P1.02-060 EGFR Mediates Activation of RET in Lung Adenocarcinoma with Neuroendocrine Differentiation Characterized by ASCL1 Expression

Abstract

Achaete-scute homolog 1 (ASCL1) is a neuroendocrine transcription factor expressed in 10-20 % of lung adenocarcinomas (AD) with neuroendocrine (NE) differentiation. Previously, we demonstrated that ASCL1 functions as an upstream regulator of the RET oncogene in AD with high ASCL1 expression (A+AD). In this study, we examined the potential role of wild type RET in influencing the oncogenic properties of A+AD. We also screened for drugs that could selectively target RET signaling and examined the role of the two RET isoform separately. The association of the mRNA expression for the long (RET51) and short (RET9) RET isoforms with overall survival (OS) were assessed in a case-control study of stage-1 A+AD patients surgically resected at the Mayo Clinic (1994-2007). Cases and controls were defined as patients who survived < 3.5 years after surgery (n= 29) and > 5 years after surgery (n=38), respectively. mRNA was isolated from FFPE tissue and analyzed by a nanostring assay. Associations of each isoform mRNA with the OS was determined by the area under the receiver operative characteristics (AUC). For drug screening, HCC1833 lung AD cells with endogenously high expression of ASCL1 were stably transfected with either empty vector or an ASCL1-shRNA. Differential sensitivities of tyrosine kinase inhibitors (TKIs) in the pair of syngeneic cell lines were measured by Cell-Titer Glo (Promega). Interactions between EGFR and RET was examined by co-immunoprecipitation. Expression of RET51 mRNA was associated with poor OS (p=0.005, AUC 0.71). We detected modestly increased sensitivity to sunitinib and vandetanib in A+AD compared with A-AD cells. However, the EGFR inhibitors gefitinib and the dual EGFR and HER2 inhibitor lapatinib resulted in ≥ 10 fold higher cytotoxicity in A+AD cells than in A-AD cells. Subsequent experiments demonstrated that EGF stimulation of EGFR mediates the phosphorylation of RET in multiple A+AD cells. RET and EGFR were found to interact only in presence of EGF and predominantly through the long RET isoform (RET51). Herein we demonstrate that wild type EGFR predominantly interacts with the long isoform of RET (RET51) in A+AD cells. In the presence of EGF this results in activation of RET. High RET51 is associated with worse OS. Furthermore, compared to A-AD cells, A+AD cells appear to be more sensitivity to EGFR inhibitors. In summary, our results suggest that A+AD patients may benefit from treatment with EGFR inhibitors even in the absence of an EGFR mutation.