Patient-Derived Organoids Recapitulate Pathological Intrinsic and Phenotypic Features of Fibrous Dysplasia.

Fibrous dysplasia (FD) is a rare bone disorder characterized by the replacement of normal bone with benign fibro-osseous tissue. Developments in our understanding of the pathophysiology and treatment options are impeded by the lack of suitable research models. In this study, we developed an in vitro organotypic model capable of recapitulating key intrinsic and phenotypic properties of FD. Initially, transcriptomic profiling of individual cells isolated from patient lesional tissues unveiled intralesional molecular and cellular heterogeneity. Leveraging these insights, we established patient-derived organoids (PDOs) using primary cells obtained from patient FD lesions. Evaluation of PDOs demonstrated preservation of fibrosis-associated constituent cell types and transcriptional signatures observed in FD lesions. Additionally, PDOs retained distinct constellations of genomic and metabolic alterations characteristic of FD. Histological evaluation further corroborated the fidelity of PDOs in recapitulating important phenotypic features of FD that underscore their pathophysiological relevance. Our findings represent meaningful progress in the field, as they open up the possibility for in vitro modeling of rare bone lesions in a three-dimensional context and may signify the first step towards creating a personalized platform for research and therapeutic studies.

Transcriptomic Signature and Pro-Osteoclastic Secreted Factors of Abnormal Bone-Marrow Stromal Cells in Fibrous Dysplasia.

Fibrous dysplasia (FD) is a mosaic skeletal disorder caused by somatic activating variants of GNAS encoding for Gαs and leading to excessive cyclic adenosine monophosphate signaling in bone-marrow stromal cells (BMSCs). The effect of Gαs activation in the BMSC transcriptome and how it influences FD lesion microenvironment are unclear. We analyzed changes induced by Gαs activation in the BMSC transcriptome and secretome. RNAseq analysis of differential gene expression of cultured BMSCs from patients with FD and healthy volunteers, and from an inducible mouse model of FD, was performed, and the transcriptomic profiles of both models were combined to build a robust FD BMSC genetic signature. Pathways related to Gαs activation, cytokine signaling, and extracellular matrix deposition were identified. To assess the modulation of several key secreted factors in FD pathogenesis, cytokines and other factors were measured in culture media. Cytokines were also screened in a collection of plasma samples from patients with FD, and positive correlations of several cytokines to their disease burden score, as well as to one another and bone turnover markers, were found. These data support the pro-inflammatory, pro-osteoclastic behavior of FD BMSCs and point to several cytokines and other secreted factors as possible therapeutic targets and/or circulating biomarkers for FD.

Mandibular rhabdomyosarcoma with TFCP2 rearrangement and osteogenic differentiation: a case misdiagnosed as fibrous dysplasia or low-grade central osteosarcoma.

Rhabdomyosarcoma with TFCP2-related fusions (TFCP2-RMS) is a rare entity that commonly affects young adults with a predilection for skeletal involvement. We herein report a 40-year-old female patient with TFCP2-RMS who was misdiagnosed as fibrous dysplasia or low-grade central osteosarcoma of the mandible by referring institutions. Histologically, the tumor showed dominant spindle cells and focal epithelioid cells with marked immature woven bone formation. Immunophenotypically, in addition to the characteristic expression of myogenic markers, ALK, and cytokeratins, tumor cells also unusually expressed osteogenic markers, such as MDM2 and SATB2. Through fluorescence in situ hybridization, the tumor cells showed EWSR1::TFCP2 gene fusion and no MDM2 gene amplification. This is a rare case of TFCP2-RMS, which was misdiagnosed as low-grade central osteosarcoma due to its presenting immunophenotype of MDM2 and SATB2, as well as extensive osteoid matrix formation.

A pathogenic role for brain-derived neurotrophic factor (BDNF) in fibrous dysplasia of bone.

