Solitary fibrous tumor (SFT) is characterized by the inv12(q13q13)-derived NAB2-STAT6 fusion, which exhibits variable breakpoints and drives STAT6 nuclear expression. The implications of NAB2-STAT6 fusion variants in pathological features and clinical behavior remain to be characterized in a large cohort of SFTs. We investigated the clinicopathological correlates of this genetic hallmark and analyzed STAT6 immunoexpression in 28 intrathoracic, 37 extrathoracic, and 23 meningeal SFTs. These 88 tumors were designated as histologically nonmalignant in 75 cases and malignant in 13, including 1 dedifferentiated SFT. Eighty cases had formalin-fixed and/or fresh samples to extract assessable RNAs for RT-PCR assay, which revealed NAB2-STAT6 fusion variants comprising 12 types of junction breakpoints in 73 fusion-positive cases, with 65 (89%) falling into 3 major types. The predominant NAB2ex4-STAT6ex2 (n = 33) showed constant breakpoints at the ends of involved exons, whereas the NAB2ex6-STAT6ex16 (n = 16) and NAB2ex6-STAT6ex17 (n = 16) might exhibit variable breakpoints and incorporate NAB2 or STAT6 intronic sequence. Including 73 fusion-positive and 7 CD34-negative SFTs, STAT6 distinctively labeled 87 (99%) SFTs in nuclei, exhibited diffuse reactivity in 73, but did not decorate 98 mimics tested. In seven fusion-negative cases, 6 were STAT6-positive, suggesting rare fusion variants not covered by RT-PCR assay. Regardless of histological subtypes, intrathoracic SFTs affected older patients (P = 0.035) and tended to be larger in size (P = 0.073). Compared with other variants, NAB2ex4-STAT6ex2/4 fusions were significantly predominant in the SFTs characterised by intrathoracic location (Po 0.001), older age (P = 0.005), decreased mitoses (P = 0.0028), and multifocal or diffuse STAT6 staining (P = 0.013), but not found to correlate with disease-free survival. Conclusively, STAT6 nuclear expression was distinctive in the vast majority of SFTs, including all fusion-positive tumors, and exploitable as a robust diagnostics of CD34-negative cases. Despite the associations of NAB2-STAT6 fusion variants with several clincopathological factors, their prognostic relevance should be further validated in large-scale prospective studies of SFTs.

Sclerosing and spindle cell rhabdomyosarcoma (SRMS) have been recently re-classified as a stand-alone pathologic entity, separate from embryonal RMS (ERMS). Genetically a subset of the congenital cases display NCOA2 gene rearrangements, while tumors occurring in older children or adults harbor MYOD1 gene mutations with or without coexisting PIK3CA mutations. Despite these recent advances, a significant number of tumors lack known genetic alterations. In this study we sought to investigate a large group of pediatric spindle cell/sclerosing RMS, spanning a diverse clinical and pathologic spectrum, by using a combined FISH, targeted DNA and whole transcriptome sequencing methodology for a more definitive molecular classification. A total of 26 spindle and sclerosing RMS cases were selected from the two participating Institutions for the molecular analysis. Ten of the 11 congenital/infantile SRMS showed recurrent fusion genes: with novel VGLL2 rearrangements seen in 7 (63%), including VGLL2-CITED2 fusion in 4 and VGLL2-NCOA2 in 2 cases. Three (27%) cases harbored the previously described NCOA2 gene fusions, including TEAD1-NCOA2 in 2, and SRF-NCOA2 in 1. All fusion positive congenital/infantile SRMS patients with available long term follow-up were alive and well, none developing distant metastases. Among the remaining 15 SRMS patients older than age of one, 10 (67%) showed MYOD1 L122R mutations, most of them following a fatal outcome despite an aggressive multi-modality treatment. All 4 cases harboring co-existing MYOD1/PIK3CA mutations shared sclerosing morphology. All 5 fusion/mutation-negative SRMS cases presented as intra-abdominal or para-testicular lesions.

Nuclear STAT6 immunoexpression is sensitive and specific for distinguishing SFT from mimics. However, considerable heterogeneity exists in the head and neck SFTs regarding the locations, histological patterns, and NAB2-STAT6 fusion variants.