Cancers are caused by a series of mutations. Each mutation alters the behavior of the cell somewhat.Image credit: Jpbrody, Wikimedia CommonsDNA mutations—long known to fuel cancer as well as evolutionary changes in a living organism—had been thought to be rare events that occur randomly throughout the genome.

However, recent studies have shown that cancer development frequently involves the formation of multiple mutations that arise simultaneously and in close proximity to each other. These groups of clustered mutations are frequently found in regions where chromosomal rearrangements take place.

The discovery, published in the journal Cell Reports, may one day lead to new cancer therapies, according to a University of Iowa biologist and her colleagues, and a group of researchers from the National Institute of Environmental Health Sciences led by Senior Associate Scientist Dmitry Gordenin.

The formation of clustered mutations may result from the process of DNA repair.

Anna Malkova, associate professor of biology in the UI College of Liberal Arts and Sciences, notes that the DNA repair pathway, known as break-induced replication (BIR), can promote clusters of DNA mutations.

“Previously, we have shown that double-strand DNA breaks, which can result from oxidation, ionizing radiation and replication errors, can be repaired by BIR,” says Malkova.

“During BIR, one broken DNA end is paired with an identical DNA sequence on another chromosome and initiates an unusual type of replication, which proceeds as a migrating bubble and is associated with the accumulation of large amounts of single-strand DNA,” she says.

In the Cell Reports study, researchers subjected yeast cells undergoing BIR to alkylating (cancer cell-killing agents) damage. “We found that the single-stranded DNA regions that accumulate during BIR are susceptible to damage that leads to the formation of mutation clusters,” explains Cynthia Sakofsky, postdoctoral fellow at the UI and one of two co-first authors on the paper. “These clusters are similar to those found in human cancer,” she says.

Importantly, say the researchers, the paper provides a mechanism to potentially explain how genetic changes form in human cancers. Thus, it will be critical for future research to determine whether BIR can form clustered mutations that lead to cancer in humans. If this turns out to be true, it may lead to the discovery of new targets for developing therapies against human cancers.

In addition to Malkova, Gordenin and Sakofsky, co-authors of the paper are: Steven A. Roberts, co-first author, and Michael A. Resnick, both of the National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina; and Ewa Malc and Piotr A. Mieczkowski, both of the Department of Genetics, Lineberger Comprehensive Cancer Center and Carolina Center for Genome Sciences, University of North Carolina, Chapel Hill, North Carolina.

The paper, titled “Break-Induced Replication Is a Source of Mutation Clusters Underlying Kataegis,” was published in the May 29 issue of Cell Reports.

The research was funded by NIH grants R01GM084242 (to A.M.) and 5R01GM052319-17 (to P.A.M.) and by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences (project ES065073, to M.A.R). S.A.R. was supported by NIH Pathway to Independence Award K99ES022633-01.