BRCA Mutation Risk May Not Be as High as Thought
BRCA Mutation Risk May Not Be as High as Thought
BETHESDA, Md--Women of Ashkenazi Jewish lineage appear to have a lower risk of breast and ovarian cancer from mutations in the BRCA1 and BRCA2 genes than was first thought, a study from the National Institutes of Health has found.
Earlier studies suggested that women with a mutation in either gene had a 76% to 87% chance of developing breast cancer and an 11% to 84% likelihood of developing ovarian cancer by age 70, compared with just an 8% risk of breast cancer and 2% risk of ovarian cancer for women as a whole. The new study suggests the risk for Ashkenazi carriers is 56% and 16%, respectively.
The mutation carrier rate among Ashkenazim is more than 2%--the highest known in any ethnic group. Of the six million Jews in the US, about 90% trace their ancestry to eastern or central Europe, indicating Ashkenazi descent.
Jeffrey P. Struewing, MD, and his NIH colleagues saw the high Ashkenazi prevalence as an opportunity to begin to answer an important question: What percentage of the enhanced susceptibility to breast and ovarian cancer in those who carry mutations of BRCA1 or BRCA2 is actually due to the genetic alteration?
Speaking at a press briefing held to coincide with publication of the data (New England Journal of Medicine 336:1401-1408, 1997), Dr. Struewing, senior investigator at the National Cancer Institute, said that the women in the previous studies of these gene mutations--Jewish or not--had extensive family histories of breast cancer, ovarian cancer, or both, and many had, themselves, had these tumors.
Unknown, therefore, was whether their elevated risk was entirely explained by BRCA mutations or whether some of it was due to other genes they and their afflicted relatives had in common or to environmental influences--dietary patterns, for instance--they had shared.
Rationale for Studying Ashkenazim
The challenge, in other words, was to assess the degree of risk conferred by a BRCA mutation when women had little or no family history of these cancers. In theory, that could have been done with studies of any population that might have the mutations in its midst. In reality, Dr. Struewing doubts that would have been useful.
"For one thing," he explained," more than 140 different BRCA mutations have been identified to date because both genes are very large. Also, it was learned from the original studies--which were done without regard to ethnicity--that very rarely is a BRCA mutation in one cancer-prone family the same as in another such family."
By contrast, three specific mutations seem to be unique to Ashkenazi Jews: 185delAG and 5382insC in BRCA1, and 6174delT in BRCA2. It thus made sense to focus on this trio of mutations in this particular ethnic group.
A further rationale for this strategy was the suggestion from earlier data that the mutations--besides predisposing women to breast and ovarian cancer-- predispose men to prostate cancer and both sexes to colon cancer. By including Jews of both genders in their study, the researchers would be able to get more information about those issues as well.
The researchers appealed to the Jewish community in and around Washington, DC, for 5,000 people who would fill out a questionnaire about their personal and family cancer histories and have blood drawn for BRCA analysis.
So enthusiastic was the response that it took only nine weeks to recruit 5,318 volunteers. Nearly 30% were men, and 75% had no personal or family history of breast cancer.
The testing showed that 120 of the volunteers (2.3%) were positive for one of the three BRCA mutations (none were positive for more than one mutation). The family histories of these volunteers were than compared with those of the noncarrier volunteers.
As family history is the single strongest predictor for many cancers--including breast cancer--this made it possible to compare the average risk, by age 70, for carriers of the mutations and non-mutation carriers.
For breast cancer, that risk proved to be 56%; for ovarian cancer, 16%. Moreover, Lawrence C. Brody, PhD--a co-author of the study--cautions that "even those estimates may be inflated because there was a larger percentage of volunteers in the study with a family history of breast cancer than there is in the Jewish population overall."
Dr. Brody is senior investigator at the National Human Genome Research Institute in its Laboratory of Gene Transfer.
By the researchers' calculations, in fact, BRCA mutations could account for only about 7% of all breast cancers in Jewish women. Still, as another of the study's co-authors, Margaret A. Tucker, MD, notes, "an individual carrier's risk may be smaller or larger than the average risk of 56%." Dr. Tucker heads the NCI's Genetic Epidemiology Branch.
Risk for Other Cancers
For colon cancer, the study found that the estimated average risk was not significantly different between noncarriers and carriers of the mutation. Thus, it would seem that none of the three BRCA mutations examined is implicated in that malignancy in either sex.
On the other hand, the study provided convincing evidence that the suspected link between BRCA mutations and an increased risk of prostate cancer is real. For a man with any of the three mutations, the risk by age 70 was 16%, the same as for ovarian cancer in female mutation carriers.
The study also found that none of the three Ashkenazi-specific mutations studied was associated with a significantly higher risk of cancer than the others.
There are as yet no data on the average risk for non-Jewish carriers of such mutations. Still, the researchers' educated guess is that just as Jewish carriers with little or no family history of breast, ovarian, and prostate cancer have a lower risk than those from cancer-ridden families, so will their counterparts in other population groups.
Follow-up Study Planned
Dr. Struewing and his colleagues are planning a follow-up study in the greater Washington, DC, Jewish community. Its volunteers, like their predecessors, will be tested for BRCA mutations. Unlike them, however, volunteers in the new study will be told the results of their tests if they so choose. For that reason, all participants will get genetic counseling before being tested.
The new study will search for factors that modify a carrier's risk of developing cancer. Among the factors being considered for inclusion in the search are hormonal influences and interactions between BRCA mutations and mutations in other genes.