The contribution of BRCA1 and BRCA2 mutations that predispose to triple-negative breast cancer development in Indian women is largely unexplored. Thus, there is an urgent necessity for population-specific studies to understand the underlying pathogenic variants that contribute to TNBC development in South Indian women. Hence, the present study screened the entire BRCA1 and BRCA2 genes for pathogenic variants in 59 TNBC patients using NGS. In the current study, 6 BRCA1 mutations were identified in TNBC patients. The most common BRCA1 pathogenic variant was c.68_69delAG (p.Glu23ValfsTer17) mutation, which was observed in three out of six BRCA1Mut positive TNBC patients. Among the three c.68_69delAG (also known as 185delAG) mutation-positive TNBC cases, two patients were observed to have a positive family history of cancer. Reports globally suggest that the frequency of BRCA1 and BRCA2 mutations vary widely across different ethnicities and geographic regions. The presence of this deleterious variant in the Indian population was first identified in a South Indian family from Trivandrum [27]. Later, two South Indian families from Kerala [23] and two sisters from Goa [28] were reported to have 185delAG mutation. A previous South Indian study by Vaidyanathan et al. [25] explored the role of BRCA1/2 mutations in hereditary breast and ovarian (HBOC) cases in South India. They observed 28% BRCA mutations in HBOC patients (24.6% BRCA1Mut and 3.3% BRCA2Mut). In line with our reports, they observed the presence of BRCA1Mut in all age groups; however, BRCA2Mut was observed only in women with ≤ 40 years of age. Similarly, they also observed a higher occurrence of 185delAG BRCA1 (frameshift deletion) mutation in South Indian women. Remarkably, the 185delAG mutation is the founder mutation of the Ashkenazi Jewish (AJ) population and is responsible for the development of HBOC in AJ women. Besides, North Indian [24] and North-East Indian [29] studies also reported the presence of 185delAG mutation in breast cancer patients (non-Jewish ancestry) with no family history of cancer. Similarly, this mutation was also observed in the non-Jewish American population [30]. Interestingly, the 185delAG mutation was reported to have developed around 61 generations ago and arose independently not less than twice in the non-Jewish population [31]. Notably, Drost et al. [32] reported that tumors of both murine and breast cancer cell line with BRCA1185stop and BRCA1185delAG mutations, respectively, expressed a new domain-less BRCA1 (RING-less) protein that mediates resistance to homologous recombination deficiency (HRD) therapies. Hence, investigation of 185delAG mutation in South Indian women might have better clinical and public health connotations. Moreover, the Indian population is not a completely uni-ethnic population, and it has a high prevalence of conservative mating habits, which prevailed for numerous generations. Thus, the likelihood of founder mutations cannot be entirely ruled out in such a population [33].
The other BRCA1Mut observed in a TNBC patient of the present study is the c.3607C > T (p.Arg1203Ter) mutation, which results in BRCA1 protein truncation. This mutation was reported in a North Indian study by Mehta et al. [34], and was observed in two patients with ovarian cancer. Similarly, another Indian study observed the prevalence of the Arg1203Ter mutation in both breast and ovarian cancer patients, irrespective of family history of cancer [35]. Remarkably, in line with our observation, this variation was also reported in Turkish TNBC patients [36]. Besides, this mutation was previously reported in Italian women with breast and/or ovarian cancer [37]. We also observed a splicing variant (c.4357 + 1G > T; IVS13 + 1G > T) in one of the TNBC patients. We did not find any Indian study reporting this variant; however, Ahmad et al. [35] reported a nearby intronic variant (c.4357 + 2T > G) in a North Indian breast cancer patient. Nevertheless, c.4357 + 1G > T intronic variant was reported in breast cancer patients from Latvia [38], China [39], mixed ethnicity [40], Brazil [41], and America [42]. Moreover, Lindor et al. [43] had classified this variant under the class 5 variant. Individuals carrying class 5 BRCA1 variant should be considered as a carrier of completely pathogenic mutation and genetic testing should be recommended for all the at-risk relatives. Similarly, we observed the c.220C > T (p.Gln74Ter) BRCA1Mut in a TNBC patient. Interestingly, this variant was recently reported in a TNBC patient from North India [44]. This variant was also reported in Chinese breast cancer patients [39].
Regarding the BRCA2 variants observed in the present study, one patient was observed to carry the BRCA2 c.5718_5719CT (p.Leu1908fs) pathogenic variant. This variant was reported in a male breast cancer patient recruited in the UNCseq™ study. The patient had a family history of cancer, where his sister and paternal aunt were affected with breast cancer [45]. This is a class 4 variant (likely-pathogenic) and genetic testing is recommended for at-high risk relatives. Moreover, this BRCA2 variant was also observed in an Italian ovarian cancer patient resistant to platinum therapy [46]. This variant was also reported in two Chinese breast cancer patients [39, 47], Algerian HBOC patients [48], Macedonian [49], and German [50] breast cancer patients. The other BRCA2 variant observed in the present study is the c.5635G > A (p.Glu1879Lys) missense variant. Even though this variant has been categorized under class 2 (likely not pathogenic) mutation, Zuntini et al. [51] observed several families with BRCA2 Glu1879Lys variant. Moreover, a report by Thirthagiri et al. [52] observed the presence of Glu1879Lys missense variant in an Indian breast cancer patient. Interestingly, the WECARE study by Borg et al. [53] reported the presence of BRCA2 Glu1879Lys variant in contralateral breast cancer patients. Pertaining to the clinical characteristics of TNBC patients with BRCA mutation, similar to our observation Thirthagiri et al. [52] observed invasive ductal carcinoma as the prevalent histological breast cancer subtype in both BRCA1 and BRCA2 mutation carriers. They also reported that all of the BRCA mutant tumors were mostly grade II or III, and none of them were grade I tumors. Furthermore, in agreement with our report, several studies observed the higher prevalence of BRCA1 mutant tumors in TNBC patients [52, 54]. Strikingly, Lang et al. [39] reported that Chinese women with BRCA mutations are at reduced risk of developing breast cancer, suggesting the effect of specific BRCA mutations and ethnic background in modulating breast cancer risk. Noteworthily, for BRCA1/2 mutation detection, NGS confers a comparably accurate and rapid yield compared to Sanger sequencing [55]. This might help the clinicians to make much informed and timely treatment decisions, such as usage of PARP inhibitors; thereby, increasing the survival of TNBC patients. Up to our knowledge, this is the first study to perform BRCA mutation screening in TNBC patients, without hereditary breast and ovarian cancer in South India. Hence, our study suggests that BRCA1 mutation testing could be incorporated for South Indian women diagnosed with TNBC, particularly for patients with a family history of cancer