The effect of disease penetrance, family size, and age of onset on family history with application to setting eligibility criteria for genetic testing

Prevalence of BRCA1 and BRCA2 pathogenic variants in a large, unselected breast cancer cohort

Breast cancer patients with BRCA1/2-driven tumors may benefit from targeted therapy. It is not clear whether current BRCA screening guidelines are effective at identifying these patients. The purpose of our study was to evaluate the prevalence of inherited BRCA1/2 pathogenic variants in a large, clinically representative breast cancer cohort and to estimate the proportion of BRCA1/2 carriers not detected by selectively screening individuals with the highest probability of being carriers according to current clinical guidelines. The study included 5,122 unselected Swedish breast cancer patients diagnosed from 2001 to 2008. Target sequence enrichment (48.48 Fluidigm Access Arrays) and sequencing were performed (Illumina Hi-Seq 2,500 instrument, v4 chemistry). Differences in patient and tumor characteristics of BRCA1/2 carriers who were already identified as part of clinical BRCA1/2 testing routines and additional BRCA1/2 carriers found by sequencing the entire study population were compared using logistic regression models. Ninety-two of 5,099 patients with valid variant calls were identified as BRCA1/2 carriers by screening all study participants (1.8%). Only 416 study participants (8.2%) were screened as part of clinical practice, but this identified 35 out of 92 carriers (38.0%). Clinically identified carriers were younger, less likely postmenopausal and more likely to be associated with familiar ovarian cancer compared to the additional carriers identified by screening all patients. More BRCA2 (34/42, 81.0%) than BRCA1 carriers (23/50, 46%) were missed by clinical screening. In conclusion, BRCA1/2 mutation prevalence in unselected breast cancer patients was 1.8%. Six in ten BRCA carriers were not detected by selective clinical screening of individuals.

A population-based assessment of the clustering of breast cancer in families eligible for testing of BRCA1 and BRCA2 mutations

BACKGROUND

The prevalence of families eligible for BRCA1/2 mutation testing in the population burden of breast cancer was analysed and the aggregation of breast cancer in these families was explored.

PATIENTS AND METHODS

The families of the Swedish Family-Cancer Database with at least three generations (N=944 723) were classified according to the criteria proposed by the German Consortium for Hereditary Breast and Ovarian Cancer for BRCA1/2 mutation testing. We calculated the proportion of women with breast cancer in the classified families and used standardised incidence ratios (SIRs) to estimate the risk of histology specific breast cancers in families with suspected BRCA1/2 mutations.

RESULTS

Families with two breast cancers before the age of 50 years included 1.8% of the breast cancer patients; 1% of the women with breast cancer belonged to families with breast and ovarian cancers. The SIR of female breast cancer was lowest in families with male breast cancer and highest in families with two women affected by breast cancer under the age of 50 years. The SIRs of medullary breast cancer agreed with the BRCA1 mutation prevalences detected by the German Consortium for Hereditary Breast and Ovarian Cancer.

CONCLUSIONS

Most of the breast malignancies in families with male breast cancer are likely to be related to BRCA2 mutations. Non-BRCA1/2 related effects are probably involved in the strong clustering of breast cancer in families with early onset breast and ovarian cancers.

Contribution of the Defective BRCA1, BRCA2 and CHEK2 Genes to the Familial Aggregation of Breast Cancer: a Simulation Study Based on the Swedish Family-Cancer Database

The known breast cancer susceptibility genes only account for 20% to 25% of the excess familial risk of the disease [1]. The present study assessed the contribution of BRCA1/2 mutations and CHEK2 variants to the relative risk of breast cancer for women with affected mothers or sisters. The familial relative risks were estimated by Poisson regression based on the Swedish Family-Cancer Database. The Database was also used to calculate the distribution of life expectancy, the number of daughters per family and the age specific cumulative risk of female breast cancer. This information, together with the penetrances of BRCA1, BRCA2 and CHEK2 from the literature, was used to simulate the familial clustering of breast cancer under different scenarios. The excess risk explained by BRCA1, BRCA2 and CHEK2 decreased steeply with the age at diagnosis of the cancers. Around 40% of the familial risk for cases diagnosed before the age of 50 years was associated with BRCA1/2 mutations. In contrast, roughly 85% of the familial risk of breast cancer diagnosed before the age of 69 years remained unexplained. The contribution of CHEK2 to familial breast cancer was small.