P21. Immunohistochemistry versus DNA testing for detection of EGFR mutations and ALK rearrangements in lung adenocarcinoma patients

Background: The molecular diagnostics of epidermal growth factor receptor (EGFR) mutations and ALK rearrangements in non-small cell lung cancer (NSCLC) is currently based on DNA assays. We evaluated the diagnostic effectiveness of immunohistochemistry (IHC) with monoclonal antibodies specific for mutant EGFR and ALK proteins in lung adenocarcinoma samples.

Methods: A total of 45 lung adenocarcinoma tissue samples were analyzed by IHC using antibodies specific for mutant EGFR: L858R (SP125) and E746_A750del (SP111) and/or ALK protein (D5F3). DNA-based EGFR mutations detection was performed using PNA-LNA PCR clamp method and Sanger sequencing. ALK IHC+ samples were reevaluated with Vysis LSI ALK Break Apart FISH probe assay.

Results: EGFR mutations (four deletions in exon 19 and eight substitutions in exon 21) were detected in DNA isolated from 12 samples (27%). IHC assay with mutant EGFR-specific antibody presented 75% sensitivity and 100% specificity of exon 19 deletions detection for E746_A750del antibody and 100% sensitivity and 97% specificity of exon 21 mutations detection for L858R antibody with a cut-off of 1+ staining. Interestingly, 2+ staining was present forE749_P753del, but L747_P753delinsS was not detected. 1+ staining was also present in sample harboring L858M+L861Q double mutation. ALK positive staining was detected in five samples (11%) with 1+ positive cut-off. FISH analysis confirmed ALK rearrangements in 2 samples, while 3 others were noted as not evaluable.

Conclusions: IHC assay proved high sensitivity and specificity in detection of most common exon 19 deletions and L858R substitutions as well as possible detection of some rare EGFR variants. ALK IHC could possibly be cost-effective and robust prescreening tool for selection samples for FISH testing. The study is on-going.

Keywords: Adenocarcinoma; immunohistochemistry; epidermal growth factor receptor (EGFR) mutation; ALK