TRPV4 and KRAS and FGFR1 gain-of-function mutations drive giant cell lesions of the jaw

Tenosynovial giant cell tumor: case report and molecular investigation

Tenosynovial giant cell tumor is a benign neoplasm arising from the synovium of joints, including the temporomandibular joint (TMJ). Despite its benign nature, these tumors may exhibit aggressive behavior. A 57-year-old woman with a swollen, hardened area in the left TMJ was referred to the university´s clinic. The diagnosis of tenosynovial giant cell tumor was made based on the presence of hyperplastic synovial lining containing mononuclear and giant cells, hemorrhagic areas, hemosiderin deposits, and calcification foci in the biopsy. A low condylectomy was performed, and histopathologic analysis of the surgical piece upheld the diagnosis. Due to histopathologic resemblance with other giant cell-rich lesions (giant cell granuloma of the jaws, brown tumor of hyperparathyroidism, and non-ossifying fibroma) for which signature mutations are known, mutational analysis of KRAS, FGFR1, and TRPV4 genes was conducted. The results revealed wild-type sequences for all the mutations tested, thereby supporting the diagnosis of tenosynovial giant cell tumor.

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).

Pediatric Gnathic Bony and Mesenchymal Tumors

Evaluation of bone pathology within the head and neck region, particularly the gnathic bonesis is complex, demonstrating unique pathologic processes. In part, this variation is due to odontogenesis and the embryological cells that may be involved, which can contribute to disease development and histologic variability. As with any boney pathosis, the key is to have clinical correlation, particularly with radiographic imaging prior to establishing a definitive diagnosis. This review will cover those entities that have a predilection for the pediatric population, and while it is not all inclusive, it should serve as a foundation for the pathologist who is evaluating bony lesions involving the craniofacial skeleton.

Management of a rare mandibular giant cell tumor of bone by neoadjuvant denosumab therapy and surgery: A 4-year follow-up case report

INTRODUCTION

Giant cell tumor of bone (GCTB) is a very rare tumor encountered in the jaws and its histology is quite similar to the more common giant cell granuloma of the jaws (GCGJ). These two entities can be easily confused in maxillofacial region. They are classically managed surgically, but in some localizations and in specific medical-surgical contexts, neoadjuvant therapy with denosumab may be indicated. This report tends to reinforce existing evidence in favor of the use of a neoadjuvant approach, particularly for localization of GCTB in the orofacial region.

PRESENTATION OF CASE

This is a 57-year-old female patient, an alcoholic smoker, in whom a voluminous mandibular radiolucent lesion was discovered during a routine X-ray by her dentist. After medical imaging assessment and incisional biopsy, diagnosis of GCTB was established. A neoadjuvant denosumab therapy was proposed first followed by a secondary surgical curettage. After 4 years' follow-up, complete healing was observed with no recurrence of the lesion.

DISCUSSION

Surgical management of aggressive GCTB is risky particularly in localizations involving the sacrum, spine or craniofacial skeleton with a high residual recurrence rate. The use of denosumab to stop tumor progression and facilitate secondary excision surgery is a recent approach that is now well documented in the literature showing promising results with a low rate of side effects.

CONCLUSION

This case of mandibular GCTB is to our knowledge the unique case described in this localization and treated by denosumab neoadjuvant therapy followed by surgery with a 4-year follow-up showing a complete healing.

Developing Models to Predict BRAFV600E and RAS Mutational Status in Papillary Thyroid Carcinoma Using Clinicopathological Features and pERK1/2 Immunohistochemistry Expression

The Cancer Genome Atlas (TCGA) has classified papillary thyroid carcinoma (PTC) into indolent RAS-like and aggressive BRAF-like based on its distinct driver gene mutations. This retrospective study aimed to assess clinicopathology and pERK1/2 expression variations between BRAF-like and RAS-like PTCs and establish predictive models for BRAFV600E and RAS-mutated PTCs. A total of 222 PTCs underwent immunohistochemistry staining to assess pERK1/2 expression and Sanger sequencing to analyze the BRAF and RAS genes. Multivariate logistic regression was employed to develop prediction models. Independent predictors of the BRAFV600E mutation include a nuclear score of 3, the absence of capsules, an aggressive histology subtype, and pERK1/2 levels exceeding 10% (X2 = 0.128, p > 0.05, AUC = 0.734, p < 0.001). The RAS mutation predictive model includes follicular histology subtype and pERK1/2 expression > 10% (X2 = 0.174, p > 0.05, AUC = 0.8, p < 0.001). We propose using the prediction model concurrently with four potential combination group outcomes. PTC cases included in a combination of the low-BRAFV600E-scoring group and high-RAS-scoring group are categorized as RAS-like (adjOR = 4.857, p = 0.01, 95% CI = 1.470-16.049). PTCs included in a combination of the high-BRAFV600E-scoring group and low-RAS-scoring group are categorized as BRAF-like PTCs (adjOR = 3.091, p = 0.001, 95% CI = 1.594-5.995). The different prediction models indicate variations in biological behavior between BRAF-like and RAS-like PTCs.

Brown Tumour in Chronic Kidney Disease: Revisiting an Old Disease with a New Perspective

Osteitis fibrosa cystica (OFC) and Brown Tumours are two related but distinct types of bone lesions that result from the overactivity of osteoclasts and are most often associated with chronic kidney disease (CKD). Despite their potential consequences, these conditions are poorly understood because of their rare prevalence and variability in their clinical manifestation. Canonically, OFC and Brown Tumours are caused by secondary hyperparathyroidism in CKD. Recent literature showed that multiple factors, such as hyperactivation of the renin-angiotensin-aldosterone system and chronic inflammation, may also contribute to the occurrence of these diseases through osteoclast activation. Moreover, hotspot KRAS mutations were identified in these lesions, placing them in the spectrum of RAS-MAPK-driven neoplasms, which were until recently thought to be reactive lesions. Some risk factors contributed to the occurrence of OFC and Brown Tumours, such as age, gender, comorbidities, and certain medications. The diagnosis of OFC and Brown Tumours includes clinical symptoms involving chronic bone pain and laboratory findings of hyperparathyroidism. In radiological imaging, the X-ray and Computed tomography (CT) scan could show lytic or multi-lobular cystic alterations. Histologically, both lesions are characterized by clustered osteoclasts in a fibrotic hemorrhagic background. Based on the latest understanding of the mechanism of OFC, this review elaborates on the manifestation, diagnosis, and available therapies that can be leveraged to prevent the occurrence of OFC and Brown Tumours.

