Glycobiology and the growth plate: current concepts in multiple hereditary exostoses

A novel role of exostosin glycosyltransferase 2 (EXT2) in glioblastoma cell metabolism, radiosensitivity and ferroptosis

Glioblastoma (GBM) employs various strategies to resist therapy, resulting in poor patient survival. A key aspect of its survival mechanisms lies in metabolic regulation, maintaining rapid growth and evading cell death. Recent studies revealed the connection between therapy resistance and ferroptosis, a lipid peroxidation-dependent cell death mechanism triggered by metabolic dysfunction. Our aim was to identify novel regulators of therapy resistance in GBM cells. We conducted a comprehensive analysis combining RNA-sequencing data from a panel of human GBM cell models and TCGA GBM patient datasets. We focused on the top-12 differentially expressed gene candidates associated with poor survival in GBM patients and performed an RNA interference-mediated screen to uncover the radiochemosensitizing potential of these molecules and their impact on metabolic activity, DNA damage, autophagy, and apoptosis. We identified exostosin glycosyltransferase 2 (EXT2), an enzyme previously described in heparan sulfate biosynthesis, as the most promising candidate. EXT2 depletion elicited reduced cell viability and proliferation as well as radiochemosensitization in various GBM cell models. Mechanistically, we explored EXT2 function by conducting untargeted and targeted metabolomics and detected that EXT2-depleted GBM cells exhibit a differential abundance of metabolites belonging to S-adenosylmethionine (SAM) metabolism. Considering these metabolic changes, we determined lipid peroxidation and found that the diminished antioxidant capacity resulting from decreased levels of metabolites in the transsulfuration pathway induces ferroptosis. Moreover, modifications of specific SAM and transsulfuration metabolism associated enzymes revealed a prosurvival and ferroptosis-reducing function when EXT2 is depleted. Collectively, our results uncover a novel role of EXT2 in GBM cell survival and response to X-ray radiation, which is controlled by modulation of ferroptosis. These findings expand our understanding of how GBM cells respond to radio(chemo)therapy and may contribute to the development of new therapeutic approaches.

Distraction osteogenesis at the proximal third of the ulna for the treatment of Masada type I/IIb deformities in children with hereditary multiple exostoses: a retrospective review of twenty cases

BACKGROUND

Ulna distraction by monolateral external fixator (MEFix) is a good option for the treatment of Masada type I and IIb deformities in children with hereditary multiple exostoses (HMEs). However, there is no consensus regarding where to perform ulnar osteotomy. Our hypothesis is that osteotomy at the proximal third of the ulna and progressive distraction with MEFix can simultaneously correct elbow and wrist deformities in patients with HME.

METHODS

We retrospectively reviewed patients with HME who underwent ulna distraction osteogenesis from June 2014 to March 2019. The carrying angle (CA), radial articular angle (RAA), ulnar variance (UV), radial variance (RV) and range of motion (ROM) of the affected forearm and elbow were clinically assessed before lengthening and at the last follow-up visit. The total ulna lengthening distance (LD) and radiographic outcome were also recorded.

RESULTS

Nineteen patients (20 forearms) with HME aged 9.1 ± 2.4 years at the time of surgery were retrospectively reviewed. The mean follow-up period was 26.1 ± 5.6 months. There were 11 patients (12 forearms) with Masada type I deformities and eight patients (8 forearms) with Masada type IIb deformities. Patients with type IIb deformity had higher RV, lower CA values, less elbow flexion and forearm pronosupination than those with type I deformity (p < 0.05); RV was an independent risk factor for radial head dislocation, with the cut off at RV > 15.5 mm. The mean LDs in patients with type I and type IIb deformities were 33.6 ± 6.6 mm and 41.4 ± 5.4 mm, respectively. The mean CA, UV, RV, forearm pronation and ulna deviation at the wrist improved significantly following surgery in all patients. In particular, five of eight patients (62.5%) with type IIb deformities had concentric reduction of the radiocapitellar joint, while no radial head subluxation was detected in patients with type I deformities at the last follow-up. Three complications were recorded: two pin-track infections and one delayed union.

CONCLUSIONS

Distraction osteogenesis at the proximal third of the ulna provides satisfactory clinical and radiological outcomes in patients with Masada type I and IIb deformities. Early treatment of Masada type I deformities is indicated before progression to more complex type IIb deformities.

