Abstract P4-06-15: Targeted sequencing of BRCA1/2 across a large unselected breast cancer cohort in Sweden
Authors: Li J , Wen SWX , Eriksson M , Kvist A , Christensen HN , Torstensson A , Easton DF , Teo S-H , Borg Å , Grönberg H , Czene K 
Published: February 2018

Abstract
Background
Observed BRCA1/2 mutation frequencies can vary depending on the population screened, screening criteria, founder effects, and methods used for testing. We aimed at examining BRCA1/2 germline mutations, identified through targeted sequencing, in a large unselected breast cancer population in Sweden.
Methods
Sequencing libraries of germline DNA from 5122 breast cancer patients diagnosed between 2001-2008 (LIBRO1) were prepared (48.48 Fluidigm Access Array system, Fluidigm Corp, USA) and sequenced (Next-generation sequencing [NGS], Illumina HiSeq 2500, v2 chemistry, University of Cambridge, UK). A total of 5099 samples (97.8%) passed quality control and were analysed.
BRCA1/2 carriers identified by NGS were compared with those who were already identified by clinical BRCA screening (n=418) performed by the BRCA Lab (Lund University). This unit carries out mutation screening for all oncogenetic clinics in Sweden using denaturing high performance liquid chromatography (DHPLC) and multiple ligation-dependent probe amplification (MLPA).
Results
A total of 50 BRCA1 mutation carriers were identified, of which 28 were unique pathogenic germline mutations. Frameshift insertions and deletions made up 34/50 (68%) of the BRCA1 mutations. Exon 11 harbored 33/50 (66%) of the BRCA1 mutations. The most common mutation was c.3048_3052dupTGAGA (n=8), which is a founder mutation originating from the West coast of Sweden. Three other Swedish founder mutations were also identified (c.1082_1092del [n=5], c.3626delT [n=3] and c.2475delC [n=2]). For BRCA2, 42 mutation carriers were identified; 33 unique deleterious BRCA2 mutations (27 frameshift deletions, 3 frameshift insertions, 9 truncating and 3 splice sites). More than half of the mutations (24/42, 57%) were found on exon 11. Of the 418 women who had attended clinical BRCA testing, 38 deleterious mutations were found. Our screening method confirmed 34 of these mutations, as two each of BRCA1 and BRCA2 mutations were missed, since NGS was proven unsuitable for the detection of large exon duplications. NGS did, however, identify three more carriers not previously identified by clinical testing (c.3048_3052dup, c.2577delA and c.7442delT). Overall, 55/92 BRCA1/2 mutation carriers (59.8%) identified in the present study by NGS were not clinically tested.
Conclusion
NGS is comparable with current BRCA testing tools for the identification of BRCA1/2 germline mutations, suggesting that the technology has the potential to be used in BRCA1/2 clinical testing in unselected breast cancer patients.
Citation Format: Li J, Wen SWX, Eriksson M, Kvist A, Christensen HN, Torstensson A, Easton DF, Teo S-H, Borg Å, Grönberg H, Czene K. Targeted sequencing of BRCA1/2 across a large unselected breast cancer cohort in Sweden [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-06-15.