific antigen (PSA). The problem is, because benign changes in the prostate can also produce elevated levels of PSA, the test has a fairly high rate of false positives. The results of a new study suggest that a more specific test may lie within reach, however. According to a report published today in the Journal of the American Medical Association, researchers have identified a gene that is overexpressed in prostate cancer tissue and found only in malignant cells. The protein produced by this gene, they say, could serve as the basis for a more accurate and sensitive screening test for the cancer, which affects one in nine men over the age of 65.

Using so-called DNA microarray technology, Mark A. Rubin and Arul M. Chinnaiyan of the University of Michigan identified 20 genes that are consistently overexpressed in cancer. High levels of one resulting protein--an enzyme called a-methyl-CoA racemase, or AMACR--showed up in roughly 95 percent of more than 300 prostate tissue samples containing localized cancer, they report. Conversely, the protein did not appear in benign prostate tissue. Subsequent needle biopsies of 94 of the samples showed AMACR expression to have 97 percent sensitivity and 100 percent specificity for detecting prostate cancer.

"The results of this study suggest that AMACR may be a useful addition to current diagnostic tools for detecting prostate cancer, although these findings require further evaluation in larger studies," Rubin and Chinnaiyan write. They add that although the AMACR turns up in the highest amounts in prostate and colorectal cancer, they also found it in ovarian, breast, bladder and lung cancer, suggesting that it could serve as a diagnostic biomarker for these types as well.