Abstract A27: The genomic profile of BRCA1-associated estrogen-receptor positive breast cancer does not resemble BRCA1-associated triple negative cancers, but is more similar to BRCA2-associated breast cancer

Background: Most breast cancers arising in BRCA1-mutation carriers are triple negative (TN). Therefore, it has been suggested that TN status is intrinsic to BRCA1-mutated tumors. However, 10 to 20% of BRCA1-associated breast cancers are estrogen receptor positive (ER+). As these tumors arise more often in older patients, and have less aggressive tumor characteristics, it has been suggested that these tumors are ‘sporadic’ and not related to BRCA1 deficiency. With the introduction of targeted treatments specific for tumors with a non-functioning BRCA1 or BRCA2 gene (i.e. PARP-inhibitors), the question whether the BRCA genes are impaired in the tumor, is highly relevant. Therefore, we performed a genomic analysis of a series of BRCA1-associated ER+ tumors, and compared them with BRCA1-associated TN tumors, as well as BRCA2-associated and sporadic breast cancers. Material and Methods: Genomic profiling of 18 BRCA1-associated ER+ tumors was performed using Nimblegen 135K arrays. We previously developed a BRCA1-like and BRCA2-like genomic profile, containing specific copy number alterations for respectively BRCA1-associated and BRCA2-associated breast cancer. These genomic profiles were applied on the current series. In addition, the BRCA1 promoter methylation status was determined, and LOH analysis was performed to assess if the wildtype or mutant allele was lost in the tumor. Results were compared with BRCA1 associated ER- tumors, BRCA2 associated tumors, and sporadic ER+ and TN tumors. Results: Only 2 out of 18 ER+ BRCA1 associated tumors showed a BRCA1-like genomic profile, while 90% of all ER- BRCA1-associated tumors show this genomic profile. Interestingly, 11 out of 18 ER+ BRCA1-associated tumors showed a BRCA2-like genomic profile. BRCA1 promoter methylation was absent in ER+ BRCA1-associated tumors, similar to TN BRCA1-associated tumors. LOH analysis was possible in 12 tumors, loss of the wildtype BRCA1 allele was shown in 10 out of 12 ER+ BRCA1-mutated tumors. One sample showed loss of the mutant allele, while another sample did not show loss at the BRCA1 locus. The genomic profiles of ER+ BRCA1-associated tumors were more similar to BRCA2-associated breast cancers and sporadic ER+ breast tumors than to BRCA1-associated TN tumors. Conclusion: The majority of BRCA1-associated ER+ tumors did not show a BRCA1-like genomic profile, even though LOH analysis indicated that loss of the wildtype BRCA1 allele was a frequent event. Therefore, it is likely that the complete loss of the BRCA1 gene plays a crucial role in tumorgenesis in this group of breast tumors. Remarkably, a BRCA2-like genomic profile was observed in the majority of ER+ BRCA1 associated tumors in this series. A clinical consequence of our findings is that ER+ BRCA1-associated breast tumors are probably highly sensitive to PARP inhibitors, similar to TN BRCA1-associated breast cancers. However, as ER+ BRCA1-mutated tumors clearly have different tumor characteristics compared to TN BRCA1-mutated tumors, they should be considered as a special group, and response to therapies exploiting the BRCA1 gene defect should be specifically monitored in this subgroup. Citation Format: Esther H. Lips, Rashmie Debipersad, Esther Scheerman, Lennart Mulder, Lizet E. van der Kolk, Jelle Wesseling, Frans B.L. Hogervorst, Petra M. Nederlof. The genomic profile of BRCA1-associated estrogen-receptor positive breast cancer does not resemble BRCA1-associated triple negative cancers, but is more similar to BRCA2-associated breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A27.