Tumor offers unsafe home for cell's genes.

Cells inside a tumor have trouble catching their breath. Though tumors secrete chemicals that trigger new blood vessel formation, these cancerous masses grow faster than their blood supply. Consequently, cells deep within the solid mass face a serious lack of oxygen, a condition known as hypoxia.

By counting mutations in genes that were added to mouse cancer cells, researchers have now discovered that the oxygen-poor environment of a tumor can cause DNA damage. This finding may explain, in part, why tumor cells gradually accumulate multiple gene mutations and chromosomal abnormalities and thus why tumors tend to become more malignant with time.

The new study, conducted by Toni Y. Reynolds, Sara Rockwell, and Peter M. Glazer, all of Yale University School of Medicine, appears in the Dec. 15 Cancer Research.

To monitor the frequency of DNA damage within a tumor, the investigators turned to a bacterial gene called supF. They inserted multiple copies of this gene into mouse cancer cells. The group then either grew those cells in dishes filled with an oxygen- and nutrient-rich solution or injected the cells into the flanks of mice and allowed tumors to develop.

Many weeks later, the researchers removed the bacterial genes from the dish-grown cells and from the mouse tumors and inserted them back into bacteria. Under the test conditions, bacteria that received a normal supF gene generated blue colonies, while those getting a mutated gene produced white ones. The experiments showed consistently that cells grown in a tumor had about five times as many supF mutations as cancer cells raised in a dish.

To examine what aspects of tumor environment might trigger mutations, the researchers again grew mouse cancer cells in dishes but deprived the cells of almost all oxygen. Like their counterparts grown inside mouse tumors, these hypoxic cells experienced an increased frequency of supF mutations, says Glazer.

This "profound result" adds to the growing link between hypoxia and tumor development, notes Amato J. Giaccia of Stanford University School of Medicine. In the past year or so, Giaccia and his colleagues have shown that the oxygen-poor environment within a tumor favors the survival of cells possessing a mutated form of p53, a gene that normally regulates cell growth and division (SN: 4/6/96, p. 216). Now, says Giaccia, the Yale group has shown that hypoxia actively encourages mutations, perhaps even those that alter p53.

How the tumor environment, or more specifically hypoxia, harms DNA is unclear. The low oxygen content of tumor cells may somehow damage genes directly, or the mutations may be a secondary result. For example, hypoxic cells experience a rise in internal acidity, much as a runner's muscles build up lactic acid when they have used up all available oxygen. This increased acid content may directly injure DNA or may impair the enzymes used to correct DNA mutations, says Glazer.

He and his colleagues are now examining whether cell acidity influences mutation frequency.