Brain derived neurotrophic factor (BDNF) is a neurotrophin, expressed in the central nervous system and in peripheral tissues, that is regulated by the Gsα/cAMP pathway. In bone, it regulates osteogenesis and stimulates RANKL secretion and osteoclast formation in osteolytic tumors such as Multiple Myeloma. Fibrous dysplasia (FD) of bone is a rare genetic disease of the skeleton caused by gain-of-function mutations of the Gsα gene in which RANKL-dependent enhanced bone resorption is a major cause of bone fragility and clinical morbidity. We observed that BDNF transcripts are expressed in human FD lesions. Specifically, immunolocalization studies performed on biopsies obtained from FD patients revealed the expression of BDNF in osteoblasts and, to a lower extent, in the spindle-shaped cells within the fibrous tissue. Therefore, we hypothesized that BDNF can play a role in the pathogenesis of FD by stimulating RANKL secretion and bone resorption. To test this hypothesis, we used the EF1α-GsαR201C mouse model of the human disease (FD mice). Western blot analysis revealed a higher expression of BDNF in bone segments of FD mice compared to WT mice and the immunolabeling pattern within mouse FD lesions was similar to that observed in human FD. Treatment of FD mice with a monoclonal antibody against BDNF reduced the fibrous tissue along with the number of osteoclasts and osteoblasts within femoral lesions. These results reveal BDNF as a new player in the pathogenesis of FD and a potential molecular mechanism by which osteoclastogenesis may be nourished within FD bone lesions. They also suggest that BDNF inhibition may be a new approach to reduce abnormal bone remodeling in FD.

Fibrous dysplasia in children and its management.

PURPOSE OF REVIEW: The purpose of this review is to provide a comprehensive overview into the diagnosis and management of fibrous dysplasia (FD) in children. RECENT FINDINGS: FD is a mosaic disorder arising from somatic Gα s variants, leading to impaired osteogenic cell differentiation. Fibro-osseous lesions expand during childhood and reach final disease burden in early adulthood. The mainstay of treatment focuses on surgical correction of skeletal deformities, physiatric care, and medical management of associated hyperfunctioning endocrinopathies. Bisphosphonates may be helpful to treat bone pain, but do not alter lesion quality or progression. Emerging evidence suggests that the RANKL inhibitor denosumab may be effective in improving lesion activity and mineralization, however further studies are needed to determine the potential utility of this and other novel therapies, particularly in children with FD. SUMMARY: Management of children with FD has unique challenges related to skeletal growth and age-related lesion progression. Inclusion of children in clinical research is critical to develop effective treatment strategies to treat FD lesions and prevent their development.

PTHrP Modulates the Proliferation and Osteogenic Differentiation of Craniofacial Fibrous Dysplasia-Derived BMSCs.

Fibrous dysplasia (FD) is a skeletal stem cell disease caused by mutations in the guanine nucleotide-binding protein, alpha-stimulating activity polypeptide (GNAS) gene, which results in the abnormal accumulation of cyclic adenosine monophosphate (cAMP) and hyperactivation of downstream signaling pathways. Parathyroid hormone-related protein (PTHrP) is secreted by the osteoblast lineage and is involved in various physiological and pathological activities of bone. However, the association between the abnormal expression of PTHrP and FD, as well as its underlying mechanism, remains unclear. In this study, we discovered that FD patient-derived bone marrow stromal cells (FD BMSCs) expressed significantly higher levels of PTHrP during osteogenic differentiation and exhibited greater proliferation capacity but impaired osteogenic ability compared to normal control patient-derived BMSCs (NC BMSCs). Continuous exogenous PTHrP exposure on the NC BMSCs promoted the FD phenotype in both in vitro and in vivo experiments. Through the PTHrP/cAMP/PKA axis, PTHrP could partially influence the proliferation and osteogenesis capacity of FD BMSCs via the overactivation of the Wnt/ß-Catenin signaling pathway. Furthermore, PTHrP not only directly modulated cAMP/PKA/CREB transduction but was also demonstrated as a transcriptional target of CREB. This study provides novel insight into the possible pathogenesis involved in the FD phenotype and enhances the understanding of its molecular signaling pathways, offering theoretical evidence for the feasibility of potential therapeutic targets for FD.

