A mutant PTH/PTHrP type I receptor in enchondromatosis

Etiological Mechanisms and Genetic/Biological Modulation Related to PTH1R in Primary Failure of Tooth Eruption

Primary failure of eruption (PFE) is a rare disorder that is characterized by the inability of a molar tooth/teeth to erupt to the occlusal plane or to normally react to orthodontic force. This condition is related to hereditary factors and has been extensively researched over many years. However, the etiological mechanisms of pathogenesis are still not fully understood. Evidence from studies on PFE cases has shown that PFE patients may carry parathyroid hormone 1 receptor (PTH1R) gene mutations, and genetic detection can be used to diagnose PFE at an early stage. PTH1R variants can lead to altered protein structure, impaired protein function, and abnormal biological activities of the cells, which may ultimately impact the behavior of teeth, as observed in PFE. Dental follicle cells play a critical role in tooth eruption and root development and are regulated by parathyroid hormone-related peptide (PTHrP)-PTH1R signaling in their differentiation and other activities. PTHrP-PTH1R signaling also regulates the activity of osteoblasts, osteoclasts and odontoclasts during tooth development and eruption. When interference occurs in the PTHrP-PTH1R signaling pathway, the normal function of dental follicles and bone remodeling are impaired. This review provides an overview of PTH1R variants and their correlation with PFE, and highlights that a disruption of PTHrP-PTH1R signaling impairs the normal process of tooth development and eruption, thus providing insight into the underlying mechanisms related to PTH1R and its role in driving PFE.

Hedgehog activation promotes osteogenic fates of growth plate resting zone chondrocytes through transient clonal competency

The resting zone of the postnatal growth plate is organized by slow-cycling chondrocytes expressing parathyroid hormone-related protein (PTHrP), which include a subgroup of skeletal stem cells that contribute to the formation of columnar chondrocytes. The PTHrP-Indian hedgehog feedback regulation is essential for sustaining growth plate activities; however, molecular mechanisms regulating cell fates of PTHrP+ resting chondrocytes and their eventual transformation into osteoblasts remain largely undefined. Here, in a mouse model, we specifically activated Hedgehog signaling in PTHrP+ resting chondrocytes and traced the fate of their descendants using a tamoxifen-inducible Pthrp-creER line with patched-1-floxed and tdTomato reporter alleles. Hedgehog-activated PTHrP+ chondrocytes formed large, concentric, clonally expanded cell populations within the resting zone ("patched roses") and generated significantly wider columns of chondrocytes, resulting in hyperplasia of the growth plate. Interestingly, Hedgehog-activated PTHrP+ cell descendants migrated away from the growth plate and transformed into trabecular osteoblasts in the diaphyseal marrow space in the long term. Therefore, Hedgehog activation drives resting zone chondrocytes into transit-amplifying states as proliferating chondrocytes and eventually converts these cells into osteoblasts, unraveling a potentially novel Hedgehog-mediated mechanism that facilitates osteogenic cell fates of PTHrP+ skeletal stem cells.

Identification of a novel heterozygous PTH1R variant in a Chinese family with incomplete penetrance

BACKGROUND

Mutations in PTH1R are associated with Jansen-type metaphyseal chondrodysplasia (JMC), Blomstrand osteochondrodysplasia (BOCD), Eiken syndrome, enchondroma, and primary failure of tooth eruption (PFE). Inheritance of the PTH1R gene can be either autosomal dominant or autosomal recessive, indicating the complexity of the gene. Our objective was to identify the phenotypic differences in members of a family with a novel PTH1R mutation.

METHODS

The proband was a 13-year, 6-month-old girl presenting with short stature, abnormal tooth eruption, skeletal dysplasia, and midface hypoplasia. The brother and father of the proband presented with short stature and abnormal tooth eruption. High-throughput sequencing was performed on the proband, and the variant was confirmed in the proband and other family members by Sanger sequencing. Amino acid sequence alignment was performed using ClustalX software. Three-dimensional structures were analyzed and displayed using the I-TASSER website and PyMOL software.

RESULTS

High-throughput genome sequencing and Sanger sequencing validation showed that the proband, her father, and her brother all carried the PTH1R (NM_000316) c.1393G>A (p.E465K) mutation. The c.1393G>A (p.E465K) mutation was novel, as it has not been reported in the literature database. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, the p.E465K variant was considered to have uncertain significance. Biological information analysis demonstrated that this identified variant was highly conserved and highly likely pathogenic.

CONCLUSIONS

We identified a novel heterozygous mutation in the PTH1R gene leading to clinical manifestations with incomplete penetrance that expands the spectrum of known PTH1R mutations.

A rare presentation of Maffucci syndrome: A case report and literature review

Maffucci syndrome is an extremely rare disease which can manifest symptoms as early as childhood. It is estimated that there have been <300 cases reported globally; however, this number is likely to be an underestimate. Maffucci syndrome is characterized by multiple enchondromas and soft tissue hemangiomas, which can cause growth and developmental malformations. In addition to bone deformities, pathological fractures and a loss of mobility, patients with Maffucci syndrome may develop secondary central chondrosarcoma and have a higher risk of developing non-skeletal malignant tumors, such as gliomas and mesenchymal ovarian tumors. The present study provides information for clinicians about this disease through the use of imaging, physical examinations, clinical manifestations and the treatment strategy used. There is need to summarize the existing cases of this disease around the world and produce an effective framework for the diagnosis, treatment and prevention of Maffucci syndrome, in order to better understand this disease. The present study reports on a 15-year-old male diagnosed with Maffucci syndrome. . Due to the risk of malignant tumor development in the absence of effective treatment, regular and careful observation through monitoring of tumor markers and imaging studies is important for patients with Maffucci syndrome. As cases of this disease are rare and case data is limited, it is difficult to create a clear treatment plan. There is an urgent need to establish a case database of Maffucci syndrome patients and explore its pathogenesis for early diagnosis, treatment and prevention of disease.

New bone formation accelerates during lower limb lengthening and deformity correction in children with Ollier's disease

BACKGROUND

Ollier's disease can cause severe length discrepancy of the lower extremities and deformity in children. Osteotomy and limb lengthening with external fixation can correct the limb deformity. This study evaluated (1) whether the duration of external fixation was reduced in patients with Ollier's disease, and (2) the incidence of complications such as pin tract infection, external fixation loosening, and joint stiffness.

METHODS

Two groups were compared with respect to age, angular correction (AC), lengthening gap (LG), distraction index (DI), lengthening length (LL), lengthening length percentage (L%), lengthening index (LI), bone healing index (BHI), and external fixation index (EFI). Group 1 (Ollier's disease) comprised nine patients undergoing 11 lower limb lengthening procedures using external fixators; group 2 (control, normal lengthened bone) comprised 28 patients undergoing 29 lengthening procedures with external fixators.

RESULTS

In patients with Ollier's disease, full correction of the deformity and full restoration of length were achieved in all cases. In the femur, the mean AC (15.97° vs. 6.72°) and DI (1.11 mm/day vs. 0.78 mm/day) were significantly larger, while the LI (9.71 days/cm vs. 13.49 days/cm), BHI (27.00 days/cm vs. 42.09 days/cm), and EFI (37.86 days/cm vs. 56.97 days/cm) were all significantly shorter in group 1 than in group 2 (p < 0.05). In the tibia, the mean AC and L% were larger, while the LG, LI, BHI, and EFI were all shorter in group 1 than in group 2. There was no significant difference between the two groups in the incidence of complications.

