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Two large studies give a much more accurate picture of the inherited mutations that increase the risk of breast cancer in women without a family history of the disease, and the frequency of these defective genes in the general population.
Doctors say the results published Wednesday by the New England Journal of Medicine may help women make better decisions about screening, preventive surgery or other steps.
Although this type of genetic testing is not currently recommended for the general population, its use is increasing and many people are obtaining it from tests sold directly to consumers.
The new work shows that the risk conferred by certain genes “is very high,” wrote Mary-Claire King in an email. King, a scientist at the University of Washington, played no role in the new studies, but discovered the first breast cancer predisposing gene, BRCA1.
“The lives of many women could be saved if all women had the opportunity to learn if they carry mutations in these genes before being diagnosed with cancer,” she wrote.
The American Cancer Society estimates that 276,000 new cases of breast cancer were diagnosed in the United States last year. The new work suggests that at least 13,800 of them occur in women with inherited genetic mutations that increase their risk of developing the disease.
Much of what is known about hereditary risk so far has come from studies of women with a family history of breast cancer or unusual situations such as catching it at a very young age. There has also been little work on specific mutations in these genes and the extent to which each affects the chances of developing disease. The new studies fill some of these gaps.
One was led by Fergus Couch, a pathologist at the Mayo Clinic and included researchers from the National Institutes of Health, who sponsored the study with the Breast Cancer Research Foundation.
They looked for mutations in 12 genes linked to breast cancer in more than 64,000 women, about half with the disease and the other half without it, bringing together the results of studies conducted across the United States, including including some in specific minority groups such as blacks.
They found troublesome mutations in about 5% of women with the disease and in 1.63% of the comparison group.
“Now we realize that 2% of women walking around the United States could have mutations in these genes,” Couch said.
There were no differences between racial groups in the likelihood of having a mutation overall, but some mutations were more common in certain groups. For example, black women were more likely to have cancers linked to “triple negative” cancers – tumors that are not fueled by estrogen or progesterone, or the gene targeted by the drug Herceptin.
The study also found that having a mutation in the BRCA1 gene increased the risk of developing breast cancer almost eight-fold, and in the BRCA2 gene, more than five-fold.
Conversely, another gene was considered to be of great concern, but “what we found is that it’s really low risk… people really shouldn’t act on it,” Couch said.
Actions might include more frequent mammograms or other screening tests, removal of breasts or ovaries, screening of family members, or other steps.
With the new work, “we’re providing more accurate risk estimates” to guide such decisions, Couch said.
The second study by researchers at the University of Cambridge in England looked at 34 genes in women in the UK, Europe, Australia and Asia – around 60,000 with breast cancer and 53,000 similar without him.
“They found what we found” – an increased risk of certain genes and a similar prevalence of them in the general population, Couch said.
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