This study provides such evidence.
Begg and his colleagues genotyped more than 2,000 participants in the Women's Environmental Cancer and Radiation Epidemiology (WECARE) Study who had been diagnosed with breast cancer. Of the total, 1,394 women had unilateral breast cancer (one breast only), while 704 had contralateral breast cancer (an additional malignancy in the opposite breast).
This study focused on the 181 women who were shown to be mutation carriers.
First-degree relatives of BRCA1 and 2 carriers had a higher risk of developing breast cancer if their proband (the relative whose breast cancer diagnosis first brought the family history to light) was diagnosed at a younger age (35 or under). The overall study only looked at women who had been diagnosed under the age of 55.
There was also a non-statistically significant trend toward higher risk for first-degree relatives of women with contralateral breast cancer as opposed to unilateral breast cancer.
The results strongly indicate that factors other than the BRCA mutations influence risk.
"If everyone with a mutation has exactly the same risk, then the presence of the mutation is the only factor that influences risk in carriers, [and] there are no other factors that can influence risk," Begg explained. "The fact that we see that age at diagnosis in the proband influences risk in the relatives implies that there is something else about the carrier families with a "young" proband that must influence risk. The presumption is that these families are passing on other genetic characteristics that positively influence breast cancer risk."
Although the authors state there is no need for wide screening of women yet, future technological advances may make such screening feasible.
"Lots of research is being done on the whole genom
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