A de Novo BRCA1 Pathogenic Variant in a 29-Year-Old Woman with Triple-Negative Breast Cancer

Abstract

Introduction

Germline pathogenic variants in the BRCA1 and BRCA2 genes lead to a highly increased lifetime risk for breast and ovarian cancer. These variants are usually inherited and reports of de novo occurrences are a very rare phenomenon.

Case Presentation

We report on a breast cancer patient with a de novo BRCA1 variant c.121C>T (p.His41Tyr). The pathogenic variant was detected in leukocyte DNA of a patient with negative family history who had developed early onset, triple-negative breast cancer. The variant was not found in any of the maternal and paternal tissues tested, but it was detected in multiple samples representing all three germ layers of the affected carrier, which renders somatic mosaicism unlikely.

Conclusion

This case highlights the importance of including early onset of disease and triple negativity of the tumor as criteria for genetic testing, even in patients without family history. Considering the availability of effective breast cancer treatments in patients with pathogenic variants in the BRCA genes, this finding underscores the importance of genetic testing in breast cancer patients.

Established Facts

• •Candidates for BRCA testing are most frequently identified based on a strong family history.
• •Next to this case, only 14 other cases of de novo BRCA pathogenic variants have been reported to date.

Novel Insights

• •This is the first documented case of a de novo pathogenic variant in BRCA, where somatic mosaicism was ruled out by genetic testing of tissues derived from all 3 germ layers in the patient and both of the parents.
• •Therapeutic BRCA germline testing identifies patients that have been overlooked due to the absence of family history.

Introduction

It is estimated that approximately 10% of all breast cancers (BC) are hereditary, with germline pathogenic variants (PVs) in the BRCA1 and BRCA2 tumor-suppressor genes being the cause for the vast majority of these cases [1, 2]. After the discovery of the BRCA1 and BRCA2 PVs in 1994 and 1995, genetic testing for these genes was introduced for women at high risk for breast or ovarian cancer (OC) [3]. Initially, BRCA testing was only available to patients with a very early onset of disease or a profound family history of BC and/or OC. The clinical utility of the test results was mainly focused on the indication of strategies for early cancer detection and prevention for the patients and their healthy family members [4, 5]. Eventually, evidence indicated that a significant number of patients with triple-negative breast cancer (TNBC) also harbor BRCA1 PVs [6, 7]. Therefore, TNBC in women younger than 50 was first added as criterion for genetic testing [8, 9].

As drugs targeted at the presence of BRCA PVs became more widely available for BC treatment, the focus of BRCA1 and BRCA2 analysis began to shift from a mainly predictive to a therapeutic approach [10, 11]. The prospect of a therapeutic impact has recently led to the recommendation of therapeutic germline testing in TNBC cases and all patients with metastatic BC, regardless of age and family history [12]. However, it is important to note that regional and organizational guidelines vary for genetic testing as well as BRCA-related cancer screening and treatment. Together with the international disparities in healthcare models and attitudes toward risk-reduction strategies, this has led to significantly different rates of BRCA testing [13, 14].

In the traditional approach to genetic testing, personal as well as familial cancer cases and disease characteristics were factored into risk prediction models, which recommended testing in individuals with a pretest probability of >10% [15, 16]. This strategy inherently overlooked individuals with a lower pretest probability and did not account for family size, nonpaternity, adoption, and the possibility of de novo PVs in BRCA [17]. Although next-generation sequencing has improved speed and efficacy of genetic testing, the chances of finding a BRCA PV in women without a distinctive family history are still low [11]. In fact, the rate of de novo PVs in the BRCA genes has been estimated to be at about 0.3% [18].

Case Report

We report the case of a female patient who developed early onset triple-negative breast cancer at the age of 29. In January of 2013, the patient was diagnosed with a 10 mm large, high-grade invasive ductal carcinoma. Immunohistochemical analysis revealed that the tumor was of the triple-negative subtype with a high Ki-67 (70%). MR imaging of the breast showed no signs of regional lymph node involvement.