CONCLUSION

In children with Ollier's disease, new bone formation accelerated and the healing speed of the lengthened segments was faster throughout the whole lengthening period with external fixation, and full correction of the deformity and full restoration of length could be achieved.

LEVEL OF EVIDENCE III

Mutant IDH regulates glycogen metabolism from early cartilage development to malignant chondrosarcoma formation

Chondrosarcomas are the most common malignancy of cartilage and are associated with somatic mutations in isocitrate dehydrogenase 1 (IDH1) and IDH2 genes. Somatic IDH mutations are also found in its benign precursor lesion, enchondromas, suggesting that IDH mutations are early events in malignant transformation. Human mutant IDH chondrosarcomas and mutant Idh mice that develop enchondromas investigated in our studies display glycogen deposition exclusively in mutant cells from IDH mutant chondrosarcomas and Idh1 mutant murine growth plates. Pharmacologic blockade of glycogen utilization induces changes in tumor cell behavior, downstream energetic pathways, and tumor burden in vitro and in vivo. Mutant IDH1 interacts with hypoxia-inducible factor 1α (HIF1α) to regulate expression of key enzymes in glycogen metabolism. Here, we show a critical role for glycogen in enchondromas and chondrosarcomas, which is likely mediated through an interaction with mutant IDH1 and HIF1α.

Growth plate resting zone chondrocytes acquire transient clonal competency upon Hedgehog activation and efficiently transform into trabecular bone osteoblasts

The resting zone of the postnatal growth plate is organized by slow-cycling chondrocytes expressing parathyroid hormone-related protein (PTHrP), which include a subgroup of skeletal stem cells that contribute to the formation of columnar chondrocytes. The PTHrP-indian hedgehog (Ihh) feedback regulation is essential for sustaining growth plate activities; however, molecular mechanisms regulating cell fates of PTHrP + resting chondrocytes and their eventual transformation into osteoblasts remain largely undefined. Here, in a mouse model, we utilized a tamoxifen-inducible PTHrP-creER line with Patched-1 ( Ptch1 ) floxed and tdTomato reporter alleles to specifically activate Hedgehog signaling in PTHrP + resting chondrocytes and trace the fate of their descendants. Hedgehog-activated PTHrP + chondrocytes formed large concentric clonally expanded cell populations within the resting zone (' patched roses ') and generated significantly wider columns of chondrocytes, resulting in hyperplasia of the growth plate. Interestingly, Hedgehog-activated PTHrP + cell-descendants migrated away from the growth plate and eventually transformed into trabecular osteoblasts in the diaphyseal marrow space in the long term. Therefore, Hedgehog activation drives resting zone chondrocytes into transit-amplifying states as proliferating chondrocytes and eventually converts these cells into osteoblasts, unraveling a novel Hedgehog-mediated mechanism that facilitates osteogenic cell fates of PTHrP + skeletal stem cells.

Hedgehog signaling orchestrates cartilage-to-bone transition independently of Smoothened

Although recent lineage studies strongly support a chondrocyte-to-osteoblast differentiation continuum, the biological significance and molecular basis remain undetermined. In silico analysis at a single-cell level indicates a transient shutdown of Hedgehog-related transcriptome during simulated cartilage-to-bone transition. Prompted by this, we genetically induce gain- and loss-of function to probe the role of Hedgehog signaling in cartilage-to-bone transition. Ablating Smo in hypertrophic chondrocytes (HCs) does not result in any phenotypic outcome, whereas deleting Ptch1 in HCs leads to disrupted formation of primary spongiosa and actively proliferating HCs-derived osteogenic cells that contribute to bony bulges seen in adult mutant mice. In HCs-derived osteoblasts, constitutive activation of Hedgehog signaling blocks their further differentiation to osteocytes. Moreover, ablation of both Smo and Ptch1 in HCs reverses neither persistent Hedgehog signaling nor bone overgrowths. These results establish a functional contribution of extended chondrocyte lineage to bone homeostasis and diseases, governed by an unanticipated mode of regulation for Hedgehog signaling independently of Smo.

The role of the protein tyrosine phosphatase SHP2 in ossification

SHP2, encoded by the PTPN11 gene, participates in multiple cell functions including cell proliferation, movement, and differentiation. PTPN11 loss-of-function and gain-of-function mutations are both associated with diseases, such as Noonan syndrome, whose manifestations include bone defects, suggesting a crucial role for SHP2 in the skeleton. However, the exact mechanisms by which SHP2 regulates bone development remain unclear. This review focuses on the current understanding of the regulation of SHP2 and highlights the vital roles of SHP2 in skeletal development, especially its roles in ossification. Overall, a better understanding of the functions of SHP2 in ossification will provide a new avenue to treat-related skeletal diseases.

SAG therapy restores bone growth and reduces enchondroma incidence in a model of skeletal chondrodysplasias caused by Ihh deficiency

Inactivation mutations in the Indian hedgehog (Ihh) gene in humans cause numerous skeletal chondrodysplasias, including acrocapitofemoral dysplasia, brachydactyly type A1, and human short stature. The lack of an appropriate human-relevant model to accurately represent these chondrodysplasias has hampered the identification of clinically effective treatments. Here, we established a mouse model of human skeletal dysplasia induced by Ihh gene mutations via ablation of Ihh in Aggrecan-positive (Acan+) cells using Aggrecan (Acan)-creERT transgenic mice. Smoothen agonist (SAG) promoted Hh activity and rescued chondrocyte proliferation and differentiation by stimulating smoothened trafficking to the cilium in Ihh-silenced cells. SAG treatment corrected mouse stature and significantly decreased mortality without evidence of toxicity. Moreover, Ihh ablation in Acan+ cells produced enchondroma-like tissues near the growth plates that were significantly reduced by SAG treatment. These results demonstrated that SAG effectively treats skeletal dysplasia caused by Ihh gene mutations in a mouse model, suggesting that SAG may represent a potential drug for the treatment of these diseases and/or enchondromas.

PTHR1 in osteosarcoma: Specific molecular mechanisms and comprehensive functional perspective

Osteosarcoma occurs largely in children and adolescents and is the most common primary malignant tumour of bone. Although surgical advances and neoadjuvant chemotherapy have made great strides in recent years, rates of local recurrence and lung metastasis remain high, with a plateau in overall survival during the past decade. It is thus urgent to explore the pathogenesis of osteosarcoma and identify potential therapeutic targets. Parathyroid hormone receptor 1 (PTHR1) belongs to the broad family of G protein–coupled receptors, binding both parathyroid hormone (PTH) and parathyroid hormone–related peptide (PTHrP, a paracrine factor). Previous studies have shown that in tissues and cells of osteosarcoma, expression of PTHR1 is markedly increased, correlating with aggressive biologic behaviour and a poor prognosis. PTHR1 expression also correlates closely with epigenetic regulation, transcriptional regulation, post‐translational modification and protein interaction. Herein, we have summarized the latest research on the role played by PTHR1 in progression of osteosarcoma, assessing its clinical utility as a novel biomarker and its therapeutic ramifications.