Osteochondroma formation is independent of heparanase expression as revealed in a mouse model of hereditary multiple exostoses

Hereditary multiple exostoses (HME) is a rare, pediatric disorder characterized by osteochondromas that form along growth plates and provoke significant musculoskeletal problems. HME is caused by mutations in heparan sulfate (HS)-synthesizing enzymes EXT1 or EXT2. Seemingly paradoxically, osteochondromas were found to contain excessive extracellular heparanase (Hpse) that could further reduce HS levels and exacerbate pathogenesis. To test Hpse roles, we asked whether its ablation would protect against osteochondroma formation in a conditional HME model consisting of mice bearing floxed Ext1 alleles in Agr-CreER background (Ext1f/f ;Agr-CreER mice). Mice were crossed with a new global Hpse-null (Hpse-/- ) mice to produce compound Hpse-/- ;Ext1f/f ;Agr-CreER mice. Tamoxifen injection of standard juvenile Ext1f/f ;Agr-CreER mice elicited stochastic Ext1 ablation in growth plate and perichondrium, followed by osteochondroma formation, as revealed by microcomputed tomography and histochemistry. When we examined companion conditional Ext1-deficient mice lacking Hpse also, we detected no major decreases in osteochondroma number, skeletal distribution, and overall structure by the analytical criteria above. The Ext1 mutants used here closely mimic human HME pathogenesis, but have not been previously tested for responsiveness to treatments. To exclude some innate therapeutic resistance in this stochastic model, tamoxifen-injected Ext1f/f ;Agr-CreER mice were administered daily doses of the retinoid Palovarotene, previously shown to prevent ectopic cartilage and bone formation in other mouse disease models. This treatment did inhibit osteochondroma formation compared with vehicle-treated mice. Our data indicate that heparanase is not a major factor in osteochondroma initiation and accumulation in mice. Possible roles of heparanase upregulation in disease severity in patients are discussed.

Hereditary Multiple Exostoses-A Review of the Molecular Background, Diagnostics, and Potential Therapeutic Strategies

Hereditary multiple exostoses (HMEs) syndrome, also known as multiple osteochondromas, represents a rare and severe human skeletal disorder. The disease is characterized by multiple benign cartilage-capped bony outgrowths, termed exostoses or osteochondromas, that locate most commonly in the juxta-epiphyseal portions of long bones. Affected individuals usually complain of persistent pain caused by the pressure on neighboring tissues, disturbance of blood circulation, or rarely by spinal cord compression. However, the most severe complication of this condition is malignant transformation into chondrosarcoma, occurring in up to 3.9% of HMEs patients. The disease results mainly from heterozygous loss-of-function alterations in the EXT1 or EXT2 genes, encoding Golgi-associated glycosyltransferases, responsible for heparan sulfate biosynthesis. Some of the patients with HMEs do not carry pathogenic variants in those genes, hence the presence of somatic mutations, deep intronic variants, or another genes/loci is suggested. This review presents the systematic analysis of current cellular and molecular concepts of HMEs along with clinical characteristics, clinical and molecular diagnostic methods, differential diagnosis, and potential treatment options.

Heparan sulfate deficiency leads to hypertrophic chondrocytes by increasing bone morphogenetic protein signaling

OBJECTIVE

Exostosin-1 (EXT1) and EXT2 are the major genetic etiologies of multiple hereditary exostoses and are essential for heparan sulfate (HS) biosynthesis. Previous studies investigating HS in several mouse models of multiple hereditary exostoses have reported that aberrant bone morphogenetic protein (BMP) signaling promotes osteochondroma formation in Ext1-deficient mice. This study examined the mechanism underlying the effects of HS deficiency on BMP/Smad signaling in articular cartilage in a cartilage-specific Ext^-/- mouse model.

METHOD

We generated mice with a conditional Ext1 knockout in cartilage tissue (Ext1-cKO mice) using Prg4-Cre transgenic mice. Structural cartilage alterations were histologically evaluated and phospho-Smad1/5/9 (pSmad1/5/9) expression in mouse chondrocytes was analyzed. The effect of pharmacological intervention of BMP signaling using a specific inhibitor was assessed in the articular cartilage of Ext1-cKO mice.

RESULTS

Hypertrophic chondrocytes were significantly more abundant (P = 0.021) and cartilage thickness was greater in Ext1-cKO mice at 3 months postnatal than in control littermates (P = 0.036 for femur; and P < 0.001 for tibia). However, osteoarthritis did not spontaneously occur before the 1-year follow-up. matrix metalloproteinase (MMP)-13 and adamalysin-like metalloproteinases with thrombospondin motifs(ADAMTS)-5 were upregulated in hypertrophic chondrocytes of transgenic mice. Immunostaining and western blotting revealed that pSmad1/5/9-positive chondrocytes were more abundant in the articular cartilage of Ext1-cKO mice than in control littermates. Furthermore, the BMP inhibitor significantly decreased the number of hypertrophic chondrocytes in Ext1-cKO mice (P = 0.007).

CONCLUSIONS

HS deficiency in articular chondrocytes causes chondrocyte hypertrophy, wherein upregulated BMP/Smad signaling partially contributes to this phenotype. HS might play an important role in maintaining the cartilaginous matrix by regulating BMP signaling.