[The new WHO classification of jaw tumours]

Maxillofacial tumours cover a broad spectrum of lesions, including neoplasms, hamartomatous changes and developmental disorders. Since the beginning of 2022, a beta version of the 5th edition of the WHO classification for head and neck tumours has been available online, and a print version is expected to be published in mid-2023. From a conceptual point of view, little has been changed compared to the 4th edition; the sort order of lesions is more rigorously arranged according to benign and malignant behaviour and identical tumour types are no longer described redundantly in different chapters depending on their location. The diagnostic criteria are now summarized as "essential" and "desirable", and in addition to the clinical features, imaging is now also incorporated, providing an interdisciplinary approach to the classification. A few new entities are included for the first time. This article gives an overview of the main changes introduced in the new WHO classification with a special emphasis on fibro-osseous lesions of the craniofacial skeleton.

Benign fibrohistiocytic jaw lesions: a 48-year clinicopathologic analysis and review of the literature

OBJECTIVE

Intra-osseous fibrohistiocytic lesions have long been reported in the literature; evidence suggests they represent a heterogeneous group of reactive and neoplastic processes. This study evaluated a series of gnathic fibrohistiocytic lesions to identify and categorize their clinical, radiographic and morphologic spectrum.

STUDY DESIGN

A retrospective case search over 48 years was conducted for maxillary and mandibular intra-bony fibrohistiocytic lesions. Diagnoses were confirmed and demographic, radiographic, clinical and follow-up data was analyzed.

RESULTS

Fifty cases met the inclusion criteria. Most cases (80%) were found in the second through fourth decades (mean, 29 years). The most common location (86%) was the posterior mandible. Radiographic presentations varied, but a few patterns emerged, including a distinctive mottled, honeycomb type with punctate lucencies. All cases demonstrated fibrous components admixed with variable histiocytes. Eight cases (16%) were histiocyte-rich with dominant sheets of xanthoma cells. Immunohistochemical staining revealed strong CD68 and CD163 expression, along with variable smooth muscle actin staining. The vast majority (92%) of cases were treated conservatively. Available follow-up showed lesional stability in 17 cases (average, 85 months) with 2 recurrences (24 months each) and no evidence of malignant transformation.

CONCLUSIONS

This study is the largest to date of fibrohistiocytic gnathic lesions, revealing distinctive radiographic and histologic findings and characteristic clinical and immunophenotypic features. Available evidence suggests that most of these are indolent, slow-growing lesions amenable to conservative therapy.

Cemento-osseous dysplasia is caused by RAS-MAPK activation

Cemento-osseous dysplasia (COD) belongs to the spectrum of benign fibro-osseous lesions occurring exclusively in the tooth-bearing areas of the jaws. Depending on site and extent of involvement, periapical, focal and florid subtypes can be distinguished that share an identical histomorphology. Most cases are asymptomatic and follow a self-limited course requiring no specific treatment. Over time, lesions progressively mineralise while the cellularity decreases. However, the molecular pathogenesis of COD, has not yet been explored. We analysed a series of 31 COD samples by targeted sequencing and detected pathogenic hotspot mutations involving the RAS-MAPK signalling pathway in 5/18 evaluable cases (28%). The mutations were found in the BRAF, HRAS, KRAS, NRAS, and FGFR3 genes. Our findings suggest that COD is driven by RAS-MAPK activation; however, the mechanism underlying the spontaneous growth arrest typically occuring in most of the lesions remains elusive.

Piezo1-mediated mechanotransduction promotes entheseal pathological new bone formation in ankylosing spondylitis

OBJECTIVE

The aim of this study was to identify the role of Piezo1-mediated mechanotransduction in entheseal pathological new bone formation and to explore the underlying molecular mechanism.

METHODS

Spinal ligament tissues were collected from 14 patients with ankylosing spondylitis (AS) and 14 non-AS controls and bulk RNA sequencing was conducted. Collagen antibody-induced arthritis models were established to observe pathological new bone formation. Pharmacological inhibition and genetic ablation of Piezo1 was performed in animal models to identify the essential role of Piezo1. Entheseal osteo-chondral lineage cells were collected and in vitro cell culture system was established to study the role and underlying mechanism of Piezo1 in regulation of chondrogenesis, osteogenesis and its own expression.

RESULTS

Piezo1 was aberrantly upregulated in ligaments and entheseal tissues from patients with AS and animal models. Pharmaceutical and genetic inhibition of Piezo1 attenuated while activation of Piezo1 promoted pathological new bone formation. Mechanistically, activation of CaMKII (Calcium/calmodulin dependent protein kinase II) signalling was found essential for Piezo1-mediated mechanotransduction. In addition, Piezo1 was upregulated by AS-associated inflammatory cytokines.

CONCLUSION

Piezo1-mediated mechanotransduction promotes entheseal pathological new bone formation through CaMKII signalling in AS.

Significant association between FGFR1 mutation frequency and age in central giant cell granuloma

Central giant cell granulomas (CGCG) are rare intraosseous osteolytic lesions of uncertain aetiology. Despite the benign nature of this neoplasia, the lesions can rapidly grow and become large, painful, invasive, and destructive. The identification of molecular drivers could help in the selection of targeted therapies for specific cases. TRPV4, KRAS and FGFR1 mutations have been associated with these lesions but no correlation between the mutations and patient features was observed so far. In this study, we analysed 17 CGCG cases of an Italian cohort and identified an interesting and significant (p=0.0021) correlation between FGFR1 mutations and age. In detail, FGFR1 mutations were observed frequently and exclusively in CGCG from young (<18 years old) patients (4/5 lesions, 80%). Furthermore, the combination between ours and previously published data confirmed a significant difference in the frequency of FGFR1 mutations in CGCG from patients younger than 18 years at the time of diagnosis (9/23 lesions, 39%) when compared to older patients (1/31 lesions, 0.03%; p=0.0011), thus corroborating our observation in a cohort of 54 patients. FGFR1 variants in young CGCG patients could favour fast lesion growth, implying that they seek medical attention earlier. Our observation might help prioritise candidates for FGFR1 testing, thus opening treatment options with FGFR inhibitors.