The patient was treated with breast conserving surgery and sentinel node biopsy, followed by adjuvant chemotherapy with three cycles of 5-fluorouracil, epirubicin, and cyclophosphamide followed by three cycles of docetaxel and radiation therapy. Histopathological examination of the surgical specimen confirmed the TNBC at stage pT1c, pN0.

The patient´s family history was completely unremarkable with no case of breast or ovarian cancer being reported in either the maternal or paternal lineage (shown in Fig. 1). However, due to the young age at disease onset and the triple-negativity of the tumor, the patient was still genetically tested for PVs in the BRCA genes.

Fig. 1.

Anonymized family pedigree of the patient. Squares = males; circles = females; crossed = deceased. The patient is arrowed. Black shading indicates breast cancer. Blue shading indicates other cancers. Diagnosis of cancer and age at diagnosis are indicated below.

Sanger sequencing of the patient’s leukocyte DNA revealed a BRCA1 missense variant (c.121C>T), leading to change of histidine to tyrosine at codon 41. This BRCA1 variant was categorized as a variant of uncertain significance in 2013. Therefore, no further genetic counseling was conducted on other members of the patient’s immediate family at that time.

However, several years after primary treatment, the detected BRCA1 variant was reclassified as pathogenic [19]. This prompted genetic testing of the patient and her immediate family. After appropriate counseling, both parents underwent genetic testing with Sanger sequencing for the variant in question. Neither of the parents was shown to carry the BRCA1 PV identified in the daughter. The patient’s brother also had predictive testing and does not carry the PV either. To rule out the possibility of somatic mosaicism, samples from all three germ layers were sampled from both the patient, and her parents: ectodermal tissue (skin) was represented by hair follicles; entodermal tissue (urogenital tract) was represented by cytospin from cells released into the urine, and from vaginal swab-derived cells. Mesodermal tissue was represented by white blood cells. In the patient, the presence of the BRCA1 PV was detected in each of the different tissues, whereas in the parents, the variant in question was not found in any of the samples (shown in Table 1).

Table 1.

Results of DNA sequencing

Proband	Sample-material	BRCA1 status
Patient	Blood	Missense Mutation c.121C>T, p.(His41Tyr)
	Hair	Missense Mutation c.121C>T, p.(His41Tyr)
	Urine	Missense Mutation c.121C>T, p.(His41Tyr)
	Vaginal swab	Missense Mutation c.121C>T, p.(His41Tyr)
Mother	Blood	Wild-type
	Hair	Wild-type
	Urine	Wild-type
	Vaginal swab	Wild-type
Father	Blood	Wild-type
	Hair	Wild-type
	Urine	Wild-type
	Sperm	Wild-type
Brother	Blood	Wild-type

The absence of the germline c.121C>T BRCA1 variant in both parents indicated either a de novo origin of the alteration or non-paternity. The latter event was ruled out by paternity testing. The results excluded the possibility of non-paternity, confirming that the genetic alteration identified in the patient indeed represents a de novo germline PV in BRCA1.

After follow-up care for primary breast cancer treatment was completed, the patient was included in the screening program for high-risk patients at our department. Moreover, the patient elected to undergo prophylactic surgery (risk-reducing mastectomy and bilateral salpingo-oophorectomy) in the near future.

Discussion

BRCA variants are inherited in an autosomal dominant manner. It has been shown that the haplotypes of frequently occurring BRCA PVs share a common ancestral origin. Some of these PVs are estimated to have emerged several hundred to thousand years ago [20, 21].

Since the BRCA genes were discovered, many distinct PVs and a large number of genetic variants of uncertain significance have been described [22]. Nevertheless, only very few cases of de novo PVs have been identified. To our knowledge, only 14 other cases of de novo BRCA PVs have been reported to date, although, in these cases, no comprehensive evaluation of potential somatic mosaicism in all three germ layers has been conducted [18, 23]. In addition, seven cases of constitutional mosaicism have been described [24].

