148 Background: Molecular characterization of metastatic castration resistant prostate cancer (mCRPC) is limited by tumor tissue availability. The analysis of circulating tumor cells (CTC) offers an attractive noninvasive surrogate option to analyze molecular alterations. We report whole exome sequencing (WES) of CTCs at the single cell level in mCRPC patients. Methods: Blood samples were drawn from 11 enzalutamide or abiraterone pre-treated mCRPC patients enrolled in the clinical program MOSCATO (NCT02613962). CTC enrichment, immunofluorescent detection and single cell isolation were performed using three methods (ISET filtration, CellSearch and the VyCap puncher system and RosetteSep enrichment) to obtain pools of 1-10 CTCs with distinct epithelial or mesenchymal phenotypes. After Whole Genome Amplification (WGA), WES was performed on the Illumina HiSeq 2000 platform. GATK Haplotype Caller enabled identification of germline polymorphisms from each patient in normal DNA, metastatic sample and CTCs in order to consider WGA induced bias. The detection of sSNV in tumor biopsies and CTCs was assessed with Mutect and IndelGenotyper respectively. Results: 189 WGA of CTC pools were performed. 34 pools of phenotypically different CTCs from 7 patients were selected and sequenced. Mean coverage of 51% was obtained at a sequencing depth of 10X. Allelic drop out was lower for CTC pools containing 5-10 cells. 17/34 (50%) CTC samples had shared sSNV with the paired tumor sample (range 0.35%-68%) Epithelial CTCs had more shared sSNV with metastatic biopsies than CTCs of other phenotypes but shared sSNV were also detected in large non epithelial CTC pointing out a high level of genetic heterogeneity between CTC. Overall, 89 deleterious protein-coding mutations were found only in pools of CTC, including mutations affecting oncogenic drivers such as MAPK1, HSP90AB1 or KDM5B. Conclusions: We present single cell WES of CTCs harboring distinct phenotypes. The detection of shared sSNV between CTC pools and corresponding biopsy could validate the use of CTCs as a liquid biopsy. The finding of sSNV specific to CTCs could offer additional data on tumor heterogeneity.
Reliability of Whole-Exome Sequencing for Assessing Intratumor Genetic Heterogeneity