Identification of GNAS Variants in Circulating Cell-Free DNA from Patients with Fibrous Dysplasia/McCune Albright Syndrome.

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare mosaic bone and endocrine disorder. Although most variants affect the GNAS R201 codon, obtaining a genetic diagnosis is difficult because not all cells harbor the variant, and an invasive biopsy may be required. We explored the presence of GNAS p.R201 variants in blood circulating cell free DNA (ccfDNA) using sensitive techniques of digital droplet polymerase chain reaction (PCR) (ddPCR) and competitive allele-specific TaqMan PCR (castPCR) in an effort to improve the genetic diagnosis of FD/MAS. We isolated ccfDNA from the plasma of 66 patients with a wide range of disease severity and performed both ddPCR and castPCR mutation analysis to search for GNAS p.R201H or R201C variants. We detected R201 variants in ccfDNA samples of 41 of 66 (62.1%) patients by either castPCR or ddPCR, and 45 of 66 (68.2%) of patients if the techniques were combined. Variant detection was more likely in patients with more severe disease. Skeletal disease burden score (SBS) was significantly higher in patients who had detectable variants, and SBS was a predictor of variant allele frequency. By ddPCR analysis, patients aged ≤30 years had higher detection rates, and higher variant allele frequencies, independent of disease burden. We detected variant DNA in only one patient with monostotic FD by ddPCR only. In summary, we have demonstrated that ccfDNA containing variant GNAS can be isolated from the plasma of patients with FD/MAS and that ddPCR and castPCR methods have similar variant detection rates. This methodology represents an important potential advancement in diagnosis for patients with FD/MAS, especially those younger than 30 years or with more severe disease. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

GNAS mutation analysis assists in differentiating chronic diffuse sclerosing osteomyelitis from fibrous dysplasia in the jaw.

Chronic diffuse sclerosing osteomyelitis of the mandible (DSOM) and fibrous dysplasia (FD) are distinct lesions with overlapping clinicopathological features that complicate their diagnosis. This study aimed to evaluate the efficacy of GNAS mutation analysis in differentiating between these two conditions. DNA samples from patients with DSOM (n = 35) and FD (n = 29) were collected to analyze the presence of GNAS mutations in exons 8 and 9, the two previously reported hotspot regions, using polymerase chain reaction and direct sequencing. Twenty-four of 29 patients (83%) with FD showed missense mutations in codon 201 in exon 8, whereas no mutation was detected in exon 9. No mutations were found in any of the 35 cases with DSOM. We also identified one case with an uncertain diagnosis due to overlapping clinicopathological features of DSOM and FD. A Q227H mutation was detected in this case, that confirmed the diagnosis of FD. Taken together, the findings indicate that mutational analysis of the GNAS is a reliable approach to differentiate between DSOM and FD of the jaw.

Fibrous dysplasia animal models: A systematic review.

BACKGROUND: Fibrous dysplasia (FD) is a rare genetic bone disorder resulting in an overproduction of cAMP leading to a structurally unsound tissue, caused by a genetic mutation in the guanine nucleotide-binding protein gene (GNAS). In order to better understand this disease, several animal models have been developed with different strategies and features. OBJECTIVE: Conduct a systematic review to analyze and compare animal models with the causative mutation and features of FD. METHODS: A PRISMA search was conducted in Scopus, PubMed, and Web of Science. Studies reporting an in vivo model of FD that expressed the causative mutation were included for analysis. Models without the causative mutation, but developed an FD phenotype and models of FD cell implantation were included for subanalysis. RESULTS: Seven unique models were identified. The models were assessed and compared for their face validity, construct validity, mosaicism, and induction methods. This was based on the features of clinical FD that were reported within the categories of: macroscopic features, imaging, histology and histomorphometry, histochemical and cellular markers, and blood/urine markers. LIMITATIONS: None of the models reported all features of FD and some features were only reported in one model. This made comparing models a challenge, but indicates areas where further research is necessary. CONCLUSION: The benefits and disadvantages of every model were assessed from a practical and scientific standpoint. While all published reports lacked complete data, the models have nonetheless informed our understanding of FD and provided meaningful information to guide researchers in bench and clinical research.