Tendon-derived cathepsin K-expressing progenitor cells activate Hedgehog signaling to drive heterotopic ossification

Heterotopic ossification (HO) is pathological bone formation characterized by ossification within muscle, tendons, or other soft tissues. However, the cells of origin and mechanisms involved in the pathogenesis of HO remain elusive. Here we show that deletion of suppressor of fused (Sufu) in cathepsin K-Cre-expressing (Ctsk-Cre-expressing) cells resulted in spontaneous and progressive ligament, tendon, and periarticular ossification. Lineage tracing studies and cell functional analysis demonstrated that Ctsk-Cre could label a subpopulation of tendon-derived progenitor cells (TDPCs) marked by the tendon marker Scleraxis (Scx). Ctsk+Scx+ TDPCs are enriched for tendon stem cell markers and show the highest self-renewal capacity and differentiation potential. Sufu deficiency caused enhanced chondrogenic and osteogenic differentiation of Ctsk-Cre-expressing tendon-derived cells via upregulation of Hedgehog (Hh) signaling. Furthermore, pharmacological intervention in Hh signaling using JQ1 suppressed the development of HO. Thus, our results show that Ctsk-Cre labels a subpopulation of TDPCs contributing to HO and that their cell-fate changes are driven by activation of Hh signaling.

Enchondromatosis and Growth Plate Development

PURPOSE OF REVIEW

Enchondroma is a common cartilage benign tumor that develops from dysregulation of chondrocyte terminal differentiation during growth plate development. Here we provide an overview of recent progress in understanding causative mutations for enchondroma, dysregulated signaling and metabolic pathways in enchondroma, and the progression from enchondroma to malignant chondrosarcoma.

RECENT FINDINGS

Several signaling pathways that regulate chondrocyte differentiation are dysregulated in enchondromas. Somatic mutations in the metabolic enzymes isocitrate dehydrogenase 1 and 2 (IDH1/2) are the most common findings in enchondromas. Mechanisms including metabolic regulation, epigenetic regulation, and altered signaling pathways play a role in enchondroma formation and progression. Multiple pathways regulate growth plate development in a coordinated manner. Deregulation of the process can result in chondrocytes failing to undergo differentiation and the development of enchondroma.

Diagnostic Performance and Radiation Dose of the EOS System to Image Enchondromatosis: A Phantom Study

Background: Radiation doses and capability of EOS, conventional radiography (CR), and computed tomography (CT) to detect and measure enchondromas in a dedicated five-year-old anthropomorphic phantom were compared. Methods: To simulate enchondromas, minced pieces of chicken bone and cartilage were packed in conventional kitchen plastic foil to create ovoidal/rounded masses and randomly hung on the phantom. The phantom was imaged five times with CR, CT, and EOS, each time changing the number and position of inserts. All images were reviewed by a senior radiologist and a radiology resident. Results: EOS and CR detected all inserts in 4/5 cases (80%), while in one case 1/17 inserts was not seen. Excellent agreement of EOS with CR (88% reproducibility; bias = 14 mm; repeatability coefficient (CoR) 2.9; 95% CI from −2.8 to 3.1 mm; p = 0.5) and CT (81% reproducibility; bias = 15 mm; CoR 5.2; 95% CI from −5.5 to 5.2 mm; p = 0.7) was found. EOS showed 71% interobserver reproducibility (CoR 7.2; bias = 0.6 mm; 95% CI from −6.6 to 7.8 mm; p = 0.25). The EOS-Fast radiation dose was also significantly lower than the median radiation dose of CR (644.7 (599.4–651.97) mGy•cm2, p = 0.004). Conclusions: Low-dose EOS has the same capability as CR to detect and measure enchondroma-like inserts on a phantom and may be considered to monitor patients with multiple enchondromas.

Ollier disease: the first report in Syria

Ollier disease is a rare nonhereditary skeletal disorder, characterized by multiple enchondromas, which are noncancerous growth of cartilage. In this report, we present a case of Ollier disease in a 10-year-old Syrian boy. The patient presented with multiple boney masses on hands; he had a history of pathological fractures when he was 5, which caused crippling. We analyzed the clinical, radiographical and pathological characteristics of our patient, which helped us to reach the final diagnosis. Ollier disease is a benign bone tumor, but it has a risk of malignant transformation into chondrosarcoma. The aim of this report is to document the presence of Ollier disease in Syria to help other Syrian physicians considering this disease in the differential diagnosis if they face similar presentations.

Recent Insights into Long Bone Development: Central Role of Hedgehog Signaling Pathway in Regulating Growth Plate

The longitudinal growth of long bone, regulated by an epiphyseal cartilaginous component known as the “growth plate”, is generated by epiphyseal chondrocytes. The growth plate provides a continuous supply of chondrocytes for endochondral ossification, a sequential bone replacement of cartilaginous tissue, and any failure in this process causes a wide range of skeletal disorders. Therefore, the cellular and molecular characteristics of the growth plate are of interest to many researchers. Hedgehog (Hh), well known as a mitogen and morphogen during development, is one of the best known regulatory signals in the developmental regulation of the growth plate. Numerous animal studies have revealed that signaling through the Hh pathway plays multiple roles in regulating the proliferation, differentiation, and maintenance of growth plate chondrocytes throughout the skeletal growth period. Furthermore, over the past few years, a growing body of evidence has emerged demonstrating that a limited number of growth plate chondrocytes transdifferentiate directly into the full osteogenic and multiple mesenchymal lineages during postnatal bone development and reside in the bone marrow until late adulthood. Current studies with the genetic fate mapping approach have shown that the commitment of growth plate chondrocytes into the skeletal lineage occurs under the influence of epiphyseal chondrocyte-derived Hh signals during endochondral bone formation. Here, we discuss the valuable observations on the role of the Hh signaling pathway in the growth plate based on mouse genetic studies, with some emphasis on recent advances.

Activation of hedgehog signaling in mesenchymal stem cells induces cartilage and bone tumor formation via Wnt/β-Catenin

Indian Hedgehog (IHH) signaling, a key regulator of skeletal development, is highly activated in cartilage and bone tumors. Yet deletion of Ptch1, encoding an inhibitor of IHH receptor Smoothened (SMO), in chondrocyte or osteoblasts does not cause tumorigenesis. Here, we show that Ptch1 deletion in mice Prrx1⁺mesenchymal stem/stromal cells (MSCs) promotes MSC proliferation and osteogenic and chondrogenic differentiation but inhibits adipogenic differentiation. Moreover, Ptch1 deletion led to development of osteoarthritis-like phenotypes, exostoses, enchondroma, and osteosarcoma in Smo-Gli1/2-dependent manners. The cartilage and bone tumors are originated from Prrx1⁺ lineage cells and express low levels of osteoblast and chondrocyte markers, respectively. Mechanistically, Ptch1 deletion increases the expression of Wnt5a/6 and leads to enhanced β-Catenin activation. Inhibiting Wnt/β-Catenin pathway suppresses development of skeletal anomalies including enchondroma and osteosarcoma. These findings suggest that cartilage/bone tumors arise from their early progenitor cells and identify the Wnt/β-Catenin pathway as a pharmacological target for cartilage/bone neoplasms.

Bone and cartilage tumors are among the most common tumors in the skeleton, often affecting the limbs. Bone tumors, also called osteosarcomas, usually occur in growing children and teenagers, and they are often resistant to conventional chemo- and radio-therapies. Surgery is the only treatment option, but this can lead to long-lasting damage that impairs the quality of life of these patients. Thus, there is a need to find new drug targets for these diseases. Unfortunately, no good laboratory-based systems exist that mimic these human cancers, hindering research into these tumors.

One way to create a laboratory-based model for cartilage tumors and osteosarcomas is to reproduce the signaling that is present in the human tumors in a mouse. A signaling pathway called Hedgehog signaling is overactive in human cartilage and bone tumors. The activity of this pathway can be increased by deleting a gene called Ptch1; but mice do not form tumors when this gene is deleted in their mature cartilage and bone cells.