A new TRPV4 mutation in a case of multiple central giant cell granulomas of the jaws

Sporadic central giant cell granulomas of the jaws (GCGJ) are often solitary lesions, characterized by KRAS, FGFR1, and TRPV4 somatic mutations. Multifocal lesions may occur and are associated with hyperparathyroidism or underlying syndromes such as cherubism, which is marked by SH3BP2 mutations, and RASopathies, which are caused by mutations in the FGFR-RAS-RAF-MEK-ERK signaling cascade. The diagnosis of multiple GCGJ can be challenging. The present case reports a 14-year-old boy with multiple central GCGJ and no obvious syndromic trait. Sanger sequencing-based analysis revealed wild-type sequences for SH3BP2 (exon 9), KRAS (exons 2-4), and FGFR1 (exons 9 and 10) genes. A rare TRPV4 somatic mutation (p.Val708Met) was detected in the lesion on the right side of the mandible, whereas the other tumor and the normal oral mucosa revealed wild-type TRPV4 sequences. This report expands the spectrum of TRPV4 somatic mutations in central GCGJ.

Brown Tumors Belong to the Spectrum of KRAS -driven Neoplasms

Brown tumors are rare and generally self-limiting mass lesions of bone occurring in the context of hyperparathyroidism. Although commonly regarded as endocrine-driven tumor-like lesions, we detected pathogenic hotspot KRAS mutations in 10/16 brown tumors (62%) with similar frequencies found in cases affecting the peripheral and axial skeleton. Pathogenic mutations in other driver genes of the RAS-MAPK pathway were not identified. Our findings suggest brown tumors to represent true neoplasms driven by the activation of the RAS-MAPK signaling pathway. The frequent regression of brown tumors after normalization of hyperparathyroidism points to a second hit mediated by endocrine stimulation to be required for tumor development. Our findings underline the pathogenic relation of brown tumors to nonossifying fibroma and giant cell granuloma of the jaws which both appear histologically similar to brown tumors and are also driven by RAS-MAPK signaling pathway activation.

RANKL inhibition for giant cell lesions of the jaw: A retrospective cohort analysis

BACKGROUND

In all giant-cell-rich lesions (GCRL) occurring in bone, a common underlying excessive RANKL expression is held responsible for the osteolytic activity. Apart from giant cell tumour of bone (GCTB), systematic outcome analysis of RANKL inhibition in other GCRL is unavailable. The aim of this study is to assess the efficacy and safety of a 1-year denosumab protocol in giant cell lesions of the jaw (GCLJ).

METHODS

A retrospective cohort study was conducted compromising patients treated with a 1-year protocol of monthly subcutaneously administered 120 mg denosumab. Objective tumour response based on histology and imaging was used to calculate objective tumour response rate, progression-free survival (PFS) and time to progression. Type, severity and frequency of adverse events were recorded in a standardised way to assess safety.

RESULTS

Twenty patients, predominantly female (90%), were included. Fifty-five per cent of lesions were located in the mandible; most classified as aggressive lesions (90%). Thirty-five per cent (7/20) of cases were either recurrent after prior treatment or progressive, while on other drug treatment. Objective tumour response rate was 100% after 12 months of treatment. Median PFS was 50.4 months (95% CI 38.0-62.8) with a cumulative PFS rate of 22.6% (95% CI 1.8-43.4) at 5 years follow-up. Median time to progression was 38.4 months (95% CI 26.0-50.8). Treatment was well tolerated, and none of the patients had to interrupt therapy for toxicity.

CONCLUSION

High-dose denosumab is effective and safe in achieving a complete response in GCLJ within 12 months. The high long-term relapse rate after treatment cessation is the main obstacle for denosumab to become standard treatment for GCLJ.

Integrated proteomics, phosphoproteomics and metabolomics analyses reveal similarities amongst giant cell granulomas of the jaws with different genetic mutations

BACKGROUND

Giant cell granuloma of the jaws are benign osteolytic lesions of the jaws. These lesions are genetically characterized by mutually exclusive somatic mutations at TRPV4, KRAS, and FGFR1, and a fourth molecular subgroup which is wild-type for the three mutations. Irrespective of the molecular background, giant cell granulomas show MAPK/ERK activation. However, it remains unclear if these mutations lead to differences in their molecular signaling in giant cell granulomas.

METHODS

Metabolomics, proteomics and phosphoproteomics analyses were carried out in formalin-fixed paraffin-embedded samples of giant cell granuloma of the jaws. The study cohort consisted of five lesions harboring mutations in FGFR1, six in KRAS, five in TRPV4 and five that were wild-type for these mutations.

RESULTS

Lesions harboring KRAS or FGFR1 mutations showed overall similar proteomics and metabolomics profiles. In all four groups, metabolic pathways showed similarity in apoptosis, cell signaling, gene expression, cell differentiation and erythrocyte activity. Lesions harboring TRPV4 mutations showed a greater number of enriched pathways related to tissue architecture. On the other hand, the wild-type group presented increased number of enriched pathways related to protein metabolism compared to the other groups.

CONCLUSION

Despite some minor differences, our results revealed an overall similar molecular profile among the groups with different mutational profile at the metabolic, proteic and phosphopeptidic levels. This article is protected by copyright. All rights reserved.

Oestrogen receptor expression distinguishes non-ossifying fibroma from other giant cell containing bone tumours

Non-ossifying fibroma (NOF) and central giant cell granuloma (CGCG) are both benign tumours of bone with overlapping morphology and similar mutations in the RAS/MAPK pathway. However, NOF is located in the long bones with regression after puberty in contrast to CGCG which is located in the jaw bones and does not regress spontaneously. We hypothesised that endocrine regulation by oestrogen plays a role in the spontaneous regression in NOF. Therefore, we examined the expression of ERα in a series of NOF and CGCG. ERα expression (EP1) was determined using immunohistochemistry on 16 NOFs (whole slides), and 47 CGCGs (tissue microarrays (TMA’s n = 41 and whole slide n = 6)). As comparison, we included TMAs of other giant cell containing bone lesions: giant cell tumour of bone (n = 75), chondroblastoma (n = 12), chondromyxoid fibroma (n = 12), aneurysmal bone cyst (n = 6) and telangiectatic osteosarcoma (n = 6). All 16 NOF samples demonstrated ERα protein expression, while all 47 CGCG and all other giant cell containing bone tumours were negative. Most NOF samples had moderate staining intensity and between 24 and 49% of the spindle cells were ERα-positive. Our findings further support the role of endocrine regulation via oestrogen in the spontaneous regression in NOF. Whether oestrogen signalling at puberty is involved in the induction of senescence in the neoplastic cells of NOF harbouring RAS/MAPK pathway mutations needs further research. Since ERα expression was not observed in other giant cell containing bone lesions with overlapping morphological features, positive ERα expression may favour the diagnosis of NOF in challenging diagnostic cases.

