In new research carrying implications for human disease development, University of North Carolina scientists and others have linked gene defects to the inability of cells to repair damaged DNA.
The findings published April 13 in the journal Cell offer new insights into how cells repair the DNA damage that may occur during normal metabolism. Such naturally occurring oxidative damage promotes tissue changes thought to be associated with disease development, including cancer, heart disease and rheumatoid arthritis.
Study co-author Steven A. Leadon, Ph.D., head of molecular radiobiology at the UNC Lineberger Comprehensive Cancer Center said "oxidative damage is probably the most common type of damage humans are exposed to because we live and breath oxygen, and oxygen can be reactive and cause DNA damage." Within cells, oxidation leads to formation of free radicals, short-lived highly active particles that occur naturally during metabolism. These are also introduced into the body through smoking, inhaling environmental pollutants, or exposure to the sun's ultraviolet (UV) radiation. From numerous studies, free radicals are known to interact readily with nearby molecules to cause cellular damage, including damage to genetic material.
Damage repair to an active gene, one that continually produces proteins crucial to cellular function, normally occurs through a process known as transcription coupled repair. "We were trying to understand how defects in one of the proteins required for this process works," Leadon said. "And what we were looking at was whether or not the defect has something to do with the inability to repair oxidative damage that occurs in a gene."
In 1998, Leadon and his UNC colleagues were the first to link the defective breast cancer gene BRCA1 to the inability of cells to correct DNA oxidative damage. His work directly demonstrated that BRCA1 was required for transcription-coupled repair. Those studies involved altered cells derived from mice that were deficient in BRCA1.
In the new experiments, the study collaborators focused on human cells that carried defects in genes associated with the hereditary condition known as Cockayne's syndrome. This is a rare disease characterized by sun sensitivity, small size at birth, poor postnatal development, and early death. The scientists also studied cells from a subset of individuals who had defects in a gene for another sun-sensitive condition, xeroderma pigmentosum. Individuals with this condition are incredibly sensitive to sunlight and develop skin cancers in pigmented skin areas. These individuals had also developed Cockayne's syndrome.
The cells were tested in several experimental conditions, including exposure to DNA-damaging gamma rays and to hydrogen peroxide, which generates high levels of free radicals.
"These cells all showed defects in the ability to target repair to oxidative damage when it occurs in a gene," Leadon said. "This indicates that individuals whose cells lack this repair will clinically present with Cockayne's syndrome."
The findings are suggestive of broader implications. In cells carrying genes associated with hereditary disease, a mutation to any one of the proteins involved in transcription coupled repair -- particularly a mutation occurring in a region where they interact -- may lead to disease expression. The study also suggests the importance of this repair process to development and why a rare disease such as Cockayne's syndrome occurs so rarely. (Only 140 people have been diagnosed worldwide.)
"If the developing embryo is unable to repair endogenous types of DNA damage, normal damage that cells are always exposed to, the likelihood of going to full term is reduced," Leadon explained.
"I think one of the goals of a number of labs including ours is to figure out why you can have these specific mutations leading clinically to disease and what it tells you about the protein to protein interactions." Along with UNC, co-authors of this report are from the Laboratory of Molecular Genetics, Villejuif, France, and Lawrence Berkeley National Laboratory, Berkeley, California.
The research in Dr. Leadon's laboratory is funded by grants from the National Institutes of Health.