Mutationally-activated PI3'-kinase-a promotes de-differentiation of lung tumors initiated by the BRAFV600E oncoprotein kinase

Human lung adenocarcinoma exhibits a propensity for de-differentiation, which complicates diagnosis and treatment, and predicts for poor overall patient survival. In genetically engineered mouse (GEM) models of lung cancer, expression of the BRAFV600E oncoprotein kinase initiates the growth of benign tumors that retain characteristics of their cell of origin, alveolar type II (ATII) pneumocytes. Cooperating genetic alterations such as silencing of the PTEN tumor suppressor or expression of mutationally-activated PI3-kinase-a (PIK3CAH1047R) promote malignant progression of such benign tumors to malignant adenocarcinoma, though their effects on differentiation status are unknown. To address this in vivo, we generated a new conditional BrafCAT allele in which Cre-mediated recombination leads to expression of a bi-cistronic mRNA encoding both BRAFV600E and the tdTomato fluorescent protein. Using this model, we demonstrate that coincident expression of BRAFV600E and PIK3CAH1047R in ATII pneumocytes leads to rapid and widespread cell de-differentiation. Surprisingly, the combined effects of BRAFV600E and PIK3CAH1047R on ATII pneumocyte identity occurred without loss of expression of the lung lineage transcription factors NKX2.1, FOXA1, or FOXA2. Instead, we demonstrate a novel role of PGC1a in maintaining pneumocyte identity, which is lost upon PIK3CAH1047R expression. These findings provide additional insight into how two of the most commonly mutated growth factor signaling pathways contribute to the pathogenesis of lung adenocarcinoma. Overall design: BRAFV600E mutant mouse lung adenocarcinoma (n=6) vs BRAFV600E;PIK3CAH1047R mutant lung adenocarcinoma (n= 8), and BRAFV600E;PGC1aHET (n=5) vs BRAFV600E;PGC1aNULL tumors (n=4)

23

van Veen JE, Scherzer M, Boshuizen J, Chu M, Liu A, Landman A, Green S, Trejo C, McMahon M