[Giant-cell tumor of bone in 2022]

Giant cell tumors of bone (GCTs) are rare mesenchymal tumors classified as intermediate in the WHO 2020 classification, i.e. neither completely benign nor definitely malignant, due to recurrence (frequent) and pulmonary metastases (rare). They involve the end of long bones as well as the axial bones of mature skeletons. They are made of mononuclear stromal tumor cells of (pre-) osteoblastic phenotype, mononuclear cells of the monocyte-macrophage lineage and osteoclast-like multinuclear giant cells responsible for tumor osteolysis. In 95% of cases, the stromal cells have a specific mutation in the H3F3A gene which encodes histone H3.3. The mutated H3.3 G34W protein (90% of cases) can be easily detected by immunohistochemistry, even on small samples. Many tumors or bone pseudotumors contain osteoclast-like giant cells, cells of the bone microenvironment, and should not be confused with GCT: mainly brown tumor of hyperparathyroidism, aneurysmal bone cyst, chondroblastoma, non-ossifying fibroma and central giant cell granuloma.

Manifestations of hyperparathyroidism in the jaws: Concepts, mechanisms, and clinical aspects

Hyperparathyroidism is one of the most common endocrine disorders worldwide. In countries where routine biochemical screening is not common, symptomatic hyperparathyroidism predominates. Its manifestations include skeletal alterations, calcification of soft tissues, kidney stones, and functional alterations in other systems. Notably, jaw alterations can be the first clinical sign of hyperparathyroidism, including brown tumor, renal osteodystrophy, osteitis fibrosa, and leontiasis ossea, and knowing such conditions is of core importance for the multidisciplinary diagnosis and management of hyperparathyroidism. We aimed to perform a concise review, systematizing the concepts and mechanisms underlying hyperparathyroidism and associated gnathic alterations. In addition, a detailed description of the clinical aspects of the jaw manifestations is presented.

Clinical and Radiologic Response of Central Giant Cell Granuloma to Denosumab: A 6-Year Prospective Observational Study

Central giant cell granuloma (CGCG) is a rare lesion of the jaw occurring in young adults and adolescents. Surgery, the traditional mainstay of therapy, is associated with significant morbidity. Denosumab, a humanised monoclonal antibody to RANKL, is effective in a related entity, giant cell tumour of bone (GCTB), but experience in the more indolent CGCG is limited. This prospective observational study of all denosumab-treated CGCG at a tertiary referral centre (2015-2021) aimed to evaluate the safety, efficacy and recurrence risk using denosumab in CGCG at lower-frequency dosing than used for GCTB. All received standardised, time-limited courses of denosumab 120 mg with stepwise increase in dosing interval based on response. They were followed for up to 75 months using a radiation-minimising protocol: 3-monthly clinical, biochemical and radiological assessment (orthopantomograms, cone beam CT). Eight patients, median age 20.5 years [IQR 6], received 13 initial doses [IQR 10] of denosumab 120 mg. Radiologic response was seen after 5.5 doses [IQR 4.5]: ossification in all and size reduction in three. Recurrence occurred in four of seven completing therapy, observed 12 months post-cessation [IQR 6.5]. Larger baseline size, aggressive subtype and fewer than 12 initial doses were more common in the recurrence group. There was no osteonecrosis of the jaw. Hypocalcaemia occurred in one receiving modified dosing. This study represents the largest, most diverse cohort of denosumab-treated CGCG with the longest follow-up in literature. It demonstrates the efficacy of lower-frequency, time-restricted course of denosumab but highlights the risk of recurrence. Long-term follow-up is critical.

Update from the 5th Edition of the World Health Organization Classification of Head and Neck Tumors: Odontogenic and Maxillofacial Bone Tumours

The 5^th edition of the World Health Organization (WHO) Classification of Head and Neck Tumours (2022) comes out only five years after the previous edition, however it presents important updates that run in parallel with the rapid progression involving the increasingly sophisticated molecular investigation and its interpretation, some of which already have therapy-related impact. This manuscript provides an overview of the leading changes introduced in the classification of Odontogenic and Maxillofacial Bone Tumours that encompasses cysts of the jaws, odontogenic tumours, giant cell lesions and bone cysts, and bone and cartilage tumours. This is the first edition that Essential and Desirable Diagnostic Features were added for each entity, so that the most important clinical, microscopic and/or radiologic features were encapsulated and briefly highlighted. Surgical ciliated cyst was added to the group of odontogenic cysts, adenoid ameloblastoma was a newly recognized benign epithelial odontogenic tumour, and segmental odontomaxillary dysplasia was introduced in the group of fibro-osseous tumours and dysplasia. In addition, rhabdomyosarcoma with TFCP2 rearrangement, was introduced into the group of malignant jawbone tumours. The unique genetic aberrations distinguish it from other types of rhabdomyosarcomas. On the other hand, melanotic neuroectodermal tumour of infancy and osteoid osteoma were deleted from the benign bone and cartilageneous tumours, as was the hematolymphoid tumour of solitary plasmacytoma of bone. We systematically reviewed each entity in this chapter and provided important updated findings for selected topics that can further aid in the diagnostic process for challenging cases, broaden insights on the logic of the present classification, and finally, emphasize the potential that some of the molecular results may have in the near future to set new treatment approaches.

