Histones and their practical application in bone tumors: Do I always need them?

Historically, the diagnosis of giant cell-rich neoplasms arising in bone has been challenging owing to overlapping clinical and radiographic findings resulting in the difficult separation of several neoplasms, particularly when biopsy material is limited. However, with the discovery of the driver histone mutations in giant cell tumor of bone (GCTB) and chondroblastoma, as well as USP6 rearrangements in aneurysmal bone cyst, pathologists now have objective ancillary tools to aid in the separation of several histologically similar giant cell-rich neoplasms. Furthermore, the recognition of histone mutations has allowed pathologists to revisit several entities, such as "malignant chondroblastoma," and furthered our understanding of phenomena such as "aneurysmal bone cyst-like change," formerly recognized as "secondary aneurysmal bone cyst." Herein, the evolution of testing for histone mutations in bone tumors is considered; the sensitivity and specificity of the histone antibodies is reviewed; and a practical guide for the use of these ancillary tests is offered.

DNA methylation profile discriminates sporadic giant cell granulomas of the jaws and cherubism from their giant cell-rich histological mimics.

Sporadic giant cell granulomas (GCGs) of the jaws and cherubism-associated giant cell lesions share histopathological features and microscopic diagnosis alone can be challenging. Additionally, GCG can morphologically closely resemble other giant cell-rich lesions, including non-ossifying fibroma (NOF), aneurysmal bone cyst (ABC), giant cell tumour of bone (GCTB), and chondroblastoma. The epigenetic basis of these giant cell-rich tumours is unclear and DNA methylation profiling has been shown to be clinically useful for the diagnosis of other tumour types. Therefore, we aimed to assess the DNA methylation profile of central and peripheral sporadic GCG and cherubism to test whether DNA methylation patterns can help to distinguish them. Additionally, we compared the DNA methylation profile of these lesions with those of other giant cell-rich mimics to investigate if the microscopic similarities extend to the epigenetic level. DNA methylation analysis was performed for central (n = 10) and peripheral (n = 10) GCG, cherubism (n = 6), NOF (n = 10), ABC (n = 16), GCTB (n = 9), and chondroblastoma (n = 10) using the Infinium Human Methylation EPIC Chip. Central and peripheral sporadic GCG and cherubism share a related DNA methylation pattern, with those of peripheral GCG and cherubism appearing slightly distinct, while central GCG shows overlap with both of the former. NOF, ABC, GCTB, and chondroblastoma, on the other hand, have distinct methylation patterns. The global and enhancer-associated CpG DNA methylation values showed a similar distribution pattern among central and peripheral GCG and cherubism, with cherubism showing the lowest and peripheral GCG having the highest median values. By contrast, promoter regions showed a different methylation distribution pattern, with cherubism showing the highest median values. In conclusion, DNA methylation profiling is currently not capable of clearly distinguishing sporadic and cherubism-associated giant cell lesions. Conversely, it could discriminate sporadic GCG of the jaws from their giant cell-rich mimics (NOF, ABC, GCTB, and chondroblastoma).

Chondroblastoma: clinicopathological analyses of 307 cases from a single institution in China and the diagnostic value of the H3F3 K36M mutant antibody.

AIMS: To elucidate the clinicopathological features and the diagnostic value of mutation specific antibody H3F3 K36M of chondroblastoma (CB) in China. METHODS: Clinicopathological profiles were retrieved, and immunohistochemistry was performed on 185 CB specimens and the control group. RESULTS: Our series included 307 patients with a mean age of 22.1 years. Long tubular bones (63.8%, 196/307) were most commonly involved, followed by short bones of the hands and feet (22.1%, 68/307), sesamoid bones (8.1%, 25/307), flat bones and irregular bones (5.9%, 18/307). The most commonly involved site was the proximal femur, followed by distal femur, proximal humerus and calcaneus. The average age in the long bones group (20.3 years) was significantly younger than the short bones group (24.9 years) (p<0.001), sesamoid bones group (24.4 years) (p=0.02) and flat bones and irregular bones group (29.1 years) (p<0.001). Microscopically, aneurysmal bone cyst-like change (63.6%, 117/184), necrosis (43.5%, 80/184) and chicken-wire calcification (26.1%, 48/184) were variably noted. In rare cases, cortical destruction, soft tissue and lymphovascular invasion were identified. Positive immunoreaction with H3F3 K36M was examined in all non-decalcified, all EDTA decalcified, 87.1% hydrochloric acid (HCl) decalcified CB samples and the high-grade sarcoma secondary to CB, but not the control group. CONCLUSIONS: CB usually involves the long tubular bones in younger age group. H3F3 K36M can identify K36M mutation with 100% specificity and 100% sensitivity in non-decalcified and EDTA decalcified samples, more than 80% sensitivity in HCl decalcified samples. Virtually, all CBs harbour an H3K36M mutation.