Now, Deng, Li et al. report that deleting Ptch1 in mesenchymal stem cells, early-stage cells that can give rise to cartilage and bone cells, generates a mouse model for osteosarcoma and cartilage tumors. The mice with these Ptch1 deficient cells developed tumors with overactive Hedgehog signaling in cartilage and bone. Deng, Li et al. also performed biochemical experiments to show that Hedgehog signaling turned on another signaling pathway called Wnt signaling. Treating the mice that had mesenchymal cells lacking Ptch1 with a drug that inhibits Wnt signaling reduced the growth of cartilage and bone tumors.

These data suggest that deleting Ptch1 in mouse mesenchymal stem cells can mimic human cartilage tumors and osteosarcomas. More experiments will be needed to explain how the Hedgehog and Wnt signaling pathways interact in these tumors. Finally, further studies will need to investigate if inhibiting Wnt signaling might become a useful therapy for human patients with osteosarcoma in the future.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Inactivation of Lkb1 in postnatal chondrocytes leads to epiphyseal growth-plate abnormalities and promotes enchondroma-like formation

Liver serine-threonine kinase B1 (LKB1) is a tumor suppressor that has been linked to many types of tumors. However, the role of LKB1 in cartilaginous tumorigenesis is still poorly understood. In this study, we find that cartilage-specific, tamoxifen-inducible Lkb1 knockout results in multiple enchondroma-like lesions adjacent to the disorganized growth plates. We showed that chondrocytes retain an immature status caused by loss of Lkb1, which may lead to the dramatic expansion of growth-plate cartilage and the formation of enchondroma-like lesions. Additionally, increased mammalian target of rapamycin complex 1 (mTORC1) activity is observed in the Lkb1 conditional knockout (cKO) chondrocytes, and rapamycin (mTORC1 inhibitor) treatment significantly alleviates the expansion of growth-plate cartilage and eliminates the enchondroma-like lesions in Lkb1 cKO mice. Thus, our findings indicate that loss of Lkb1 leads to the expansion of chondrocytes and the formation of enchondroma-like lesions during postnatal cartilage development, and that the up-regulated mTORC1-signaling pathway is implicated in this process. Our findings suggest that modulation of LKB1 and related signaling is a potential therapy in cartilaginous tumorigenesis.-Zhou, S., Li, Y., Qiao, L., Ge, Y., Huang, X., Gao, X., Ju, H., Wang, W., Zhang, J., Yan, J., Teng, H., Jiang, Q. Inactivation of Lkb1 in postnatal chondrocytes leads to epiphyseal growth-plate abnormalities and promotes enchondroma-like formation.

Unique and overlapping GLI1 and GLI2 transcriptional targets in neoplastic chondrocytes

Excessive Hedgehog (Hh) signaling in chondrocytes is sufficient to cause formation of enchondroma-like lesions which can progress to chondrosarcoma. To elucidate potential underlying mechanisms, we identified GLI1 and GLI2 target genes in human chondrosarcoma. Using chromatin immunoprecipitation (ChIP) sequencing and microarray data, in silico analyses were conducted to identify and characterize unique and overlapping GLI1 and GLI2 binding regions in neoplastic chondrocytes. After overlaying microarray data from human chondrosarcoma, 204 upregulated and 106 downregulated genes were identified as Hh-responsive Gli binding targets. After overlaying published Gli ChIP-on-chip data from mouse, 48 genes were identified as potential direct downstream targets of Hedgehog signaling with shared GLI binding regions in evolutionarily conserved DNA elements. Among these was BMP2, pointing to potential cross-talk between TGF beta signaling and Hh signaling. Our identification of potential target genes that are unique and common to GLI1 and GLI2 in neoplastic chondrocytes contributes to elucidating potential pathways through which Hh signaling impacts cartilage tumor biology.

An update of molecular pathology of bone tumors. Lessons learned from investigating samples by next generation sequencing

The last decade has seen the majority of primary bone tumor subtypes become defined by molecular genetic alteration. Examples include giant cell tumour of bone (H3F3A p.G34W), chondroblastoma (H3F3B p.K36M), mesenchymal chondrosarcoma (HEY1-NCOA2), chondromyxoid fibroma (GRM1 rearrangements), aneurysmal bone cyst (USP6 rearrangements), osteoblastoma/osteoid osteoma (FOS/FOSB rearrangements), and synovial chondromatosis (FN1-ACVR2A and ACVR2A-FN1). All such alterations are mutually exclusive. Many of these have been translated into clinical service using immunohistochemistry or FISH. 60% of central chondrosarcoma is characterised by either isocitrate dehydrogenase (IDH) 1 or IDH2 mutations distinguishing them from other cartilaginous tumours. In contrast, recurrent alterations which are clinically helpful have not been found in high grade osteosarcoma. High throughput next generation sequencing has also proved valuable in identifying germ line alterations in a significant proportion of young patients with primary malignant bone tumors. These findings will play an increasing role in reaching a diagnosis and in patient management.

Widespread and debilitating hemangiomas in a patient with enchondromatosis and D-2-hydroxyglutaric aciduria

Metaphyseal chondromatosis with D-2-hydroxyglutaric aciduria (MC-HGA) (OMIM 614875) is a severe chondrodysplasia combined with a urinary excretion of D-2-hydroxyglutaric acid. Here, we reported the tenth case of this disease. A 15-year-old boy had symmetric radiolulencies in the metaphyses of the long bones suggesting enchondromatosis and platyspondyly. Remarkably, he manifested widespread cavernous hemangiomas including scalp, lips, tongue, larynx, and prepuce, with the onset of 3 years of age. Hemangiomas at the larynx had caused dyspnea and those in the oral cavity led to recurrent bleeding, requiring several surgical removals. These multiple and debilitating hemangiomas have never been previously reported in patients with MC-HGA. Mutation analyses including Sanger sequencing of genes involving in enchondromatosis and the metabolic pathway of D-2-hydroxyglutarate including PTHR1, D2HGDH, HOT, and IDH1, as well as whole-exome sequencing for proband-parent trio analysis and paired blood versus hemangioma studies showed no pathogenic variants. In summary, we reported the tenth patient with MC-HGA who manifested widespread and debilitating hemangiomas in several organs, expanding the clinical spectrum of MC-HGA.

Surgical Treatment of Enchondromas of the Hand During Childhood in Ollier Disease

PURPOSE

The objective of the present study was to evaluate the outcome of surgical treatment of hand lesions in Ollier disease (OD) carried out during childhood.

METHODS

A retrospective review was carried out of 10 pediatric patients with hand involvement of OD, who had undergone surgery for metacarpal or phalangeal enchondromas. The technique comprised curettage and cortical bone reconstruction with corticoplasty, to restore a near-normal phalangeal volume. The range of finger motion (pulp-to-palm distance), the shortened Disabilities of the Arm, Shoulder, and Hand (QuickDASH) questionnaire score, cosmetic improvement, radiological findings (according to Tordai's classification), and recurrence were recorded after a mean follow-up of 7.5 years (range, 4-11.3 years).

RESULTS

The mean age at surgery was 10.7 years (range, 6-14 years). Curettage was performed on 35 enchondromas, and 9 cavities were filled with a bone graft. The mean pulp-to-palm distance was significantly lower after surgery (from 1.5 cm to 0.25 cm; P < .05). The mean QuickDASH score was 3.84 (range, 0-11.4). A marked cosmetic improvement was noted for 83% of the hands. Three enchondromas recurred in 1 patient, requiring a second curettage. Fifty-seven percent of the cavities were completely filled with bone (Tordai stage 1) at last follow-up. The outcome did not depend on the presence or absence of a bone graft.

