P1.02-009 High Concordance of ALK Rearrangement Testing between ALK RNA-In Situ Hybridization and IHC/FISH in Patients with Lung Adenocarcinoma

Abstract

Patients with anaplastic lymphoma kinase (ALK) gene rearrangements manifest good responses to ALK inhibitors and thus, accurate and rapid identification of ALK gene rearrangements is essential for the clinical application of ALK-targeted therapies. The aim of this study was to investigate the diagnostic accuracy of the recently developed ALK RNA-in situ hybridization (RNA-ISH) assay, using formalin fixed paraffin embedded samples of lung adenocarcinoma tissue. We first tested whether ALK RNA-ISH could be performed in 11 resected lung adenocarcinomas in which ALK gene rearrangements were confirmed by immunohistochemistry (IHC, D5F3), and/or fluorescence in situ hybridization (FISH), and also clarified the intra-tumor heterogeneity of ALK RNA-ISH by counting 100 tumor cells in 10 different loci (total, 1000 tumor cells) in each tumor. Secondly, we analyzed the diagnostic accuracy of ALK RNA-ISH in tissue microarrays (TMAs) containing 294 lung adenocarcinoma cores (by counting 100 tumor cells in each core) with no information about ALK gene rearrangements and compared these results with those of conventional IHC and FISH tests. ALK mRNA expression was observed in all 11 resected lung adenocarcinomas by the ALK RNA-ISH assay and the median of positive tumor cells was 67.7%, whereas ALK mRNA expression was not observed in normal lung cells in the background. Next, 5 ALK positive cases were found by IHC and/or FISH in the 294 cases of lung adenocarcinoma. The median of positive cells by ALK RNA-ISH in these 5 cases was 75.6% (range: 40-94%), whereas the median of positive cells by ALK RNA-ISH in the remaining 289 cases was 0.3% (range: 0-15%). When the cutoff value was set as 15% based on the first test, the ALK RNA-ISH–positive and ALK RNA-ISH–negative cases were readily distinguishable with 100% sensitivity and specificity compared with the results of IHC and/or FISH. Our findings suggest that the ALK RNA-ISH assay is useful for detecting ALK positive lung adenocarcinomas with high sensitivity and specificity compared with the conventional IHC and FISH test. Thus, this study provides important and timely insight into the clinical testing of ALK in lung cancer because the RNA-ISH assay detects the target mRNA easily and rapidly.