Most cases of breast cancer are not hereditary, but breast cancer without an inherited component appears both sporadically and in multiply affected families [1
]. Both breast cancer susceptibility genes (BRCA)1 and BRCA2 proteins are critical to the repair of double-strand DNA breaks due to their function in homologous recombination repair (HRR), a form of DNA repair that uses a homologous DNA sequence to guide repair at double-stranded DNA breaks [2
]. Mutations in these genes are considered to be responsible for approximately 40% of familial breast cancers and for the majority of familial ovarian cancers, and account for 5% to 20% of all breast and ovarian cancers [3
]. Germline mutations in BRCA1 confer a cumulative breast and ovarian cancer risk by age 80 of 72% and 44%, respectively, while BRCA2 mutations confer a 69% and 17% increased risk of breast and ovarian cancer by this age, respectively [4
]. BRCA1 is ubiquitously expressed, and it remains a mystery why BRCA1 mutation leads specifically to breast and ovarian cancer. BRCA2 mutation is associated with increased risks of pancreatic cancer and high-grade prostate cancer [1
Mutations in BRCA1 and BRCA2 are found in approximately 1 in 300 individuals in the general population and 1 in 40 individuals of Ashkenazi Jewish descent [5
]. In the Korean hereditary breast cancer (KOHBRA) study, the incidence of BRCA mutation was found to be 24.8% (106/428) in breast cancer patients with a family history of breast/ovarian cancers [6
]; However, few studies have investigated breast cancer patients who carry germline mutations in both BRCA1 and BRCA2. In this study, we describe the clinical, pathological, and genomic characteristics of Korean breast cancer patients with germline mutations of both BRCA1 and 2 based on a single-center experience.
All four patients with both gBRCA1 and gBRCA2 mutations had mutations that have been described previously; However, double BRCA1 and BRCA2 mutations are very rare [7
]. We found that patients who carry gBRCA1 and gBRCA2 mutations are very rare and are diagnosed with breast cancer at a young age.
In both gBRCA1 and gBRCA2, mutations in coding areas were distributed evenly along the length of each gene in approximate proportion to the relative size of the exons [9
]. TNBCs are enriched for HRR gene defects, and there is evidence that somatic alterations in TNBCs share molecular features of ovarian cancer, including an elevated level of genomic instability and specific chromosomal gains on 1q, 3q, 8q, and 12p and losses on 4q, 5q, and 8p [10
]. BRCA1 and BRCA2 mutated breast cancers have been reported to have different molecular characteristics from each other, for example, a correlation between TNBC and BRCA1 but not BRCA2 [11
]. Breast cancer cells with deleterious mutations in BRCA1/2 are deficient in the repair mechanism for DNA double-strand breaks, leaving these tumors highly dependent on the repair pathway for single-strand breaks [12
]. In Asian countries, including Korea, the most common BRCA1 mutation is 185delAG (c.68_69delAG), and the most common BRCA2 mutation is BRCA2 c.7480C > T [13
BRCA1 plays a major role in DNA repair through homologous recombination (HR). While BRCA2 is directly involved in RAD51 protein-mediated repair, BRCA1 appears to act via a more complex mechanism through interaction with other proteins [14
]. The question then becomes how BRCA1 and BRCA2 mutations lead to genetic instability? BRCA1 mutation carriers develop breast and ovarian cancer at a younger age than BRCA2 mutation carriers, and BRCA2 is associated with fewer cases of breast cancer than BRCA1 [15
]. Three of the four patients who had germline mutations in both BRCA1 and BRCA2 were TNBC patients. Most patients with pathogenic gBRCA1 mutation and gBRCA2 VUS also had TNBC. Given that a large proportion of BRCA1-mutated tumors are TNBC, we hypothesize that BRCA1 plays a leading role in the clinical presentation of patients who carry germline mutations in both BRCA1 and BRCA2 [16
]. In the previous study, more TNBC cases were found to be associated with BRCA1 mutation than BRCA2 mutation [13
]. It is likely that mutations in both gBRCA1 and gBRCA2 genes increase the risk for cancer development, possibly at a younger age [17
There were no patients with gBRCA2 mutation and gBRCA1 VUS. Given this, we hypothesize that gBRCA2 mutations have different functional consequences than gBRCA1 mutations. gBRCA2 mutations increase the lifetime risk of breast cancer development in women by 50–60%, whereas the lifetime risk of breast cancer in women with gBRCA1 mutations is 70–80% and the lifetime risk of ovarian cancer is 50% [18
]. Further research is needed to determine the roles of gBRCA1 and gBRCA1 and their interaction.
