Abstract 505: Optimized 7-Plex cell-free DNA assay for clinically relevant KRAS mutations in colorectal cancer
Authors: Avula Rajeswari , Kipp Benjamin R. , Voss Jesse S. , Haselkorn Keegan E. , Highsmith W. Edward , Halling Kevin C. , Jones Jeremy C. , Alberts Steven R. , Campion Michael B. , Nelson Cassandra J. , Jen Jin , Wieben Eric D. , Cunningham Julie M. , Liu Minetta C. 
Published: July 2016

Background: KRAS gene mutations are present in approximately 40% of colorectal adenocarcinomas and predict for nonresponse to the anti-epidermal growth factor receptor antibodies used in routine clinical practice. The most common activating mutations in this malignancy occur in codons 12 and 13 of exon 2. Determination of KRAS mutational status from tumor samples is an essential tool for managing patients with colorectal cancer (CRC) but requires tissue obtained by invasive percutaneous or surgical biopsies. Recent advances allow for the isolation and analysis of cell-free DNA (cfDNA) from the peripheral blood. A highly sensitive multiplex technology that uses low amounts of input DNA is needed to establish cfDNA as a liquid biopsy to personalize care.

Methods: Digital droplet PCR (ddPCR) is a particularly sensitive method for detecting rare and low copy targets. Related work led to 7 individual assays, and to companion 4-plex and 3-plex assays, to accommodate the 7 common KRAS mutations and internal controls. A single 7-plex assay is favored to maximize efficiency and minimize cost. We optimized the RainDrop Digital PCR System to collectively detect the G12/G13 KRAS mutations, given that documentation of a mutation (not the specific mutation) is the factor of clinical relevance. Commercially available cell lines and genomic DNA aliquots were used as reference standards and for determinations of sensitivity and limit of blank (LOB). Initial clinical validation samples included tumor DNA derived from FFPE tissue (n = 10), cfDNA from healthy donors (n = 10), and cfDNA from metastatic CRC patients with tumors of known KRAS mutation status (n = 4). All assays were run in triplicate to confirm reproducibility.

Results: Custom Taqman MGB probes were selected for KRAS wild type (WT), G12A, G12C, G12D, G12R, G12S, G12V, and G13D to allow for testing in a single reaction. LOB was 9 per 1×10(6) droplets on the basis of WT controls, including cfDNA samples from healthy donors. Evaluation of cell line DNA, genomic DNA aliquots, and FFPE samples demonstrates that the assay detects mutant KRAS with 100% accuracy and 0.1% mutant DNA in a total input of 10 ng WT/mutant DNA. With the clinical samples, concordant KRAS findings between cfDNA and tissue were obtained for two patients with WT tumors and one patient with a KRAS G12S mutant tumor. cfDNA analysis did not detect the G12V mutation identified in the colon primary of the other patient. Evaluation of additional clinical samples is in progress.

Conclusions: Our 7-plex G12/G13 KRAS assay has excellent accuracy and is able to detect the KRAS mutations of interest at low (0.1%) mutation frequencies. Additional studies are being performed on prospectively collected plasma samples from patients with metastatic CRC of known KRAS mutation status. Large scale correlation between tumor mutation status and cfDNA findings will be performed to validate our assay for clinical implementation.

Citation Format: Rajeswari Avula, Benjamin R. Kipp, Jesse S. Voss, Keegan E. Haselkorn, W. Edward Highsmith, Kevin C. Halling, Jeremy C. Jones, Steven R. Alberts, Michael B. Campion, Cassandra J. Nelson, Jin Jen, Eric D. Wieben, Julie M. Cunningham, Minetta C. Liu. Optimized 7-Plex cell-free DNA assay for clinically relevant KRAS mutations in colorectal 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 505.