Abstract 1359: Somatic involution of pathogenic BRCA1 germline mutations
Authors: Kotoula Vassiliki , Demiri Efterpi , Fostira Florentia , Vrettou Eleni , Papadopoulou Kyriaki , Tikas Ioannis , Papazisis Konstantinos , Zaramboukas Thomas , Asimaki-Vlachopoulou Asimina , Miliaras Spyridon , Fountzilas Elena , Ananiadis Ananias , Chrisafi Sofia , Poulios Christos , Natsiopoulos Ioannis , Tsiftsoglou Aris , Fountzilas George 
Published: August 2018

Abstract
There is currently a wealth of data on the tumor genomic contexture from BRCA1/2 carriers, particularly breast and ovarian carcinomas. By contrast, little attention has as yet been paid to the genomic status of cancer-related normal tissues from these individuals. Here, we investigated the status of pathogenic BRCA1 germline mutations (GM) in breast (B) and gynecologic (GYN) tissues.
Methods: We examined 121 DNA samples (48 B; 36 GYN; 37 tumors) from an equal number of paraffin blocks obtained upon prophylactic or debulking surgery from 44 BRCA1/2 carriers (mean age 38 yrs, range 24-62; 43 BRCA1 carriers). Six women had never had cancer manifestation (CM). At the time of surgery, 32 were cancer-free but had received neo- or adjuvant chemotherapy, and 6 had concurrent cancer without prior treatment. Following multimethod DNA quality control, mutation validation and sample identity match to exclude false negatives, we interrogated GM presence in tissues in comparison to clinicopathologic data and tumor genotypes (60-gene panel; mean read depth over 800).
Results: In 19 samples from 13 BRCA1 carriers, including 13 normal B/GYN and 6 tumors, the germline mutation was present at frequencies lower than 5% (observed with the integrated genome viewer) up to 12%, or it was undetectable with Sanger sequencing and multiplex PCR. This condition, termed GM-loss, was present in 13 GYN, 12 of which in the histologically normal tube, and in only 3 B with fibrocystic disease (p=0.0210). It was also present in one ovarian thecoma but it was absent in usual or atypical hyperplasia in B. GM-loss was observed in 9/55 normal tissues from breast cancer and in 4/6 normal tissues from ovarian cancer patients, while it was absent in the normal tissues from women without CM (p=0.0002). GM-loss mostly affected the BRCA1 BRCT functional domain (p&lt;0.0001) and concerned large deletions (10/17 samples), small indels (5/32; 15.6%) and less so single-nucleotide substitutions (4/66; 6.0%; p&lt;0.0001). Normal tissue GM-loss was rather localized, e.g., in 10 patients with multiple samples it was present in only one B or fallopian tube. It was, however, related to the status of concurrent ovarian tumors in 3/6 cases. In the normal tube of these patients, next to the low-frequency GM a somatic pathogenic BRCA1 mutation was present at 25-41% allelic frequency; the same was observed in one breast tumor after neoadjuvant chemotherapy. Three of the replaced GMs were indels, two corresponded to the well-known BRCA1 p.Q1777fs, and all novel mutations were missense.
Conclusions: We observed BRCA1 GM-loss in normal tissues, breast and mostly fallopian tube, in about 30% of the carriers in the present cohort, all with previous or concurrent cancer. The phenomenon seems analogous to the GM reversion described in tumors but, as shown, it may also occur in the absence of prior treatment. Its origin and impact on cancer dynamics and therapeutic approaches seem worth further pursuing with functional studies.
Citation Format: Vassiliki Kotoula, Efterpi Demiri, Florentia Fostira, Eleni Vrettou, Kyriaki Papadopoulou, Ioannis Tikas, Konstantinos Papazisis, Thomas Zaramboukas, Asimina Asimaki-Vlachopoulou, Spyridon Miliaras, Elena Fountzilas, Ananias Ananiadis, Sofia Chrisafi, Christos Poulios, Ioannis Natsiopoulos, Aris Tsiftsoglou, George Fountzilas. Somatic involution of pathogenic BRCA1 germline mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1359.