© 1998 by Oxford University Press
One of the lessons from tumor suppressor research is that many years usually elapse between the cloning of a gene and identifying its function. Translating the gene's function into effective therapy is even more difficult. At this point, however, the breast cancer susceptibility gene BRCA2 seems to be the exception to that norm.
Even though the first breast cancer susceptibility gene, BRCA1, was cloned more than a year before BRCA2, scientists seem to be having a much easier time trying to figure out the function of BRCA2's protein. Furthermore, what is being discovered about its function has led to several ideas for rational therapy.
"The BRCA2 story has fallen out fast," said Jeffrey T. Holt, M.D., at Vanderbilt University Medical Center in Nashville, Tenn. "It's almost too good to be true. There are projects in the lab where every time you try something it works. The BRCA2 story feels like that. BRCA1 has been difficult."
On the surface, there are many similarities between the two genes: Both are extremely large; mutations in each gene result in similar clinical symptoms; both are expressed in most tissues during cell proliferation, peaking at similar phases of the cell cycle; both are nuclear proteins; and both seem to be involved in transcriptional regulation and DNA repair.
But the similarities end there. From the beginning, BRCA1 research, unlike BRCA2, has been plagued with difficulties -- problems such as locating where the BRCA1 protein resides in the cell, identifying which proteins interact with BRCA1, and making knock-out mouse models. (See sidebar.) All of these provide important clues to a protein's function in the cell.
In the meantime, the BRCA2 story seems to be moving forward quickly. In 1997 several groups found a direct association with Rad51, a key protein in both the DNA double-strand break repair pathway and recombination in yeast. (In contrast, the BRCA1/Rad51 interaction is generally thought to be indirect.)
Direct evidence for function came in 1997 in a paper in the April 24 issue of Nature. The article's lead author, Shyam K. Sharan, Ph.D., now at the Mammalian Genetics Laboratory at the Frederick Cancer Research and Development Center in Frederick, Md., showed that knock-out mouse embryos lacking both functional copies of BRCA2 were sensitive to gamma radiation, known to create double-strand breaks in DNA. Further proof that BRCA2's function is linked to Rad51 was that both are expressed simultaneously in the embryo in rapidly dividing cells and that both Rad51 and BRCA2 knock-out mouse embryos die during development at 6.5 days when the genes are normally expressed.
Because these data strongly suggest that the normal function of BRCA2 is involved in repair of double-strand DNA breaks, researchers in Holt's lab in Nashville decided to see if BRCA2-defective cancer cells are killed more easily than cells with normal BRCA2 when exposed to drugs that induce double-strand breaks.
Therapy Clue
Using a BRCA2-defective human pancreatic cancer cell line, the scientists reported in the July 1 issue of this journal that the cell line is both deficient in double-strand break repair and killed more easily by chemotherapy drugs that cause double-strand DNA breaks. Similar results were seen when the cell line was injected into the thighs of nude mice. BRCA2-defective tumors were shrunk significantly by therapies causing double-strand breaks.
These results have led Holt and others to see if the results in mice hold up in humans. Several groups have begun studies to see if patients with BRCA2 mutations will have a different response to radiation therapy than those with sporadic cancers.
Another therapeutic approach that may turn out to be important in many more types of tumors, including those that are sporadic, has been proposed by Wen-Hwa Lee, Ph.D., at the University of Texas Health Science Center in San Antonio. Lee sees the most important clinical pay-offs coming from the BRCA2/Rad51 interaction.
In a recent article in Proceedings of the National Academy of Sciences, researchers in Lee's lab identified the portions of BRCA2 protein that associate directly with Rad51. His strategy is to synthesize a small peptide that will block this interaction. Because most breast tumors, as well as other tumors, have an intact BRCA2 protein, a drug that blocks this interaction might leave the tumors more susceptible to damage from radiation or drugs causing double-stranded DNA breaks.
Even though the data for BRCA2 seems to be stronger, some would argue that it's difficult to imagine that BRCA1 and BRCA2 are not both involved in at least some of the same pathways. Lewis Chodosh, M.D, Ph.D., at the University of Pennsylvania Medical Center in Philadelphia, is one of those.
Chodosh has published a series of articles during the last three years looking at the pattern of expression of both genes during embryogenesis, in a variety of adult tissues, and in mammary glands during different developmental stages.
"Basically you can't tell them apart," remarked Chodosh. "It's remarkable. We looked at over 20 tissues, and what's extraordinary is not just that BRCA1 and BRCA2 are expressed in the same tissues, but that they're expressed at the same relative levels within the same cellular compartments and at the same developmental stages within those tissues. You just don't find that for very many genes in nature."
Because of these data, he believes that BRCA1 and BRCA2 are likely to be interconnected. His sense is that within the nucleus are dynamic macromolecular complexes that include BRCA1/2, Rad51, and other DNA damage-related proteins.
"My guess is that there are probably three, four, five or more proteins in these complexes. The particular proteins present or absent will be different, depending on the state of the cell and the state of DNA damage," he speculated.
Ralph Scully, M.D., Ph.D., a researcher in David Livingston's lab at the Dana-Farber Cancer Institute at the Harvard Medical School, agrees that BRCA1/2 functions are probably intertwined. In support of this, the Livingston group has recently shown in a paper published in September in Molecular Cell that BRCA1 and BRCA2 interact biochemically, although it is not yet clear whether this interaction is direct or indirect.
One hypothesis advanced by the group is that both genes are involved in maintaining the integrity of the DNA into breast tissue, which is uniquely vulnerable to assaults from carcinogens throughout life -- during menarche, breast development, and pregnancy. One candidate is a metabolite of estrogen.
For the moment, however, until a consensus emerges, the data linking BRCA2's function to the repair of DNA breaks offers the most promise for translating knowledge of function into effective rational therapy. If progress in the BRCA2 field continues at its present pace, the answers from the clinic should be in relatively soon.
-- Nancy J. Nelson
BRCA2 Research Is On a Fast Track to The Clinic
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