Plasma ctDNA enables early detection of temozolomide resistance mutations in glioma

Liquid biopsy based on circulating tumor DNA (ctDNA) is a novel tool in clinical oncology, however, its use has been limited in glioma to date, due to low levels of ctDNA. In this study we aimed to demonstrate that sequencing techniques optimised for liquid biopsy in glioma patients can detect ctDNA in plasma with high sensitivity and with potential clinical utility. We investigated ten glioma patients with tumor tissue available from at least two surgical operations, who had 49 longitudinally collected plasma samples available for analysis. Plasma samples were sequenced with CAPP-seq (AVENIO) and tissue samples with TSO500. Glioma-derived ctDNA mutations were detected in 93.8% of plasma samples. 25% of all mutations detected were observed in plasma only. Mutations of the mismatch repair (MMR) genes MSH2 and MSH6 were the most frequent circulating gene alterations seen after temozolomide treatment and were frequently observed to appear in plasma prior to their appearance in tumor tissue at the time of surgery for recurrence. This pilot study suggests that plasma ctDNA in glioma is feasible and may provide sensitive and complementary information to tissue biopsy. Furthermore, plasma ctDNA detection of new MMR gene mutations not present in the initial tissue biopsy may provide an early indication of the development of chemotherapy resistance. Additional clinical validation in larger cohorts is needed.