Distal Femoral Non-Epiphyseal Cortical Chondroblastoma Confirmed with H3F3B p. Lys36Met Mutation.

Background: Chondroblastoma is a primary bone tumor typically arising from the intramedullary space of the epiphysis or epimetaphysis. A non-epiphyseal chondroblastoma is uncommon. Case report: An 11-year-old girl presented with an eccentric cortical osteolytic lesion in the distal femur metaphysis. The typical morphology, diffuse H3.3 K36M immunohistochemical expression and H3F3B point mutation (c. 110A > T) unequivocally supported the diagnosis of chondroblastoma. Discussion: We described a non-epiphyseal cortical-based chondroblastoma involving the distal femur harboring the typical H3F3B mutation. Non-epiphyseal chondroblastoma may harbor the H3F3B mutation.

Chondroblastoma-like osteosarcoma: a clinicopathological and molecular study of a rare osteosarcoma variant.

OBJECTIVE: Chondroblastoma-like osteosarcoma (CBLOS) is a rare and poorly understood variant of OS. We examined the clinicopathological, immunohistochemical and molecular features of six CBLOSs to highlight the differences with conventional high-grade OS (CHGOS) and CB, including CB with aggressive features. METHODS: We performed histone 3.3 mutation analysis by gene sequencing and/or immunohistochemistry in all cases, while whole exome sequencing (WES) was performed on two CB-like osteosarcomas and 11 conventional high-grade OS. RESULTS: CBLOSs were predominantly localised at acral sites and involved mainly male subjects with a mean age of 29 years. One patient who had metastases at presentation died of disease, while another patient who developed multiple local recurrences and lung metastases was alive with no evidence of disease (ANED) at 294 months. The remaining patients were ANED after a mean interval of 70.8 months. Histologically, all CBLOS presented aggressive features, including nuclear atypia and infiltrative growth. Immunohistochemistry with H3F3 K36M mutant antibody was negative in all CBLOSs, and none of the five tumours tested by gene sequencing had H3F3B mutations. Conversely, all CBs presented the H3F3B K36M variant and were positive for immunostaining with the H3F3 K36M antibody. Two CBLOSs analysed by WES differed in amount and type of mutation from 11 cases of CHGOS. Moreover, CBLOSs showed lower copy number alteration (CNA) score values than CHGOSs. CONCLUSIONS: CBLOS presents a different genetic background and a less aggressive clinical behaviour in comparison with CHGOS. Search of the H3F3B K36M mutation is useful in the differential diagnosis with CB.

Epigenetic lockdown of CDKN1A (p21) and CDKN2A (p16) characterises the neoplastic spindle cell component of giant cell tumours of bone.

