Histone 3.3 mutations in giant cell tumor and giant cell-rich sarcomas of bone

Mutually exclusive histone 3.3 gene mutations have been recognized in chondroblastoma and giant cell tumor of bone (GCTB), which may be useful for differential diagnostic purposes in morphologically ambiguous cases. Although more than 90% of GCTBs present histone 3.3 variants exclusively in the H3F3A gene, chondroblastoma is mutated mainly in H3F3B. In this study, we examined a series of giant cell-rich primary bone tumors, aiming to evaluate the possible diagnostic role of histone 3.3 mutations in the differential diagnosis between GCTB and giant cell-rich sarcomas. Sixteen cases of nonmetastatic GCTB, 9 GCTBs with lung metastases, and 35 giant cell-rich sarcomas were selected from our institutional archives. Eight chondroblastomas were used as controls. Direct sequencing for the presence of H3F3A and H3F3B variants in coding region between codons 1 and 42, including the hotspot codons (28, 35, and 37), was performed on DNA extracted from formalin-fixed, paraffin-embedded tissue using conventional polymerase chain reaction and fast coamplification at lower denaturation temperature-polymerase chain reaction. Overall, 24 GCTBs (96%) presented a mutation in the H3F3A gene (15 of 16 nonmetastatic and 9 of 9 metastatic). Five sarcomas harbored an H3F3A mutation (3 p.G35W, 1 p.G35L, and 1 p.G35E), and these were all secondary malignant GCTBs. In conclusion, we confirm that H3F3A mutational testing may be a useful adjunct to differentiate GCTB from giant cell-rich sarcomas. Although the presence of H3F3A mutations does not exclude with certainty a diagnosis of sarcoma, the possibility of a malignant evolution of GCTB should also be considered.