BRCA Breakthrough
Authors: Leslie Mitch 
Published: August 2003

Cracking a stubborn mystery, a new study pins down the function of BRCA2, the DNA-repair protein that goes awry in many cases of breast cancer. The mammoth molecule holds onto a cracked DNA strand while a protein partner fixes the damage. The findings promise to illuminate how flaws in BRCA2 provoke cancer.

Mutations in
BRCA2
show up in inherited cancers of the breast, ovaries, prostate, and pancreas. Although scientists sequenced the gene 7 years ago, they've made little progress in understanding how the protein works. They do know that BRCA2 belongs to a repair gang that heals DNA fractures using a matching chromosome as a pattern. When the protein is disabled, chromosomes break, shuffle segments, and sometimes disappear altogether--types of damage that can provoke cancer (see
"Dangerous Liaisons"
). Some scientists suspected that BRCA2 regulated the DNA-refurbishing process. Others believed that it was just a scaffold that supported another DNA-knitting enzyme, known as RAD51, but that it didn't attach to the DNA directly. The protein's enormous size--more than 3400 amino acids--stymied experiments to divine its function.

Trying a novel approach, Yang and colleagues used x-ray crystallography to deduce the architecture of BRCA2's tail, which mutational analysis had shown was essential for restoring broken DNA. "When we saw the structure, it was one of those ah-ha moments," says co-author Nikola Pavletich, a structural biologist at the Memorial Sloan-Kettering Cancer Center in New York City. Before a cell mends a broken piece of DNA, its enzymes peel away one of the strands from the damaged section, leaving a single strand to which fresh nucleotides are added. The researchers spotted three segments in BRCA2 that could bind to single-stranded DNA and one that could hold double-stranded DNA. The arrangement suggests that the double-strand-binding region of BRCA2 grasps DNA on one side of the break, and the other three segments cradle the single strand that is undergoing repair. In test tube experiments, the researchers confirmed that short DNA strands bound to BRCA2. They also mixed RAD51, which clings to BRCA2 in cells, with another fix-it enzyme and DNA fragments. The combination started to mend the strands only when the researchers also added the BRCA2 tail. The evidence suggests that BRCA2 grips a severed piece of DNA and brings RAD51 into position to begin repairs, says Pavletich.
The work demystifies this cancer-fighting protein, says molecular biologist Simon Powell of Harvard University: The study "takes BRCA2 above and beyond a protein scaffold and gives it a direct functional role." The next step is to determine whether BRCA2 latches onto broken chromosomes inside living cells, which is easier to determine with the structure in hand, says geneticist Shyam Sharan of the National Cancer Institute's branch in Frederick, Maryland. For example, scientists can meddle with BRCA2's DNA-binding regions to find out if the changes upset repair. The work will also help clarify how the different mutations seen in breast cancer patients hamstring the protein, he says. After years of struggling, researchers might finally get a grip on this slippery protein.
--Mitch Leslie

H. Yang, P. D. Jeffrey, J. Miller, E. Kinnucan, Y. Sun, N. H. Thomä, N. Zheng, P.-L. Chen, W.-H. Lee, N. P. Pavletich, BRCA2 function in DNA binding and recombination from a BRCA2-DSS1-ssDNA structure.
Science
297
, 1837-1848 (2002).
[Abstract]
[Full Text]