Abstract 743: Detection of TERT C228T and C250T promoter mutations in melanoma tumor and plasma samples using novel mutation-specific droplet digital PCR assays

Purpose: Detecting mutations in the plasma of patients with solid tumors is becoming a valuable method of diagnosing and monitoring cancer. Mutations in 1 of 2 hot spots in the TERT promoter sequence are found in several cancers, including up to 85% of melanomas and the majority of cases that lack BRAF or NRAS mutations (about one-third of melanomas). Due to the high G-C content of the TERT promoter sequence these mutations can be difficult to detect using NGS approaches. We developed novel droplet digital PCR (ddPCR) assays to detect these 2 mutations with high sensitivity and specificity, and demonstrate the application of these assays in melanoma clinical samples. Methods: Assays were optimized using cell lines with Sanger sequencing-confirmed mutations: glioblastoma A172 (C228T), and melanoma NYU12-126 (C250T). We varied assay designs and amplification conditions to optimize probe-based detection using the Bio-Rad QX-200 ddPCR system. Assay sensitivities and specificities at various DNA input levels were determined using serial dilutions with 3 replicate wells for each condition. Sensitivity is defined as the lowest mutant allele dilution for which the confidence interval did not overlap with that of the 0% mutant wells. We used normal and cancer-derived DNA sources of different quality (e.g. normal human DNA (Promega), cancer cell lines, plasma and FFPE-derived DNAs) with and without the mutations, and compared the efficiency of detection of amplicons of 88, 113 and 163 base-pairs. We compared efficiencies to assays of similar size for RPP30, a housekeeping gene. Patient-matched metastatic melanoma tumors and plasma samples were analyzed to explore the clinical utility of these assays. Results: The assays showed greater sensitivity when higher amounts of DNA were analyzed. For C228T the limit of detection (LOD) of the mutant allele was 1%, 0.25% and 0.1% for 6.6ng/well, 33ng/well and 66ng/well respectively; for C250T the LODs were 0.25%, 0.05% and 0.05% respectively. Using normal human DNA, the efficiency of the TERT assays averaged approximately 90% of that for RPP30 across assays of similar size, and no decrease in assay efficiency was observed as amplicon length increased. In contrast, whereas amplicon size had only a modest effect on assay efficiency in plasma cfDNA, it gave a more pronounced effect on FFPE DNA’s, decreasing to 38% for the 163bp amplicon. We observed 100% concordance between TERT mutation detection by SNaPshot and ddPCR in 10 FFPE tumor samples, and in plasma samples from 4 metastatic melanoma patients with matching tumor samples. Conclusion: We developed robust ddPCR assays to detect TERT promoter mutations with high sensitivity and specificity. Mutated TERT DNA can be detected and quantitated in the plasma of patients with metastatic melanoma, and is likely to be present in the plasma of other cancer patients in whom TERT mutations occur. Citation Format: Broderick Corless, Gregory Chang, Samantha Cooper, Mahrukh Syeda, Iman Osman, George Karlin-Neumann, David Polsky. Detection of TERT C228T and C250T promoter mutations in melanoma tumor and plasma samples using novel mutation-specific droplet digital PCR assays [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 743. doi:10.1158/1538-7445.AM2017-743