Recurrent driver mutations in benign tumors

The understanding of the molecular pathogenesis of benign tumors may bring essential information to clarify the process of tumorigenesis, and ultimately improve the understanding of events such as malignant transformation. The definition of benign neoplasia is not always straightforward and herein the issues surrounding this concept are discussed. Benign neoplasms share all cancer hallmarks with malignancies, except for metastatic potential. Recently, next-generation sequencing has provided unprecedented opportunities to unravel the genetic basis of benign neoplasms and, so far, we have learned that benign neoplasms are indeed characterized by the presence of genetic mutations, including genes rearrangements. Driver mutations in advanced cancer are those that confer growth advantage, and which have been positively selected during cancer evolution. Herein, some discussion will be brought about this concept in the context of cancer prevention, involving precursor lesions and benign neoplasms. When considering early detection and cancer prevention, a driver mutation should not only be advantageous (i.e., confer survival advantage), but predisposing (i.e., promoting a cancer phenotype). By including the benign counterparts of malignant neoplasms in tumor biology studies, it is possible to evaluate the risk posed by a given mutation and to differentiate advantageous from predisposing mutations, further refining the concept of driver mutations. Therefore, the study of benign neoplasms should be encouraged because it provides valuable information on tumorigenesis central for understanding the progression from initiation to malignant transformation.

Malignant Giant Cell Tumor of Bone With a KRAS G12V Mutation

Malignant giant cell tumor of bone (GCTB) is a rare, aggressive, sarcoma occurring in adolescent and young adults. It is characterized by the presence of multinucleated giant cells and an aggressive clinical course. Because of the rarity of this tumor, no standard therapies have been identified. Current treatment regimens often include osteosarcoma chemotherapy protocols. We present a case of a malignant GCTB with a KRAS G12V mutation. This mutation is a known oncogenic driver that has not previously been reported on patients with malignant GCTB.

Giant Cell-Rich Tumors of Bone

The term giant cell-rich tumors of bone refers to a shared morphologic pattern in a group of different osseous lesions, that is, the abundance of osteoclastlike giant cells. Fitting with a broad spectrum of clinical presentations and biological behavior, the recent detection of characteristic molecular alterations in giant cell tumor of bone (H3-3), nonossifying fibroma (KRAS, FGFR1), giant cell granuloma of the jaws (KRAS, FGFR1, TRPV4), and aneurysmal bone cyst (USP6) have contributed significantly to the biological understanding of these morphologically related but clinically distinct lesions and their systematic classification, highlighting differences and pathogenic relationships.

TRPV4 induces apoptosis via p38 MAPK in human lung cancer cells

Lung cancer is one of the most common cancers worldwide. TRPV4 belongs to the ‘transient receptor potential' (TRP) superfamily. It has been identified to profoundly affect a variety of physiological processes, including nociception, heat sensation, and inflammation. Unlike other TRP superfamily channels, its roles in cancers are unknown. Here, we elucidated the effects of TRPV4 and molecular mechanisms in human lung cancer cells. The levels of TRPV4 were detected in human lung cancer tissues and the paired paracarcinoma tissues by real-time PCR and western blotting analysis. The proliferation of human lung cancer cells was determined by MTT assay. Cell apoptosis was determined by FACS assay. The results demonstrated that low levels of TRPV4 were detected in clinical lung carcinoma specimens. Over-expression of TRPV4 induced cell death and inhibited cell proliferation and migration in A549 cells and H460 cells. Moreover, over-expression of TRPV4 enhanced the activation of p38 MAPK signal pathway. Inhibition of p38 MAPK abolished the effects of TRPV4 on cell proliferation, apoptosis, and migration in A549 cells. Collectively, our findings indicated that TRPV4 induced apoptosis via p38 MAPK in human lung cancer cells and suggested that TRPV4 was a potential target for therapy of human lung cancers.

Multiple versus solitary giant cell lesions of the jaw: Similar or distinct entities?

The majority of giant cell lesions of the jaw present as a solitary focus of disease in bones of the maxillofacial skeleton. Less frequently they occur as multifocal lesions. This raises the clinical dilemma if these should be considered distinct entities and therefore each need a specific therapeutic approach. Solitary giant cell lesions of the jaw present with a great diversity of symptoms. Recent molecular analysis revealed that these are associated with somatic gain-of-function mutations in KRAS, FGFR1 or TRPV4 in a large component of the mononuclear stromal cells which all act on the RAS/MAPK pathway. For multifocal lesions, a small group of neoplastic multifocal giant cell lesions of the jaw remain after ruling out hyperparathyroidism. Strikingly, most of these patients are diagnosed with jaw lesions before the age of 20 years, thus before the completion of dental and jaw development. These multifocal lesions are often accompanied by a diagnosis or strong clinical suspicion of a syndrome. Many of the frequently reported syndromes belong to the so-called RASopathies, with germline or mosaic mutations leading to downstream upregulation of the RAS/MAPK pathway. The other frequently reported syndrome is cherubism, with gain-of-function mutations in the SH3BP2 gene leading through assumed and unknown signaling to an autoinflammatory bone disorder with hyperactive osteoclasts and defective osteoblastogenesis. Based on this extensive literature review, a RAS/MAPK pathway activation is hypothesized in all giant cell lesions of the jaw. The different interaction between and contribution of deregulated signaling in individual cell lineages and crosstalk with other pathways among the different germline- and non-germline-based alterations causing giant cell lesions of the jaw can be explanatory for the characteristic clinical features. As such, this might also aid in the understanding of the age-dependent symptomatology of syndrome associated giant cell lesions of the jaw; hopefully guiding ideal timing when installing treatment strategies in the future.

Towards better understanding of giant cell granulomas of the oral cavity

Giant cell granulomas are enigmatic lesions of the oral cavity characterised by a peculiar combined proliferation of mononuclear and multinucleated giant cells in a mesenchymal stromal background. Central and peripheral giant cell granulomas may have similar pathogenesis and histology but differ in their location and biological behaviour. It is important to differentiate them from other giant cell lesions that can occur in the oral cavity, such as giant cell tumour of the bone, aneurysmal bone cyst, brown tumour of hyperparathyroidism, and giant cell lesions of Ramon syndrome, Noonan syndrome, neurofibromatosis and Jaffe-Campanacci syndrome. A recent insight into their molecular genetics and pathogenesis, with identification of KRAS, FGFR1 and TRPV4 mutations, allows for better diagnostic differentiation and opens the door to the use of pathway inhibitors in the treatment of recurrent or dysmorphic lesions. In this review, we provide an updated summary of the clinical and pathological features of oral cavity giant cell granulomas that help with their precise diagnosis and management.

Successful denosumab treatment for central giant cell granuloma in a 9-year-old child

BACKGROUND

Denosumab is a nonsurgical treatment option for central giant cell granulomas (CGCG), especially in aggressive lesions.