Giant cell tumour of bone (GCTB) comprises the eponymous osteoclastic multinucleated giant cells eliciting bone lysis, an H3F3A-mutated neoplastic mononucleated fibroblast-like cell population, and H3F3A wild-type mononucleated stromal cells. In this study, we characterised four new cell lines from GCTB. Furthermore, we compared the genome-wide DNA methylation profile of 13 such tumours and three further cell lines with giant cell-rich lesions comprising three H3F3B-mutated chondroblastomas, three USP6-rearranged aneurysmal bone cysts, three non-ossifying fibromas, two hyperparathyroidism-associated brown tumours as well as mesenchymal stem cells, osteoblasts, and osteoclasts. In an unsupervised analysis, we delineated GCTB and chondroblastomas from the other analysed tumour entities. Using comparative methylation analysis, we demonstrated that the methylation pattern of the cell lines approximately equals that of H3F3A-mutated stromal cells in tissue. These patterns more resemble that of osteoblasts than that of mesenchymal stem cells, which argues for the osteoblast as the cell of origin of giant cell tumours of bone. Using enrichment analysis, we detected distinct hypermethylated clusters containing histone and collagen genes as well as target genes of the tumour suppressor p53. We found that the promotor regions of CDKN1A, CDKN2A, and IGFBP3 are methylated more strongly in GCTB than in the other giant cell-containing lesions, mesenchymal stem cells, osteoblasts, and osteoclasts (p < 0.001). This hypermethylation correlates with the lower gene expression at the mRNA level for these three genes in the cell lines, the lack of p16 and p21 in these cell lines, and the lower expression of p16 and p21 in GCTB. Overall, our analysis reveals characteristic DNA methylation patterns of giant cell tumours of bone and chondroblastomas and shows that cell lines of giant cell tumours of bone are a valid model for further analysis of H3F3A-mutated tumour cells. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Epithelioid Fibrous Histiocytoma With Chondroblastoma-Like Features: A Report of a Rare Entity and Discussion of Related Diagnostic Challenges.

ABSTRACT: Epithelioid fibrous histiocytoma (EFH) is an uncommon benign skin lesion. It is distinct from FH by virtue of its recurrent anaplastic lymphoma kinase (ALK) gene rearrangements and immunohistochemical expression of ALK protein. It often poses a challenge in interpretation. Clinically, it is characterized by a flesh-colored papule/nodule on an extremity of a young to middle-aged individual. Microscopically, it is represented by a circumscribed dermal papule/nodule composed of sheets of plump epithelioid cells, forming whorled aggregates around numerous intralesional vessels. Immunohistochemistry, notably ALK positivity and relevant negative stains, serves to distinguish EFH from its morphological mimics. Rare examples of chondroblastoma-like EFH and EFH with osseous metaplasia are recorded in the literature. Our case is of a 58-year-old man who attended an oculoplastic surgeon because of an exophytic cutaneous nodule on the right upper eyelid. The lesion was excised. Microscopically, it displayed morphological and immunohistochemical features of EFH. Of interest, discrete foci of chondro-osseous change, including chondroblastoma-like pericellular calcification, osteoid formation, and osteoclast-like giant cells, were noted throughout the lesion. A diagnosis of EFH with chondroblastoma-like features was made. Of interest, the changes observed in this EFH serve to link the previously reported examples of pure chondroblastoma-like EFH and EFH with osseous metaplasia. This morphological variant of EFH adds to the existing diagnostic challenge presented by these lesions, particularly in the distinction from other calcifying tumors of the skin.

Gene of the month: H3F3A and H3F3B.

H3F3A and H3F3B genes are located at 1q42.12 and 17q25.1, respectively, and encode identical H3.3 core histone proteins which form part of the histone hetero-octamer complex. Histones function by packaging DNA into small units, the nucleosome, and are highly susceptible to epigenetic post-translational modification. H3 K27 mutations have been shown to inhibit the polycomb repressive complex 2, which is normally involved in epigenetic gene silencing. Mutations in H3F3A and H3F3B are increasingly recognised in a variety of solid tumours. Point mutations in H3F3A have been described in giant cell tumour of bone and paediatric-type diffuse high-grade gliomas. Mutations in H3F3B have been described in chondroblastoma. Loss of trimethylation of H3 K27 is characteristic of most sporadic and radiation-associated malignant peripheral nerve sheath tumours. Immunohistochemistry with a variety of novel antibodies directed against specific mutations, as well as loss of H3K27me3 staining, may be useful in specific settings and in diagnostically challenging cases.

The incorporation loci of H3.3K36M determine its preferential prevalence in chondroblastomas.

