The Canadian-Polish research team found recurrent mutations in the RECQL gene among the women, who have a strong family history of breast cancer but do not carry one of the more common BRCA mutations.
One type of RECQL mutation occurred 50 times more frequently among Quebec women with familial breast cancer compared to control subjects.
Another RECQL mutation put affected Polish women at a five-fold increased risk for developing breast cancer compared to women without the mutation.
While RECQL mutations appear to be quite rare, researchers estimate that up to half of all women who have the mutation will get breast cancer.
About one in 10 breast cancer cases are hereditary, but it's believed that only half of all susceptibility genes for the disease have been discovered.
Up to 20 per cent of inherited breast cancers are caused by the BRCA1 or BRCA2 genetic mutations. Actress Angelina Jolie, for instance, had a double-mastectomy and her ovaries removed after testing showed she carried the BRCA1 mutation, which put her at high risk for both breast and ovarian cancer.
"Our work is an exciting step in identifying all of the relevant genes that are associated with inherited breast cancer," said Dr. Mohammad Akbari of Women's College Hospital in Toronto.
The study published Monday in the journal Nature Genetics involved more than 25,000 women with and without breast cancer within the French-Canadian and Polish "founder" populations, which have similar genetic profiles.
"This study showed that studying specific founder populations like Polish and French-Canadian women is an excellent approach for identifying disease-associated genes," said Dr. Steven Narod, director of familial breast cancer research at the Women's College Research Institute.
Akbari believes all women with breast cancer should be tested for genetic mutations, so treatment might possibly be targeted based on their genetic profile.
"In the future, we might be able to select or develop treatments that can work around or correct relevant genetic mutations that are linked to breast cancer," he said. "This opens the door for new and better ways of approaching treatment."
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