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Study subjects were French-Canadian women with ductal carcinoma in situ ( DCIS) or invasive breast cancer (incident or prevalent) who were treated and followed at a single breast cancer clinic affiliated with the Research Center of University of Montreal ( CRCHUM), who were either aged less than 50 years at diagnosis or who were 50 years or older and with at least two affected first- or second-degree relatives. Subjects were tested for six founder mutations (three in BRCA1 and three in BRCA2); 1093 eligible cases were tested. Of these, 56 women (5.1%) were mutation carriers, including 43 BRCA2 carriers and 13 BRCA1 carriers. The prevalence of mutations was 5.3% for unselected women aged 50 and less and was 4.6% for familial cases over age 50. The prevalence of mutations was 3.3% for women with DCIS and was 5.3% for women with invasive cancer. It is rational to offer genetic testing to all French-Canadian women diagnosed recently or in the past with either DCIS or invasive breast cancer before age 50 or with familial breast cancer above age 50. [ABSTRACT FROM AUTHOR]

In an ethnically-homogeneous population, it is valuable to identify founder mutations in cancer-predisposing genes. Founder mutations have been found in four breast-cancer-predisposing genes in French-Canadian breast cancer families. The frequencies of the mutant alleles have been measured neither in a large series of unselected breast cancer patients from Quebec, nor in healthy controls. These estimates are necessary to measure their contribution to the hereditary burden of breast cancer in Quebec and to help develop genetic screening policies which are appropriate for the province. We studied 564 French-Canadian women with early-onset invasive breast cancer who were treated at a single Montreal hospital. Patients had been diagnosed at age 50 or less, and were ascertained between 2004 and 2008. We screened all 564 patients for nine founder mutations: four in BRCA1, three in BRCA2 and one each in PALB2 and CHEK2. We also studied 6433 DNA samples from newborn infants from the Quebec City area to estimate the frequency of the nine variant alleles in the French-Canadian population. We identified a mutation in 36 of the 564 breast cancer cases (6.4%) and in 35 of 6443 controls (0.5%). In the breast cancer patients, the majority of mutations were in BRCA2 (54%). However, in the general population (newborn infants), the majority of mutations were in CHEK2 (54%). The odds ratio for breast cancer to age 50, given a BRCA1 mutation, was 10.1 (95% CI: 3.7–28) and given a BRCA2 mutation was 29.5 (95% CI: 12.9–67). The odds ratio for breast cancer to age 50, given a CHEK2 mutation, was 3.6 (95% CI: 1.4–9.1). One-half of the women with a mutation had a first- or second-degree relative diagnosed with breast or ovarian cancer. Thus, it can be concluded that a predisposing mutation in BRCA1, BRCA2, CHEK2 or PALB2 is present in approximately 6% of French-Canadian women with early-onset breast cancer. It is reasonable to offer screening for founder mutations to all French-Canadian women with breast cancer before age 50. The frequency of these mutations in the general population (0.5%) is too low to advocate population-based screening. [ABSTRACT FROM AUTHOR]