The histone H3.3K36M mutation, identified in over 90% of chondroblastoma cases, reprograms the H3K36 methylation landscape and gene expression to promote tumorigenesis. However, it's still unclear how the H3K36M mutation preferentially occurs in the histone H3 variant H3.3 in chondroblastomas. Here, we report that H3.3K36M-, but not H3.1K36M-, mutant cells showed increased colony formation ability and differentiation defects. H3K36 methylations and enhancers were reprogrammed to different status in H3.3K36M- and H3.1K36M-mutant cells. The reprogramming of H3K36 methylation and enhancers was depended on the specific loci at which H3.3K36M and H3.1K36M were incorporated. Moreover, targeting H3K36M-mutant proteins to the chromatin inhibited the H3K36 methylation locally. Taken together, these results highlight the roles of the chromatic localization of H3.3K36M-mutant protein in the reprogramming of the epigenome and the subsequent induction of tumorigenesis, and shed light on the molecular mechanisms by which the H3K36M mutation mainly occurs in histone H3.3 in chondroblastomas.

H3.3 K36M Mutation as a Clinical Diagnosis Method of Suspected Chondroblastoma Cases.

OBJECTIVE: Whether H3.3 K36M mutation (H3K36M) could be an approach if the diagnosis of chondroblastoma (CB) patients was indistinct and it was suspected to be unclear clinically. METHODS: We reviewed and compared our clinical experiences of CB cases and some suspected cases, which were not diagnosed distinctly, between 2013 to 2019. A total of 15 male and four female cases included in this study were seperated into two groups, CB group and suspected case (SC) group. The CB group included 13 men and 3 women, with an age range from 9 to 54 (mean age, 22 years old). The SC group included two men and one woman, with the age range from 13 to 25 (mean age, 19 years old). In both groups the patients had been followed-up until December 2019 and none of the patients had prior treatment history. We evaluated the clinical complaints, radiological features, and clinical-histological features of the cases and performed an immunohistochemical (IHC) study to detect whether the H3K36M expression of cases was different, consistent with a gene-mutation analysis. RESULTS: In both groups, the radiologic features of both groups appeared as round low-density shadow with a clear edge, pathologic features showed diffuse proliferation of neoplastic cells with multinuclear giant cells. The radiological tumor size of CB group and SC group showed little difference, which was about 29.0*21.6 mm. Clinical-immunohistochemical features of both groups showed chondroid matrix inside with naïve tumor cells, multinucleated giant cells, and ground substance cells. Most of them showed chondro-related antibody positive (12 cases) but some of them showed S-100 negative (four cases). The clear difference of both groups was the result of H3K36M IHC study and gene analysis. In our cases, the CB group showed diffuse H3K36M positive and the SC group showed negative. The gene mutation analysis revealed that H3K36M-positive CB patients had K36M mutation, which were not found in the SC group. Sanger sequencing showed an A > T substitution at codon 36 of histone H3F3B. No other types of histone H3 mutation was detected in the CB group. Particularly, one of the suspected cases showed a G34W mutation was confirmed to be a giant cell tumor of bone (GCTB). CONCLUSIONS: Our study showed H3K36M immunohistochemistry and gene mutation analysis were specific clinical diagnostic tools to distinguish suspected CB from other giant cell-rich or cartilage matrix-diffuse bone tumors. The clinical-radiological and histomorphological features of patients gave suggestions on whether the H3K36M IHC and gene analysis should be required.

Histone Mutations and Bone Cancers.

