Abstract B117: Analysis of secondary mutations in BRCA1/2 genes as a mechanism of resistance to PM01183 in BRCA-mutation carriers.
Authors: Cruz Cristina , Vivancos Ana , Caratu Ginevra , Ibrahim Yasir , Serra Violeta , Balmana Judith 
Published: August 2014

Introduction: PM01183 is a modified analogue of trabectedine currently being evaluated in clinical trials, including germline BRCA mutation carriers. PM01183 binds to the DNA minor groove forming PM01183-DNA adducts that provoke DNA double strand (ds) breaks. The homologous recombination (HR) DNA repair pathway is the major mechanism involved in the repair of these DNA ds breaks, and BRCA mutated cells, which are HR deficient, show great sensitivity to these agents. In BRCA carriers, the somatic loss of the wild type copy of BRCA through loss-of-heterozygosity (LOH) is proposed to be an early event in tumorigenesis. Genomic characterization of somatic BRCA status can yield valuable information for therapeutic decisions and the development of new therapeutic strategies in BRCA carriers. Secondary mutations in BRCA1/2 genes restoring the gene and the HR function have been identified in up to 46% of cisplatin-resistant BRCA-associated ovarian tumors. Secondary mutations have also been identified in breast cancers with acquired resistance to olaparib. Based on the sensitivity of BRCA mutated cells to ds DNA-damaging, we hypothesized that BRCA genetic reversion could appear as one of the mechanism of resistance to PM01183.

Methods: Patients with metastatic BRCA-associated breast cancer treated at our institution with PM01183 were enrolled in this study after signing the specifically designed, IRB-approved informed consent form. A blood sample was drawn prior to receiving treatment. Tumor biopsies were performed at baseline and at the time of progression to PM01183 from two patients exhibiting an initial clinical response. Whole exome sequencing (Illumina) was performed on DNA extracted from blood and from fresh-frozen paired tumor samples in order to analyze BRCA1/2 status, BRCA1/2 genetic reversion, and to detect novel genetic events acquired at disease progression.

Findings: Sequencing of the BRCA genes in blood confirmed that one patient carried a germline mutation in BRCA1 (c5123C&gt;A), with concomitant LOH found in both tumor samples. The second patient was confirmed a BRCA2-mutation carrier (9610 C&gt;T), without LOH of BRCA2. Secondary mutations in BRCA1/2 genes were not identified in either tumor after progression to PM01183. Other de novo genetic events upon PM01183 progression are under review.

Conclusions: Secondary functional-reverting mutations in BRCA1/2 genes have not been identified as a mechanism of resistance to PM01183 in any of the two patients analyzed. LOH of BRCA was not found in the metastatic disease of one patient, thus warranting further investigation of the mechanisms of tumorigenesis in BRCA tumors.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B117.

Citation Format: Cristina Cruz, Ana Vivancos, Ginevra Caratu, Yasir Ibrahim, Violeta Serra, Judith Balmana. Analysis of secondary mutations in BRCA1/2 genes as a mechanism of resistance to PM01183 in BRCA-mutation carriers. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B117.