CONCLUSIONS

Our results suggest that early surgical treatment comprising curettage and corticoplasty leads to good clinical, cosmetic, and radiological outcomes. Early surgical treatment of well-developed and/or symptomatic enchondromas of the hand in OD should be considered.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic V.

Intracellular biosynthesis of lipids and cholesterol by Scap and Insig in mesenchymal cells regulates long bone growth and chondrocyte homeostasis

During enchondral ossification, mesenchymal cells express genes regulating the intracellular biosynthesis of cholesterol and lipids. Here, we have investigated conditional deletion of Scap or of Insig1 and Insig2 (Scap inhibits intracellular biosynthesis and Insig proteins activate intracellular biosynthesis). Mesenchymal condensation and chondrogenesis was disrupted in mice lacking Scap in mesenchymal progenitors, whereas mice lacking the Insig genes in mesenchymal progenitors had short limbs, but normal chondrogenesis. Mice lacking Scap in chondrocytes showed severe dwarfism, with ectopic hypertrophic cells, whereas deletion of Insig genes in chondrocytes caused a mild dwarfism and shortening of the hypertrophic zone. In vitro studies showed that intracellular cholesterol in chondrocytes can derive from exogenous and endogenous sources, but that exogenous sources cannot completely overcome the phenotypic effect of Scap deficiency. Genes encoding cholesterol biosynthetic proteins are regulated by Hedgehog (Hh) signaling, and Hh signaling is also regulated by intracellular cholesterol in chondrocytes, suggesting a feedback loop in chondrocyte differentiation. Precise regulation of intracellular biosynthesis is required for chondrocyte homeostasis and long bone growth, and these data support pharmacological modulation of cholesterol biosynthesis as a therapy for select cartilage pathologies.

Summary: Conditional deletion of genes that regulate intracellular cholesterol biosynthesis in mesenchymal cells or chondrocytes shows that precise regulation of biosynthesis is required for chondrocyte homeostasis and long bone growth.

Signaling systems affecting the severity of multiple osteochondromas

Multiple osteochondromas (MO) syndrome is a dominant autosomal bone disorder characterized by the formation of cartilage-capped bony outgrowths that develop at the juxtaposition of the growth plate of endochondral bones. MO has been linked to mutations in either EXT1 or EXT2, two glycosyltransferases required for the synthesis of heparan sulfate (HS). The establishment of mouse mutants demonstrated that a clonal, homozygous loss of Ext1 in a wild type background leads to the development of osteochondromas. Here we investigate mechanisms that might contribute to the variation in the severity of the disease observed in human patients. Our results show that residual amounts of HS are sufficient to prevent the development of osteochondromas strongly supporting that loss of heterozygosity is required for osteochondroma formation. Furthermore, we demonstrate that different signaling pathways affect size and frequency of the osteochondromas thereby modulating the severity of the disease. Reduced Fgfr3 signaling, which regulates proliferation and differentiation of chondrocytes, increases osteochondroma number, while activated Fgfr3 signaling reduces osteochondroma size. Both, activation and reduction of Wnt/β-catenin signaling decrease osteochondroma size and frequency by interfering with the chondrogenic fate of the mutant cells. Reduced Ihh signaling does not change the development of the osteochondromas, while elevated Ihh signaling increases the cellularity and inhibits chondrocyte differentiation in a subset of osteochondromas and might thus predispose osteochondromas to the transformation into chondrosarcomas.

Induced GnasR201H expression from the endogenous Gnas locus causes fibrous dysplasia by up-regulating Wnt/β-catenin signaling

Fibrous dysplasia (FD; Online Mendelian Inheritance in Man no. 174800) is a crippling skeletal disease caused by activating mutations of the GNAS gene, which encodes the stimulatory G protein Gα_s FD can lead to severe adverse conditions such as bone deformity, fracture, and severe pain, leading to functional impairment and wheelchair confinement. So far there is no cure, as the underlying molecular and cellular mechanisms remain largely unknown and the lack of appropriate animal models has severely hampered FD research. Here we have investigated the cellular and molecular mechanisms underlying FD and tested its potential treatment by establishing a mouse model in which the human FD mutation (R201H) has been conditionally knocked into the corresponding mouse Gnas locus. We found that the germ-line FD mutant was embryonic lethal, and Cre-induced Gnas FD mutant expression in early osteochondral progenitors, osteoblast cells, or bone marrow stromal cells (BMSCs) recapitulated FD features. In addition, mosaic expression of FD mutant Gα_s in BMSCs induced bone marrow fibrosis both cell autonomously and non-cell autonomously. Furthermore, Wnt/β-catenin signaling was up-regulated in FD mutant mouse bone and BMSCs undergoing osteogenic differentiation, as we have found in FD human tissue previously. Reduction of Wnt/β-catenin signaling by removing one Lrp6 copy in an FD mutant line significantly rescued the phenotypes. We demonstrate that induced expression of the FD Gα_s mutant from the mouse endogenous Gnas locus exhibits human FD phenotypes in vivo, and that inhibitors of Wnt/β-catenin signaling may be repurposed for treating FD and other bone diseases caused by Gα_s activation.

[Hereditary bone tumors]

Hereditary bone tumors are rare and result from mutations affecting cell cycle regulation (e.g. retinoblastoma syndrome/RB1 and Li-Fraumeni syndrome/TP53, Gardner syndrome/APC), energy metabolism (enchondromatosis/IDH1/2), complex signaling cascades (multiple hereditary exostoses/EXT1/2) and DNA integrity (Rothmund-Thomson/RECQL4, Werner/WRN and Bloom syndromes/BLM). The majority of syndromes are incompletely understood and can lead to multiple benign tumors, of which some might undergo secondary malignant transformation over time (enchondromatosis: enchondromas, multiple hereditary exostoses: osteochondromas, Gardner syndrome: osteomas) or bone sarcomas, primarily osteosarcomas as primary (Li-Fraumeni, Rothmund-Thomson, Werner and Bloom syndromes) or secondary manifestation (retinoblastoma syndrome) of the disease. Some syndromes additionally predispose to the development of a variety of other malignant tumors during life. Compared to sporadically occurring tumors, syndrome-related neoplasms can differ in the time of manifestation, site and histology, which can help in recognizing a specific tumor predisposition syndrome.

Pseudohypoparathyroidism type I-b with neurological involvement is associated with a homozygous PTH1R mutation

Pseudohypoparathyroidism type 1b (PHP1b) is characterized by hypocalcemia, hyperphosphatemia, increased levels of circulating parathyroid hormone (PTH), and no skeletal or developmental abnormalities. The goal of this study was to perform a full characterization of a familial case of PHP1b with neurological involvement and to identify the genetic cause of disease. The initial laboratory profile of the proband showed severe hypocalcemia, hyperphosphatemia and normal levels of PTH, which was considered to be compatible with primary hypoparathyroidism. With disease progression the patient developed cognitive disturbance, PTH levels were found to be slightly elevated and a picture of PTH resistance syndrome seemed more probable. The diagnosis of PHP1b was established after the study of family members and blunted urinary cAMP results were obtained in a PTH stimulation test. Integration of whole genome genotyping and exome sequencing data supported this diagnosis by revealing a novel homozygous missense mutation in PTH1R (p.Arg186His) completely segregating with the disease. Here, we demonstrate segregation of a novel mutation in PTH1R with a phenotype of PHP1b presenting with neurological symptoms, but no bone defects. This case represents the extreme end of the spectrum of cognitive impairment in PTH dysfunction and defines a possible novel form of PHP1b resulting from the impaired interaction between PTH and PTH1R.

