Comparison of biomarker assays for EGFR: Implications for precision medicine in patients with glioblastoma

Purpose: Patients with glioblastoma (GBM) have a poor prognosis and are in desperate need of better therapies. As therapeutic decisions are increasingly guided by biomarkers, and EGFR abnormalities are common in GBM, thus representing a potential therapeutic target, we systematically evaluated methods of assessing EGFR amplification by multiple assays. Specifically, we evaluated correlation among fluorescence in situ hybridization (FISH), a standard assay for detecting EGFR amplification, with other methods. Experimental Design: Formalin-fixed, paraffin-embedded tumor samples were used for all assays. EGFR amplification was detected using FISH (N ¼ 206) and whole-exome sequencing (WES, N ¼ 74). EGFR mRNA expression was measured using reverse transcription-polymerase chain reaction (RT-PCR, N ¼ 206) and transcriptome profiling (RNAseq, N ¼ 64). EGFR protein expression was determined by immunohistochemistry (IHC, N ¼ 34). Significant correlations among various methods were determined using Cohen's kappa (k ¼ 0.61–0.80 defines substantial agreement) or R² statistics. Results: EGFR mRNA expression levels by RNA sequencing (RNAseq) and RT-PCR were highly correlated with EGFR amplification assessed by FISH (k ¼ 0.702). High concordance was also observed when comparing FISH to WES (k ¼ 0.739). RNA expression was superior to protein expression in delineating EGFR amplification. Conclusions: Methods for assessing EGFR mRNA expression (RT-PCR, RNAseq) and copy number (WES), but not protein expression (IHC), can be used as surrogates for EGFR amplification (FISH) in GBM. Collectively, our results provide enhanced understanding of available screening options for patients, which may help guide EGFR-targeted therapeutic approaches.