Medicine: Finding a Cancer Clue

Of the numerous types of cancer, few are more dreaded than acute leukemia. The disease, which often afflicts the young, is characterized by an uncontrolled proliferation of certain white blood cells, which gradually crowd out the vital red blood cells. The cause of this lethal rampage is not yet clear, but what may be a crucial clue has just been reported in Nature by Dr. Robert C. Gallo of the National Cancer Institute. His findings could point the way to a leukemia cure.

Carefully examining the white blood cells of 48 healthy people and three leukemia patients, Gallo and two colleagues−Stringner S. Yang and Robert C. Ting of the Bionetics Research Laboratories−discovered a small but possibly critical difference. The white cells of the leukemia victims showed the presence of an enzyme known as RNA-dependent DNA polymerase; the cells of the normal people did not. The presence of the enzyme suggested that it may play a key role in the development of the disease.

Deadly Message. Gallo’s hypothesis tends to support the iconoclastic ideas of Howard Temin, a University of Wisconsin molecular biologist who long espoused what his colleagues considered a major heresy. According to accepted theory, the hereditary information in the chromosomes of all cells passes in the same direction. Double-stranded DNA molecules make single-stranded messenger RNA molecules, which then direct the production of proteins, the basic building blocks of every cell. Temin contended that the process is sometimes reversed: RNA, he insisted, could make DNA. Otherwise, he asked, how could cancer-causing viruses−which consist of bundles of RNA sheathed in protein−inject their deadly message into normal cells?

Last summer Temin and other molecular biologists produced strong experimental evidence that RNA viruses may indeed be capable of producing their own DNA (TIME, July 20). Columbia University’s Sol Spiegelman confirmed it. He demonstrated how an enzyme, or natural chemical catalyst, can cause tumors in laboratory animals by a DNA-RNA reversal. As Temin had postulated, the enzyme turned out to be RNA-dependent DNA polymerase. But a question remained: Was the same enzyme also present in human cancer?

By detecting the enzyme in human leukemia cells−something that has also been done by Spiegelman’s team−the scientists may have discovered an important diagnostic tool. Testing for the presence of the enzyme may now help doctors to identify leukemia in its earliest stages. And early identification is almost always the first step toward a cure. If the enzyme is proved to be at the heart of the process resulting in leukemia, it should be possible to find chemicals that suppress it.

Gallo and other investigators are already searching. One likely candidate is an antibiotic called N-dimethylrifampicin. This chemical has already proved effective in the laboratory in inhibiting the activity of the suspect enzyme. In the future, such chemicals may be able to work their magic in man.