Novel Yeast Protein Plays a Key Role in Repairing Double-Strand DNA Breaks

By Eric Sauter

Scientists at The Scripps Research Institute have uncovered a novel protein in yeast that plays a key role in controlling the repair of double-strand breaks in DNA. The discovery of the protein, Ctp1, and its role, strongly suggests that the same mechanism works in regulating DNA damage repair in human cells and may point the way to future cancer therapies.

The study was published in the October 12 issue (Volume 27, Issue 7) of the journal Molecular Cell.

"Our study shows that this Ctp1 protein is the underlying factor in cell cycle control in one type of double-strand DNA break repair, homologous recombination," said Paul Russell, a Scripps Research scientist and professor of molecular biology, whose laboratory conducted the study in collaboration with Scripps Research Professor Curt Wittenberg and his group. "Because our yeast protein resembles the human protein CtIP—which has been proven to be a tumor suppressor in mice—we believe that CtIP is crucial in humans for repairing DNA damage that if left unrepaired could lead to cancer."

The study showed that when a double-strand break occurs, the CtP1 protein goes directly to the site of the break and is required for resection, part of the repair process.

DNA repair is crucial to normal cell function—without it, any number of things can go wrong. Double-strand breaks, the kind described in the study, are especially damaging because they often lead to a rearrangement of the entire genome. Research has traced some breast cancer tumors, for example, to a disruption in the repair of double-strand breaks.

Homologous recombination makes up one of two primary mechanisms for repairing DNA double-strand breaks in both yeast and human cells (the other mechanism is non-homologous end joining). It involves DNA strand exchange in which an undamaged chromosome is used as a template for repair of a damaged homologous chromosome. This "error-free" mechanism of DNA repair prevents mutations that can lead to cancer or other diseases. Homologous recombination also occurs naturally during meiosis and is commonly used for gene modification in the laboratory. In homologous recombination, the Mre11-Rad50-Nbs1 (MRN) complex is the main sensor of breaks in double-strand DNA and plays a critical role in catalyzing single-strand resection.

The study's findings suggest that Ctp1 and the MRN complex have mutually dependent functions in the repair of double-strand breaks, including those breaks that occur naturally during meiosis.

"We made a series of predictions about phenotypes of cells lacking the Ctp1 protein—that they would be sensitive to ionizing radiation and would be unable to undergo meiosis," Russell said. "And those predictions proved to be true. In fact, mutant cells that lack Ctp1 are hypersensitive to a variety of DNA-damaging agents. Our conclusion is that Ctp1 is essential for homologous recombination."

In addition to Russell and Wittenberg, other authors of the study, "Ctp1 is a Cell Cycle-Regulated Protein that Functions with Mre11 Complex to Control Double-Strand Break Repair by Homologous Recombination," include Oliver Limbo, Charly Chahwan, Yoshiki Yamada, and Robertus A. M. de Bruin of The Scripps Research Institute.

The study was supported by the National Institutes of Health and The Uehara Memorial Foundation.

Send comments to: mikaono[at]scripps.edu

"We made a series of predictions about phenotypes of cells lacking the Ctp1 protein—that they would be sensitive to ionizing radiation and would be unable to undergo meiosis—and those predictions proved to be true."

—Paul Russell