H3F3A mutation in giant cell tumour of the bone is detected by immunohistochemistry using a monoclonal antibody against the G34W mutated site of the histone H3.3 variant

Aims Giant cell tumour of the bone (GCTB) is a neoplasm predominantly of long bones characterized by the H3F3A mutation G34W. Conventional diagnosis is challenged by the tumour's giant cell‐rich morphology, which overlaps with other giant cell‐containing lesions of the bone. Recently, a monoclonal antibody specific for the H3F3A mutation has been generated. Our aim was to test this antibody on a cohort of giant cell‐containing lesions. Methods and results We used the antibody for analysis of 22 H3F3A‐mutated GCTB, including two patients with recurrences; for comparison we analysed a cohort of 36 H3F3A wild‐type giant cell‐rich lesions of the bone and soft tissue, containing one brown tumour, six aneurysmal bone cysts (ABC), six chondroblastomas, five non‐ossifying‐fibromas, two fibrous dysplasias, nine tenosynovial giant cell tumours, one giant cell‐rich sarcoma and six osteosarcomas. Furthermore, among the 22 mutated cases, we included one GCTB with two recurrences and lung metastases; the patient was treated with the anti‐receptor activator of nuclear factor κB (RANK) ligand denosumab. We show that all 22 H3F3A‐mutated GCTB display strong nuclear H3.3 G34W staining in the neoplastic component, while the osteoclastic giant cells are negative. 36 H3F3A wild‐type lesions are negative. The GCTB treated with denosumab revealed a reduction in the H3.3 G34W‐positive tumour cells and a decrease in osteoclastic giant cells accompanied by matrix and osteoid formation. Conclusions We conclude that positive H3.3 G34W staining is a specific and sensitive method for detection of H3F3A‐mutated GCTB. Denosumab treatment leads to a pathomorphosis of the lesion characterized by matrix and osteoid producing H3.3 G34W‐negative stromal cells.