CASE REPORT

We describe a 9-year-old girl with an aggressive maxillary CGCG successfully treated with denosumab, avoiding a mutilating surgery after intralesional corticosteroid injections failed, and the lesion continued to rapidly grow. During denosumab treatment, she developed a self-limiting area of osteonecrosis in the maxillary alveolar bone, which rapidly resolved after antibiotic therapy. Six months after denosumab discontinuation, a maxillary surgical recontour was performed. Two weeks after surgery, the patient presented vomiting, pallor, dehydration, but no fever. Blood tests revealed severe hypercalcemia and acute renal dysfunction. After discarding thyroid, parathyroid, and adrenal alterations, a diagnosis of severe rebound hypercalcemia after denosumab treatment was made. Treatment consisted of hyperhydration, calcium pamidronate, and methylprednisolone, restoring calcium levels to normal.

CONCLUSION

After 2 years of follow-up, she remains on orthodontic treatment, with no recurrences or other episodes of hypercalcemia.

De novo TRPV4 Leu619Pro variant causes a new channelopathy characterised by giant cell lesions of the jaws and skull, skeletal abnormalities and polyneuropathy

BACKGROUND

Pathogenic germline variants in Transient Receptor Potential Vanilloid 4 Cation Channel (TRPV4) lead to channelopathies, which are phenotypically diverse and heterogeneous disorders grossly divided in neuromuscular disorders and skeletal dysplasia. We recently reported in sporadic giant cell lesions of the jaws (GCLJs) novel, somatic, heterozygous, gain-of-function mutations in TRPV4, at Met713.

METHODS

Here we report two unrelated women with a de novo germline p.Leu619Pro TRPV4 variant and an overlapping systemic disorder affecting all organs individually described in TRPV4 channelopathies.

RESULTS

From an early age, both patients had several lesions of the nervous system including progressive polyneuropathy, and multiple aggressive giant cell-rich lesions of the jaws and craniofacial/skull bones, and other skeletal lesions. One patient had a relatively milder disease phenotype possibly due to postzygotic somatic mosaicism. Indeed, the TRPV4 p.Leu619Pro variant was present at a lower frequency (variant allele frequency (VAF)=21.6%) than expected for a heterozygous variant as seen in the other proband, and showed variable regional frequency in the GCLJ (VAF ranging from 42% to 10%). In silico structural analysis suggests that the gain-of-function p.Leu619Pro alters the ion channel activity leading to constitutive ion leakage.

CONCLUSION

Our findings define a novel polysystemic syndrome due to germline TRPV4 p.Leu619Pro and further extend the spectrum of TRPV4 channelopathies. They further highlight the convergence of TRPV4 mutations on different organ systems leading to complex phenotypes which are further mitigated by possible post-zygotic mosaicism. Treatment of this disorder is challenging, and surgical intervention of the GCLJ worsens the lesions, suggesting the future use of MEK inhibitors and TRPV4 antagonists as therapeutic modalities for unmet clinical needs.

H3.3 G34W Promotes Growth and Impedes Differentiation of Osteoblast-Like Mesenchymal Progenitors in Giant Cell Tumor of Bone

Glycine 34-to-tryptophan (G34W) substitutions in H3.3 arise in approximately 90% of giant cell tumor of bone (GCT). Here, we show H3.3 G34W is necessary for tumor formation. By profiling the epigenome, transcriptome, and secreted proteome of patient samples and tumor-derived cells CRISPR-Cas9-edited for H3.3 G34W, we show that H3.3K36me3 loss on mutant H3.3 alters the deposition of the repressive H3K27me3 mark from intergenic to genic regions, beyond areas of H3.3 deposition. This promotes redistribution of other chromatin marks and aberrant transcription, altering cell fate in mesenchymal progenitors and hindering differentiation. Single-cell transcriptomics reveals that H3.3 G34W stromal cells recapitulate a neoplastic trajectory from a SPP1+ osteoblast-like progenitor population toward an ACTA2+ myofibroblast-like population, which secretes extracellular matrix ligands predicted to recruit and activate osteoclasts. Our findings suggest that H3.3 G34W leads to GCT by sustaining a transformed state in osteoblast-like progenitors, which promotes neoplastic growth, pathologic recruitment of giant osteoclasts, and bone destruction. SIGNIFICANCE: This study shows that H3.3 G34W drives GCT tumorigenesis through aberrant epigenetic remodeling, altering differentiation trajectories in mesenchymal progenitors. H3.3 G34W promotes in neoplastic stromal cells an osteoblast-like progenitor state that enables undue interactions with the tumor microenvironment, driving GCT pathogenesis. These epigenetic changes may be amenable to therapeutic targeting in GCT.See related commentary by Licht, p. 1794.This article is highlighted in the In This Issue feature, p. 1775.

RNA binding proteins: Linking mechanotransduction and tumor metastasis

Mechanotransduction is the leading cellular process that mammalian cells adopted to receive and respond to various mechanical cues from their local microenvironment. Increasing evidence suggests that mechano-transduction is involved in many physiological and disease conditions, ranging from early embryonic development, organogenesis, to a variety of human diseases including cancer. Mechanotransduction is mediated through several classes of senor proteins on the cell surface, intracellular signaling mediators, and core transcriptional regulation networks. Dissecting the molecular mechanisms regulating mechanotransduction and their association with cancer metastasis has received much attention in recent years. RNA binding proteins (RBPs) are a special group of nucleic acid interacting factors that participate in many important cellular processes. In this review, we would like to summarize recent research progresses in understanding the role of RBPs-mediated regulation in mechanotransduction and cancer metastasis. Those intriguing findings will provide novel insights for the disease and guide the potential development of new therapeutic approaches.

KRAS mutations in brown tumor of the jaws in hyperparathyroidism

BACKGROUND

Brown tumors are giant cell-rich lesions that result from abnormal bone metabolism in hyperparathyroidism, one of the most common endocrine disorders worldwide. Brown tumors occasionally affect the jaws and, despite well-known clinical and microscopic features, their molecular pathogenesis remains unclear. We investigated the presence of pathogenic activating mutations in TRPV4, FGFR1, and KRAS in a cohort of brown tumors since these have recently been reported in giant-cell lesions of the jaws and non-ossifying fibromas of the bones (FGFR1 and KRAS), which are histologic mimics of brown tumors.

