PO-344 Osteoblastoma is characterised by recurrent rearrangements of FOS and FOSB

IntroductionOsteoblastoma, and the related entity osteoid osteoma, are the most common benign bone-forming tumours. Large, inaccessible, and recurrent tumours can cause considerable morbidity. On occasion, there can be diagnostic uncertainty with osteosarcoma, a malignant tumour that requires multi-modal therapy. We sought to define the somatic changes that underpin osteoblastoma.Material and methodsWe analysed the whole genomes of 5 osteoblastomas and 1 osteoid osteoma and catalogued all somatic variants. RNA-seq was used to corroborate DNA changes and call gene fusions. FOS fusions were validated with Sanger sequencing. We used FISH and IHC to validate the finding of FOS/FOSB rearrangements in 55 osteoblastomas, 17 angiosarcomas and 183 osteosarcomas. We analysed 55 osteosarcoma and 2652 pan-cancer whole genomes for similar rearrangements.Results and discussionsThere was a paucity of somatic alterations in osteoblastoma, with a median mutation burden of 319 substitutions (range 123–700) and 28 indels (range 14–50) per genome. Copy number analyses demonstrated diploid tumours with few aberrations. Only a small number of mutations affected the coding sequence of genes, none of which were plausible driver events.Analysis of structural variants revealed breakpoints in the AP-1 transcription factor FOS, in 5/6 cases, and its paralogue FOSB in the sixth case. All were validated with RNA-seq reads and FOS fusions were validated with Sanger sequencing.FOSB fusion brought expression under the control of the PPP1R10 promotor. FOS fusions were all between exon 4 and intronic or intergenic regions. FOS fusions resulted in the introduction of a stop codon within 30 bp of the breakpoint.In a validation cohort of 55 tumours, FISH identified FOSB or FOS breakapart signals in 1 and 48 tumours respectively (89%). Osteoblastoma cellularity is low, hampering FISH sensitivity. IHC for the preserved N-terminus of FOS revealed marked immunoreactivity in all FOS rearranged cases, including the 3/6 FOS FISH negative cases with available material. Only 1/183 osteosarcoma cases had comparable FOS immunoreactivity. No osteosarcoma or pan-cancer whole genome harboured similar rearrangements.ConclusionFOS or FOSB rearrangements define osteoblastoma and osteoid osteoma. The truncated FOS transcript and protein bears a striking resemblance to the retroviral oncogenic form of v-fos,known to induce osteosarcoma in mice. This is the first report of FOS alteration in a bone-forming tumour.