RREB1-MKL2 fusion in a spindle cell sinonasal sarcoma: biphenotypic sinonasal sarcoma or ectomesenchymal chondromyxoid tumor in an unusual site?

Abstract

Biphenotypic sinonasal sarcoma (BSNS) is a rare, low grade spindle cell sarcoma, recently recognized in the WHO classification of head and neck tumors, which is characterized by a dual myogenic and neural differentiation and recurrent gene fusions, often involving PAX3-MAML3, and less commonly PAX3 fusions with other partners such as NCOA1, NCOA2, or WWTR1. Yet, in about 4% of tumors no gene rearrangements are identified. Herein, we describe a RREB1-MKL2 fusion in a BSNS lesion occurring in a 73-year-old female patient with a right maxillo-ethmoidal angle lesion. The polypoid, moderately cellular tumor with infiltrative submucosal growth was composed of fascicles of relatively bland spindle cells embedded in a loose collagenous matrix. The tumor cells showed moderate amounts of eosinophilic cytoplasm with indistinct borders and uniform, pale, ovoid to slender nuclei. The slowly proliferating neoplastic cells co-expressed smooth muscle actin and S100, and showed focal nuclear positivity for ß-catenin, while lacking staining for cytokeratins, desmin, myogenin, caldesmon, glial fibrillary acid protein, and SOX-10. Molecular analysis by targeted RNA-based next-generation sequencing identified an in-frame fusion between exon 8 of RREB1 and exon 11 of MKL2, a genetic event that was reported to be a molecular hallmark of ectomesenchymal chondromyxoid tumor. Gene rearrangements in both genes were independently verified by fluorescence in situ hybridization (FISH). To evaluate its recurrent potential an additional group of 15 fusion negative BSNS were tested for abnormalities in RREB1 and MKL2 genes by FISH, but no additional positive cases were identified.

Keywords: MKL2; RREB1; biphenotypic sinonasal sarcoma; gene fusion.

FIGURE 1

Computed tomographic (CT), histopathological, and immunohistochemical findings. A, Axial CT and B, coronal CT shows a unilateral, rather sharply demarcated polypoid mass located within the right maxillo-ethmoidal angle and the nasal cavity; the base of the lesion was in the posterior third of the middle right turbinate. C, Polypoid tumor with infiltrative submucosal growth and slight reactive hyperplasia of surface respiratory epithelium. D, Moderately dense intersecting fascicles of monomorphic spindle cells intermingled by a loose network of collagen fibers. E, The tumor cells show moderate amounts of pale eosinophilic cytoplasm with indistinct borders and uniform, ovoid to slender nuclei. F, Diffuse expression for S100 and G, α-smooth muscle actin, and H, multifocal positivity for CD34-positive. I, H3K27-me3 expression is retained. J, In contrast to slightly hyperplastic surface respiratory epithelium, the neoplastic cells are cytokeratin (AE1/AE3)-negative. K, The tumor is desmin-negative throughout. L, The Ki67 labeling index accounts for less than 5%

FIGURE 2

Schematic representation of the RREB1 (green)–MKL2 (blue) fusion gene detected in our patient. Given are the positions on chromosome 6 (RREB1) and 16 (MKL2) as well as the break point (BP) of the gene fusion in the exon-intron sequence of the mRNA of the respective genes. In contrast to the case described by Siegfried et al, the break point is located intronically and the exon boundaries in both genes (RREB1 exon 8 and MKL2 exon 11) remain intact. The consensus sequence of the NGS is shown underneath the hg38 human genome sequence for both genes. Underneath, the coverage of the gene fusion and representative NGS reads are depicted

FIGURE 3

Fluorescence in situ hybridization (FISH) in the index biphenotypic sinonasal sarcoma (BSNS) case showing break-apart signals in keeping with gene rearrangements in MKL2 (A) and RREB1 (B) (both images: red, centromeric; green, telomeric)