Forearm Deformity and Radial Head Dislocation in Pediatric Patients with Hereditary Multiple Exostoses: A Prospective Study Using Proportional Ulnar Length as a Scale to Lengthen the Shortened Ulna

BACKGROUND

Ulnar lengthening is a consensus treatment for hereditary multiple exostoses with radial head dislocation in pediatric patients; however, the optimal amount of ulnar lengthening remains unclear. It is necessary to look for a parameter to decide the amount of ulnar lengthening that will avoid recurrence and complications. The purpose of the present study was to confirm that proportional ulnar length (ulnar length/radial length) can be used as a scale for ulnar lengthening in pediatric patients.

METHODS

The normal lengths of the ulna and radius in the pediatric population were measured in different age groups. The proportional ulnar length was calculated as ulnar length/radial length in each group. Thirty forearms in 26 patients with radial head dislocation were treated with ulnar lengthening and deformity correction. The goal of ulnar lengthening was to recover the normal proportional ulnar length. The function of the forearm was evaluated at the time of the latest follow-up. The preoperative and postoperative values for radial bowing, radioarticular angle, ulnar variance, and carpal slip were also compared.

RESULTS

The value of proportional ulnar length in the normal population consistently averaged approximately 1.1. In all cases, proportional ulnar length recovered to the normal value of 1.1. Reduction of the dislocated radial head was achieved in 28 forearms (93%); in 16 of these forearms, good-quality reduction was achieved and no recurrence of radial head dislocation was observed during follow-up. The function of the forearm improved markedly (p < 0.001). Ulnar variance improved from 2.51 to -0.79 cm (p < 0.001). Radial deformities improved according to measurements of radial bowing and the radioarticular angle (p < 0.001). All parents were satisfied with the postoperative appearance and function of the forearm.

CONCLUSIONS

Proportional ulnar length could be used as a scale to decide the amount of ulnar lengthening for radial head dislocation in pediatric patients with hereditary multiple exostoses. Ulnar lengthening according to proportional ulnar length and deformity correction can prevent recurrence of ulnar variance and avoid impingement of the wrist.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Hereditary Multiple Exostoses: Current Insights

Hereditary multiple exostoses (HME), also called hereditary multiple osteochondromas, is a rare genetic disorder characterized by multiple osteochondromas that grow near the growth plates of bones such as the ribs, pelvis, vertebrae and especially long bones. The disease presents with various clinical manifestations including chronic pain syndromes, restricted range of motion, limb deformity, short stature, scoliosis and neurovascular alteration. Malignant transformation of exostosis is rarely seen. The disease has no medical treatment and surgery is only recommended in symptomatic exostoses or in cases where a malignant transformation is suspected. HME is mainly caused by mutations and functional loss of the EXT1 and EXT2 genes which encode glycosyltransferases, an enzyme family involved in heparan sulfate (HS) synthesis. However, the peculiar molecular mechanism that leads to the structural changes of the cartilage and to osteochondroma formation is still being studied. Basic science studies have recently shown new insights about altering the molecular and cellular mechanism caused by HS deficiency. Pediatricians, geneticists and orthopedic surgeons play an important role in the study and treatment of this severe pathology. Despite the recent significant advances, we still need novel insights to better specify the role of HS in signal transduction. The purpose of this review was to analyze the most relevant aspects of HME from the literature review, give readers an important tool to understand its clinical features and metabolic-pathogenetic mechanism, and to identify an effective treatment method. We focused on the aspects of the disease related to clinical management and surgical treatment in order to give up-to-date information that could be useful for following best clinical practice.

A novel splice mutation induces exon skipping of the EXT1 gene in patients with hereditary multiple exostoses

The molecular mechanism of hereditary multiple exostoses (HME) remains ambiguous and a limited number of studies have investigated the pathogenic mechanism of mutations in patients with HME. In the present study, a novel heterozygous splice mutation (c.1284+2del) in exostosin glycosyltransferase 1 (EXT1) gene was identified in a three-generation family with HME. Bioinformatics and TA clone-sequencing indicated that the splice site mutation would result in exon 4 skipping. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) revealed that the expression levels of wild-type EXT1/EXT2 mRNA in patients with HME were significantly decreased, compared with normal control participants (P<0.05). Abnormal EXT1 transcript lacking exon 4 (EXT1-DEL) and full-length EXT1 mRNA (EXT1-FL) were overexpressed in 293-T cells and Cos-7 cells using lentivirus infection. RT-qPCR demonstrated that the expression level of EXT1-DEL was significantly increased, compared with EXT1-FL (17.032 vs. 6.309, respectively; P<0.05). The protein encoded by EXT1-DEL was detected by western blot analysis, and the level was increased, compared with EXT1-FL protein expression. Immunofluorescence indicated that the protein encoded by EXT1-DEL was located in the cytoplasm of Cos-7 cells, which was consistent with the localization of the EXT1-FL protein. In conclusion, the present study identified a novel splice mutation that causes exon 4 skipping during mRNA splicing and causes reduced expression of EXT1/EXT2. The mutation in EXT1-DEL generated a unique peptide that is located in the cytoplasm in vitro, and it expands the mutation spectrum and provides molecular genetic evidence for a novel pathogenic mechanism of HME.