Gynecologic Manifestations of Less Commonly Encountered Hereditary Syndromes

This review covers gynecologic manifestations that may occur in rare hereditary syndromes. Recent advances in disorders, such as hereditary leiomyomatosis, renal cell carcinoma syndrome and tuberous sclerosis complex, are discussed as well as lesions that occur in von Hippel-Lindau syndrome, nevoid basal cell carcinoma syndrome, Cowden syndrome, Ollier disease/Maffucci syndrome, and Carney complex. Characteristic clinicopathologic features of each of these syndromes are discussed with an emphasis on the key features that enable pathologists to identify patients at highest risk for these diseases.

Regulation of Cholesterol Homeostasis by Hedgehog Signaling in Osteoarthritic Cartilage

Objective

With no effective therapies to attenuate cartilage degeneration in osteoarthritis (OA), the result is pain and disability. Activation of hedgehog (HH) signaling causes changes related to the progression of OA, with higher levels of Gli‐mediated transcriptional activation associated with increased disease severity. To elucidate the mechanism through which this occurs, this study sought to identify genes regulated by HH signaling in human OA chondrocytes.

Methods

Using human OA cartilage samples, microarray analyses were performed to detect changes in gene expression when the HH pathway was modulated. Results were analyzed for differentially expressed genes, grouped into functional networks, and validated in independent samples. To investigate the effects of chondrocyte‐specific sterol accumulation, we generated mice lacking Insig1 and Insig2, which are major negative regulators of cholesterol homeostasis, under Col2a1 regulatory elements.

Results

HH signaling was found to regulate genes that govern cholesterol homeostasis, and this led to alterations in cholesterol accumulation in chondrocytes. A higher level of Gli‐mediated transcription resulted in accumulation of intracellular cholesterol. In genetically modified mice, chondrocyte‐specific cholesterol accumulation was associated with an OA phenotype. Reducing cholesterol accumulation attenuated the severity of OA in mice in vivo and decreased the expression of proteases in human OA cartilage in vitro.

Conclusion

HH signaling regulates cholesterol homeostasis in chondrocytes, and intracellular cholesterol accumulation contributes to the severity of OA. Our findings have therapeutic implications, since reduction of HH signaling reversed cholesterol accumulation and statin treatment attenuated cartilage degeneration.

A novel homozygous PTH1R variant identified through whole-exome sequencing further expands the clinical spectrum of primary failure of tooth eruption in a consanguineous Saudi family

OBJECTIVES

The present study aimed to identify the genetic cause of non-syndromic primary failure of tooth eruption in a five-generation consanguineous Saudi family using whole-exome sequencing (WES) analysis.

DESIGN

The family pedigree and phenotype were obtained from patient medical records. WES of all four affected family members was performed using the 51 Mb SureSelect V4 library kit and then sequenced using the Illumina HiSeq2000 sequencing system. Sequence alignment, variant calling, and the annotation of single nucleotide polymorphisms and indels were performed using standard bioinformatics pipelines. The genotype of candidate variants was confirmed in all available family members by Sanger sequencing.

RESULTS

Pedigree analysis suggested that the inheritance was autosomal recessive. WES of all affected individuals identified a novel homozygous variant in exon 8 of the parathyroid hormone 1 receptor gene (PTH1R) (NM_000316: c.611T>A: p.Val204Glu).

CONCLUSION

To the best of our knowledge, this is the first report of primary failure of eruption caused by a homozygous mutation in PTH1R. Our findings prove the application of WES as an efficient molecular diagnostics tool for this rare phenotype and further broaden the clinical spectrum of PTH1R pathogenicity.

Hyperplasia in glands with hormone excess

Five syndromes share predominantly hyperplastic glands with a primary excess of hormones: neonatal severe primary hyperparathyroidism, from homozygous mutated CASR, begins severely in utero; congenital non-autoimmune thyrotoxicosis, from mutated TSHR, varies from severe with fetal onset to mild with adult onset; familial male-limited precocious puberty, from mutated LHR, expresses testosterone oversecretion in young boys; hereditary ovarian hyperstimulation syndrome, from mutated FSHR, expresses symptomatic systemic vascular permeabilities during pregnancy; and familial hyperaldosteronism type IIIA, from mutated KCNJ5, presents in young children with hypertension and hypokalemia. The grouping of these five syndromes highlights predominant hyperplasia as a stable tissue endpoint and as their tissue stage for all of the hormone excess. Comparisons were made among this and two other groups of syndromes, forming a continuum of gland staging: predominant oversecretions express little or no hyperplasia; predominant hyperplasias express little or no neoplasia; and predominant neoplasias express nodules, adenomas, or cancers. Hyperplasias may progress (5 of 5) to neoplastic stages while predominant oversecretions rarely do (1 of 6; frequencies differ P<0.02). Hyperplasias do not show tumor multiplicity (0 of 5) unlike neoplasias that do (13 of 19; P<0.02). Hyperplasias express mutation of a plasma membrane-bound sensor (5 of 5), while neoplasias rarely do (3 of 14; P<0.002). In conclusion, the multiple distinguishing themes within the hyperplasias establish a robust pathophysiology. It has the shared and novel feature of mutant sensors in the plasma membrane, suggesting that these are major contributors to hyperplasia.

International Union of Basic and Clinical Pharmacology. XCIII. The parathyroid hormone receptors--family B G protein-coupled receptors

The type-1 parathyroid hormone receptor (PTHR1) is a family B G protein-coupled receptor (GPCR) that mediates the actions of two polypeptide ligands; parathyroid hormone (PTH), an endocrine hormone that regulates the levels of calcium and inorganic phosphate in the blood by acting on bone and kidney, and PTH-related protein (PTHrP), a paracrine-factor that regulates cell differentiation and proliferation programs in developing bone and other tissues. The type-2 parathyroid hormone receptor (PTHR2) binds a peptide ligand, called tuberoinfundibular peptide-39 (TIP39), and while the biologic role of the PTHR2/TIP39 system is not as defined as that of the PTHR1, it likely plays a role in the central nervous system as well as in spermatogenesis. Mechanisms of action at these receptors have been explored through a variety of pharmacological and biochemical approaches, and the data obtained support a basic "two-site" mode of ligand binding now thought to be used by each of the family B peptide hormone GPCRs. Recent crystallographic studies on the family B GPCRs are providing new insights that help to further refine the specifics of the overall receptor architecture and modes of ligand docking. One intriguing pharmacological finding for the PTHR1 is that it can form surprisingly stable complexes with certain PTH/PTHrP ligand analogs and thereby mediate markedly prolonged cell signaling responses that persist even when the bulk of the complexes are found in internalized vesicles. The PTHR1 thus appears to be able to activate the Gα(s)/cAMP pathway not only from the plasma membrane but also from the endosomal domain. The cumulative findings could have an impact on efforts to develop new drug therapies for the PTH receptors.

Genochondromatosis type I: A clinicoradiological study of four family members

Genochondromatosis is an extremely rare autosomal dominant disorder, which manifests during childhood and tends to regress in adult life. The bony lesions are symmetrically distributed with characteristic localization at the metaphysis of proximal humerus and distal femur. Two types have been described based on the involvement of clavicle. Usually asymptomatic, sometimes patients may present with pathological fractures. In this communication, we describe four members of a family with Genochondromatosis type I, with some additional clinical and radiological findings not reported previously.

