Malignant Transformation in a Chondroblastoma-Does it Exist?

Chondroblastoma is currently described as a benign bone tumor, histopathologically characterized by its classical features including chondroblasts, pink cartilage, and a variable number of osteoclast-like giant cells with foci of dystrophic calcification. Although recurrent and metastasizing chondroblastomas are reported, a malignant chondroblastoma is exceedingly rare and somewhat a contentious entity. A 35-year-old male presented with a lump in his ankle of 15 years' duration. Imaging disclosed a lytic destructive lesion involving the lower ends of the tibia and fibula with a soft tissue component, indicative of atypical/"worrisome" features. Microscopic examination of the biopsy revealed distinct foci of chondroblastoma, transitioning to areas of high-grade sarcoma, including pleomorphic cells, increased mitoses, and prominent stromal hyalinization. Immunohistochemically, the entire tumor was positive for H3K36M, while DOG1 highlighted the areas of chondroblastoma. SATB2 highlighted the areas of high-grade sarcoma, sparing the areas of chondroblastoma. Additionally, the areas of a high-grade sarcoma showed multifocal desmin immunostaining. A diagnosis of a malignant transformation in a chondroblastoma was offered. The patient defaulted to further treatment and unfortunately died 8 months, post-diagnosis. The conceptual evolution of a malignant chondroblastoma with H3K36M immunostaining in the few reported tumors is described herewith.

Methylation of histone H3 lysine 36 is a barrier for therapeutic interventions of head and neck squamous cell carcinoma.

Approximately 20% of head and neck squamous cell carcinomas (HNSCCs) exhibit reduced methylation on lysine 36 of histone H3 (H3K36me) due to mutations in histone methylase NSD1 or a lysine-to-methionine mutation in histone H3 (H3K36M). Whether such alterations of H3K36me can be exploited for therapeutic interventions is still unknown. Here, we show that HNSCC models expressing H3K36M can be divided into two groups: those that display aberrant accumulation of H3K27me3 and those that maintain steady levels of H3K27me3. The former group exhibits reduced proliferation, genome instability, and heightened sensitivity to genotoxic agents like PARP1/2 inhibitors. Conversely, H3K36M HNSCC models with constant H3K27me3 levels lack these characteristics unless H3K27me3 is elevated by DNA hypomethylating agents or inhibiting H3K27me3 demethylases KDM6A/B. Mechanistically, H3K36M reduces H3K36me by directly impeding the activities of the histone methyltransferase NSD3 and the histone demethylase LSD2. Notably, aberrant H3K27me3 levels induced by H3K36M expression are not a bona fide epigenetic mark because they require continuous expression of H3K36M to be inherited. Moreover, increased sensitivity to PARP1/2 inhibitors in H3K36M HNSCC models depends solely on elevated H3K27me3 levels and diminishing BRCA1- and FANCD2-dependent DNA repair. Finally, a PARP1/2 inhibitor alone reduces tumor burden in a H3K36M HNSCC xenograft model with elevated H3K27me3, whereas in a model with consistent H3K27me3, a combination of PARP1/2 inhibitors and agents that up-regulate H3K27me3 proves to be successful. These findings underscore the crucial balance between H3K36 and H3K27 methylation in maintaining genome instability, offering new therapeutic options for patients with H3K36me-deficient tumors.

Chondroblastoma of foot bones; a clinicopathological study of 29 cases confirming the diagnostic utility of H3K36M and H3G34W antibodies at an uncommon site.

OBJECTIVE: Chondroblastoma (CB) is a benign cartilaginous bone neoplasm which commonly occurs in long bones of adolescents. CB can uncommonly involve foot. Its mimics include both benign and malignant lesions. H3K36M immunohistochemical (IHC) stain is a helpful tool for establishing the diagnosis of CB in such challenging situations. In addition, H3G34W IHC stain helps to rule out giant cell tumor which is the closest differential of CB. Our objective was to describe the clinicopathological features and frequencies of H3K36M, H3G34W and SATB2 IHC stains in CB of foot. MATERIALS AND METHODS: We reviewed H&E slides and blocks of 29 cases diagnosed as "chondroblastoma" of foot at our institutions. RESULTS: Patient's age ranged from 6 to 69 (mean: 23.3 and median: 23) years. Males were almost 5 times more commonly affected than females. Talus and calcaneum were involved in 13 (44.8 %) cases each. Microscopically, tumors were composed of polygonal mononuclear cells and multinucleated giant cells and chondroid matrix. Other histological features included aneurysmal bone cyst-like (ABC-like) change (44.8 %), osteoid matrix (31 %), chicken-wire calcification (20.7 %), and necrosis (10.3 %). H3K36M was expressed in 100 % and SATB2 in 91.7 % cases. H3G34W was negative in all cases, where performed. One out of 11 patients with follow up information developed local recurrence after 48 months. CONCLUSION: CB in foot occur at an elder age and show more frequent ABC-like changes as compared to long bones. Males are affected ~5:1 as compared to 2:1 in long bones. H3K36M are H3G34W are extremely useful diagnostic markers for CB, especially elderly (aged or higher) patients and we report the largest series of foot CB cases confirmed by immunohistochemistry.