Copy number alteration profiling facilitates differential diagnosis between ossifying fibroma and fibrous dysplasia of the jaws.

Ossifying fibroma (OF) and fibrous dysplasia (FD) are two fibro-osseous lesions with overlapping clinicopathological features, making diagnosis challenging. In this study, we applied a whole-genome shallow sequencing approach to facilitate differential diagnosis via precise profiling of copy number alterations (CNAs) using minute amounts of DNA extracted from morphologically correlated microdissected tissue samples. Freshly frozen tissue specimens from OF (n = 29) and FD (n = 28) patients were obtained for analysis. Lesion fibrous tissues and surrounding normal tissues were obtained by laser capture microdissection (LCM), with ~30-50 cells (5 000-10 000 µm2) per sample. We found that the rate of recurrent CNAs in OF cases was much higher (44.8%, 13 of 29) than that in FD cases (3.6%, 1 of 28). Sixty-nine percent (9 of 13) of the CNA-containing OF cases involved segmental amplifications and deletions on Chrs 7 and 12. We also identified eight CNA-associated genes (HILPDA, CALD1, C1GALT1, MICALL2, PHF14, AIMP2, MDM2, and CDK4) with amplified expression, which was consistent with the copy number changes. We further confirmed a jaw lesion with a previous uncertain diagnosis due to its ambiguous morphological features and the absence of GNAS mutation as OF based on the typical Chr 12 amplification pattern in its CNA profile. Moreover, analysis of a set of longitudinal samples collected from an individual with a cellular lesion in suspicion of OF at the first surgery, recurrence and the latest malignant transformation revealed identical CNA patterns at the three time points, suggesting that copy number profiling can be used as an important tool to identify borderline lesions or lesions with malignant potential. Overall, CNA profiling of fibro-osseous lesions can greatly improve differential diagnosis between OF and FD and help predict disease progression.

Frontal bone fibrous dysplasia in a 6-months-old boy: A distinctive entity.

Fibrous Dysplasia (FD) is a non-malignant condition caused by post-zygotic, activating mutations of the GNAS gene that results in inhibition of the differentiation and proliferation of bone-forming stromal cells and leads to the replacement of normal bone and marrow by fibrous tissue and woven bone. The clinical behavior and progression of FD is variable. The management of this condition is difficult and in every case is strictly individualized. We report a case of frontal fibrous dysplasia in a 6month's old boy who underwent a successfully resection of the lesion with an excellent cosmetic effect.

Genotype-Phenotype Correlation in Fibrous Dysplasia/McCune-Albright Syndrome.

CONTEXT: Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare bone and endocrine disorder resulting in fractures, pain, and disability. There are no targeted or effective therapies to alter the disease course. Disease arises from somatic gain-of-function variants at the R201 codon in GNAS, replacing arginine by either cysteine or histidine. The relative pathogenicity of these variants is not fully understood. OBJECTIVE: This work aimed 1) to determine whether the most common GNAS variants (R201C and R201H) are associated with a specific clinical phenotype, and 2) to determine the prevalence of the most common GNAS variants in a large patient cohort. METHODS: This retrospective cross-sectional analysis measured the correlation between genotype and phenotype characterized by clinical, biochemical, and radiographic data. RESULTS: Sixty-one individuals were genotyped using DNA extracted from tissue or circulating cell-free DNA. Twenty-two patients (36.1%) had the R201C variant, and 39 (63.9%) had the R201H variant. FD skeletal disease burden, hypophosphatemia prevalence, fracture incidence, and ambulation status were similar between the 2 groups. There was no difference in the prevalence of endocrinopathies, ultrasonographic gonadal or thyroid abnormalities, or pancreatic involvement. There was a nonsignificant association of cancer with the R201H variant. CONCLUSION: There is no clear genotype-phenotype correlation in patients with the most common FD/MAS pathogenic variants. The predominance of the R201H variant observed in our cohort and reported in the literature indicates it is likely responsible for a larger burden of disease in the overall population of patients with FD/MAS, which may have important implications for the future development of targeted therapies.

