Abstract 1371: Assessing the utility of cell-free DNA in identifying prostate cancer and characterizing tumor heterogeneity via whole exome and whole genome, multi-region sequencing
Authors: Chen Emmalyn , Cario Clinton , Leong Lancelote , Lopez Karen , Li Patricia , Oropeza Erica , Tenggara Imelda , Cowan Janet , Simko Jeffry , Wells Daniel , Kageyama Robin , Chan June , Friedlander Terence , Paris Pamela , Carroll Peter , Witte John 
Published: August 2019

Early cancer diagnosis, especially while the disease is still localized and before symptoms appear, results in significantly higher survival rates compared to late-stage diagnosis. At the time of diagnosis, it is also common to find multiple foci within a single prostate gland in men with localized disease. Cell-free DNA (cfDNA) may not only reflect underlying disease biology, but also simultaneously allow for the identification of genetically distinct tumor subclones. The objectives of this study are to determine if 1) cfDNA levels are able to distinguish between healthy individuals from patients with localized or metastatic castration-resistant prostate cancer (mCRPC), and 2) if somatic mutations identified in tumor tissue are detectable in cfDNA and representative of the distribution observed in tumor tissue.

This study includes samples from 130 individuals at UCSF: 21 healthy donors, 100 patients with localized prostate cancer who underwent radical prostatectomy (RP), and 9 mCRPC patients. Blood samples and matched tissue from adjacent normal seminal vesicles and multiple tumor regions (1-9 samples per patient) were collected from patients undergoing radical prostatectomy. CfDNA was extracted from plasma, and the concentration and fragment length distribution were measured with a Bioanalyzer 2100. Comparisons of cfDNA levels between groups were assessed with a Welch’s t-test due to the potential for unequal variances. Fifty-seven samples from nine patients have been subjected to whole exome sequencing, and 22 samples from five patients have been subjected to whole genome sequencing at ~40x coverage. Somatic variant calling was performed with Broad Institute’s Firecloud platform (GATK4/MuTect2) for tumor tissue and with the Curio platform for cfDNA to build consensus sequences leveraging unique molecular tags.

CfDNA levels are able to distinguish between healthy and localized (p = 0.005), as well as healthy and metastatic groups (p = 0.043). Tumor foci within a patient’s prostate gland are genetically heterogeneous, with the majority of somatic mutations private to tumor regions and a subset of mutations at the intersection of these regions. Preliminary analyses result in an average of 72 somatic SNVs and indels per tumor tissue region, with ~10% overlap between regions in the same patient. Additional mCRPC and follow-up blood samples are being collected from all patients. The association between cfDNA levels prior to RP surgery and biochemical recurrence will be investigated when follow-up collection concludes. Further analysis of mutational concordance, clonality, and copy number variation between tumor tissue DNA and cfDNA, along with clinical data, will be performed.

Citation Format: Emmalyn Chen, Clinton Cario, Lancelote Leong, Karen Lopez, Patricia Li, Erica Oropeza, Imelda Tenggara, Janet Cowan, Jeffry Simko, Daniel Wells, Robin Kageyama, June Chan, Terence Friedlander, Pamela Paris, Peter Carroll, John Witte. Assessing the utility of cell-free DNA in identifying prostate cancer and characterizing tumor heterogeneity via whole exome and whole genome, multi-region sequencing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1371.