Expression of Paired box 9 defines an aggressive subset of lung adenocarcinoma preferentially occurring in smokers.

Abstract

A distinct subset of lung adenocarcinomas (LADs), arising from a series of peripheral lung cells defined as the terminal respiratory unit (TRU), is characterized by thyroid transcription factor 1 (TTF-1) expression. The clinical relevance of transcription factors (TFs) other than TTF-1 remains unknown in LAD and was explored in the present study. Seventy-one LAD samples were subjected to high-throughput transcriptome screening of LAD using cap analysis gene expression (CAGE) sequencing data; CAGE provides genome-wide expression levels of the transcription start sites (TSSs). In total, 1083 invasive LAD samples were subjected to immunohistochemical examination for paired box 9 (PAX9) and TTF-1 expression levels. PAX9 is an endoderm development-associated TF that most strongly and inversely correlates with the expression of TTF-1 TSS subsets. Immunohistochemically, PAX9 expression was restricted to the nuclei of ciliated epithelial and basal cells in the bronchi and bronchioles and the nuclei of epithelial cells of the bronchial glands; moreover, PAX9 expression was observed in 304 LADs (28%). PAX9-positive LADs were significantly associated with heavy smoking, non-lepidic subtype, EGFR wild-type tumors, and PD-L1 expression (all p < 0.0001). All these characteristics were opposite to those of TRU-type LADs with TTF-1 expression. PAX9 expression was an independent prognostic factor for decreased overall survival (p = 0.022). Our results revealed that PAX9 expression defines an aggressive subset of LADs preferentially occurring in smokers that may arise from bronchial or bronchiolar cells.