Multiple Hereditary Exostoses: Report of an EXT2 Gene Mutation in a Colombian Family

Multiple hereditary exostoses (MHE) is a rare disease with autosomal dominant inheritance, caused by heterozygous germline mutations in the EXT1 or EXT2 genes. This disorder is characterized by the growth of prominences surrounded by cartilage in the growth plates and the long bones. Here, we report a family affected by MHE. In this family, a pathogenic variant c.544C > T (p. Arg182Ter) was identified in the EXT2 gene. This variant has been previously described in the literature, and here we are reporting the relationship with clinical findings. MHE is suspected according to the clinical manifestations; molecular research should be performed to establish the most frequent mutations. A support, diagnosis, and follow-up group should be created, and genetic counseling should be available for patients and families.

Domains with highest heparan sulfate-binding affinity reside at opposite ends in BMP2/4 versus BMP5/6/7: Implications for function

Signaling proteins, including bone morphogenetic proteins (BMPs), specifically interact with heparan sulfate (HS). These interactions regulate protein distribution and function and are largely mediated by domains rich in basic amino acids. The N-terminal region of BMP2 and BMP4 contains one such domain with a typical Cardin-Weintraub (CW) motif, but it is unclear whether the same occurs in BMP5, BMP6, and BMP7 that constitute a separate evolutionary subgroup. Peptides spanning the N-terminal domain of BMP2/4 interacted with substrate-bound HS with nanomolar affinity, but peptides spanning BMP5/6/7 N-terminal domain did not. We re-examined the entire BMP5/6/7 sequences and identified a novel CW-like motif at their C terminus. Peptides spanning this domain displayed high-affinity HS binding, but corresponding BMP2/4 C-terminal peptides did not, likely because of acidic or noncharged residue substitutions. Peptides pre-assembled into NeutrAvidin tetramers displayed the same exact binding selectivity of respective monomers but bound HS with greater affinity. Tests of possible peptide biological activities showed that the HS-binding N-terminal BMP2/4 and C-terminal BMP5/6/7 peptides stimulated chondrogenesis in vitro, potentially by freeing endogenous BMPs. Thus, HS interactions appear largely ascribable to domains at opposite ends of BMP2/4 versus BMP5/6/7, reiterating the evolutionary distance of these BMP subgroups and possible functional diversification.

Hereditary multiple exostoses: are there new plausible treatment strategies?

Introduction

Hereditary multiple exostoses (HME) is a rare congenital pediatric disorder characterized by osteochondromas forming next to the growth plates in young patients. The osteochondromas cause multiple health problems that include skeletal deformities and chronic pain. Surgery is used to remove the most symptomatic osteochondromas but because of their large number, many are left in place, causing life-long problems and increasing the probability of malignant transformation. There is no other treatment to prevent or reduce osteochondromas formation at present.

Areas covered

Recent studies reviewable through PubMed are providing new insights into cellular and molecular mechanisms of osteochondroma development. The resulting data are suggesting rational and plausible new therapeutic strategies for osteochondroma prevention some of which are being tested in HME animal models and one of which is part of a just announced clinical trial.

Expert Commentary

This section summarizes and evaluates such strategies and points also to possible future alternatives.

Current paediatric orthopaedic practice in hereditary multiple osteochondromas of the forearm: a systematic review

Introduction: This systematic review aims to answer three research questions concerning the management of hereditary multiple osteochondromas of forearm in children: What is the best available evidence for the currently employed surgical procedures? What patient characteristics are associated with better prognosis? What disease characteristics are associated with better prognosis?

Methods: We searched the literature using three major databases with no publication date restrictions. To enhance search sensitivity and maintain precision we used keywords/subject terms correlating with patient population, problem and interventions. We used strict inclusion/exclusion criteria to improve validity evidence.

Results: The search process yielded 34 eligible studies with a total of 282 patients (315 forearms). We comprehensively analysed study and patient demographics and interventions and outcomes. Eleven studies (32%) had a long-term follow-up and 31 studies (91%) were retrospective. Of the total number of forearms, ulnar lengthening +/− associated procedures was used in 210 forearms (66.7%), isolated osteochondroma excision in 65 forearms (20.6%) and isolated distal radius hemiepiphysiodesis in 15 forearms (4.7%) among others.

