The CircSarc study aims to provide new insights into the clinical utility of liquid biopsies in soft tissue sarcomas. At present, mutational profiles of solid tumours are obtained from tissue biopsies or surgical specimens. Recent advances in technology now allows to use blood plasma as a “liquid biopsy”, examining circulating tumour DNA (ctDNA) shed by the tumour cells into peripheral blood. ctDNA in plasma carries tumour-specific alterations that can be used to monitor minimal residual disease, response to therapy, tumour burden and evolution throughout the course of the disease. In CircSarc, we have enrolled 30 high-grade soft tissue sarcoma patients, with localised disease, that are being followed throughout the course of their disease. Plasma from each patient is collected longitudinally; before and after surgery, at each routine control and before and after each treatment cycle. Patient’s tumour and germline DNA were exome sequenced to identify tumour-specific mutations, and ctDNA is being sequenced using a comprehensive 900 cancer gene panel. The level of ctDNA in plasma, represented by the tumour-specific biomarkers, are then monitored throughout the course of the treatment, and acts as an indicator of tumour burden. In addition, we are analysing plasma samples from 70 Gastro Intestinal Stroma Tumour samples using targeted resequencing with Anchored Multiplex PCR. The levels of KIT and PDGFRA mutations in ctDNA are being correlated with clinical and pathological features. The established targeted resequencing protocols for ctDNA provide different levels of complexity and sensitivity. In the first screening, we have successfully detected somatic mutations in plasma at time of surgery in 70% of the samples analysed. Mutations present in the tumour at an allele frequency large than 20% can be robustly identified in plasma. In addition, analysis of plasma samples identified novel mutations not detected in the primary tumour, possibly reflecting intra-tumour heterogeneity. These mutations are being followed throughout the course of the disease to monitor tumour evolution and resistance. In a proof-of-concept study, we have shown that that levels of tumour-specific mutations in liquid biopsies correlated to clinical manifestation of metastatic disease in aggressive sarcoma, and have the potential to detect disease progression at an early stage. Our work provides new insights into the clinical significance of ctDNA in sarcomas. By repeated sampling of liquid biopsies, somatic mutations identified in ctDNA can be used as unique non-invasive tumour-specific biomarkers for monitoring tumour burden and disease evolution throughout the course of the disease.

Citation Format: Heidi M. Namløs, Seyed Hossein Moosavi, Bodil Bjerkehagen, Olga Zaikova, Synnøve Granlien, Stine Næss, Nina Louise Jebsen, Skyler Mishkin, Eva W. Stratford, Else Munthe, Kirsten S. Hall, Lars B. Aasheim, Eivind Hovig, Brian Kudlow, Ola Myklebost, Kjetil Boye, Leonardo A. Meza-Zepeda. CircSarc: Disease monitoring by liquid biopsies in sarcomas [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 5700. doi:10.1158/1538-7445.AM2017-5700