The microRNA Expression Signature of Lung Adenocarcinoma Harboring EGFR Mutations: Identification of Therapeutic Targets for EGFR-TKI Combination Therapy.

Approximately half of Japanese patients with lung adenocarcinoma (LUAD) have epi-dermal growth factor receptor (EGFR) gene mutations. Therefore, EGFR is the most critical therapeutic target in treating LUAD. This study's purpose is to explore novel therapeutic targets for LUAD and further improve EGFR inhibitor combination therapy. We created microRNA (miRNA) expression signatures from clinical specimens of LUAD patients harboring EGFR gene mutations using RNA sequencing. Based on the miRNA signature, we focused on miR-206, which had the most downregulated expression. We searched for its target gene in LUAD cells by facilitating transfection of miR-206 into the EGFR mutant cell line PC9 and searching for genes with suppressed expression. A total of 821 genes were downregulated in PC9 cells. Through molecular classification of these genes, we revealed that 18 genes were closely related to the cell cycle. Among these genes, we focused on cyclin-dependent kinase 4 (CDK4) and investigated the synergistic effects of a CDK4/6 inhibitor (abemaciclib or palbociclib) and osimertinib, an EGFR inhibitor in LUAD cells. In vitro and three-dimensional culture analyses showed that combination therapy with a CDK4/6 inhibitor and an EGFR inhibitor synergistically suppressed proliferation of LUAD cells with EGFR mutations. Our miRNA-based analysis is an excellent strategy for identifying therapeutic targets for LUAD. Continued analysis will reveal target molecules that enhance the therapeutic effects of EGFR inhibitors.