Highly Sensitive Detection of EGFR T790M Mutation Using Colony Hybridization Predicts Favorable Prognosis of Patients with Lung Cancer Harboring Activating EGFR Mutation

Introduction: Approximately 50% of lung cancer patients with epidermal growth factor receptor (EGFR)-mutations (deletion in exon 19 or L858R) who develop acquired resistance to EGFR tyrosine kinase inhibitors (TKIs) reportedly carry a secondary EGFR T790M mutation. This mutation has been suggested to be present in tumor cells before EGFR-TKI treatment in a small population of individuals. Here, we use a highly sensitive colony hybridization technique in an attempt to evaluate the actual incidence of T790M in pretreatment tumor specimens. Methods: DNA was extracted from surgically resected tumor tissues of 38 patients with the EGFR mutation and examined for the presence of T790M, using a standard polymerase chain reaction based method followed by a modified colony hybridization (CH) technique with an analytical sensitivity of approximately 0.01%. Associations between the T790M status and clinical characteristics including time to treatment failure (TTF) for EGFR-TKI were evaluated. Results: The T790M mutation analysis of the specimens from the 38 patients detected 30 mutants (79%). The median TTF was 9 months for the patients with pretreatment T790M and 7 months for the patients without the T790M mutation (p = 0.44). When the patients with T790M were divided into strongly positive and modestly positive subgroups in terms of the frequency of positive signals observed using CH technique, the 7 patients with strong positivity had a TTF that was significantly longer than that of the 8 patients without T790M (p = 0.0097) and of the 23 patients with modest positivity (p = 0.0019). Conclusions: Our highly sensitive CH method showed that a subgroup of non-small-cell lung cancer patients with the EGFR mutation harbored the rare T790M allele before EGFR-TKI treatment. A high proportion of T790M allele may define a clinical subset with a relatively favorable prognosis.