Discussion: Ulnar lengthening can restore radiologic anatomy, improve appearance and to a lesser extent objective clinical parameters like joint range of motion on the short/intermediate term. Isolated osteochondroma excision can relief pain and satisfy cosmetic concerns occasionally. There is poor evidence to suggest that surgery improves quality of life or function. Predictors of surgical success in regard to patient and disease characteristics remain elusive. Natural history and prospective randomized control studies where the control group receives no treatment should be rethought. They have the potential for bias control and identification of the ideal surgical candidate. The complex interplay between the confounding variables has undermined the capability of most studies to provide well-grounded evidence to support and generalize their conclusions. Valid quality of life scales should supplement objective outcome measures.

Advances in the pathogenesis and possible treatments for multiple hereditary exostoses from the 2016 international MHE conference

Multiple hereditary exostoses (MHE) is an autosomal dominant disorder that affects about 1 in 50,000 children worldwide. MHE, also known as hereditary multiple exostoses (HME) or multiple osteochondromas (MO), is characterized by cartilage-capped outgrowths called osteochondromas that develop adjacent to the growth plates of skeletal elements in young patients. These benign tumors can affect growth plate function, leading to skeletal growth retardation, or deformations, and can encroach on nerves, tendons, muscles, and other surrounding tissues and cause motion impairment, chronic pain, and early onset osteoarthritis. In about 2-5% of patients, the osteochondromas can become malignant and life threatening. Current treatments consist of surgical removal of the most symptomatic tumors and correction of the major skeletal defects, but physical difficulties and chronic pain usually continue and patients may undergo multiple surgeries throughout life. Thus, there is an urgent need to find new treatments to prevent or reverse osteochondroma formation. The 2016 International MHE Research Conference was convened to provide a forum for the presentation of the most up-to-date and advanced clinical and basic science data and insights in MHE and related fields; to stimulate the forging of new perspectives, collaborations, and venues of research; and to publicize key scientific findings within the biomedical research community and share insights and relevant information with MHE patients and their families. This report provides a description, review, and assessment of all the exciting and promising studies presented at the Conference and delineates a general roadmap for future MHE research targets and goals.

The pathogenic roles of heparan sulfate deficiency in hereditary multiple exostoses

Heparan sulfate (HS) is an essential component of cell surface and matrix proteoglycans (HS-PGs) that include syndecans and perlecan. Because of their unique structural features, the HS chains are able to specifically interact with signaling proteins -including bone morphogenetic proteins (BMPs)- via their HS-binding domain, regulating protein availability, distribution and action on target cells. Hereditary Multiple Exostoses (HME) is a rare pediatric disorder linked to germline heterozygous loss-of-function mutations in EXT1 or EXT2 that encode Golgi-resident glycosyltransferases responsible for HS synthesis, resulting in a systemic HS deficiency. HME is characterized by cartilaginous/bony tumors -called osteochondromas or exostoses- that form within perichondrium in long bones, ribs and other elements. This review examines most recent studies in HME, framing them in the context of classic studies. New findings show that the spectrum of EXT mutations is larger than previously realized and the clinical complications of HME extend beyond the skeleton. Osteochondroma development requires a somatic "second hit" that would complement the germline EXT mutation to further decrease HS production and/levels at perichondrial sites of osteochondroma induction. Cellular studies have shown that the steep decreases in local HS levels: derange the normal homeostatic signaling pathways keeping perichondrium mesenchymal; cause excessive BMP signaling; and provoke ectopic chondrogenesis and osteochondroma formation. Data from HME mouse models have revealed that systemic treatment with a BMP signaling antagonist markedly reduces osteochondroma formation. In sum, recent studies have provided major new insights into the molecular and cellular pathogenesis of HME and the roles played by HS deficiency. These new insights have led to the first ever proof-of-principle demonstration that osteochondroma formation is a druggable process, paving the way toward the creation of a clinically-relevant treatment.

Outcomes of Hemiepiphyseal Stapling for Genu Valgum Deformities in Patients With Multiple Hereditary Exostoses: A Comparative Study of Patients With Deformities of Idiopathic Cause

BACKGROUND

Patients with multiple hereditary exostoses (MHE) frequently present with a genu valgum deformity. Temporary hemiepiphysiodesis, such as hemiepiphyseal stapling, is a relatively safe surgical method to correct angular deformities in skeletally immature patients, but its outcomes for genu valgum deformity in MHE patients have not been extensively reported. We investigated the outcomes of hemiepiphyseal stapling in MHE patients (MHE group) and compared those with the outcomes in patients with idiopathic deformities (idiopathic group) after adjusting for potential bias.

METHODS

Data from 70 limbs with genu valgum deformity (15 MHE and 55 idiopathic), which had undergone hemiepiphyseal stapling, were retrospectively reviewed. The outcomes were focused on the achievement of satisfactory correction and the velocity of correction. The independent effects of each characteristic on each outcome were investigated using multivariate analyses. The outcomes between the groups were also compared after 1:2 matching using propensity score analysis.

