In laboratory experiments using budding yeast, the same type used in baking and brewing, scientists at the National Cancer Institute (NCI), part of the National Institutes of Health, developed a new approach to determine the location of unrepaired breaks in DNA. This new approach should better inform research as unrepaired DNA damage often underlies the development of cancer. The research findings appear in the December, 2007, issue of PloS Biology.

The investigators, from NCI's Center for Cancer Research (CCR), examined meiosis, a form of cell division that produces sperm and eggs in animals. During meiosis, a process called recombination may occur that involves the swapping of genetic material between chromosomes. Chromosomes are molecules of DNA that carry genes and function in the transmission of genetic information. For recombination to occur, chromosomal DNA must first be broken and then spliced together in new combinations, which creates genetic diversity as new combinations of genes are passed from parent to child.

"Our new method to detect where DNA is purposefully broken during meiosis should be a useful tool in understanding the events that start cells on the road to cancer," said study author Michael Lichten, Ph.D., of NCI's Laboratory of Biochemistry and Molecular Biology.

Recent research has shown that recombination is initiated during meiosis when a protein called Spo11 breaks both strands of the DNA molecule present in a chromosome. These double-strand breaks (DSBs) are then efficiently repaired by recombination. While these DSBs are useful during meiosis, DSBs formed by accident or by chemical damage can be harmful because they are often incorrectly repaired, creating the kind of genetic rearrangements that can cause cancer and other diseases. By studying how yeast efficiently repair the DSBs that occur during meiosis, researchers aim to develop ways of reducing the impact of cancer-causing, unrepaired or improperly repaired DNA damage.

When a DSB is caused by the Spo11 protein, the protein sometimes remains attached to the end of the DSB. Previous methods of detecting DSBs involved seeking the Spo11 protein to see what DNA was attached. These methods are not sensitive, and do not detect all of the DSBs that are formed during meiosis.

Instead of searching for the Spo11 protein, the researchers examined the single strands of DNA that accumulate at DSBs in mutants that lack critical recombination proteins. By purifying this single-stranded DNA, they were able to map yeast DSBs during meiosis at the whole-genome level. Because this approach finds breaks using a feature common to all DSBs, it can be used in circumstances where Spo11 is not involved, such as DSBs that are caused by chemical agents.

Using this new detection approach, the authors took a whole-genome snapshot of DSB locations. While the previous, less sensitive studies had suggested that DSBs did not occur in up to 40 percent of the genome during meiosis, this more sensitive method showed that DSBs were occurring at similar levels throughout the yeast genome.

The researchers concluded that recombination in yeast is distributed much more uniformly than previously believed. They predict that their new mapping method will be useful for studying recombination and DNA damage in other organisms.

Buhler C, Borde V, Lichten M. Mapping meiotic single-strand DNA reveals a new landscape of DNA double-strand breaks in Saccharomyces cerevisiae. PLoS Biology December 2007. Vol. 5, No. 12: e324.

For more information on Lichten's laboratory, please go to http://ccr.cancer.gov/staff/staff.asp?profileid=5611.

For a basic tutorial on Cancer Genomics, please visit http://www.cancer.gov/cancertopics/understandingcancer/cancergenomics/.

For more information about cancer, please visit the NCI Web site at http://www.cancer.gov, or call NCI's Cancer Information Service at 1-800-4-CANCER (1-800-422-6237).

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