Microarray analyses have revealed significantly elevated expression of the proto-oncogene ROS1 receptor tyrosine kinase in 20–30 % of non-small cell lung carcinomas (NSCLC). Selective and potent ROS1 kinase inhibitors have recently been developed and oncogenic rearrangement of ROS1 in NSCLC identified.

We performed immunohistochemical evaluation of expression of ROS1 kinase and its downstream molecules in 399 NSCLC cases. ROS1 expression in primary and recurring lesions of 92 recurrent NSCLC cases was additionally analyzed. To elucidate mechanism of expression, two ROS1-nonexpressing NSCLC cell lines (Calu6 and H358) and fresh frozen tissues from 28 consecutive NSCLC patients were examined for ROS1 promoter methylation status and ROS1 expression.

Overall expression rate of ROS1 was 22 % (19 % for adenocarcinomas and 25 % for nonadenocarcinomas) in NSCLC. ROS1 expression was a worse prognostic factor for overall survival in adenocarcinomas of stage I NSCLC. In recurred NSCLC, ROS1 expression was significantly higher in recurring tumors (38 %) than primary tumors (19 %). Two NSCLC cell lines showed increased ROS1 expression after treatment with 5-aza-2′deoxycytidine and/or trichostatin A. Among the 14 adenocarcinomas examined, two (14 %) showed more than twice the level of ROS1 expression in tumor tissue than was observed in matched normal tissue and statistically significant differences in the ROS1 promoter methylation level.

A subset of NSCLC revealed overexpression of ROS1 receptor tyrosine kinase, possibly in relation to epigenetic changes. ROS1 expression was an independent prognostic factor for overall survival in adenocarcinomas of stage I NSCLC. Further studies are needed to validate our results.