RESULTS

The mean valgus angle of the MHE group was 7.4±4.1 degrees at stapling and was corrected to 1.3±3.0 degrees at staple removal. The rate of satisfactory corrections was not different between the MHE and idiopathic groups (67% and 70%, respectively, P=0.820). However, the correction velocity was significantly lower in the MHE group than in the idiopathic group on both multivariate analysis (P=0.001) and matching comparison (4.4 vs. 7.9 degrees/y, P<0.001). The duration of correction was longer in the MHE group than in the idiopathic group by approximately half a year (1.5±0.6 vs. 0.9±0.3 y, respectively, P=0.003).

CONCLUSIONS

In MHE patients with genu valgum deformity, satisfactory correction can be achieved by hemiepiphyseal stapling and is comparable with that seen in idiopathic patients. However, the MHE group showed lower correction velocity and required a longer time by about one half year for correction compared with the idiopathic group. Temporary hemiepiphysiodesis should be considered at an earlier age for patients with MHE compared with those with idiopathic deformity.

LEVEL OF EVIDENCE

Level III-prognostic study.

Hereditary Multiple Exostoses: New Insights into Pathogenesis, Clinical Complications, and Potential Treatments

PURPOSE OF REVIEW

Hereditary multiple exostoses (HME) is a complex musculoskeletal pediatric disorder characterized by osteochondromas that form next to the growth plates of many skeletal elements, including long bones, ribs, and vertebrae. Due to its intricacies and unresolved issues, HME continues to pose major challenges to both clinicians and biomedical researchers. The purpose of this review is to describe and analyze recent advances in this field and point to possible targets and strategies for future biologically based therapeutic intervention.

RECENT FINDINGS

Most HME cases are linked to loss-of-function mutations in EXT1 or EXT2 that encode glycosyltransferases responsible for heparan sulfate (HS) synthesis, leading to HS deficiency. Recent genomic inquiries have extended those findings but have yet to provide a definitive genotype-phenotype correlation. Clinical studies emphasize that in addition to the well-known skeletal problems caused by osteochondromas, HME patients can experience, and suffer from, other symptoms and health complications such as chronic pain and nerve impingement. Laboratory work has produced novel insights into alterations in cellular and molecular mechanisms instigated by HS deficiency and subtending onset and growth of osteochondroma and how such changes could be targeted toward therapeutic ends. HME is a rare and orphan disease and, as such, is being studied only by a handful of clinical and basic investigators. Despite this limitation, significant advances have been made in the last few years, and the future bodes well for deciphering more thoroughly its pathogenesis and, in turn, identifying the most effective treatment for osteochondroma prevention.

Extracellular Signal-Regulated Kinase Signaling in CD4-Expressing Cells Inhibits Osteochondromas

Defects in cartilage homeostasis can give rise to various skeletal disorders including osteochondromas. Osteochondromas are benign bone tumors caused by excess accumulation of chondrocytes, the main cell type of cartilage. The extracellular signal-regulated kinase (ERK) pathway is a major signaling node that functions within chondrocytes to regulate their growth and differentiation. However, it is not known whether the ERK pathway in other cell types regulates cartilage homeostasis. We show here that mice with a germline deficiency of Erk1 and a conditional deletion of Erk2 in cells that express CD4, or expressed CD4 at one point in development, unexpectedly developed bone deformities. The bone lesions were due to neoplastic outgrowths of chondrocytes and disordered growth plates, similar to tumors observed in the human disease, osteochondromatosis. Chondrocyte accumulation was not due to deletion of Erk2 in the T cells. Rather, CD4cre was expressed in cell types other than T cells, including a small fraction of chondrocytes. Surprisingly, the removal of T cells accelerated osteochondroma formation and enhanced disease severity. These data show for the first time that T cells impact the growth of osteochondromas and describe a novel model to study cartilage homeostasis and osteochondroma formation.

Unsuspected osteochondroma-like outgrowths in the cranial base of Hereditary Multiple Exostoses patients and modeling and treatment with a BMP antagonist in mice

