Giant Cell Tumor of Bone Without Giant Cells with a Long Clinical Course: A Case Report

Giant cell tumor of bone (GCTB) consists of a mixture of neoplastic mononuclear cells and non-neoplastic cells, including polynuclear giant cells. Recently, with the spread of the immunohistochemical staining marker H3.3 G34W corresponding to specific genetic abnormalities, the histological diversity of GCTB has been recognized. GCTB without giant cells is uncommon, although it has also been reported previously. Herein, we describe a 45-year-old man with GCTB without giant cells who was successfully diagnosed using H3.3 G34W immunohistochemistry. Other unusual findings in GCTB that were identified in this patient include bone and osteoid formation with a long clinical course of 13 years. We also compared the histological findings of the current patient to those who received denosumab therapy.

The H3.3 G34W oncohistone mutation increases K36 methylation by the protein lysine methyltransferase NSD1

The H3.3 G34W mutation has been observed in 90% of the patients affected by giant cell tumor of bone (GCTB). It had been shown to reduce the activity of the SETD2 H3K36 protein lysine methyltransferase (PKMT) and lead to genome wide changes in epigenome modifications including a global reduction in DNA methylation. Here, we investigated the effect of the H3.3 G34W mutation on the activity of the H3K36me2 methyltransferase NSD1, because NSD1 is known to play an important role in the differentiation of chondrocytes and osteoblasts. Unexpectedly, we observed that H3.3 G34W has a gain-of-function effect and it stimulates K36 methylation by NSD1 by about 2.3-fold with peptide substrates and 6.3-fold with recombinant nucleosomal substrates. This effect is specific for NSD1, as NSD2 and SETD2 show only a very mild stimulation and even reduced activity on G34W substrates. The potential downstream effects of the G34W induced hyperactivity of NSD1 on DNA methylation, H3K27me3, histone acetylation and splicing are discussed.

[Expression of H3.3 G34W mutant-specific antibody in giant cell tumors of bone and its diagnostic value]

Objective: To investigate the expression of H3.3 G34W mutant-specific antibody in giant cell tumors of bone (GCTB), and its value in the diagnosis of GCTB. Methods: Immunohistochemical (IHC) EnVision method was used to detect the expression of H3.3 G34W mutant-specific antibody and p63 in 83 GCTBs, 18 aneurysmal bone cysts, 23 chondroblastomas and 28 osteosarcomas diagnosed at Nanjing Jinling Hospital from June 2001 to April 2019. Results: Among the 83 cases of GCTB, 69 cases (69/83, 83.1%) expressed H3.3 G34W. H3.3 G34W expression was found exclusively in the mononuclear cell population with strong and diffuse nuclear staining. H3.3 G34W was expressed in 55 of 57 (96.5%) cases of GCTB in long bones, but only 14 of 26 (53.8%) cases of non-long bone GCTB. All recurrent (9/9)/metastatic GCTB (2/2), post-denosumab GCTB (3/3), primary malignant GCTB (3/3) and secondary malignant GCTB (5/5) also expressed H3.3 G34W. H3.3 G34W was negative in all aneurysmal bone cysts and chondroblastomas. H3.3 G34W was positive in 3 of 28(10.7%) cases of osteosarcomas, and giant cell-rich osteosarcoma(GCRO) was the only histological subtype of osteosarcoma that expressed H3.3 G34W. p63 was expressed in 71.1%(59/83) of GCTB, while the positive rates of p63 in aneurysmal bone cysts,chondroblastomas and osteosarcomas were 3/18, 43.5% (10/23) and 21.4% (6/28) respectively. The sensitivity and specificity of H3.3 G34W mutant-specific antibody in the diagnosis of GCTB were 83.1% and 95.7%. Conclusions: H3.3 G34W mutant-specific antibody is a highly sensitive and specific marker for GCTB and helpful for the diagnosis of GCTB and its variants. The limitation of this antibody is that as a mall number of GCTB harbor G34 mutation other than G34W, and thus that cannot be detected. The incidental expression of H3.3 G34W mutant protein in osteosarcoma could be a potential diagnostic dilemma, and the results of H3.3 G34W IHC staining needs careful interpretation.