Abstract LB-136: High concordance of whole-exome sequencing of cell-free DNA and matched biopsies enables genomic discovery in metastatic cancer

Background: Circulating cell-free DNA (cfDNA) has largely been used to monitor blood for specific tumor mutations, but genome-wide discovery from cfDNA has not been well established. Here, we establish a scalable approach for whole-exome sequencing (WES) of cfDNA, making it possible to perform comprehensive genomic characterization of metastatic cancer in a routine and minimally-invasive manner. Comprehensive genomic characterization of metastatic cancer stands to uncover novel alterations of clinical significance. A major challenge is that metastatic tumors are infrequently biopsied. Cell-free DNA is shed abundantly into the bloodstream from metastatic tumors, presenting an opportunity for genomic discovery in advanced cancers that are rarely biopsied in routine clinical care. We report an efficient process to qualify and sequence whole-exomes from cfDNA at scale and systematically compare the somatic mutations, indels, and copy number alterations detected in WES of cfDNA to WES of matched tumor biopsies. Methods: We consented 86 patients with metastatic breast or prostate cancers for blood collection. We isolated cfDNA and germline DNA from blood and performed low coverage sequencing to estimate tumor content based on genome-wide copy number. We screened patient blood samples and prioritized those with higher tumor fractions for WES. In parallel, we analyzed cfDNA and germline DNA from healthy donors to calibrate our methods and assess false positive rate for genomic alterations. Results: We found the vast majority of patients with metastatic prostate or breast cancer to have detectable tumor-derived cfDNA. WES of cfDNA from healthy donors revealed very low false positive rates for somatic mutations, indels and copy number alterations (SCNAs). By analyzing WES of cfDNA and tumor biopsies from dozens of patients with metastatic breast or prostate cancers, we established guidelines for the coverage and tumor fraction required for mutation discovery in WES of cfDNA. We found WES of cfDNA to uncover 91% of the clonal mutations, 59% of the subclonal mutations, and 75% of the SCNAs detected in WES of matched tumor biopsies. In several cases, we observed mutations exclusive to cfDNA that were confirmed in later blood draws, suggesting that cfDNA-exclusive mutations may be derived from unsampled metastases. In some cases, cfDNA revealed clinically actionable mutations that were not detected in matched tumor biopsies. Conclusions: WES of cfDNA uncovers the majority of somatic mutations, indels, and SCNAs found in matched tumor biopsies of metastatic cancer. The high degree of concordance suggests that comprehensive sequencing of cfDNA can be leveraged for genomic discovery in settings where conventional biopsies are difficult to access. Furthermore, the detection of mutations in cfDNA that are not detected in concurrent biopsies suggests that cfDNA may be complementary to tumor biopsies for both translational studies and precision cancer medicine. Citation Format: Viktor A. Adalsteinsson, Gavin Ha, Sam Freeman, Atish D. Choudhury, Daniel G. Stover, Heather A. Parsons, Gregory Gydush, Sarah Reed, Denis Loginov, Dimitri Livitz, Daniel Rosebrock, Ignat Leshchiner, Ofir Cohen, Coyin Oh, Jaegil Kim, Chip Stewart, Mara Rosenberg, Huiming Ding, Maxwell R. Lloyd, Sairah Mahmud, Karla E. Helvie, Margaret S. Merrill, Rebecca A. Santiago, Edward P. O’Connor, Seong H. Jeong, Joseph F. Kramkowski, Jens G. Lohr, Laura Polacek, Nelly Oliver, Lori Marini, Joshua Francis, Lauren C. Harshman, Eliezer M. Van Allen, Eric P. Winer, Nancy U. Lin, Mari Nakabayashi, Mary-Ellen Taplin, Levi A. Garraway, Todd R. Golub, Jesse S. Boehm, Nikhil Wagle, Gad Getz, Matthew Meyerson, Christopher J. Love. High concordance of whole-exome sequencing of cell-free DNA and matched biopsies enables genomic discovery in metastatic cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-136.