Novel PAX3-NCOA1 Fusions in Biphenotypic Sinonasal Sarcoma With Focal Rhabdomyoblastic Differentiation

Abstract

Sarcomas arising in the sinonasal region are uncommon and encompass a wide variety of tumor types, including the newly described biphenotypic sinonasal sarcoma (BSNS), which is characterized by a monomorphic spindle cell proliferation with dual neural and myogenic phenotypes. Most BSNSs harbor a pathognomonic PAX3-MAML3 fusion driven by t(2;4)(q35;q31.1), whereas the alternative fusion partner gene remains unidentified in a subset of PAX3-rearranged cases. As NCOA1 on 2p23 is a known partner in PAX3-related fusions in other tumor types (ie, alveolar rhabdomyosarcoma), we investigated its status by fluorescence in situ hybridization (FISH) and reverse transcription polymerase chain reaction assays in 2 BSNS cases showing only PAX3 gene rearrangements. Novel PAX3-NCOA1 fusions were identified in these 2 index cases showing an inv(2)(q35p23) by FISH and confirmed by reverse transcription polymerase chain reaction. Five additional BSNS cases with typical morphology were studied by FISH, revealing a PAX3-MAML3 fusion in 4 cases and only PAX3 rearrangement in the remaining case without abnormalities in MAML3 or NCOA1 gene. Except for 1 case with surface ulceration, all other tumors lacked increased mitotic activity or necrosis, and all cases immunohistochemically coexpressed S100 protein and actin, but lacked SOX10 reactivity. Interestingly, the 2 PAX3-NCOA1-positive cases showed desmin reactivity and displayed a small component of rhabdomyoblastic cells, which were not seen in the more common PAX3-MAML3 fusion cases. In conclusion, we report a novel PAX3-NCOA1 fusion in BSNS, which appears to be associated with focal rhabdomyoblastic differentiation and should be distinguished from PAX3-NCOA1-positive alveolar rhabdomyosarcoma or malignant Triton tumor. SOX10 immunohistochemistry is a useful marker in distinguishing BSNS from peripheral nerve sheath tumors.

Figure 1. Pathologic features of PAX3-NCOA1-related biphenotypic sinonasal sarcoma

Case #1 displays interlacing fascicles of monomorphic spindle cells (A), with a focal component of epithelioid cells, displaying rhabdomyoblastic-like features (B). Tumor cells are diffusely positive for S100 protein (C) and show patchy desmin expression (D). Desmin immunostain highlights the epithelioid cell component (D, inset). Case #2 exhibits a cellular spindle cell proliferation with interspersed thick collagen fibers (E), focal round cell features (F) and scattered well-differentiated rhabdomyoblasts displaying cytoplasmic cross-striations (G) in a loose-textured stroma. MyoD1 (H) and myogenin (I) positivity was observed in the rhabdomyoblastic elements.

Figure 2. Novel PAX3-NCOA1 fusion in BSNS

Schematic diagram of inv(2)(p23q35) and the corresponding FISH probes applied, flanking the PAX3 and NCOA1 genes (arrows indicate the direction of transcription)(A). FISH fusion assay showed the PAX3 (red) and NCOA1 (green) signals joined together (B), as well as the individual break-apart assays showing PAX3 (C, left) and NCOA1 (C, right) rearrangements in case #1 (red, centromeric; green, telomeric). RT-PCR chromatogram of case #2 demonstrates exon 7 of PAX3 gene joined to exon 14 of NCOA1 gene (D), resulting in a PAX3 ex1-7 - NCOA1 ex14-23 chimeric transcript (E). The predicted fusion protein retained the DNA-binding domain (DBD), including PD and HD of PAX3, and the transcriptional activation domain (TAD) of NCOA1 (F). PD, paired domain; HD, homeodomain; bHLH/PAS, basic helix-loop-helix-Per/Ah receptor nuclear translocation/Sim motif; NRID, nuclear receptor interacting domain.

Figure 3. PAX3-MAML3 fusion positive BSNS

All three cases displayed similar histology with sweeping or intersecting bundles (A) of monotonous and mitotically inert spindle cells (B) as exemplified by case #3. Case #5 showed respiratory type glandular hyperplasia and invaginations, mimicking inverted papilloma (C), and had scattered mast cells and rare pleomorphic hyperchromatic cells (D). The canonical PAX3-MAML3 fusion is illustrated by the break-apart signals of PAX3 (E, left) and MAML3 (E, right) by FISH in case #3 (red, centromeric; green, telomeric). The RT-PCR chromatogram of case #4 confirmed the classic fusion variant of PAX3 exon 7 to MAML3 exon 2 (F).