Tumor-specific genetic variants can be detected in circulating cell-free DNA of malignant pleural mesothelioma patients

Objectives: Patients diagnosed with malignant pleural mesothelioma (MPM) face a poor prognosis, with an overall survival plateauing at a median of one year. This can be explained by difficulties in early diagnosis, effective treatment and treatment monitoring. Circulating cell-free tumor DNA (ctDNA) is emerging as an interesting biomarker addressing some of these issues. So far, the development of ctDNA in MPM lags behind that in other tumors. In this study, the possibility of tracing tumor-specific genetic variants, identified in MPM tissue, in circulating DNA of the corresponding patients is investigated. Materials and methods: Whole exome sequencing was performed on paired tumor and germline DNA of ten MPM patients, of which five were treatment naive. For each patient, a tumor-specific variant was selected and traced in tumor, germline and circulating DNA using droplet digital PCR in two independent runs. Results: All but one tumor-specific variants, selected after whole exome sequencing, were validated on primary tumor tissue using droplet digital PCR analysis. Patient-specific, selected variants could be detected in circulating DNA of three MPM patients, either in one or both independent droplet digital PCR runs. Mutated fractions in circulating DNA ranged from 0.28 to 0.9%. Interestingly, all patients whose circulating DNA samples contained tumor-specific variants, were treatment naive. Conclusion: We demonstrated for the first time the presence of ctDNA within circulating DNA of treatment naive MPM patients. This finding opens perspectives towards the use of ctDNA as a biomarker for (early and differential) diagnosis, treatment and treatment monitoring of MPM, which all remain challenging.