Osteosarcoma of the Mandible in a Patient with Florid Cemento-Osseous Dysplasia and Li-Fraumeni Syndrome: A Rare Coincidence.

Cemento-osseous dysplasia (COD) is the most common benign fibro-osseous lesion of the jaws and generally considered non-neoplastic and self-limited. Here, we present a 30-year old female who noticed a bilateral swelling of her posterior mandible with irregular periapical mineralization and incomplete root resorption on panoramic radiographs. A biopsy revealed florid COD and no further treatment was initiated. 9 years later, she presented with a progressive expansion of her left posterior mandible after being treated for bilateral breast cancer 4 and 8 years before. CT scans showed expansile and densely mineralized lesions in all four quadrants with the left posterior mandible showing a focal penetration of the buccal cortical bone. Biopsies revealed an osteoblastic high-grade osteosarcoma in the left and a COD in the right mandible, notably with cellular atypia in the spindle cell component. The patient underwent segmental resection of the left mandible with clear margins and adjuvant chemotherapy. Subsequent genetic testing identified a heterozygous germline TP53 mutation (p.V173G) which confirmed the clinically suspected Li-Fraumeni syndrome (LFS). 3 years after the resection, the patient is free of disease and the other foci of COD remained stable in size on follow-up imaging analyses. Our case illustrates LFS-related osteosarcoma developing within florid COD. Given the rarity of this coincidence, a causative relation between the two lesions seems unlikely but in patients with tumor predisposition syndromes it might be advisable to closely monitor even benign lesions like COD.

Role of Genetic Mutations of the Na+/H+ Exchanger Isoform 1, in Human Disease and Protein Targeting and Activity.

The mammalian Na+/H+ exchanger isoform one (NHE1) is a plasma membrane protein that is ubiquitously present in human cells. It functions to regulate intracellular pH removing an intracellular proton in exchange for one extracellular sodium and is involved in heart disease and in promoting metastasis in cancer. It is made of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. The membrane domain is thought to have 12 transmembrane segments and a large membrane-associated extracellular loop. Early studies demonstrated that in mice, disruption of the NHE1 gene results in locomotor ataxia and a phenotype of slow-wave epilepsy. Defects included a progressive neuronal degeneration. Growth and reproductive ability were also reduced. Recent studies have identified human autosomal homozygous recessive mutations in the NHE1 gene (SLC9A1) that result in impaired development, ataxia and other severe defects, and explain the cause of the human disease Lichtenstein-Knorr syndrome. Other human mutations have been identified that are stop codon polymorphisms. These cause short non-functional NHE1 proteins, while other genetic polymorphisms in the NHE1 gene cause impaired expression of the NHE1 protein, reduced activity, enhanced protein degradation or altered kinetic activation of the protein. Since NHE1 plays a key role in many human physiological functions and in human disease, genetic polymorphisms of the protein that significantly alter its function and are likely play significant roles in varying human phenotypes and be involved in disease.

Copy number alteration profiling facilitates differential diagnosis between ossifying fibroma and fibrous dysplasia of the jaws / 国际口腔科学杂志·英文版

Ossifying fibroma (OF) and fibrous dysplasia (FD) are two fibro-osseous lesions with overlapping clinicopathological features, making diagnosis challenging. In this study, we applied a whole-genome shallow sequencing approach to facilitate differential diagnosis via precise profiling of copy number alterations (CNAs) using minute amounts of DNA extracted from morphologically correlated microdissected tissue samples. Freshly frozen tissue specimens from OF (n = 29) and FD (n = 28) patients were obtained for analysis. Lesion fibrous tissues and surrounding normal tissues were obtained by laser capture microdissection (LCM), with ~30-50 cells (5 000-10 000 µm

