Researchers say they have discovered a major reason why women who inherit a mutated version of the gene BRCA1 run a high risk of breast cancer - and that finding might aid the search for new treatments. A second gene, called PTEN, plays a key role, scientists said in a study released Sunday. Scientists have long known that BRCA1 normally repairs damage to other genes. So if it is crippled by mutations, the un-repaired damage could be expected to lead to cancer. But the specifics of that story have been murky. The new study fingers PTEN, which normally acts as a brake on cancer. The researchers found evidence that in breast cancers associated with a BRCA1 mutation, PTEN is often broken and does not get repaired. That sets off a chemical cascade that leads to malignancy.