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Background  Inherited pathogenic variants in  BRCA1  and  BRCA2  are the most common causes of hereditary breast and ovarian cancer (HBOC). The risk of developing breast cancer by age 80 in women carrying a  BRCA1  pathogenic variant is 72%. The lifetime risk varies between families and even within affected individuals of the same family. The cause of this variability is largely unknown, but it is hypothesized that additional genetic factors contribute to differences in age at onset (AAO). Here we investigated whether truncating and rare missense variants in genes of different DNA-repair pathways contribute to this phenomenon.    Methods  We used extreme phenotype sampling to recruit 133  BRCA1 -positive patients with either early breast cancer onset, below 35 (early AAO cohort) or cancer-free by age 60 (controls). Next Generation Sequencing (NGS) was used to screen for variants in 311 genes involved in different DNA-repair pathways.    Results  Patients with an early AAO (73 women) had developed breast cancer at a median age of 27 years (interquartile range (IQR); 25.00–27.00 years). A total of 3703 variants were detected in all patients and 43 of those (1.2%) were truncating variants. The truncating variants were found in 26 women of the early AAO group (35.6%; 95%-CI 24.7 - 47.7%) compared to 16 women of controls (26.7%; 95%-CI 16.1 to 39.7%). When adjusted for environmental factors and family history, the odds ratio indicated an increased breast cancer risk for those carrying an additional truncating DNA-repair variant to  BRCA1  mutation (OR: 3.1; 95%-CI 0.92 to 11.5;  p -value = 0.07), although it did not reach the conventionally acceptable significance level of 0.05.    Conclusions  To our knowledge this is the first time that the combined effect of truncating variants in DNA-repair genes on AAO in patients with hereditary breast cancer is investigated. Our results indicate that co-occurring truncating variants might be associated with an earlier onset of breast cancer in  BRCA1 -positive patients. Larger cohorts are needed to confirm these results.    Electronic supplementary material  The online version of this article (10.1186/s12885-019-5946-0) contains supplementary material, which is available to authorized users.

Investigating the effects of additional truncating variants in DNA-repair genes on breast cancer risk in BRCA1-positive women