"So there's something about the breast and ovary that they can tolerate the loss of both copies and not die, and it promotes growth and cancer. What it really means, there are secondary mutations that allow for -- probably -- for the growth of the cell," says Nussenzweig. A tumor-suppressant protein, known as p53, widely believed to play a role in preventing cancer in healthy cells, appears to be defective in BRCA tumors, too. But if that's the real culprit, no one knows. "People really don't understand it," admits Dr. Nussenzweig.

Another aspect they don't understand is whether this same HR pathway defect also causes the risk of cancer from BRCA2 gene mutations, the other main breast and ovarian cancer mutation gene.


And to answer that, Dr. Nussenzweig is experimenting with BRCA2 mice in his lab to see if knocking out 53BP1 helps them the way it does for their BRCA1 mutated cousins. But until the study's finished, the jury's out.


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