Ultra-Sensitive Detection of the Pretreatment EGFR T790M Mutation in Non-Small Cell Lung Cancer Patients with an EGFR-Activating Mutation Using Droplet Digital PCR. 
The resistance to the EGFR tyrosine kinase inhibitors (TKI) is a major concern in non-small cell lung cancer (NSCLC) treatment.  T790M mutation in EGFR accounts for nearly 50% of the acquired resistance to EGFR-TKIs.  Earlier studies suggested that T790M mutation was also detected in TKI-naïve NSCLCs in a small cohort.  Here, we use an ultra-sensitive droplet digital PCR (ddPCR) technique to address the incidence and clinical significance of pretreatment T790M in a larger cohort.  ddPCR was established as follows: wild-type or T790M mutation-containing DNA fragments were cloned into plasmids.  Candidate threshold was identified using wild-type plasmid, normal human genomic DNA, and human A549 cell line DNA, which expresses wild type.  Surgically resected tumor tissues from 373 NSCLC patients with EGFR-activating mutations were then examined for the presence of T790M using ddPCR.  Our data revealed a linear performance for this ddPCR method (R(2) = 0.998) with an analytical sensitivity of approximately 0.001%.  The overall incidence of the pretreatment T790M mutation was 79.9% (298/373), and the frequency ranged from 0.009% to 26.9%.  The T790M mutation was detected more frequently in patients with a larger tumor size (P = 0.019) and those with common EGFR-activating mutations (P = 0.022), as compared with the others.  The ultra-sensitive ddPCR assay revealed that pretreatment T790M was found in the majority of NSCLC patients with EGFR-activating mutations.  ddPCR should be utilized for detailed assessment of the impact of the low frequency pretreatment T790M mutation on treatment with EGFR-TKIs.