Hereditary Multiple Exostoses (HME) is a rare pediatric disorder caused by loss-of-function mutations in the genes encoding the heparan sulfate (HS)-synthesizing enzymes EXT1 or EXT2. HME is characterized by formation of cartilaginous outgrowths-called osteochondromas- next to the growth plates of many axial and appendicular skeletal elements. Surprisingly, it is not known whether such tumors also form in endochondral elements of the craniofacial skeleton. Here, we carried out a retrospective analysis of cervical spine MRI and CT scans from 50 consecutive HME patients that included cranial skeletal images. Interestingly, nearly half of the patients displayed moderate defects or osteochondroma-like outgrowths in the cranial base and specifically in the clivus. In good correlation, osteochondromas developed in the cranial base of mutant Ext1f/f;Col2-CreER or Ext1f/f;Aggrecan-CreER mouse models of HME along the synchondrosis growth plates. Osteochondroma formation was preceded by phenotypic alteration of cells at the chondro-perichondrial boundary and was accompanied by ectopic expression of major cartilage matrix genes -collagen 2 and collagen X- within the growing ectopic masses. Because chondrogenesis requires bone morphogenetic protein (BMP) signaling, we asked whether osteochondroma formation could be blocked by a BMP signaling antagonist. Systemic administration with LDN-193189 effectively inhibited osteochondroma growth in conditional Ext1-mutant mice. In vitro studies with mouse embryo chondrogenic cells clarified the mechanisms of LDN-193189 action that turned out to include decreases in canonical BMP signaling pSMAD1/5/8 effectors but interestingly, concurrent increases in such anti-chondrogenic mechanisms as pERK1/2 and Chordin, Fgf9 and Fgf18 expression. Our study is the first to reveal that the cranial base can be affected in patients with HME and that osteochondroma formation is amenable to therapeutic drug intervention.

Management of nerve compression in multiple hereditary exostoses: a report of two cases and review of the literature

PURPOSE

Multiple hereditary exostoses (MHE) is a rare autosomal dominant condition that results in the growth of cartilage-capped prominences that often cause nerve compression and injury. Many patients suffer from continued and debilitating chronic pain which leads some to advocate avoiding surgical intervention in patients with multiple hereditary exostoses. We present a review of the literature as well as a case series at our institution in order to evaluate the role of surgery in multiple hereditary exostoses.

METHODS

We searched the literature for reports of patients with multiple hereditary exostoses undergoing surgery for nerve compression. We then reviewed the recent experience at our institution which revealed two patients with multiple hereditary exostoses.

RESULTS

Our literature search revealed that there have been several case series and retrospective analyses in the literature that assess the benefit of surgery in the case of nerve compression caused by exostoses. The majority of these reports are of solitary exostoses. Few reports expand on the role of surgery in patients with multiple hereditary exostoses suffering from nerve compressions secondary to bony overgrowth. A recent review of the experience at our institution revealed two patients with multiple hereditary exostoses who together underwent a total of four surgeries for treatment of peripheral nerve compression resulting in pain or weakness. Postoperative evaluation revealed improvement in pain and/or motor strength following each operation.

CONCLUSION

Based on our experience and literature review, we advocate that nerve compression in selected individuals with multiple hereditary exostoses that results in neurological injury should be considered for nerve decompression and resection of the offending exostosis.

Assessing the general population frequency of rare coding variants in the EXT1 and EXT2 genes previously implicated in hereditary multiple exostoses

Hereditary multiple exostoses (HME) is a rare childhood-onset skeletal disease linked to mutations in exostosin glycosyltransferase 1 (EXT1) or 2 (EXT2). Patients are heterozygous for either an EXT1 or EXT2 mutation, and it is widely assumed that exostosis formation and associated defects, such as growth retardation and skeletal deformities, require loss-of-heterozygosity or a second hit in affected cells. However, the relevance and phenotypic impact of many presumed pathogenic EXT variants remain uncertain. We extracted all amino acid-altering (missense) and loss of function (LoF; nonsense, frameshift, or splice-site) variants from the Exome Aggregation Consortium (ExAC), a large population-based repository of exome sequence data from diverse ancestries that has screened out severe pediatric disease, to assess the overall mutation spectrum of predicted protein-damaging variants across these two genes in the general population. We then determined whether clinically-identified, presumably pathogenic variants implicated in HME exist among healthy individuals. We found six EXT1 and four EXT2 missense mutations in ExAC, suggesting that these mutations have either been misclassified as pathogenic or are not fully penetrant. Furthermore, EXT1 is heavily selectively constrained, while EXT2 is more tolerant to protein-damaging variants, especially at its C-terminus, possibly explaining the genotype-phenotype correlation that EXT1 variants usually result in more severe disease. In conclusion, population-based exome data is a useful filter for determining whether clinically detected variants are likely pathogenic, as well as revealing biological insight into rare disease genes such as EXT1 and EXT2.

