Enzyme that Digests DNA Linked to Increased Genetic Mutations

An enzyme that chops up DNA during programmed cell death, when rendered inoperable in mice, confers a higher survival rate to cells even if they have harmful genetic mutations such as those that cause cancer, Duke University Medical Center researchers have discovered.

The findings could aid in understanding how cancer-causing genetic mutations accumulate with enough frequency to cause the disease in people with no hereditary risk, said senior study author Chuan-Yuan Li, M.D., a professor of radiation oncology and cancer biology at Duke University Medical Center.

The discovery could also help explain why people develop resistance to drug or radiation treatments for cancer, he added.

Programmed cell death, or apoptosis, is the mechanism for normal culling of cells by the body when they are damaged or no longer needed. The DNA fragmentation factor (DFF) enzyme appears late in the process, Li said.

"Native DNA is densely packed and very sticky, so the job of this enzyme is chop it up into small pieces so other cells can absorb it and deal with it," Li said.

The Duke team discovered that mice lacking the gene for the DFF enzyme developed cancer at three- to four-fold higher rates than normal mice when exposed to ionizing radiation or chemicals.

Previous studies have shown the gene is missing in some human cancers, especially neuroblastomas, an aggressive childhood cancer, Li said.

Tests on cultured mouse cells with the deleted gene showed that slightly fewer damaged cells became apoptotic than in cells with the functioning gene, the Duke researchers found.

The results were published the week of January 23, 2006, in the early online edition of the Proceedings of the National Academy of Sciences.

"The small fraction of cells that survive is the one you want to worry about," Li said.

"Cells with genetic damage that are normally destined to die somehow survive, and the increase in mutations that result could lead to cancer or drug resistance," he said.

In the case of cancer, cells missing the DFF enzyme may be more likely to "evolve" resistance to drugs or radiation by surviving and accumulating mutations after treatment.

Li explains that in cells that have lost the ability to properly undergo programmed death, the genetic mutation rate could jump from one in a million cell divisions to one per thousand.

Environmental stresses could also play a role in disrupting the DFF enzyme, Li said.

"Normally, people aren't born with this deletion in their genome, but it could occur in their cells during life due to exposure to hazards such as chemicals," Li said.

"When you get increased mutation you see increased carcinogenesis. Having this gene deleted may increase your chances of getting cancer."

Because the DFF enzyme functions so late in apoptosis, Li and his colleagues were surprised to see that removing the enzyme could disrupt the death process.

"The enzyme isn't active until almost the end-stage of apoptosis, but it somehow affects overall survival in a few percent of cells," Li said.

"Nobody really understands how this works, but we think the full apoptosis process may involve a feedback mechanism," he said.