Discondrosteosis de Léri-Weill. Presentación de una familia afectada / Léri-Weill dyschondrosteosis. Presentation of an affected family

RESUMEN Fundamentación: La discondrosteosis de Léri-Weill, displasia ósea de origen genético que afecta la región mesomélica con acortamiento de las extremidades, provoca talla baja con extremidades cortas con deformidad de Madelung; esta enfermedad muestra un patrón de herencia autosómico dominante con alta penetrancia. Objetivo: Describir las deformidades de esta discondrosteosis de baja frecuencia con expresividad variable, que se presentó de la misma forma en todos los afectados de esta familia. Presentación de caso: Se reportó una familia con enfermos en tres generaciones con deformidad de Madelung de ambas muñecas y baja estatura de origen mesomélico, que se mantiene seguimiento en consultas de Genética Clínica y Ortopedia. Conclusiones: El examen físico y radiológico imprescindibles para llegar al diagnóstico clínico. El método clínico y la valoración multidisciplinaria resultaron de gran valor para definir esta enfermedad y poder brindar un adecuado asesoramiento genético a esta familia.

ABSTRACT Background: Léri-Weill dyschondrosteosis, bone dysplasia of genetic origin that affects the mesomelic region with shortening of the extremities, causes short stature with short extremities with Madelung deformity.This disease shows an autosomal dominant inheritance pattern with high penetrance. Objective: To describe the deformities of this low frequency dyschondrosteosis with variable expressivity which was presented in the same way in all those affected in this family. Case presentation: A family with sick members was reported in three generations with Madelung deformity of both wrists and short stature of mesomelic origin which is followed up in consultations of Clinical Genetics and Orthopedics. Conclusion: The essential physical and radiological examination to reach the clinical diagnosis. The clinical method and the multidisciplinary assessment were of great value to define this disease and to be able to provide adequate genetic counseling to this family.

Discondrosteosis de Léri-Weill. Presentación de una familia afectada / Léri-Weill dyschondrosteosis. Presentation of an affected family

RESUMEN Fundamentación: La discondrosteosis de Léri-Weill, displasia ósea de origen genético que afecta la región mesomélica con acortamiento de las extremidades, provoca talla baja con extremidades cortas con deformidad de Madelung; esta enfermedad muestra un patrón de herencia autosómico dominante con alta penetrancia. Objetivo: Describir las deformidades de esta discondrosteosis de baja frecuencia con expresividad variable, que se presentó de la misma forma en todos los afectados de esta familia. Presentación de caso: Se reportó una familia con enfermos en tres generaciones con deformidad de Madelung de ambas muñecas y baja estatura de origen mesomélico, que se mantiene seguimiento en consultas de Genética Clínica y Ortopedia. Conclusiones: El examen físico y radiológico imprescindibles para llegar al diagnóstico clínico. El método clínico y la valoración multidisciplinaria resultaron de gran valor para definir esta enfermedad y poder brindar un adecuado asesoramiento genético a esta familia.

ABSTRACT Background: Léri-Weill dyschondrosteosis, bone dysplasia of genetic origin that affects the mesomelic region with shortening of the extremities, causes short stature with short extremities with Madelung deformity.This disease shows an autosomal dominant inheritance pattern with high penetrance. Objective: To describe the deformities of this low frequency dyschondrosteosis with variable expressivity which was presented in the same way in all those affected in this family. Case presentation: A family with sick members was reported in three generations with Madelung deformity of both wrists and short stature of mesomelic origin which is followed up in consultations of Clinical Genetics and Orthopedics. Conclusion: The essential physical and radiological examination to reach the clinical diagnosis. The clinical method and the multidisciplinary assessment were of great value to define this disease and to be able to provide adequate genetic counseling to this family.

A Signature of Circulating miRNAs Associated With Fibrous Dysplasia of Bone: the mirDys Study.

