P1.01-56 Increased ROS1 and RET Transcripts in Fusion-Negative NSCLC Patients

Fusion involving anaplastic lymphoma kinase (ALK), RET proto-oncogene (RET) or v-ros UR2 sarcoma virus oncogene homolog 1 (ROS1) occur in non-small cell lung cancers (NSCLC) and are important biomarkers for targeted therapies. However, little is known about the RNA expression levels of these genes regardless of fusions. We used a custom nCounter panel (NanoString Technologies) designed to detect several genetic alterations, including fusions and mRNA expression levels of ALK, ROS1 and RET in formalin-fixed paraffin embedded (FFPE) samples. RNA was purified from NSCLC tumor samples and analyzed with the custom panel. The counts corresponding to the 3’ probes were normalized using the geometrical mean of the housekeeping genes and then added to evaluate total mRNA expression levels. Cut-off values for overexpression were established as the average counts for each gene plus two times the standard deviation. A total of 400 stage III-IV NSCLC patients (p) from two different institutions were retrospectively analyzed. Overexpression of ALK was found in 55 p (13.8%). Of them, 48 (87%) were also positive for EML4-ALK fusions. One ALK-translocated patient with low levels of ALK mRNA expression did not respond to therapy. Fifteen p (3.8%) showed ROS1 overexpression. In contrast with ALK, only three of them (15%) had a concomitant ROS1 fusion. Among the remaining 12 patients overexpressing ROS1, four were ALK positive, five harbored mutations in EGFR and three were non-smoker females with no known drivers. Regarding RET, high expression levels were found in 14 p (3.5%) and only one of them showed a RET fusion (7%). Among the remaining 13 p, three presented neuroendocrine features and seven were smoker or ex-smoker without other known drivers. Overexpression of ALK mRNA in NSCLC is associated with EML4-ALK translocations. In contrast, a significant number of fusion negative patients show high ROS1 or RET mRNA levels. Further research is warranted to determine the clinical relevance of this finding.