PITTSBURGH, April 21, 2014 - Malfunctions in the molecular "proofreading" machinery, which repairs structural errors in DNA caused by ultraviolet (UV) light damage, help explain why people who have the disease xeroderma pigmentosum (XP) are at an extremely high risk for developing skin cancer, according to researchers at the University of Pittsburgh School of Medicine and the University of Pittsburgh Cancer Institute (UPCI). Their findings will be published this week in the early online version of the Proceedings of the National Academy of Sciences.
Previous research has shown that a DNA-repair protein called human UV-damaged DNA-binding protein, or UV-DDB, signals for a repair when two UV-DDB molecules bind to the site of the problem, said senior investigator Bennett Van Houten, Ph.D., the Richard M. Cyert Professor of Molecular Oncology, Pitt School of Medicine, and co-leader of UPCI's Molecular and Cell Biology Program.
"Our new study shows UV-DDB makes stops along the DNA strand and transiently attaches to it, causing a proofreading change in the protein's conformation, or shape. If the DNA is damaged the protein stays, if the DNA is not damaged the protein leaves," Dr. Van Houten said. "When it comes to a spot that has been damaged by UV radiation, two molecules of UV-DDB converge and stay tightly bound to the site, essentially flagging it for the attention of repair machinery."
The researchers followed the trail of single molecules of UV-DDB by tagging them with light-emitting quantum dots, enabling them to watch the molecules jump from place to place in real time on both normal and UV-exposed DNA strands.
They also tracked a mutant UV-DDB protein associated with XP, an inherited, incurable disease of light sensitivity that affects about 1 in 250,000 people. They found that the mutant UV-DDB molecules are still capable of binding to DNA, but continued to slide along the DNA rather than staying put to signal where the fix was needed.
"Without this important damage control, UV-induced errors could accumulate to cause cell alterations that foster cancer development," Dr. Van Houten said. "Like a bus with no brakes, the XP-associated UV-DDB complex stays on the road and sees possible passengers, but keeps going past the stop."
Co-investigators include Harshad Ghodke, Ph.D., Ching L. Hsieh, Selamawit Woldemeskel, Simon C. Watkins, Ph.D., and Vesna Rapic-Otrin, Ph.D., all of the University of Pittsburgh School of Medicine; and Hong Wang, Ph.D., of North Carolina State University.
The project was funded by the University of Pittsburgh Cancer Institute Cancer Center Support grant CA 047904 and National Institutes of Health grant ES019566.
About the University of Pittsburgh School of Medicine
As one of the nation's leading academic centers for biomedical research, the University of Pittsburgh School of Medicine integrates advanced technology with basic science across a broad range of disciplines in a continuous quest to harness the power of new knowledge and improve the human condition. Driven mainly by the School of Medicine and its affiliates, Pitt has ranked among the top 10 recipients of funding from the National Institutes of Health since 1998. In rankings recently released by the National Science Foundation, Pitt ranked fifth among all American universities in total federal science and engineering research and development support.
Likewise, the School of Medicine is equally committed to advancing the quality and strength of its medical and graduate education programs, for which it is recognized as an innovative leader, and to training highly skilled, compassionate clinicians and creative scientists well-equipped to engage in world-class research. The School of Medicine is the academic partner of UPMC, which has collaborated with the University to raise the standard of medical excellence in Pittsburgh and to position health care as a driving force behind the region's economy. For more information about the School of Medicine, see http://www.
As the only NCI-designated comprehensive cancer center in western Pennsylvania, UPCI is a recognized leader in providing innovative cancer prevention, detection, diagnosis, and treatment; bio-medical research; compassionate patient care and support; and community-based outreach services. Investigators at UPCI, a partner with UPMC CancerCenter, are world-renowned for their work in clinical and basic cancer research.