Three per cent of Norwegian ovarian cancers are caused by BRCA1 1675delA or 1135insA.

Our aim was to determine the prevalence of two Norwegian BRCA1 founder mutations in ovarian cancer patients, to identify carriers and their families for medical follow-up, and to study histopathological factors. Of a cohort of 727 ovarian cancer patients, 615 gave informed consent to testing. 2.9% (18/615) of the tested patients were found to be carriers of BRCA1 1675delA (n = 13) or 1135insA (n = 5). The total frequency of the mutations was 4.7% (8/171) in patients below 50 years of age, and zero (0/144) in patients above 70 years of age. In patients below 70 years of age, the frequency of 1675delA and 1135insA mutations was 2.8% and 1.0%, respectively. Out of 13 patients with 1675delA mutation, 4 had breast cancer. 14/16 (87.5%) families fulfilled clinical criteria for familial breast-ovarian cancer. Median age of onset of ovarian and breast cancer was 51 years and 37 years, respectively. Mutation carriers tended to have tumours with unfavourable prognostic factors. This is, to our knowledge, the highest reported frequency of founder mutations in a national ovarian cancer cohort (less than in the Ashkenazis). It seems justified to offer such testing to ovarian cancer patients below 70 years of age in Norway, identify their risk of breast cancer and offer medical follow-up to the families.