By Dean Haycock
Special To BioWorld Today
For years, the discovery of disease genes has produced gratification for successful gene trackers and headlines for the mass media. Now, headlines carrying different messages are beginning to appear. They report progress in figuring out what some disease genes do and how they may contribute to the development and progression of disease.
In recent years, BRCA1 and BRCA2 genes have been the subject of many headlines because, according to the National Cancer Institute, they are responsible for up to 90 percent of inherited breast cancers. Between 5 and 10 percent of all breast cancers may be inherited. BRCA1 mutations alone may be present in half of those people with inherited breast or ovarian cancer.
Researchers learned BRCA1 encodes a zinc finger protein. These are proteins so named because a zinc atom interacts with specific amino acids in the protein's structure. Zinc finger proteins bind to nucleic acids, other proteins and even some non-proteins. Unfortunately, the exact function of the BRCA1 zinc finger protein remained unknown. Clues, however, such as its association with other molecules, including a DNA repair protein called Rad51, suggested it might play a role in repairing DNA.
New research appearing in the Aug. 14 issue of Science takes our understanding of the cancer gene a step farther. The report, "BRCA1 Required for Transcription-Coupled Repair of Oxidative DNA Damage," provides for the first time a functional assay of the BRCA1 gene. Using this assay, Steven Leadon, professor of radiation oncology at the University of North Carolina, in Chapel Hill, and his colleagues show that BRCA1 appears to be involved in a specific type of DNA repair process called "transcription-coupled repair" (TCR).
Mammalian cells have many mechanisms for repairing DNA, which can be damaged by physical and chemical agents in the environment. When one or more of these repair mechanisms breaks down, the result can be a genetic disease, compromised cell function or cancer. TCR targets DNA that is in the process of being transcribed, or having its genetic message copied into RNA prior to being translated into a protein. TCR allows transcriptionally active DNA to be repaired more quickly than DNA that is not being transcribed.
The process concentrates on repairing damage in the new, or transcribed, strand of DNA in preference to the untranscribed strand. For this reason, TCR has a particular relevance for active genes.
The North Carolina team describe results of experiments with mouse embryonic stem cells, each of which has two copies of the BRCA1 gene. In some mice, the BRCA1 genes were inactivated. In control animals, the genes functioned normally. The scientists then chose a gene that is actively transcribed, the dihydrofolate reductase gene, and measured the effects on TCR of such DNA-damaging stimuli as ionizing radiation, ultraviolet light and hydrogen peroxide.
Ionizing radiation and hydrogen peroxide severely hampered BRCA1 deficient cells from correcting DNA damage using TCR. Both of these treatments produce oxidative damage to DNA. UV light, which produces a different type of DNA damage, did not affect the ability of BRCA1 deficient cells to employ TCR to fix damaged DNA. The results suggest that cells with BRCA1 mutations are particularly susceptible to specific types of DNA damage.
It is easy to conjecture that women who inherit defective BRCA1 genes may be more susceptible to certain types of DNA damage that accumulate over time. As damage to genes involved in vital cell functions accumulates, cells may lose the ability to regulate their growth, leading to cancer.
Because BRCA1 cells might be more sensitive to ionizing radiation, the results also raise the possibility that such radiation treatment might be particularly effective in women with BRCA1 mutations. The authors acknowledge that their results cannot determine if BRCA1 acts directly in the TCR process, or whether it turns on some other genes that are required for efficient DNA repair.
"The question, with respect to its role in transcription-coupled repair, is what other proteins does [BRCA1] interact with that are involved with that specific repair process," Leadon said.
Leadon and his colleagues have begun to look at other mutations in BRCA1 that may indicate where the protein interacts with other proteins in TCR.
"By putting in specific mutations in specific parts of the BRCA1 gene such that you don't end up deleting the whole gene, you can tell, in part, where the protein-protein interactions are. Then from there you can begin to go see which proteins are interacting with that region," Leadon said. For this work, Leadon plans to extend his experiments to include other cell types, such as embryonic mouse fibroblasts.
Another important line of research will involve the BRCA2 gene. Leadon and his coworkers would like to know if it is involved in similar cellular mechanisms. "In many ways, Leadon said, "that is the next natural step, as far as piecing together protein-protein associations [goes]." *