Small cell osteosarcoma (SCOS), a variant of conventional high-grade osteosarcoma (COS), may mimic fusion-driven round cell sarcomas (FDRCS) by overlapping clinico-radiological and... Show moreSmall cell osteosarcoma (SCOS), a variant of conventional high-grade osteosarcoma (COS), may mimic fusion-driven round cell sarcomas (FDRCS) by overlapping clinico-radiological and histomorphological/immunohistochemical characteristics, hampering accurate diagnosis and consequently proper therapy. We retrospectively analyzed decalcifed formalin-fxed parafn-embedded (FFPE) samples of 18 bone tumors primarily diagnosed as SCOS by methylation profling, fusion gene analysis, and immunohistochemistry. In eight cases, the diagnosis of SCOS was maintained, and in 10 cases it was changed into FDRCS, including three Ewing sarcomas (EWSR1::FLI1 in two cases and no identifed fusion gene in the third case), two sarcomas with BCOR alterations (KMT2D::BCOR, CCNB3::BCOR, respectively), three mesenchymal chondrosarcomas (HEY1::NCOA2 in two cases and one case with insufcient RNA quality), and two sclerosing epithelioid fbrosarcomas (FUS::CREBL3 and EWSR1 rearrangement, respectively). Histologically, SCOS usually possessed more pleomorphic cells in contrast to the FDRCS showing mainly monomorphic cellular features. However, osteoid was seen in the latter tumors as well, often associated with slight pleomorphism. Also, the immunohistochemical profle (CD99, SATB2, and BCOR) overlapped. Clinically and radiologically, similarities between SCOS and FDRCS were observed, with by imaging only minimal presence or lack of (mineralized) osteoid in most of the SCOSs. In conclusion, discrimination of SCOS, epigenetically related to COS, versus FDRCS of bone can be challenging but is important due to diferent biology and therefore therapeutic strategies. Methylation profling is a reliable and robust diagnostic test especially on decalcifed FFPE material. Subsequent fusion gene analysis and/or use of specifc immunohistochemical surrogate markers can be used to substantiate the diagnosis. Show less
Saba, K.H.; Cornmark, L.; Hofvander, J.; Magnusson, L.; Nilsson, J.; Bos, H. van den; ... ; Nord, K.H. 2020
Osteoblastoma is a locally aggressive tumour of bone. Until recently, its underlying genetic features were largely unknown. During the past two years, reports have demonstrated that acquired... Show moreOsteoblastoma is a locally aggressive tumour of bone. Until recently, its underlying genetic features were largely unknown. During the past two years, reports have demonstrated that acquired structural variations affect the transcription factorFOSin a high proportion of cases. These rearrangements modify the terminal exon of the gene and are believed to stabilise both theFOStranscript and the encoded protein, resulting in high expression levels. Here, we applied in-depth genetic analyses to a series of 29 osteoblastomas, including five classified as epithelioid osteoblastoma. We found recurrent homozygous deletions of theNF2gene in three of the five epithelioid cases and in one conventional osteoblastoma. These events were mutually exclusive fromFOSmutations. Structural variations were determined by deep whole genome sequencing and the number ofFOS-rearranged cases was less than previously reported (10/23, 43%). One conventional osteoblastoma displayed a novel mechanism of FOS upregulation; bringing the entireFOSgene under the control of theWNT5Aenhancer that is itself activated by FOS. Taken together, we show thatNF2loss characterises a subgroup of osteoblastomas, distinct fromFOS-rearranged cases. BothNF2andFOSare involved in regulating bone homeostasis, thereby providing a mechanistic link to the excessive bone growth of osteoblastoma. Show less
