Detection of GRM1 gene rearrangements in chondromyxoid fibroma: a comparison of fluorescence in-situ hybridisation, RNA sequencing and immunohistochemical analysis.

AIMS: Chondromyxoid fibroma (CMF) is a rare, benign bone tumour which arises primarily in young adults and is occasionally diagnostically challenging. Glutamate metabotropic receptor 1 (GRM1) gene encodes a metabotropic glutamate receptor and was recently shown to be up-regulated in chondromyxoid fibroma through gene fusion and promoter swapping. The aim of this study was to interrogate cases of CMF for the presence of GRM1 gene rearrangements, gene fusions and GRM1 protein overexpression. METHODS AND RESULTS: Selected cases were subjected to testing by fluorescent in-situ hybridisation (FISH) with a GRM1 break-apart probe, a targeted RNA sequencing method and immunohistochemical study with an antibody to GRM1 protein. Two cases were subjected to whole transcriptomic sequencing. In 13 of 13 cases, GRM1 protein overexpression was detected by immunohistochemistry using the GRM1 antibody. Of the 12 cases successfully tested by FISH, nine of 12 showed GRM1 rearrangements by break-apart probe assay. Targeted RNA sequencing analysis did not detect gene fusions in any of the eight cases tested, but there was an increase in GRM1 mRNA expression in all eight cases. Two cases subjected to whole transcriptomic sequencing (WTS) showed elevated GRM1 expression and no gene fusions. CONCLUSION: GRM1 gene rearrangements can be detected using FISH break-apart probes in approximately 75% of cases, and immunohistochemical detection of GRM1 protein over-expression is a sensitive diagnostic method. The gene fusion was not detected by targeted RNA sequencing, due most probably to the complexity of fusion mechanism, and is not yet a reliable method for confirming a diagnosis of CMF in the clinical setting.

NTRK-rearranged spindle cell neoplasm: Initial observation of imaging appearance and clinicopathologic correlation.

OBJECTIVE: To explore pre-treatment imaging findings of neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell neoplasm, an emerging group of molecularly defined soft tissue tumors and summarize the clinical course, including TRK inhibitor therapy response. MATERIALS AND METHODS: This retrospective study included 8 women and 4 men with NTRK-rearranged spindle cell neoplasm (median age, 35.5 years, range, 0-66). Available pre-treatment MRI, CT, PET, and US imaging were reviewed. Tumor histology and the patients' clinical course were reviewed. RESULTS: Primary tumors were located within the soft tissue, lungs, kidney, and breast with soft tissue being the most prevalent site (n = 6). Pre-treatment MRI (n = 4) revealed linear hypointense signal foci and contrast enhancement in all patients with hemorrhage in half of the tumors. A tail sign (n = 1) and fluid levels (n = 1) were less frequent. Ultrasound showed well-marginated hypoechoic masses with internal flow. Primary tumors were all non-calcified on CT (4/4). Metastases were FDG-avid (4/4). Among the 8 patients who developed metastasis, 7 developed pulmonary metastases. All four patients who received NTRK inhibitor therapy showed an initial decrease in tumor size or FDG uptake. CONCLUSION: NTRK-rearranged neoplasms may occur as enhancing masses with linear hypointense signal foci on MRI and FDG avid metastases on PET. Pulmonary metastases were frequent in our study. Initial treatment response is observed in most patients.

GRM1-Rearranged Chondromyxoid Fibroma With FGF23 Expression: A Potential Pitfall in Small Biopsies.

The clinical, radiological, and histopathological features of chondromyxoid fibroma can sometimes resemble those of other benign or malignant tumors. Recently, recurrent GRM1 rearrangements have been identified in chondromyxoid fibroma, and GRM1 positivity by immunohistochemistry has emerged as a dependable surrogate marker for this molecular alteration. Phosphaturic mesenchymal tumor is a rare tumor that often exhibits overexpression of fibroblastic growth factor 23 (FGF23) through various mechanisms. In this report, we present a case of GRM1-rearranged chondromyxoid fibroma that also exhibited FGF23 expression via in situ hybridization, posing significant diagnostic challenges during workup of the initial core biopsy. We hope that this case can serve as an educational resource, shedding light on a rare diagnostic pitfall.

Modeling Extraordinary Response Through Targeting Secondary Alterations in Fusion-Associated Sarcoma.

PURPOSE: Targeted therapy in translocation-associated sarcomas has been limited to oncogenic activation of tyrosine kinases or ligands while gene fusions resulting in aberrant expression of transcription factors have been notoriously difficult to target. Moreover, secondary genetic alterations in sarcomas driven by translocations are uncommon, comprising mostly alterations in tumor suppressor genes (TP53, CDKN2A/B). Our study was triggered by an index patient showing a dramatic clinical response by targeting the secondary BRAF V600E mutation in a metastatic angiomatoid fibrous histiocytoma (AFH) harboring the typical EWSR1::CREB1 fusion. MATERIALS AND METHODS: The patient, a 28-year-old female, was diagnosed with an AFH of the thigh and followed a highly aggressive clinical course, with rapid multifocal local recurrence within a year and widespread distant metastases (adrenal, bone, liver, lung). The tumor showed characteristic morphologic features, with histiocytoid cells intermixed with hemorrhagic cystic spaces and lymphoid aggregates. In addition to the pathognomonic EWSR1::CREB1 fusion, targeted DNA sequencing revealed in both primary and adrenal metastatic sites a hot spot BRAF V600E mutation and a CDKN2A/B deletion. Accordingly, the patient was treated with a BRAF-MEK inhibitor combination (encorafenib/binimetinib) showing an excellent but short-lived response. RESULTS: Using a CRISPR-Cas9 approach, we introduced the BRAF c.1799 T>A point mutation in human embryonic stem (hES) cells harboring a conditional EWSR1 (exon7)::CREB1 (exon7) translocation and further differentiated to mesenchymal progenitors (hES-MP) before fusion expression. The cells maintained the fusion transcript expression and the AFH core gene signature while responding to treatment with encorafenib and binimetinib. CONCLUSION: These results highlight that additional targeted DNA NGS in chemotherapy-resistant translocation-associated sarcomas may reveal actionable oncogenic drivers occurring as secondary genetic events during disease progression.