21P - Second hit landscape in BRCA1/2-associated breast cancer

Date 04 May 2017
Event IMPAKT 2017
Session Welcome reception and Poster Walk
Topics Breast Cancer
Translational Research
Basic Principles in the Management and Treatment (of cancer)
Presenter Mattias Van Heetvelde
Authors M. Van Heetvelde1, M. Van Bockstal2, T. Lepez3, K. Lambein4, D. Deforce3, A. Vral5, B. Poppe1, K. De Leeneer1, J. Van Dorpe2, K. Claes1
  • 1Center For Medical Genetics Ghent, Ghent University Hospital, 9000 - Ghent/BE
  • 2Department Of Pathology, Ghent University Hospital, Ghent/BE
  • 3Laboratory Of Pharmaceutical Biotechnology, Ghent University, Ghent/BE
  • 4Department Of Pathology, AZ St Lucas Hospital, Ghent/BE
  • 5Department Of Basic Medical Sciences, Ghent University, Ghent/BE

Abstract

Body

Objective

Somatic loss of the wild type BRCA1 or 2 allele (LOH) is often reported in BRCA1/2-associated breast tumors, while hypermethylation of the promoter region is a much less frequently observed somatic event. However, little is known about the prevalence of somatic point mutations or the fact if these events are mutually exclusive. Here we explored the landscape of somatic alterations in breast tumors from patients with a germline BRCA1/2 mutation.

Methods

We obtained 84 formalin fixed and paraffin embedded (FFPE) breast tumors from proven germline BRCA1/2 mutation carriers. Using multiplex PCR derived NGS libraries, analysis of the complete coding region of BRCA1/2 was performed for all 84 FFPE breast tumors and matching blood samples to evaluate loss of heterozygosity and the presence of somatic nucleotide variations. In addition exon-spanning deletions/amplification were investigated using MLPA (n=72) and methylation-specific MLPA (n=38) was used to determine the methylation status of both BRCA gene promoters.

Results

Quality of the sequencing results was statistically significantly correlated to FFPE sample age, making LOH interpretation impossible for many of the older samples (n=28). Nonetheless, 33 tumors were suggestive for loss of the wild type allele based on one or multiple markers across the germline mutated gene. In addition, somatic truncating point mutations were observed. None of the samples showed fully methylated promoters. MLPA data suggests partial or complete deletion of BRCA1/2 alleles in several samples. Interestingly, the NGS data of four tumors suggests loss of the mutant allele.

Conclusions

This study shows that determining the BRCA genes’ functionality within breast tumors from BRCA1/2 germline mutation carriers remains challenging. Also, loss of the mutant allele implicates that use of LOH analyses in breast tumors is not informative to determine the impact of variants of unknown clinical significance. The complex combination of somatic events in this cohort could potentially explain the less successful treatment effect of PARP inhibitors in breast cancer. We aim to gain more insight in the mechanisms underlying these results after validation of our findings and correlation of the meta-data to the molecular data.

Clinical trial identification