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How Does That Work, Exactly?

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When the BRCA1 gene isn't mutated, it works to suppress breast and ovarian cancer. A new study published in Nature on how BRCA1 functions may help explain how other tumor-suppressing genes work as well, reports Nature News' Alla Katsnelson. Researchers from the Salk Institute for Biological Studies looked into cells from mice lacking BRCA1, and, along with the usual problems attributed to BRCA1 defects in areas like cell cycle regulation and DNA repair, the team found these cells had a surprisingly small number of heterochromatic centers — "dense packages of normally untranscribed, repetitive sequences of DNA near a chromosome's centromere," Katsnelson says. Instead, the cells were active, and created large numbers of satellite repeats. When BRCA1 is present and normal in a cell, it keeps these centers silent by tagging histones with ubiquitin. When the researchers added an artificial ubiquitin-histone complex to the BRCA1-deficient cells, they resumed their normal functioning, she adds. Although it's still not clear why BRCA1 defects specifically affect breast and ovarian tissue, experts say it's possible those tissues are especially sensitive to the gene's loss of function. However, Katsnelson says, "patients with BRCA1 defects initially have one mutant copy of the gene and one normal copy, then lose the functioning copy as the tumor develops. But because the researchers studied cells completely lacking BRCA1, the findings do not explain why patients are initially predisposed to tumors, but rather why tumors develop after the functioning copy of BRCA1 shuts down."

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