METHODS

We target sequenced 13 brown tumors of the jaws associated with primary or secondary hyperparathyroidism. As mutations in these genes are known to activate the MAPK/ERK signaling pathway, we also assessed the immunostaining of the phosphorylated form of ERK1/2 (pERK1/2) in these lesions.

RESULTS

KRAS pathogenic mutations were detected in seven cases (p.G12V n = 4, p.G12D n = 1, p.G13D n = 1, p.A146T n = 1). KRAS variants of unknown significance (VUS), p.A134T and p.E37K, were also detected. All samples showed wild-type sequences for FGFR1 and TRPV4 genes. The activation of the MAPK/ERK signaling pathway was demonstrated by pERK1/2 immunohistochemical positivity of the brown tumors´ mononuclear cells.

CONCLUSION

Mutations in KRAS and activation of the MAPK/ERK signaling pathway were detected in brown tumors of hyperparathyroidism of the jaws, expanding the spectrum of giant cell lesions whose molecular pathogenesis involve RAS signaling.

[Molecular pathology in the diagnosis of bone tumors: current concepts]

The integrative evaluation of histology and corresponding imaging is essential for the classification of bone tumors. Until a few years ago, there were hardly any molecular markers that could be used for diagnostic purposes. However, exome- and genome-wide sequencing analyses have since uncovered a number of tumor-specific aberrations that can be very helpful in ambiguous cases. In addition to characteristic gene mutations (e.g. H3F3A and H3F3B in giant-cell tumors and chondroblastomas), the detection of fusion transcripts (e.g. structural rearrangements in the AP‑1 transcription factors FOS and FOSB in osteoid osteomas and osteoblastomas) plays an increasing role. The article gives an overview of the current state of knowledge of the most important alterations in bone tumors.

[Giant cell tumor of bone : Morphology, molecular pathogenesis, and differential diagnosis]

The histological picture of giant cell tumor of bone is characterized by numerous osteoclast-like giant cells. However, these are not the actual tumor cells, but constitute a reactive infiltrate. Rather, the tumor cells are mononuclear mesenchymal cells, which even reveal an osteoblastic line of differentiation. The CD68-positive macrophages form the second group of mononuclear cells. The receptor activator of nuclear factor kappa-B/ligand (RANK/RANKL) system, which belongs to the tumor necrosis factor (TNF) cytokine family, is decisively involved in the activation of the giant cells. It is generally accepted that a RANKL expression of mononuclear stromal cells is responsible for the development and differentiation of osteoclast-like giant cells. Therefore, the RANKL inhibitor denosumab constituted an essential element for giant cell tumor therapy over the last several years, as it blocks the maturation of osteoclasts and thus the osteolytic activity and the spread of tumor. However, with time it became evident that the not risk-free therapy with denosumab may lead to extensive recurrences upon withdrawal, so this therapy is applied with caution today.At the molecular genetic level, the giant cell tumors of bone are characterized by point mutations in the H3F3A gene. The detection of this mutation is used for the diagnostic differentiation from other bone lesions containing giant cells. Giant cell osteosarcomas rarely contain H3F3A mutations. Chondroblastoma is characterized by mutations in the H3F3B gene.

Noonan syndrome with multiple Giant cell lesions, management and treatment with surgery and interferon alpha-2a therapy: Case report

We report the case of a 14-year-old girl that was referred to the maxillo facial surgery unit at age 11 years because she exhibited swelling in the right side of her maxilla and right mandible. After a conservative surgery, she started with interferon alpha-2a to avoid recurrence. She has remained in treatment with successful results during her follow up. Considerable reduction of both maxilla and mandible lesions and bone fill have been documented. In addition, her clinical history and phenotype were suggestive of Noonan syndrome. She has short stature, broad and short neck; hypertelorism (increased distance between the eyes); downslanting palpebral fissures; sparse eyebrows and eyelashes; posteriorly rotated ears with fleshy lobes; follicular keratosis over the face, and developmental delay. Her karyotype was 46, XX. Molecular analysis of RAS/MAPK pathway genes showed a SOS1 amino acid substitution of arginine to lysine at position 552 (p.R552K). This case presents the infrequent condition of Noonan syndrome with multiple giant cell lesions (NS/MGCL) that would be the first patient as far as we know treated with surgery and interferon alpha-2a for her giant cell lesions.

Soft Tissue Special Issue: Giant Cell-Rich Lesions of the Head and Neck Region

Giant cell-rich lesions represent a heterogeneous group of tumors and non-neoplastic lesions, usually arising in bone, which harbor varying number of reactive osteoclastic-type multinucleate giant cells as a common feature. Among these entities, some are confined to the head and neck region (e.g., central giant cell granuloma and mimicking lesions, i.e., peripheral giant cell granuloma and cherubism) or show a relative predilection for this region (e.g., aneurysmal bone cyst and brown tumor of hyperparathyroidism), while others are rare but associated with distinct underlying disease (e.g., giant cell tumor of bone) or histology (e.g., tenosynovial giant cell tumor of the temporomandibular joint and phosphaturic mesenchymal tumor of the jaws) when occurring in the head and neck. Collectively, these lesions pose great challenge in the pathologic diagnosis, which often requires combinatory assessment from the clinical, histopathologic, and/or molecular aspects. This review provides a summary of pertinent clinical and pathologic features and an update of recent molecular and genetic findings of these entities. The considerations in differential diagnosis as well as the effects of the emerging therapeutic RANKL-antagonizing antibody denosumab will also be addressed.

KRAS mutations in implant-associated peripheral giant cell granuloma

OBJECTIVES

To investigate the molecular pathogenesis of implant-associated peripheral giant cell granuloma (IA-PGCG).

METHODS

A convenience sample of 15 IA-PGCG cases was selected. Hotspot mutations of KRAS, FGFR1, and TRPV4 genes, previously reported in conventional giant cell lesions of the jaws, were investigated by Sanger sequencing. As these mutations could activate MAPK/ERK pathway, the expression of phospho-ERK1/2 was also evaluated by immunohistochemistry.