Hereditary Breast and Ovarian Cancer Syndrome (HBOC) is considered when there are multiple cases of breast cancer and/or ovarian cancer on the same side of the family. Patient No. 6 had breast cancer with gBRCA1 mutation (c.3627dupA), while her sister had breast cancer with the same gBRCA1 mutation (c.3627dupA). Both sisters had breast cancer on the left side, but one cancer was TNBC while the other was hormone receptor-positive breast cancer. This indicates that not only genetic factors but also other factors determine breast cancer phenotype.
This study had several limitations. First, BRCA mutation testing was performed by direct sequencing of all exons and flanking intronic sequences. The sensitivity of this test is known to be between 60% and 70% and cannot identify large genomic rearrangements. Additional tests to verify the deletion/duplication of BRCA genes require multiplex ligation-dependent probe amplification (MLPA). MLPA test for BRCA mutation is recommended for breast cancer patients with a familial history of breast and/or ovarian cancer, young breast cancer patients, and patients who test negative for BRCA1/2 small mutations in initial testing [19
]. There were no patients who had MLPA testing performed in this study, and thus we may have missed some patients with BRCA mutations. This could have resulted in the underestimation of the prevalence of BRCA mutation. Additionally, no patients were treated with poly-adenosine diphosphate-ribose polymerase (PARP) inhibitors. In a previous study, BRCA carriers who had ovarian, breast, or prostate cancers benefitted clinically from treatment with Olaparib [20
]. Lastly, the mechanism linking double gBRCA1/2 mutation with the phenotype of TNBC is unclear.
4. Materials and Methods
A retrospective review of medical records was done to identify breast cancer patients with gBRCA mutation who underwent surgery at Samsung Medical Center (SMC) between January 2007 to October 2018. This research was approved by the institutional review board (IRB) of SMC (SMC 2019-03-067). Informed consent for genomic analysis was obtained from all patients. All samples were obtained with the approval of the IRB (SMC 2019-03-067). For gBRCA testing, genomic DNA was extracted from peripheral blood leukocytes and sequenced by direct Sanger sequencing (total of 22 exons). Direct Sanger sequencing annotation was performed based on a pre-defined internal calling algorithm. Pathogenic mutations in gBRCA1 and gBRCA2 genes were investigated in this study in addition to other genetic VUS.
BRCA1 and BRCA2 are on chromosomes 17q21.31 and 13q12.3, respectively. The results of BRCA testing were reported accordingly to the American College of Medical Genetics (ACMG) standards and guidelines [21
], Korea ONCOgene Research and Diagnosis (KONCORD) [22
], and multifactorial probability-based model of IARC (International Agency for Research on Cancer) [23
]. Mutations were classified based on the ClinVar database maintained by the National Center for Biotechnology Information (NCBI) [24
]. The mutations or VUS are reported according to the HGVS (Human Genome Variation Society) [25
The following variables were extracted from clinical records: age at diagnosis, sex, reason for BRCA testing, marriage, parous status, family history of breast cancer or ovarian cancer, menopause status, surgical record, histopathology, and pathologic stage. Date of last follow-up or death, survival status, cause of death (if applicable), and relapse status were collected. Progression-free survival (PFS) was defined as the time from the date of first-line palliative chemotherapy to the progression of cancer or death from any cause. Overall survival (OS) was defined as the time from the date of surgery to the date of the last follow-up or of death from any cause. ER and PR expression were measured by IHC. TNBC was defined as both ER and PR negative and lacking overexpression of HER2.