FGFR3 Deficiency Causes Multiple Chondroma-like Lesions by Upregulating Hedgehog Signaling

Most cartilaginous tumors are formed during skeletal development in locations adjacent to growth plates, suggesting that they arise from disordered endochondral bone growth. Fibroblast growth factor receptor (FGFR)3 signaling plays essential roles in this process; however, the role of FGFR3 in cartilaginous tumorigenesis is not known. In this study, we found that postnatal chondrocyte-specific Fgfr3 deletion induced multiple chondroma-like lesions, including enchondromas and osteochondromas, adjacent to disordered growth plates. The lesions showed decreased extracellular signal-regulated kinase (ERK) activity and increased Indian hedgehog (IHH) expression. The same was observed in Fgfr3-deficient primary chondrocytes, in which treatment with a mitogen-activated protein kinase (MEK) inhibitor increased Ihh expression. Importantly, treatment with an inhibitor of IHH signaling reduced the occurrence of chondroma-like lesions in Fgfr3-deficient mice. This is the first study reporting that the loss of Fgfr3 function leads to the formation of chondroma-like lesions via downregulation of MEK/ERK signaling and upregulation of IHH, suggesting that FGFR3 has a tumor suppressor-like function in chondrogenesis.

Ollier Disease: Pathogenesis, Diagnosis, and Management

Ollier disease (Spranger type I) is a rare bone disease that is characterized by multiple enchondromatosis with a typical asymmetrical distribution and confined to the appendicular skeleton. The pathogenesis of enchondromatosis is not clearly understood. Recently, heterozygous mutations of PTHR1, IDH1 (most common), and/or IDH2 genes have been suggested by various authors as genetic aberrations. Genomic copy number alterations and mutations controlling many vital pathways are responsible for the pathogenesis of Ollier disease. A comprehensive description of all genetic events in Ollier disease is presented in this article. Clinically, Ollier disease has a wide variety of presentations. This article describes the plethora of clinical features, both common and rare, associated with Ollier disease. Multiple enchondromas are most commonly seen in phalanges and metacarpals. Radiologically, Ollier disease presents with asymmetrical osteolytic lesions with well-defined, sclerotic margins. In this article, various radiological features of Ollier disease, including radiographs, computed tomography, and magnetic resonance imaging, are also discussed. Gross pathology, cytological, and histological features of both Ollier disease and its malignant transformation are outlined. Although treatment is conservative in most cases, different possible treatment options for difficult cases are discussed. In the literature, there is a paucity of data about the disease, including diagnosis, management, prognostication, and rehabilitation, necessitating a comprehensive review to further define all of the possible domains related to this disease.

Mutant IDH is sufficient to initiate enchondromatosis in mice

Enchondromas are benign cartilage tumors and precursors to malignant chondrosarcomas. Somatic mutations in the isocitrate dehydrogenase genes (IDH1 and IDH2) are present in the majority of these tumor types. How these mutations cause enchondromas is unclear. Here, we identified the spectrum of IDH mutations in human enchondromas and chondrosarcomas and studied their effects in mice. A broad range of mutations was identified, including the previously unreported IDH1-R132Q mutation. These mutations harbored enzymatic activity to catalyze α-ketoglutarate to d-2-hydroxyglutarate (d-2HG). Mice expressing Idh1-R132Q in one allele in cells expressing type 2 collagen showed a disordered growth plate, with persistence of type X-expressing chondrocytes. Chondrocyte cell cultures from these animals or controls showed that there was an increase in proliferation and expression of genes characteristic of hypertrophic chondrocytes with expression of Idh1-R132Q or 2HG treatment. Col2a1-Cre;Idh1-R132Q mutant knock-in mice (mutant allele expressed in chondrocytes) did not survive after the neonatal stage. Col2a1-Cre/ERT2;Idh1-R132 mutant conditional knock-in mice, in which Cre was induced by tamoxifen after weaning, developed multiple enchondroma-like lesions. Taken together, these data show that mutant IDH or d-2HG causes persistence of chondrocytes, giving rise to rests of growth-plate cells that persist in the bone as enchondromas.

Common somatic alterations identified in maffucci syndrome by molecular karyotyping

Maffucci syndrome (MS) is a rare congenital disorder characterized by multiple central cartilaginous tumors (enchondromas) in association with cutaneous spindle cell hemangiomas. These patients have a high incidence of malignant transformation. No familial case is known and the etiopathogenic cause remains unknown. In enchondromatosis (Ollier disease, OD), which is comprised of enchondromas only, 4 mutations in the PTHR1 gene have been identified in 4 patients; 3 were somatic and 1 was germline. No PTHR1 mutations have been detected in MS, whereas somatic IDH1 and, more rarely, IDH2 mutations have been observed in 77% of patients with MS and 81% of patients with OD. These genetic alterations are shared with other tumors, including glioma, leukemia and carcinoma. To search for underlying somatic genomic causes, we screened MS tissues using Affymetrix SNP-chips. We looked for CNVs, LOH and uniparental isodisomy (UPID) by performing pairwise analyses between allelic intensities in tumoral DNA versus the corresponding blood-extracted DNA. While common chromosomal anomalies were absent in constitutional DNA, several shared CNVs were identified in MS-associated tumors. The most frequently encountered somatic alterations were localized in 2p22.3, 2q24.3 and 14q11.2, implicating these chromosomal rearrangements in the formation of enchondromas and spindle cell hemangiomas in MS. In one chondrosarcoma specimen, large amplifications and/or deletions were observed in chromosomes 3, 6, 9, 10, 12, 13, and 19. Some of these genetic changes have been reported in other chondrosarcomas suggesting an etiopathogenic role. No LOH/UPID was observed in any Maffucci tissue. Our findings identify frequent somatic chromosomal rearrangements on 2p22.3, 2q24.3 and 14q11.2, which may unmask mutations leading to the lesions pathognomonic of MS.

Unique mutation portraits and frequent COL2A1 gene alteration in chondrosarcoma

Chondrosarcoma is the second most frequent malignant bone tumor. However, the etiological background of chondrosarcomagenesis remains largely unknown, along with details on molecular alterations and potential therapeutic targets. Massively parallel paired-end sequencing of whole genomes of 10 primary chondrosarcomas revealed that the process of accumulation of somatic mutations is homogeneous irrespective of the pathological subtype or the presence of IDH1 mutations, is unique among a range of cancer types, and shares significant commonalities with that of prostate cancer. Clusters of structural alterations localized within a single chromosome were observed in four cases. Combined with targeted resequencing of additional cartilaginous tumor cohorts, we identified somatic alterations of the COL2A1 gene, which encodes an essential extracellular matrix protein in chondroskeletal development, in 19.3% of chondrosarcoma and 31.7% of enchondroma cases. Epigenetic regulators (IDH1 and YEATS2) and an activin/BMP signal component (ACVR2A) were recurrently altered. Furthermore, a novel FN1-ACVR2A fusion transcript was observed in both chondrosarcoma and osteochondromatosis cases. With the characteristic accumulative process of somatic changes as a background, molecular defects in chondrogenesis and aberrant epigenetic control are primarily causative of both benign and malignant cartilaginous tumors.

