Key Protein May Help to Repair Damaged DNA

To preserve a person's DNA, cells have developed a complex system for monitoring and repairing DNA damage. Yet, exactly how the first damage signal is converted into a repair response remains unclear. Researchers have now solved a crucial piece of the complex puzzle.

In an article published in the December 24, 2009, issue of the journal Molecular Cell, investigators from the Salk Institute for Biological Studies (La Jolla, CA, USA) demonstrated that a protein named CtIP plays a fundamental role in the DNA damage "signal-to-repair” conversion process. "Being able to repair damaged DNA is extremely important; the cell has to know when it has received this type of damage and respond appropriately,” explained Tony Hunter, Ph.D., a professor in the Molecular and Cell Biology Laboratory and director of the Salk Institute Cancer Center, who led the study. "Failure to do so can have disastrous consequences.”

The DNA in an organism's cells is under constant attack from reactive chemicals generated as byproducts of cellular metabolism. Moreover, it is battered by X-rays, ultraviolet radiation from the sun, and environmental carcinogens such as tobacco smoke. Because of this continuous assault, some studies have estimated that the DNA in a single human cell is damaged over 10,000 times every day.

If not repaired correctly, the damage leads to mutations, which over time can cause cancer. "As a result, individuals with an inherited impairment in DNA repair capability are often at increased risk of cancer,” noted first author Zhongsheng You, Ph.D., a former postdoctoral researcher at the Salk Institute and now an assistant professor at Washington University School of Medicine in St. Louis (MO, USA).

DNA consists of two intertwined strands so that when the DNA is broken, two ends are revealed, one from each strand. In order to repair the DNA break, one strand is trimmed away--or resected--similar to a loose thread, leaving only the second strand. This exposed strand then searches for a copy of itself (located on its sister chromosome), and "photocopies” past the broken region, repairing the DNA, and zipping itself back up.

In yeast, CtIP is required for resection of the broken end, and since it is also recruited to sites of DNA damage in human cells, Dr. Hunter's team wanted to figure out whether CtIP plays a similar role there. To find out, they depleted CtIP from human cells and caused DNA damage. Without the CtIP, they discovered, the cells could no longer trim back the damaged DNA strands, which brought the whole repair process to an abrupt halt.

"It looks like CtIP recruitment is a very important control point in the DNA repair process,” Dr. You observed. "Once CtIP is recruited, resection and repair begin, so regulating CtIP recruitment is one way to regulate DNA repair itself.”

In order to understand the process better, the researchers then tried to find out which regions of the CtIP protein are involved in binding it to the broken DNA ends. By looking at small portions of the protein, they found that a region in the central part of CtIP helps recruit the protein. They named this region the "damage recruitment” (DR) domain.

Further studies suggested that the DR domain within CtIP is typically hidden inside the folded protein. Only when the cell sends a DNA damage signal is CtIP's DR domain exposed, and only then can CtIP bind to the broken DNA. In this way, CtIP is like a switchblade that cells open only in the presence of DNA damage.

The scientists believe that exposure of CtIP's DR domain and its recruitment to the site of DNA damage starts a chain reaction that results in DNA repair, and they now want to understand precisely what CtIP does to start the DNA repair process.

Dr. You is also trying to understand the modifications in CtIP that cause the DR domain to be exposed, and is looking into the role of CtIP in cancer. "Mutations in CtIP have not been mapped extensively in human tumors, but from this data, we predict that mutations to the DR domain would lead to cancer,” he commented.

In the long term, the investigators hope that a better understanding of the DNA damage pathway may provide clues for cancer treatment in the future. "CtIP is another important player in the double-strand break response,” stated Dr. Hunter. "We have added another piece to the complex puzzle of DNA repair.”

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Salk Institute for Biological Studies