Droplet digital PCR based detection of EGFR mutations in advanced lung cancer patient liquid biopsies : a comparison of circulating tumour DNA extraction kits

Background: Mutations in the epidermal growth factor receptor gene, EGFR, predict response or resistance to first generation tyrosine kinase inhibitors in non-small cell lung cancer. These biomarkers can now be conveniently detected from liquid biopsies, however technical details of these assays are still being refined. Objective: To compare detection of four different non-small cell lung cancer (NSCLC) associated EGFR mutations from patient ctDNA isolated with five different ctDNA isolation kit. Methods: Droplet digital PCR (ddPCR) assays detecting four EGFR mutations were developed. ctDNA was isolated with five kits from plasma samples, one pleural and one ascites fluid from nine NSCLC patients with known EGFR mutations. ctDNA fragment sizes and concentrations were also assessed. Results: Each kit isolated DNA from all samples which contained an expected dominant DNA fragment of ~ 170 base pairs. Normalised for plasma input, one kit produced ctDNA extracts which consistently enabled the highest cop n umber detection for all EGFR variants, and importantly was able to validate mutations in all patient samples. Other kits stood out in regards to cost economy as well as ease and speed of processing but were less efficient and one kit was found to be incompatible with ddPCR. Conclusion: This study demonstrated successful ctDNA isolation from plasma, pleural fluid and ascites by four of five ctDNA isolation kits. The QIAmp circulating nucleic acid kit produced consistently the most sensitive detection of EGFR variants. While other kits allow for lower volume plasma input down to 0.1 ml, are faster, more economical and simpler to use, they are challenged by very low ctDNA concentrations in plasma.