Primary bone tumors are rare cancers that cause significant morbidity and mortality. The recent identification of recurrent mutations in histone genes H3F3A and H3F3B within specific bone cancers, namely, chondroblastomas and giant cell tumors of bone (GCTB), has provided insights into the cellular and molecular origins of these neoplasms and enhanced understanding of how histone variants control chromatin function. Somatic mutations in H3F3A and H3F3B produce oncohistones, H3.3G34W and H3.3K36M, in more than nine of ten GCTB and chondroblastomas, respectively. Incorporation of the mutant histones into nucleosomes inhibits histone methyltransferases NSD2 and SETD2 to alter the chromatin landscape and change gene expression patterns that control cell proliferation, survival, and differentiation, as well as DNA repair and chromosome stability. The discovery of these histone mutations has facilitated more accurate diagnoses of these diseases and stratification of malignant tumors from benign tumors so that appropriate care can be delivered. The broad-scale epigenomic and transcriptomic changes that arise from incorporation of mutant histones into chromatin provide opportunities to develop new and disease-specific therapies. In this chapter, we review how mutant histones inhibit SETD2 and NSD2 function in bone tumors and discuss how this information could lead to better treatments for these cancers.

Histone Lysine-to-Methionine Mutation as Anticancer Drug Target.

Histone modification stands for a vital genetic information form, which shows tight correlation with the modulation of normal physiological activities by genes. Abnormal regulation of histone methylation due to histone modification enzyme changes and histone mutations plays an important role in the development of cancer. Histone mutations, especially H3K27M and H3K36M, have been identified in various cancers such as pediatric DIPG (diffuse intrinsic pontine glioma) and chondroblastoma respectively. "K to M" mutation results overall downregulation of methylation on these lysine residues. Also, "K to M" mutant histones can inhibit the enzymatic activity of the responsible HMT (histone methyltransferase); for instance, SETD2 indicates H3K36 methylation, and Ezh2 represents H3K27 methylation. In-depth analysis of the mechanism of tumor formation triggered by the K to M mutation results in possible targeted therapies. This chapter is going to briefly introduce the mechanism of histone lysine-to-methionine mutation and review the recently identified targeted therapeutic strategies.

Chondroblastoma Expresses RANKL by RNA In Situ Hybridization and May Respond to Denosumab Therapy.

Lesions of bone featuring osteoclast-like giant cells comprise a diverse group of entities, including giant cell tumor (GCT) of bone, chondroblastoma, and aneurysmal bone cyst, among others. The receptor activator of nuclear factor-κB ligand (RANKL) has been implicated in the pathogenesis of GCT of bone and may play a role in the pathogenesis of other giant cell-rich lesions as well. In addition, RANKL inhibitors (denosumab) have also been shown to have some efficacy in treating some giant cell-rich lesions. Herein, we examine RANKL expression by RNA in situ hybridization in a total of 84 osseous lesions with a focus on chondroblastoma, GCT, fibrous dysplasia, and aneurysmal bone cyst. The lesions were tested for RANKL expression using a chromogenic RNA in situ hybridization assay. RANKL expression was identified in 24/25 (96%) GCT, 24/26 (92%) chondroblastomas, 6/7 (86%) aneurysmal bone cysts, and 3/16 (19%) patients with fibrous dysplasia. RANKL expression was statistically lower in chondroblastoma and aneurysmal bone cyst compared with GCT. RANKL reactivity in fibrous dysplasia was exclusively seen in the 3 cases with osteoclast-type giant cells. Our results indicate a high proportion of chondroblastomas, GCTs, and aneurysmal bone cysts express RANKL while reactivity in fibrous dysplasia is dependent on the presence of osteoclast-type giant cells. On the basis of the success of denosumab therapy for GCTs, our results indicate that it may be a potential therapeutic option in other primary osseous tumors.

Clinicopathologic characterization of malignant chondroblastoma: a neoplasm with locally aggressive behavior and metastatic potential that closely mimics chondroblastoma-like osteosarcoma.