The rate of de novo PVs in the BRCA genes has been estimated to be at 0.3% [18]. For some of the other high-risk genes for BC, markedly higher rates for de novo PVs have been reported.

The frequency of de novo PVs has been estimated to be about 10–40% for the PTEN gene [25] and about 14% for the TP53 gene, with approximately one-fifth of the TP53 de novo cases being mosaic [26]. As for CDH1, PALB2, and STK11, only a few occurrences of de novo PVs have been reported. The exact proportion of de novo PVs and mosaic cases for these genes is currently unknown [27–29].

Since candidates for BRCA testing are still most frequently identified based on a strong family history, it is well possible that prevalence of BRCA de novo PVs is profoundly underestimated. Notably, de novo PVs in BRCA reported in the literature have typically been described in patients with early onset cancer [23, 30, 31].

The identification of BRCA PV carriers enables adequate counseling of patients as well as their family members in regard to their high lifetime risk of BC and OC. Potential consequences include intensified screening for these cancers as well as prophylactic mastectomy and salpingo-oophorectomy [12]. Additionally, cancer patients with BRCA PVs are eligible for treatments that target cancer cells with homologous recombination deficiency, like platinum-based chemotherapy or poly (ADP-ribose) polymerase inhibitors (PARPi) [32, 33]. A phase III trial has already demonstrated that the addition of PARPi to standard treatment for early BC patients with BRCA PVs significantly improves survival for this subgroup [34, 35].

Along with our report, these cases highlight the importance of genetic testing criteria applying to the individual diagnostic criteria of the BC patient, regardless of family history. These criteria include early onset of disease and the TNBC phenotype [11]. The identification of a BRCA PV in a breast cancer patient not only has an immediate impact on the therapeutic strategy but also leads to critical consequences for long-term clinical care [12]. Additionally, first- and second-degree relatives of BRCA PV carriers may become eligible for genetic testing. The early detection of these PVs facilitates appropriate counseling and prophylactic treatment of affected female and male patients, leading to a significant reduction in their lifetime risk of cancer [1].

In summary, we herewith present the first description of a de novo PV in the BRCA1 gene in a woman with early onset breast cancer, in which somatic mosaicism was systematically ruled out. Neither of the parents was shown to carry the variant in any of the tissues tested, and paternity testing confirmed true paternity. The age of onset and the TNBC phenotype were highly indicative for the genetic predisposition.

Conclusion

Therapeutic BRCA germline testing significantly contributes to the identification of patients that have been overlooked by predictive testing guidelines due to the absence of family history.

Acknowledgments

We thank the patient and her family for their participation in this case study.

Statement of Ethics

The study was conducted in accordance with the World Medical Association Declaration of Helsinki and the Guidelines of Good Clinical Practice, consistent with the STROBE Guidelines. Written informed consent was obtained from all study participants. The study was reviewed and approved by the Ethics Committee of the Medical University of Vienna, approval number 2190/2019. Written informed consent was obtained from the patient and the tested family members for publication of this case report and any accompanying images.

Conflict of Interest Statement

The authors declare that they do not have any conflicts of interest in direct or indirect relationship with this research.

Funding Sources

This work was funded by the Medical University of Vienna.

Author Contributions

P.G.: data acquisition and draft preparation; Y.T.: writing – review and editing; D.M. and C.St.: genetic analysis (BRCA analysis); C.S.: genetic counseling, clinical management, and writing – review and editing. All authors gave final approval for the manuscript to be published.

Data Availability Statement

The data that support the findings of this study are not publicly available due to the containing information that could compromise the privacy of research participants but are available from the corresponding author P.G. upon reasonable request and with permission of the Medical University of Vienna.

Funding Statement

This work was funded by the Medical University of Vienna.