Mesenchymal Tumors with RREB1‐MRTFB Fusion Involving the Mediastinum: Extra‐Glossal Ectomesenchymal Chondromyxoid Tumors?

Mesenchymal Tumors with RREB1‐MRTFB Fusion Involving the Mediastinum: Extra‐Glossal Ectomesenchymal Chondromyxoid Tumors?:

Abstract

Aims

Ectomesenchymal chondromyxoid tumor (ECT) is a rare benign intraoral tumor, which almost exclusively presents as a small mass of the anterior dorsal tongue. Recently, the RREB1‐MRTFB (previously known as MKL2) fusion gene has been identified in 90% of ECTs, all located in the tongue, emphasizing its genetic distinctiveness. Here, we report two mesenchymal tumors involving the superior mediastinum of adult women with RREB1‐MRTFB fusions.

Methods and results

Both tumors presented as well‐circumscribed paravertebral masses that were clinically suspected to be schwannoma. After fragmented resection, recurrence was not observed at 27 and 18 months. Although tumors were originally unclassifiable, next generation sequencing detected identical RREB1 (exon 8)‐MRTFB (exon 11) fusion transcripts, which were validated by reverse transcriptase‐polymerase chain reaction, Sanger sequencing, and fluorescence in situ hybridization. Both tumors shared hyalinized areas with round cells embedded in a cord or reticular manner. The tumor cells showed mild nuclear atypia of possible degenerative type with very low mitotic activity, and were at least focally positive for S100, glial fibrillary acidic protein, smooth muscle actin, and epithelial membrane antigen. Overall, these findings suggest that they may represent previously undescribed extra‐glossal ECT involving the mediastinum. However, the histology was not classic for ECT, because that in case 2 was predominated by storiform growth of spindle cells, whereas the tumor in case 1 lacked myxoid change.

Conclusions

We have provided the first evidence that RREB1‐MRTFB fusion is not limited to tumors in the head region, and whether such tumors represent extra‐glossal ECTs requires further research.