Background and objective: Chemotherapy for advanced non-small-cell lung cancer (NSCLC) remains marginally effective, with a 5-year overall survival rate of approximately 5%. Recently, the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor gefitinib was approved in Slovakia for the treatment of metastatic NSCLC. Gefitinib is a selective EGFR inhibitor that binds to the adenosine triphosphate binding pocket of the kinase domain and blocks downstream signaling pathways. Mutations of the EGFR gene, particularly an in-frame 15 bp deletion (delE746_A750) in exon 19 and the L858R mutation in exon 21, correlate with enhanced clinical responsiveness to EGFR tyrosine kinase inhibitors. However, the detection of these mutations and thereby prediction of the therapy outcome is sometimes unreliable due to the low sensitivity of direct sequencing if the proportion of tumor cells in the tissue is less than 25%. Therefore we decided to test the applicability of other methods, particularly high-resolution melting analysis (HRMA), for detection of these mutations in clinical samples.
Methods: We analyzed 53 archival cytologic specimens for the presence of EGFR mutations, using the HRMA method. Results were verified by direct sequencing. For samples containing less than 25% tumor cells, we used mutant-enriched PCR before sequencing. We also performed a titration assay to establish the lower limit of the proportion of tumor cells for detection of EGFR mutations.
Results: EGFR mutations were detected in 13 cases (24%). In-frame deletions in exon 19 were detected in eight cases (15%) and the L858R mutation in exon 21 was detected in five cases (9%). The positive results of the HRMA were confirmed by direct sequencing only in five of 13 cases. In the remaining eight positive samples, HRMA results were confirmed by sequencing analysis after mutant-DNA enrichment. The titration assay established that the lower limit for detection of EGFR mutations by HMRA was 1% tumor cells in the clinical sample.
Conclusion: Our results indicated that HRMA in combination with mutant-enriched PCR represents a sensitive method for detection of EGFR mutations from cytologic specimens. When properly executed, this protocol allows identification of EGFR mutations in specimens containing a minimal percentage of tumor cells.