Chondroblastoma is currently classified as a benign neoplasm; however, chondroblastoma and chondroblastoma-like osteosarcoma have morphologic overlap, raising the possibility that some tumors diagnosed as chondroblastoma-like osteosarcoma might actually represent malignant chondroblastoma. The H3F3B K36M point mutation, which has not been reported in osteosarcoma, is identified in 95% of chondroblastomas and is reliably detectable by immunohistochemistry (IHC). We reviewed 11 tumors diagnosed as atypical chondroblastoma, malignant chondroblastoma, or chondroblastoma-like osteosarcoma (median follow-up: 8.8 years; range: 4 months-26.4 years). Seven chondroblastomas with cytologic atypia and permeative growth were designated "malignant chondroblastoma"; six were H3K36M-positive by IHC. Relative to conventional chondroblastoma, malignant chondroblastoma occurred in older individuals (median: 52 years; range: 29-57 years) and arose at unusual sites. Three of four tumors with long-term follow-up recurred, and one patient died of widespread metastases. One was found to have chromosomal copy number alter4ations and a SETD2 mutation in addition to H3F3B K36M. The four remaining tumors were classified as chondroblastoma-like osteosarcoma. Chondroblastoma-like osteosarcoma occurred in younger patients (median: 21 years; range: 19-40 years) than malignant chondroblastoma. In contrast to malignant chondroblastoma, all had regions of malignant cells forming bone. Two of three patients with long-term follow-up developed recurrences, and two died of disease, one with widespread metastases. No mutations in H3F3A/H3F3B were detected by Sanger sequencing. While malignant chondroblastoma and chondroblastoma-like osteosarcoma show significant morphologic overlap, they have distinct clinical presentations and genetic findings. When considering this challenging differential diagnosis, IHC using histone H3 mutation-specific antibodies is a critical diagnostic adjunct.

Mutation-driven epigenetic alterations as a defining hallmark of central cartilaginous tumours, giant cell tumour of bone and chondroblastoma.

Recently, specific driver mutations were identified in chondroblastoma, giant cell tumour of bone and central cartilaginous tumours (specifically enchondroma and central chondrosarcoma), sharing the ability to induce genome-wide epigenetic alterations. In chondroblastoma and giant cell tumour of bone, the neoplastic mononuclear stromal-like cells frequently harbour specific point mutations in the genes encoding for histone H3.3 (H3F3A and H3F3B). The identification of these driver mutations has led to development of novel diagnostic tools to distinguish between chondroblastoma, giant cell tumour of bone and other giant cell containing tumours. From a biological perspective, these mutations induce several global and local alterations of the histone modification marks. Similar observations are made for central cartilaginous tumours, which frequently harbour specific point mutations in the metabolic enzymes IDH1 or IDH2. Besides an altered methylation pattern on histones, IDH mutations also induce a global DNA hypermethylation phenotype. In all of these tumour types, the mutation-driven epigenetic alterations lead to a highly altered transcriptome, resulting for instance in alterations in differentiation. These genomic alterations have diagnostic impact. Further research is needed to identify the genes and signalling pathways that are affected by the epigenetic alterations, which will hopefully lead to a better understanding of the biological mechanism underlying tumourigenesis.

Genetic profiling of a chondroblastoma-like osteosarcoma/malignant phosphaturic mesenchymal tumor of bone reveals a homozygous deletion of CDKN2A, intragenic deletion of DMD, and a targetable FN1-FGFR1 gene fusion.

Conventional osteosarcoma is the most common primary malignancy of bone. This group of neoplasms is subclassified according to specific histological features, but hitherto there has been no correlation between subtype, treatment, and prognosis. By in-depth genetic analyses of a chondroblastoma-like osteosarcoma, we detect a genetic profile that is distinct from those previously reported in benign and malignant bone tumors. The overall genomic copy number profile was less complex than that typically associated with conventional osteosarcoma, and there was no activating point mutation in any of H3F3A, H3F3B, IDH1, IDH2, BRAF, or GNAS. Instead, we found a homozygous CDKN2A deletion, a DMD microdeletion and an FN1-FGFR1 gene fusion. The latter alteration has been described in phosphaturic mesenchymal tumor. This tumor type shares some morphological features with chondroblastoma-like osteosarcoma and we cannot rule out that the present case actually represents an FN1-FGFR1 positive malignant phosphaturic mesenchymal tumor of bone without osteomalacia.

Histone H3K36M mutation and trimethylation patterns in chondroblastoma.

