| Summary: | Introduction: In this study, we sought to further characterize ROS1 protein expression in solid tumors with the complete spectrum of ROS1 genomic alterations. Methods: ROS1 immunohistochemistry (IHC) was performed using the ROS1 (SP384) class I assay per manufacturer’s instructions on a variety of solid tumors (n = 32) with known ROS1 genomic alterations. Genomic alterations included fusions (n = 17), gene amplifications (n = 10), and short-variant mutations (n = 11). Results: Of the 32 cases with ROS1 IHC results, 100% (11 of 11) with canonical ROS1 fusions were positive for ROS1 IHC. Among noncanonical ROS1 fusions, only two (of five) cases with SQSTM1-ROS1 and RDX-ROS1 fusions were positive for ROS1 IHC whereas PTPRK-ROS1 (two) and TTC28-ROS1 fusions were negative for ROS1 IHC. One sample with a canonical ROS1 fusion and co-occurring ROS1 resistance mutation (6094G>A, p.G2032R) was positive for ROS1 IHC. A total of 10% (one of 10) of ROS1 amplified tumors were positive for ROS1 IHC. None of the cases (zero of five) with ROS1 short-variant mutations were positive for ROS1 protein expression. Conclusions: These findings suggest that if ROS1 IHC was used as a screening tool for ROS1 fusion, a subset of fusion-negative tumors will reveal positive IHC staining highlighting the value of reflexing to genomic profiling to confirm the presence of a targetable fusion-driver before the initiation of therapy. In addition, the ability of comprehensive genomic profiling to detect ROS1 resistance mutations will be important for clinical decision making.
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