Building the What Comes Next Cohort for BRCA1 and BRCA2 testing: a descriptive analysis

Background — Our understanding of how testing for and mutations of the BRCA1 and BRCA2 genes affect cancer risk and the use of risk-reduction strategies comes largely from studies of women recruited from specialized genetics clinics. Our aim was to assemble a generalizable cohort of women who underwent BRCA1/BRCA2 testing (the What Comes Next Cohort), irrespective of test result, to enable study of healthcare utilization and outcomes after testing.

Methods — This descriptive study included adult women (≥ 18 yr) who met at least 1 of 13 provincial criteria for BRCA1/BRCA2 testing and who underwent genetic testing at sites in Ontario, Canada, from 2007 to 2016. Most of the women were tested at 1 of 2 main sites, which together capture about 70% of all BRCA1/BRCA2 testing in the province. We collected detailed demographic, genetic testing and family history data through chart review for linkage with data from administrative health databases providing information on cancer history before and after testing. We followed all women to September 2019, evaluating the demographic characteristics of the cohort, indications for testing and test results.

Results — We identified 15 986 women (mean age 52.5 [standard deviation 13.9] yr) who underwent BRCA1/BRCA2 testing. Of these, 2033 women had positive results, 1175 women had variants of uncertain significance, and 12 778 women had negative results. Positive yields were 41.0% (955/2329) for predictive testing (for familial variants), 10.4% (216/2072) for Ashkenazi Jewish founder testing and 7.4% (862/11 585) for complete gene analysis. Six of the 13 provincial testing criteria had less than 10% positive yield. Among 403 women who tested negative for Ashkenazi Jewish founder mutations and subsequently underwent complete gene analysis, 12 (3.0%) tested positive for alternate pathogenic or likely pathogenic variants in the BRCA1 or BRCA2 gene.

Interpretation — Several provincial eligibility criteria for BRCA1/BRCA2 testing led to positive results in less than 10% of cases. How testing influences women’s healthcare behaviours, particularly those with negative results and those found to carry variants of uncertain significance, is unknown; the What Comes Next Cohort will be instrumental in the study of long-term implications of BRCA1/BRCA2 testing.