RESULTS

KRAS mutations were detected in 8/15 (53.4%) samples. Similar to conventional peripheral giant cell granuloma, the KRAS mutations most frequently occurred in codon 146 (p.A146V, n = 3), followed by codon 12 (p.G12A and p.G12D, n = 1 each) and codon 14 (p.V14L, n = 1). Variants of unknown significance (VUS) were also detected in two cases, affecting codons 37 (p.E37K) and 127 (p.T127I). All samples showed wild-type (WT) sequences for FGFR1 and TRPV4 genes. Consistent with MAPK/ERK pathway activation, all mononuclear cells of the lesion showed strong staining for phospho-ERK1/2 protein in the immunohistochemical analysis.

CONCLUSIONS

KRAS mutations and activation of the MAPK-ERK signaling pathway occur in IA-PGCG. This is the first study to demonstrate cancer-associated gene mutations in a non-neoplastic reactive condition associated with dental implants.

Making sense of giant cell lesions of the jaws (GCLJ): lessons learned from next-generation sequencing

Next-generation sequencing has revealed mutations in several bone-related lesions and was recently used to uncover the genetic basis of giant cell lesions of the jaws (GCLJ). Consistent with their benign nature, GCLJ show a low tumor mutation burden. They also harbor somatic, heterozygous, mutually exclusive mutations in TRPV4, KRAS, or FGFR1. These signature mutations occur only in a subset of lesional cells, suggesting the existence of a 'landscaping effect', with mutant cells inducing abnormal accumulation of non-mutant cells that form the tumor mass. Osteoclast-rich lesions with histological similarities to GCLJ can occur in the jaws sporadically or in association with genetically inherited syndromes. Based on recent results, the pathogenesis of a subgroup of sporadic GCLJ seems closely related to non-ossifying fibroma of long bones, with both lesions sharing MAPK pathway-activating mutations. In this review, we extrapolate from these recent findings to contextualize GCLJ genetics and we highlight the therapeutic implications of this new information. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Molecular findings in maxillofacial bone tumours and its diagnostic value

According to the WHO, mesenchymal tumours of the maxillofacial bones are subdivided in benign and malignant maxillofacial bone and cartilage tumours, fibro-osseous and osteochondromatous lesions as well as giant cell lesions and bone cysts. The histology always needs to be evaluated considering also the clinical and radiological context which remains an important cornerstone in the classification of these lesions. Nevertheless, the diagnosis of maxillofacial bone tumours is often challenging for radiologists as well as pathologists, while an accurate diagnosis is essential for adequate clinical decision-making. The integration of new molecular markers in a multidisciplinary diagnostic approach may not only increase the diagnostic accuracy but potentially also identify new druggable targets for precision medicine. The current review provides an overview of the clinicopathological and molecular findings in maxillofacial bone tumours and discusses the diagnostic value of these genetic aberrations.

KRAS mutant tenosynovial giant cell tumor in a pediatric patient: a case report

Tenosynovial giant cell tumors (TSGCT) are a group of rare, benign soft tissue tumors with common histologic and cytogenetic features, with a median age of diagnosis being 47 years. Generally divided into localized and diffuse subtypes, TSGCTs are typically driven by overexpression of macrophage colony stimulating factor receptor-1 (CSF1R). Treatment of TSGCT is tumor resection, followed by radiation therapy in cases of incomplete resection. Even when the tumor is completely removed, recurrence rates can be as high as 30% in some anatomical locations. Here we report the identification of a previously undescribed KRAS p.G12D activating mutation within a pediatric TSGCT patient, who clinically presented with an enlarging right lower extremity mass pathologically consistent with TSGCT. The patient continues to be in remission three years after complete surgical removal. KRAS mutations are usually found in adult cancers, such as lung and pancreatic, as well as giant cell lesion of the jaw. This case demonstrates the utility of integrative clinical sequencing in identifying lesions with aggressive potential and aiding in complex diagnoses.

Clinical factors associated with the recurrence of central giant cell lesions

Central giant cell lesion of the jaws (CGCLJ) is a destructive condition that shows a varied and unpredictable biological behaviour. In the present study, we aimed to evaluate factors associated with the recurrence of CGCLJ. Based on the data of a previous systematic review of 2270 cases of CGCLJ, we used the multiple imputation to deal with the missing data. The dependent variable was the recurrence after the first treatment (yes/no). The dichotomic covariates were sex, upper or lower jaw location, size (up to or larger than 4 cm), pain, cortical bone perforation (yes/no), locularity (uni-/multilocular), tooth displacement (yes/no), treatment type (curettage or enucleation) and root resorption (yes/no). The final logistic model indicated that the tumours associated with tooth displacement, root resorption and treated with curettage had a more significant chance of recurrence. In conclusion, our study suggests that tooth displacement, root resorption and the type of treatment are potentially useful in the future construction of an algorithm for patient's treatment.

Multiple adenomatoid odontogenic tumors in a patient with Schimmelpenning syndrome

Schimmelpenning syndrome (SS) is a congenital neurocutaneous disorder characterized by the presence of linear nevus sebaceous, ophthalmic, neurologic, skeletal, urologic, and cardiovascular alterations. Oral manifestations related to SS mainly include dental defects, papillary lesions in the oral mucosa, giant cell lesions of the jaws, and odontogenic tumors. Here, we report the first case of multiple adenomatoid odontogenic tumor observed in a patient with SS.

Non-ossifying fibroma: A RAS-MAPK driven benign bone neoplasm

Non-ossifying fibroma (NOF) has been an intriguing entity since its first description. It is the most common bone tumour, is usually asymptomatic affecting children and adolescents, is composed of a heterogeneous cell population, and undergoes spontaneous regression after puberty. In a recent article in The Journal of Pathology, Baumhoer and colleagues demonstrate mutations activating the RAS-MAPK pathway (KRAS, FGFR1 and NF1) in ∼80% of the tumours. Activation of the RAS-MAPK pathway by somatic mutations is found in a plethora of tumour types, both benign and malignant, while germline mutations cause a wide range of syndromes collectively termed the RASopathies. Their findings indicate that NOF, for long thought to be reactive, should be considered a true neoplasm. Moreover, their data suggest that only a subset of cells in the lesion contain the mutation. A second cell population consisting of histiocytes and osteoclast-like giant cells appears to be reactive. This intimate relation between WT and mutant cells is also frequently encountered in other benign and locally aggressive bone tumours and seems essential for tumourigenesis. The spontaneous regression remains enigmatic and it is tempting to speculate that pubertal hormonal signalling, especially increased oestrogen levels, affect the balance between mutant and WT cells. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.