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.

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.

Histone H3 Mutations in Cancer.

Histone modifications are one form of epigenetic information that relate closely to gene regulation. Aberrant histone methylation caused by alteration in chromatin-modifying enzymes has long been implicated in cancers. More recently, recurrent histone mutations have been identified in multiple cancers and have been shown to impede histone methylation. All three histone mutations (H3K27M, H3K36M, and H3G34V/R) identified result in amino acid substitution at/near a lysine residue that is a target of methylation. In the cases of H3K27M and H3K36M, found in pediatric DIPG (diffuse intrinsic pontine glioma) and chondroblastoma respectively, expression of the mutant histone leads to global reduction of histone methylation at the respective lysine residue. These mutant histones are termed "oncohistones" because their expression reprograms the epigenome and shapes an oncogenic transcriptome. Dissecting the mechanism of H3K27M-driven oncogenesis has led to the discovery of promising therapeutic targets in pediatric DIPG. The purpose of this review is to summarize the work done on identifying and dissecting the oncogenic properties of histone H3 mutations.

Recognition of cancer mutations in histone H3K36 by epigenetic writers and readers.

Histone posttranslational modifications control the organization and function of chromatin. In particular, methylation of lysine 36 in histone H3 (H3K36me) has been shown to mediate gene transcription, DNA repair, cell cycle regulation, and pre-mRNA splicing. Notably, mutations at or near this residue have been causally linked to the development of several human cancers. These observations have helped to illuminate the role of histones themselves in disease and to clarify the mechanisms by which they acquire oncogenic properties. This perspective focuses on recent advances in discovery and characterization of histone H3 mutations that impact H3K36 methylation. We also highlight findings that the common cancer-related substitution of H3K36 to methionine (H3K36M) disturbs functions of not only H3K36me-writing enzymes but also H3K36me-specific readers. The latter case suggests that the oncogenic effects could also be linked to the inability of readers to engage H3K36M.

Immunohistochemistry for histone H3G34W and H3K36M is highly specific for giant cell tumor of bone and chondroblastoma, respectively, in FNA and core needle biopsy.

BACKGROUND: Diagnosing giant cell-rich bone tumors can be challenging on limited biopsies. H3 histone family member 3A (H3F3A) (G34W/V/R/L) mutations are present in the majority of giant cell tumors (GCTs) of bone and H3 histone family member 3B (H3F3B) (K36M) mutations are present in nearly all chondroblastomas, but are absent in histologic mimics. Mutation-specific immunohistochemistry (IHC) is highly specific for GCT and chondroblastoma in surgical excisions. The objective of the current study was to validate H3G34W and H3K36M IHC in the diagnosis of giant cell-rich bone tumors on fine-needle aspiration and core needle biopsy specimens. METHODS: IHC was performed using monoclonal antibodies against histone H3.3 G34W and K36M in GCTs of bone (26 cases, including 2 malignant cases), GCT of Paget disease (1 case), chondroblastoma (8 cases), aneurysmal bone cyst (7 cases), and osteosarcoma (13 cases) with available fine-needle aspiration and/or core needle biopsy specimens from 2 institutions. H3F3A and H3F3B Sanger sequencing was performed on all 4 H3G34W IHC-negative GCTs. RESULTS: IHC for H3G34W was positive in 22 of 26 GCTs (85%) and negative in all histologic mimics. IHC for H3K36M was positive in all 8 chondroblastomas and negative in all histologic mimics. IHC results were concordant between biopsy and surgical specimens in 152 of 158 samples (96%). Sequencing identified alternate H3F3A G34L and G34V mutations in 1 IHC-negative GCT each, but no mutation was found in the remaining 2 cases. CONCLUSIONS: H3G34W and H3K36M IHC is highly specific for GCT and chondroblastoma, respectively, among giant cell-rich bone tumors, and is useful for confirming the diagnosis in limited biopsies. The presence of alternate H3F3A mutations accounts for the H3G34W IHC negativity in a subset of GCT of bone cases. Cancer Cytopathol 2018. © 2018 American Cancer Society.