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Associate Attending, Pathology
Diagnostic Molecular Service
Directory, Diagnostic Molecular Pathology Laboratory
Director, Molecular Hematopathology
Diagnostic Molecular Pathology and Hematopathology
Memorial Sloan Kettering Cancer Center
New York, New York
Maria E. Arcila, MD, has disclosed that she has received consulting fees from AstraZeneca, Biocartis, Invivoscribe, and Johnson & Johnson.
Several gene fusions (eg, ALK, ROS1, RET, BRAF, FGFR and NTRK) have been established as targetable oncogenic drivers across a variety of solid tumors. Another emerging and potentially targetable oncogenic fusion involves the NRG1 gene which encodes a ligand for HER3, a member of the human epidermal growth factor (EGF) receptor family. Structurally, NRG1 fusions result in the formation of chimeric oncoproteins that retain the extracellular EGF-like domain of NRG1 and the transmembrane domain of the specific fusion partner. The fusion protein then serves as a ligand for HER3 with subsequent heterodimerization and constitutive signaling of downstream pathways that lead to uncontrolled cellular proliferation. NRG1 fusions have been identified across multiple types of solid tumors including pancreatic adenocarcinoma, renal cell carcinoma, cholangiocarcinoma, non-small-cell lung cancer, breast cancer, sarcoma, bladder cancer, and colorectal cancer. NRG1 fusions are rare, with a prevalence across multiple tumor types of <1%. Of note, NRG1 fusions are enriched in invasive mucinous adenocarcinomas of the lung and have been reported in approximately 10% to 30% of cases. However, in the eNRGy1 registry of 110 patients, the largest series of patients with NRG1 fusion–positive lung cancer reported to date, this oncogenic driver event was found to occur in a heterogenous population of patients with lung cancer regardless of smoking history or tumor histology. Of importance, this study also indicated disappointing outcomes in patients with tumors harboring NRG1 fusions when treated with currently available standards of care. Similar to other fusion-positive non-small-cell lung cancer, this study demonstrated that patients with NRG1 fusion–positive disease derive limited benefit from immunotherapy.
There is early evidence of antitumor activity with investigational therapies targeting HER3 in patients with NRG1 fusion–positive cancers as well as case reports suggestive of activity of afatinib, a pan-HER family tyrosine kinase inhibitor, in this setting. Therefore, testing and identifying NRG1 fusion–positive tumors are important for appropriate patient selection and treatment in a clinical trial setting.
Testing for NRG1 Gene Fusions
From a diagnostic perspective, the detection of NRG1 fusions may be challenging, depending on the assay and testing modality selected for use. Historically, screening methods for fusion genes have consisted of low throughput methods such as immunohistochemistry as a surrogate for molecular analysis and fluorescence in situ hybridization. Given the complexity of NRG1 fusions and with the growing number of other clinically relevant genetic alterations in solid tumors, the use of next-generation sequencing (NGS) methods is the preferred approach. This approach provides a broader and more comprehensive genomic profiling as a whole. Of note, there is a significant variation in the type and design of NGS assays, and not all NGS assays are capable of adequately detecting NRG1 fusions. Structurally, NRG1 fusions may involve a wide range of partners with highly variable breakpoint regions that tend to occur in large intronic areas. This places limitations on the ability to design comprehensive DNA-based molecular assays that can cover all intronic territories and partners of the fusion isoforms. By contrast, RNA-based assays can circumvent common difficulties associated with the coverage of extensive intronic regions. In the global eNRGy1 registry that was discussed above, only 26% of the patients were positive for NRG1 fusions by DNA-based NGS assays, whereas RNA-based testing identified 74% of patients with these fusions. In this context, there are still limitations when using standard targeted RNA-based approaches due to the heterogeneity of potential partners and breakpoint regions. So, to increase diagnostic sensitivity, some RNA-based assays may be designed with a dual approach that uses fusion specific primers that target the most prevalent fusions and partners, as well as the detection of expression imbalance of the 3' and 5’ ends as a surrogate for the presence of even rarer fusion events. This approach, however, should be used with caution with the recognition that some gene fusions may have a high expression of both the 3’ and 5’ ends and may go undetected using this testing modality. Alternatively, the use of anchored multiplex polymerase chain reaction assays prior to NGS is a highly valuable testing approach that enables the detection of transcript fusion events even when the partner gene is not targeted by the gene panel.
The timing for the testing of NRG1 gene fusions depends on the type of malignancy, disease stage, and the therapeutic options available for the patient. There are 2 general approaches that may be taken depending on the amount of tumor tissue available. First, a sequential approach may be taken if a large tumor sample is available with the common genetic alterations being queried for initially. If no actionable mutations are identified, then NGS-based testing for NRG1 fusions can be performed. However, this sequential approach takes longer and may use more tissue. The second approach would be to simultaneously identify all detectable genetic alterations present in the tumor using comprehensive NGS, and preferably incorporating both DNA-based and RNA-based modalities as discussed above. This latter approach would clearly be preferred for highly symptomatic patients with lung cancer, for example, when time is of the essence for the immediate disease management, and missing the opportunity to identify potentially targetable biomarkers is not a valuable option. Moreover, this is a more suitable approach for situations when only small tumor tissue samples are available, and where multiple sequential tests would lead to tissue exhaustion and incomplete assessments.
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