Dissection of the expressed actionable fusions’ repertoire in solid tumors tested serially in the clinical setting across 14 cancer types.

e18804 Background: More frequently, select gene fusions have been tested in clinical practice as more approvals of very effective therapies targeting oncogenic chimeric fusion proteins. Large studies based on DNA-based NGS testing alongside clinical trials have established the prevalence of clinically actionable gene fusions such as NTRK1/2/3. However, much less is known about the broader repertoire and prevalence of bonafide expressed gene fusions. Methods: 6009 tumors across 14 cancer types were tested in our clinical laboratory between January and October 2021. Enrichment was performed using a custom hybridization-based capture after RNA to cDNA conversion with custom designed probes targeting > 1104 different genes and enriched for 2230 specific RNA fusions to maximize sensitivity and breadth before sequencing ̃50 million 150x150 reads/sample on a NovaSeq 6000. Data was deidentified and aggregated according an approved IRB. Statistical significance was calculated by Fisher’s exact test. Results: We found that 37% of tumors tested carry > 1 expressed chimeric transcript and that 7.2% of tumors (n = 431) express a gene fusion involving one of 19 actionable gene families/pathways targeted by an approved therapy. The most frequent involved FGFR1/2/3/4 (15%), ALK (14%), NTRK1/2/3 (12%), BRAF, ERBB2, RET and MET (6% each), RAS/RAF/MAPK and CDK4/6 pathway (5%). These fusions imply intra- and inter-chromosomal translocations on all chromosomes, especially on chromosomes 1, 3, 6, 7, 11, 12 and 17. NTRK1/2/3 fusions prevalence (0.9%) is significantly higher (p = 1.1e-10) than the 0.3% reported on a recent study from a large dataset (296K samples) based on DNA-sequencing testing (1). Moreover, we found that 32% of the NTRK partners were unique vs. 10% (p = 6.5e-08) from the Westphalen et al. study (1). 14 of 25 unique NTRK partners have not been described on large datasets (1) and only 3 have been reported on the literature as NTRK partners. Among them, we identified the hypoxia-induced neovascularization gene HIF1A, as a novel in frame 3’ fusion partner of NTRK3 [t(15;14)(15q25.3; 14q23.1). Finally, NTRK fusions were detected across 14 cancer types with higher frequency on brain tumors (17%), followed by lung cancers, colorectal and head and neck cancer (13% each) and sarcomas (12%). Conclusions: Gene fusions involving common actionable genes showed a high prevalence in the clinical setting. This study confirms that the number and breadth of gene fusions can present with common actionable information in > 7% of clinical cases. We report here 14 new unique NTRK partners. Despite a 50 times smaller dataset, 3-fold more actionable expressed NTRK fusions involving novel partners were detected with a targeted RNA based hybridization approach in lieu of DNA based testing supporting the benefit of this strategy. Overexpression and translation can be confirmed only by this approach.