Background: The mutually exclusive pattern of the major driver oncogenes in lung cancer suggests that other mutually exclusive oncogenes exist. We conducted a systematic search for tyrosine kinase fusions by screening all tyrosine kinases for aberrantly high RNA expression levels of the 3' kinase domain (KD) exons relative to more 5' exons.
Methods: We studied 69 patients (including five never smokers and 64 current or former smokers) with lung adenocarcinoma negative for all major mutations in KRAS, EGFR, BRAF, MEK1, HER2, and for ALK fusions (termed "pan-negative"). A NanoString-based assay was designed to query the transcripts of 90 tyrosine kinases at two points: 5' to the KD and within the KD or 3' to it. Tumor RNAs were hybridized to the NanoString probes and analyzed for outlier 3' to 5' expression ratios. Presumed novel fusion events were studied by rapid amplification of cDNA ends (RACE) and confirmatory reverse transcriptase PCR (RT-PCR) and FISH.
Results: We identified one case each of aberrant 3' to 5' ratios in ROS1 and RET. RACE isolated a GOPC-ROS1 (FIG-ROS1) fusion in the former and a KIF5B-RET fusion in the latter, both confirmed by RT-PCR. The RET rearrangement was also confirmed by FISH. The KIF5B-RET patient was one of only five never smokers in this cohort.
Conclusion: The KIF5B-RET fusion defines an additional subset of lung cancer with a potentially targetable driver oncogene enriched in never smokers with "pan-negative" lung adenocarcinomas. We also report in lung cancer the GOPC-ROS1 fusion originally discovered and characterized in a glioma cell line.