SHP2 regulates chondrocyte terminal differentiation, growth plate architecture and skeletal cell fates

Loss of PTPN11/SHP2 in mice or in human metachondromatosis (MC) patients causes benign cartilage tumors on the bone surface (exostoses) and within bones (enchondromas). To elucidate the mechanisms underlying cartilage tumor formation, we investigated the role of SHP2 in the specification, maturation and organization of chondrocytes. Firstly, we studied chondrocyte maturation by performing RNA-seq on primary chondrocyte pellet cultures. We found that SHP2 depletion, or inhibition of the ERK1/2 pathway, delays the terminal differentiation of chondrocytes from the early-hypertrophic to the late-hypertrophic stage. Secondly, we studied chondrocyte maturation and organization in mice with a mosaic postnatal inactivation of Ptpn11 in chondrocytes. We found that the vertebral growth plates of these mice have expanded domains of early-hypertrophic chondrocytes that have not yet terminally differentiated, and their enchondroma-like lesions arise from chondrocytes displaced from the growth plate due to a disruption in the organization of maturation and ossification zones. Furthermore, we observed that lesions from human MC patients also display disorganized chondrocyte maturation zones. Next, we found that inactivation of Ptpn11 in Fsp1-Cre-expressing fibroblasts induces exostosis-like outgrowths, suggesting that loss of SHP2 in cells on the bone surface and at bone-ligament attachment sites induces ectopic chondrogenesis. Finally, we performed lineage tracing to show that exostoses and enchondromas in mice likely contain mixtures of wild-type and SHP2-deficient chondrocytes. Together, these data indicate that in patients with MC, who are heterozygous for inherited PTPN11 loss-of-function mutations, second-hit mutations in PTPN11 can induce enchondromas by disrupting the organization and delaying the terminal differentiation of growth plate chondrocytes, and can induce exostoses by causing ectopic chondrogenesis of cells on the bone surface. Furthermore, the data are consistent with paracrine signaling from SHP2-deficient cells causing SHP2-sufficient cells to be incorporated into the lesions.

Patients with the inherited disorder, metachondromatosis (MC), develop multiple benign cartilage tumors during childhood. MC patients carry heterozygous loss-of-function mutations in the PTPN11 gene, and their cartilage tumors likely arise when the second PTPN11 allele is lost due to a somatic mutation. PTPN11 encodes a phosphatase called SHP2 that is involved in a variety of signaling pathways. Here, we use mouse models and cell culture assays to investigate the mechanisms by which loss of SHP2 promotes cartilage tumor formation. We show that cartilage tumors that form inside bones (enchondromas) likely arise due to disorganized growth and delayed terminal differentiation of growth plate chondrocytes, while cartilage tumors that form on the bone surface (exostoses) can arise due to ectopic chondrogenesis of fibroblast-like cells that surround bones. We also suggest that paracrine signals from SHP2-deficient cells cause neighboring SHP2-sufficient cells to contribute to exostoses and enchondromas. Finally, we provide in vitro data that the ERK1/2 pathway is regulated by SHP2 and promotes chondrocyte terminal differentiation. Together, our data provide insight into the mechanisms underlying cartilage tumor formation and implicate SHP2 as a key regulator of chondrocyte specification, organization and maturation.

Aberrant hedgehog signaling and clinical outcome in osteosarcoma

Despite the importance of Hedgehog signaling in bone development, the relationship between Hedgehog pathway expression and osteosarcoma clinical characteristics and outcome has not been investigated. In this study of 43 high-grade human osteosarcoma samples, we detected high expression levels of the Hedgehog ligand gene, IHH, and target genes, PTCH1 and GLI1, in most samples. Further analysis in tumors of patients with localized disease at diagnosis identified coexpression of IHH and PTCH1 exclusively in large tumors. Higher levels of IHH were observed more frequently in males and patients with higher levels of GLI1 were more responsive to chemotherapy. Subgroup analysis by tumor size and IHH expression indicated that the well-known association between survival and tumor size was further refined when IHH levels were taken into consideration.

Targeted disruption of Shp2 in chondrocytes leads to metachondromatosis with multiple cartilaginous protrusions

Metachondromatosis is a benign bone disease predominantly observed in the hands and feet of children or young adults demonstrating two different manifestations: a cartilage-capped bony outgrowth on the surface of the bone called exostosis and ectopic cartilaginous nodules inside the bone called enchondroma. Recently, it has been reported that loss-of-function mutations of the SHP2 gene, which encodes the SHP2 protein tyrosine phosphatase, are associated with metachondromatosis. The purpose of this study was to investigate the role of SHP2 in postnatal cartilage development, which is largely unknown. We disrupted Shp2 during the postnatal stage of mouse development in a chondrocyte-specific manner using a tamoxifen-inducible system. We found tumor-like nodules on the hands and feet within a month after the initial induction. The SHP2-deficient mice demonstrated an exostosis-like and enchondroma-like phenotype in multiple bones of the hands, feet, and ribs as assessed by X-ray and micro-computed tomography (CT). Histological assessment revealed the disorganization of the growth plate cartilage, a cartilaginous protrusion from the epiphyseal bone, and ectopic cartilage nodules within the bones, which is consistent with the pathological features of metachondromatosis in humans (ie, both exostosis and enchondroma). At molecular levels, we observed an abundant expression of Indian hedgehog protein (IHH) and fibroblast growth factor 2 (FGF2) and impaired expression of mitogen-activated protein kinases (MAPK) in the affected cartilage nodules in the SHP2-deficient mice. In summary, we have generated a mouse model of metachondromatosis that includes manifestations of exostosis and enchondroma. This study provides a novel model for the investigation of the pathophysiology of the disease and advances the understanding of metachondromatosis. This model will be useful to identify molecular mechanisms for the disease cause and progression as well as to develop new therapeutic strategies in the future.

Genetic alterations in chondrosarcomas - keys to targeted therapies?

BACKGROUND

Chondrosarcomas are malignant tumors of chondrocytes and represent the second most common type of primary bone tumors. Within the context of normal chondrogenesis, this review summarizes results from recent research outlining the key molecular changes that occur during the development of this sarcoma type.

RESULTS

Current data support the notion that a two-hit scenario, common to many tumors, also underlies chondrosarcoma formation. First, early-stage mutations alter the normal proliferation and differentiation of chondrocytes, thereby predisposing them to malignant transformation. These early-stage mutations, found in both benign cartilaginous lesions and chondrosarcomas, include alterations affecting the IHH/PTHrP and IDH1/IDH2 pathways. As they are not observed in malignant cells, mutations in the EXT1 and EXT2 genes are considered early-stage events providing an environment that alters IHH/PTHrP signaling, thereby inducing mutations in adjacent cells. Due to normal cell cycle control that remains active, a low rate of malignant transformation is seen in benign cartilaginous lesions with early-stage mutations. In contrast, late-stage mutations, seen in most malignant chondrosarcomas, appear to induce malignant transformation as they are not found in benign cartilaginous lesions. These late-stage mutations primarily involve cell cycle pathway regulators including p53 and pRB, two genes that are also known to be implicated in numerous other human tumor types.

CONCLUSIONS

Now the key genetic alterations involved in both early and late stages of chondrosarcoma development have been identified, focus should be shifted to the identification of druggable molecular targets for the design of novel chondrosarcoma-specific therapies.

Diagnosis and prognosis of chondrosarcoma of bone

The mechanisms of chondrosarcoma development are just beginning to be unraveled. The distinction between benign and low-grade malignant cartilaginous tumors is difficult and is based mainly on radiological and clinicopathological features. In this review, the conventional chondrosarcomas are subdivided into central and secondary peripheral chondrosarcomas, based on their different genetic and clinicopathological background. Thus far, no diagnostic markers have been identified for central tumors. Bcl-2 is a good diagnostic marker that can be used in the distinction between osteochondroma and low-grade secondary peripheral chondrosarcoma. For the prognosis of chondrosarcomas, the best and most commonly used marker at present is histological grade. Several molecular markers, recapitulated in this paper, have been tested to see if they aid in determining diagnosis and predicting prognosis, but most are not independent of histological grade.