CHAPEL HILL -- Scientists at the University of North Carolina at Chapel Hill and colleagues have developed a new microscopic assay that, for the first time, allows them to see DNA breaks in chromosomes in living cells following damage to those complex, gene-filled structures.
Their success is exciting because the assay should become a powerful new aid for boosting understanding of chromosome damage and how it is repaired naturally or might be repaired therapeutically, said Dr. Kerry S. Bloom, professor of biology at UNC. Chromosome damage leads to at least hundreds of fatal or debilitating illnesses.
"In the course of this work, we discovered that when breaks occur in either one or both strands of DNA, which is a complex, double-stranded, helical molecule, the chromosomes do not fragment," Bloom said. "Proteins are recruited very quickly to the sites of DNA damage, and they keep the chromosome intact. This was hypothesized but never shown before. We also identified some of those specific proteins."
Human cells contain an amazing amount of DNA, which produces proteins for countless tasks in the body, the scientist said. "In fact, if you took all of the DNA in your cells and stretched it out, it would go to the sun and back. That is awesome."
A report on the research appears in the latest issue of the journal Current Biology. Besides Bloom, authors are Kirill Lobachev, formerly of the National Institute of Environmental Health Sciences’ Laboratory of Molecular Genetics and now at the Georgia Institute of Technology; first-year biology graduate students Eric Vitriol of UNC and Jennifer Stemple of the University of Southern California; and Dr. Michael A. Resnick of NIEHS. Both Vitriol and Stemple were UNC undergraduates in the College of Arts and Sciences when they worked on the study.
"This is the first paper that distinguishes breaks in chromosomes versus breaks in DNA," Bloom said. "Chromosomal breaks are important in many diseases that stem from chromosomal translocation, in which genes get switched around."
Chromosomal aberrations are common outcomes of exposure to DNA-damaging
chemicals such as Agent Orange, excessive sunlight and radiation, he said. They also result from replication disrupted in some way and can lead to such conditions as leukemias, lymphomas, sarcomas and epithelial tumors.
"Aberrations can be greatly increased as a result of defects in DNA repair," Bloom said. "While there was considerable information about molecular events associated with the induction and repair of a double-stranded break, little has been known about events leading to chromosome breaks or the re-association of broken ends that result in translocations or deletions."
He likened translocations to circuits being rewired incorrectly or surgeons accidentally connecting veins to arteries.
"Our new system allows for visualizing DNA ends at the site of a double-stranded break in living cells," Bloom said. "We showed that a protein complex we call RMX holds broken ends of DNA together and counteracts forces that can be destructive to damaged chromosomes."
Grants from the National Institutes of Health and the U.S. Department of Energy supported the continuing studies. Bloom is a member of both the College of Arts and Sciences faculty and the Lineberger Comprehensive Cancer Center.
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