A trio of University of Rochester researchers has unexpectedly discovered that most mutations in yeast are caused by the activity of an enzyme that acts to salvage irreparably damaged DNA. The enzyme, REV1, helps a cell evade its own stringent quality control and opens the door to mutations. The findings are reported in the August 22 issue of Nature.
The researchers set out to understand the function of a family of DNA repair enzymes in yeast. When they damaged the yeast's DNA and then studied the yeast's coping mechanisms, they made a startling discovery: Almost all mutations in DNA, ironically, result from the activity of one of the DNA repair enzymes, REV1.
"We really found something that we weren't at all looking for," says David Hinkle, associate professor of biology. "This was a bombshell." Also working on the project, funded by the National Institutes of Health, are postdoctoral fellow John Nelson and Christopher Lawrence, professor of biophysics.
The vast majority of mutations come not from asbestos, pesticides, or other environmental factors, but from normal chemistry that goes on within our bodies. "DNA is not a stainless steel molecule," Lawrence points out. "It is susceptible to damage as a natural part of its existence."
The most common form of DNA damage is sporadic loss of individual molecules of adenine and guanine -- two of the four bases that encode genetic information along a strand of DNA. Such losses result in genetic gaps known as "abasic" sites.
"Formation of these abasic sites," says Hinkle, "is, in fact, one of the most common occurrences in our cells," taking place every day in about 10,000 to 20,000 of the three trillion DNA bases in each of our cells. Uncorrected abasic sites can be lethal to cells because they act as vast potholes, halting the polymerase enzymes that usually zip along a strand of DNA during replication. If a cell is unable to replicate its DNA, it will eventually die.
Cells usually avoid this fate -- as well as mutations -- by accurately repairing abasic sites in a variety of ways. But very rarely these cellular efforts at quality control fail, and it's at this desperate stage that REV1, the enzyme discovered by Nelson, Hinkle, and Lawrence, causes a mutagenic bypass to occur.
"REV1, in its efforts to cope with DNA damage, can actually produce mutations," Nelson says. "When exposed to DNA- damaging agents, yeast without the REV1 enzyme develop less than one percent as many mutations as yeast with the enzyme."
In the Nature paper, the Rochester researchers show that REV1 leapfrogs the gap by inserting a default base, cytosine, at the abasic site, allowing the unique zeta DNA polymerase -- recently announced by the same group of scientists in Science -- to resume copying the DNA strand. This saves the cell from certain death, but its survival carries a potentially steep price: The arbitrary placement of a cytosine base may change the genetic code and cause a mutation. While individual mutations are seldom serious, "if you get that rare mutation that causes cancer, you're in trouble," Nelson says.
"There's every reason to believe that mutagenesis in humans will prove to be very similar to that in yeast," says Lawrence, noting that Rochester scientists have already identified similar gene sequences in human DNA. "And mutagenesis is at the heart of cancer, which is thought to occur because mutations permit precancerous cells to overcome the genetic defenses against unlimited cell proliferation and migration."
The Rochester researchers believe the findings could open a major new avenue of cancer prevention. People with genetic predispositions to cancer could be given drugs to knock out the REV1 enzyme, stopping mutations before they arise.