Fibrous dysplasia (FD) is a rare bone disease caused by activating mutations of GNAS encoding the Gsα protein, enhancing cyclic adenosine monophosphate (cAMP) production by overstimulation of adenylyl cyclase and impairing osteoblastic differentiation. The clinical presentation ranges from asymptomatic to polyostotic forms with severe disability, explained by the mosaic distribution of the GNAS mutation. Physicians have to deal with the gap of knowledge in FD pathogenesis, the absence of prognostic markers and the lack of specific treatment. The identification of specific biomarkers for FD is an important step to improve the clinical and therapeutic approaches. An epigenetic regulation driven by microRNAs (miRNAs), known as promising biomarkers in bone disease, could be involved in FD. We have sought circulating miRNAs that are differentially expressed in FD patients compared to controls and would reflect dysregulations of osteogenesis-related genes and bone disorder. The global miRNA profiling was performed using Next Generation Sequencing in patient serum collected from a discovery cohort of 20 patients (10 polyostotic and 10 monostotic) and 10 controls. From these, we selected 19 miRNAs for a miRNA validation phase from serum of 82 patients and 82 controls, using real-time qPCR. Discovery screening identified 111 miRNAs differentially expressed in patient serum, after adjusting for the false discovery rate (FDR). Among the 82 patients, 55% were polyostotic, and 73% were women with a mean age of 42 years. Six miRNAs (miR-25-3p, miR-93-5p, miR-182-5p, miR-324-5p, miR-363-3p, and miR-451a) were significantly overexpressed in serum, with FDR <0.05. The expression level of these six miRNAs was not associated with the FD severity. In conclusion, we identified a signature of circulating miRNAs associated with FD. These miRNAs are potential negative regulators of gene expression in bone cell progenitors, suggesting their activity in FD by interfering with osteoblastic and osteoclastic differentiation to impair bone mineralization and remodeling processes. © 2020 American Society for Bone and Mineral Research.

Changes in gene expression in human skeletal stem cells transduced with constitutively active Gsα correlates with hallmark histopathological changes seen in fibrous dysplastic bone.

Fibrous dysplasia (FD) of bone is a complex disease of the skeleton caused by dominant activating mutations of the GNAS locus encoding for the α subunit of the G protein-coupled receptor complex (Gsα). The mutation involves a substitution of arginine at position 201 by histidine or cysteine (GsαR201H or R201C), which leads to overproduction of cAMP. Several signaling pathways are implicated downstream of excess cAMP in the manifestation of disease. However, the pathogenesis of FD remains largely unknown. The overall FD phenotype can be attributed to alterations of skeletal stem/progenitor cells which normally develop into osteogenic or adipogenic cells (in cis), and are also known to provide support to angiogenesis, hematopoiesis, and osteoclastogenesis (in trans). In order to dissect the molecular pathways rooted in skeletal stem/progenitor cells by FD mutations, we engineered human skeletal stem/progenitor cells with the GsαR201C mutation and performed transcriptomic analysis. Our data suggest that this FD mutation profoundly alters the properties of skeletal stem/progenitor cells by pushing them towards formation of disorganized bone with a concomitant alteration of adipogenic differentiation. In addition, the mutation creates an altered in trans environment that induces neovascularization, cytokine/chemokine changes and osteoclastogenesis. In silico comparison of our data with the signature of FD craniofacial samples highlighted common traits, such as the upregulation of ADAM (A Disintegrin and Metalloprotease) proteins and other matrix-related factors, and of PDE7B (Phosphodiesterase 7B), which can be considered as a buffering process, activated to compensate for excess cAMP. We also observed high levels of CEBPs (CCAAT-Enhancer Binding Proteins) in both data sets, factors related to browning of white fat. This is the first analysis of the reaction of human skeletal stem/progenitor cells to the introduction of the FD mutation and we believe it provides a useful background for further studies on the molecular basis of the disease and for the identification of novel potential therapeutic targets.