Calcified chondroid mesenchymal neoplasms with FN1-receptor tyrosine kinase gene fusions including MERTK, TEK, FGFR2, and FGFR1: a molecular and clinicopathologic analysis

Abstract

ABSTRACTTranslocations involving FN1 have been described in a variety of neoplasms, which share the presence of cartilage matrix and a variable extent of calcification. Fusions of FN1 to FGFR1 or FGFR2 have been reported in nine soft tissue chondromas, mostly demonstrated indirectly by FISH analysis. Delineation of FN1 fusions with various partner genes will facilitate our understanding of the pathogenesis and diagnostic classification of these neoplasms. In this study, we present molecular, clinical and pathologic features of 9 cartilaginous soft tissue neoplasms showing a predilection for the TMJ region and the extremities. We analyzed for gene fusions with precise breakpoints using targeted RNA-seq with a 115-gene panel, including FN1, FGFR1 and FGFR2. All 9 cases were positive for a gene fusion, including two novel fusions, FN1-MERTK and FN1-TEK, each in one case, recurrent FN1-FGFR2 in 5 cases, FN1-FGFR1 without the Ig3 domain in one case, and FGFR1-PLAG1 in one case. The breakpoints in the 5’ partner gene FN1 ranged from exons 11-48, retaining the domains of signal peptide, FN1, FN2, and/or FN3, while the 3’partner genes retained the trans-membrane domain, tyrosine kinase domains and /or Ig domain. The tumors with FN1-FGFR1, FN1-FGFR2 and FN1-MERTK fusions are generally characterized by nodular/lobular growth of polygonal to stellate cells within a chondroid matrix, often accompanied by various patterns of calcification. These features resemble those as described for the chondroblastoma-like variant of soft tissue chondroma. Additional histologic findings include calcium pyrophosphate dehydrate deposition and features resembling tenosynovial giant cell tumor. Overall, while the tumors from our series show significant morphologic overlap with chondroblastoma-like soft tissue chondroma, we describe novel findings that expand the morphologic spectrum of these neoplasms and have therefore labeled them as “calcified chondroid mesenchymal neoplasms.” These neoplasms represent a distinct pathologic entity given the presence of recurrent FN1-receptor tyrosine kinase fusions.