AIMS: Histones are essential components of chromatin, and mutations in histones lead to alterations in methylation and acetylation, which play an important role in tumorigenesis. Most of the chondroblastomas harbour the H3K36M mutation. With the availability of a mutation-specific antibody, we sought to assess the sensitivity of this antibody and the alterations of histone methylation in a series of chondroblastoma cases. METHODS AND RESULTS: Immunohistochemical staining with antibodies against H3K36M, trimethylated histones (H3K27me3 and H3K36me3) and an osteoblastic marker (SATB2) was performed on 27 chondroblastomas from 27 patients. The clinical and radiological characteristics of each patient were reviewed. All 27 tumours showed typical radiological and histological features of chondroblastoma, with a subset of cases showing secondary aneurysmal bone cyst changes (11/27), giant-cell-rich foci (4/27), and matrix-rich areas mimicking chondromyxoid fibroma (1/27). All except one case (26/27, 96%) showed positive H3K36M immunostaining (nuclear). In the majority of cases, there was a diffuse staining pattern. Immunohistochemical staining for H3K27me3 and H3K36me3 showed a heterogeneous staining pattern in all cases, regardless of mutation status. None of the cases showed loss of positivity or diffuse positivity. Focal or diffuse SATB2 expression was seen in 21 of 26 tumours (81%). CONCLUSION: Our results demonstrate that the vast majority of chondroblastomas are positive for H3K36M by immunohistochemical analysis, confirming its diagnostic value. H3K27me3 expression and H3K36me3 expression are heterogeneous in these tumours.

Diagnostic value of histone 3 mutations in osteoclast-rich bone tumors.

Differentiating osteoclast-rich lesions of bone (giant cell tumor of bone [GCTB], chondroblastoma [CBA], and aneurysmal bone cyst [ABC]) can be challenging, especially in small biopsies or fine-needle aspirations. Mutations affecting codons 34 and 36 of either H3 Histone Family Member 3A (H3F3A) and/or 3B (H3F3B) are characteristically seen in GCTB and CBAs. We devised a simple assay to identify these mutations and evaluated its applicability for routine clinical diagnosis. One hundred twenty-four tissue specimens from 108 patients (43 GCTBs, 38 CBAs and 27 ABCs) were collected from the archives of the Calgary Laboratory Services/University of Calgary and Vanderbilt University Medical Center. Histology was reviewed by an expert orthopedic pathologist. A single base extension assay (SNaPshot) is used to interrogate each nucleotide in codons 34 and 36 of H3F3A and codon 36 of H3F3B. Final diagnoses were generated after re-reviewing cases and incorporating molecular findings. Of 43 GCTBs, 38 (88%) had an H3F3A G34W mutation; 35 of 38 CBAs (92%) had a K36M mutation in either H3F3B (N = 31; 82%) or H3F3A (N = 4; 11%); none of 27 ABCs had a tested mutation. Molecular findings changed the histomorphologic diagnosis in 5 cases (3 GCTB changed to ABC, and 2 ABC changed to GCTB). These findings support the diagnostic utility of mutational analysis for this differential diagnosis in certain challenging cases when clinicoradiologic and histomorphologic features are not definitive, particularly for distinguishing cellular ABC versus GCTB with secondary ABC.

Chondroblastoma: An Update.

Chondroblastoma is a rare primary bone tumor of young people that typically arises in the ends of the long bones. Radiologic investigations show a small, circumscribed, lytic lesion. The tumor is characterized histologically by the proliferation of chondroblasts along with areas of mature cartilage, giant cells, and occasionally, secondary aneurysmal bone cyst formation. Chondroblastoma, however, may also present with atypical features, such as prominent hemosiderin deposition, numerous giant cells, or the presence of a large aneurysmal bone cyst component. Malignant entities such as clear cell chondrosarcoma and chondroblastic osteosarcoma must also be considered. Recently, immunohistochemical stains such as DOG1 and SOX9 have been described in chondroblastoma, and K36M mutations in either the H3F3A or H3F3B genes have also been identified. While generally regarded as a benign entity, chondroblastoma manifests an intermediate type of behavior, given its ability to recur locally, and rarely, metastasize.