Heparanase stimulates chondrogenesis and is up-regulated in human ectopic cartilage: a mechanism possibly involved in hereditary multiple exostoses

Hereditary multiple exostoses is a pediatric skeletal disorder characterized by benign cartilaginous tumors called exostoses that form next to growing skeletal elements. Hereditary multiple exostoses patients carry heterozygous mutations in the heparan sulfate (HS)-synthesizing enzymes EXT1 or EXT2, but studies suggest that EXT haploinsufficiency and ensuing partial HS deficiency are insufficient for exostosis formation. Searching for additional pathways, we analyzed presence and distribution of heparanase in human exostoses. Heparanase was readily detectable in most chondrocytes, particularly in cell clusters. In control growth plates from unaffected persons, however, heparanase was detectable only in hypertrophic zone. Treatment of mouse embryo limb mesenchymal micromass cultures with exogenous heparanase greatly stimulated chondrogenesis and bone morphogenetic protein signaling as revealed by Smad1/5/8 phosphorylation. It also stimulated cell migration and proliferation. Interfering with HS function both with the chemical antagonist Surfen or treatment with bacterial heparitinase up-regulated endogenous heparanase gene expression, suggesting a counterintuitive feedback mechanism that would result in further HS reduction and increased signaling. Thus, we tested a potent heparanase inhibitor (SST0001), which strongly inhibited chondrogenesis. Our data clearly indicate that heparanase is able to stimulate chondrogenesis, bone morphogenetic protein signaling, cell migration, and cell proliferation in chondrogenic cells. These properties may allow heparanase to play a role in exostosis genesis and pathogenesis, thus making it a conceivable therapeutic target in hereditary multiple exostoses.

The first Korean patient with Potocki-Shaffer syndrome: a rare cause of multiple exostoses

Potocki-Shaffer syndrome (PSS, OMIM #601224) is a rare contiguous gene deletion syndrome caused by haploinsufficiency of genes located on the 11p11.2p12. Affected individuals have a number of characteristic features including multiple exostoses, biparietal foramina, abnormalities of genitourinary system, hypotonia, developmental delay, and intellectual disability. We report here on the first Korean case of an 8-yr-old boy with PSS diagnosed by high resolution microarray. Initial evaluation was done at age 6 months because of a history of developmental delay, hypotonia, and dysmorphic face. Coronal craniosynostosis and enlarged parietal foramina were found on skull radiographs. At age 6 yr, he had severe global developmental delay. Multiple exostoses of long bones were detected during a radiological check-up. Based on the clinical and radiological features, PSS was highly suspected. Subsequently, chromosomal microarray analysis identified an 8.6 Mb deletion at 11p11.2 [arr 11p12p11.2 (Chr11:39,204,770-47,791,278)×1]. The patient continued rehabilitation therapy for profound developmental delay. The progression of multiple exostosis has being monitored. This case confirms and extends data on the genetic basis of PSS. In clinical and radiologic aspect, a patient with multiple exostoses accompanying with syndromic features, including craniofacial abnormalities and mental retardation, the diagnosis of PSS should be considered.

Molecular, phenotypic aspects and therapeutic horizons of rare genetic bone disorders

A rare disease afflicts less than 200,000 individuals, according to the National Organization for Rare Diseases (NORD) of the United States. Over 6,000 rare disorders affect approximately 1 in 10 Americans. Rare genetic bone disorders remain the major causes of disability in US patients. These rare bone disorders also represent a therapeutic challenge for clinicians, due to lack of understanding of underlying mechanisms. This systematic review explored current literature on therapeutic directions for the following rare genetic bone disorders: fibrous dysplasia, Gorham-Stout syndrome, fibrodysplasia ossificans progressiva, melorheostosis, multiple hereditary exostosis, osteogenesis imperfecta, craniometaphyseal dysplasia, achondroplasia, and hypophosphatasia. The disease mechanisms of Gorham-Stout disease, melorheostosis, and multiple hereditary exostosis are not fully elucidated. Inhibitors of the ACVR1/ALK2 pathway may serve as possible therapeutic intervention for FOP. The use of bisphosphonates and IL-6 inhibitors has been explored to be useful in the treatment of fibrous dysplasia, but more research is warranted. Cell therapy, bisphosphonate polytherapy, and human growth hormone may avert the pathology in osteogenesis imperfecta, but further studies are needed. There are still no current effective treatments for these bone disorders; however, significant promising advances in therapeutic modalities were developed that will limit patient suffering and treat their skeletal disabilities.

[Hereditary multiple exostoses]

BACKGROUND

Hereditary multiple exostosis (HME) is a hereditary autosomal dominant disease in which multiple exostoses occur. Typically, the exostoses are primarily located at the metaphysis and migrate with continued growth towards the diaphysis. Clinical problems are caused by local pain, impingement of muscle tendons and neurovascular structures, malformation - especially in the forearm - and malignant transformation - especially exostoses at the trunc and pelvic girdle.

METHODS

A causal therapy is currently not available. Mechanical irritation is an indication for resection of the exostosis. Axial deviation of the lower extremity is treated according to the same principles as primary malalignments (temporary hemiepiphysiodesis/corrective osteotomy).

RESULTS

The indication for correction of axial deviation at the upper extremity depends on age, extent as well as functional and cosmetic impairment. This should be discussed with the patient in detail. The patient has to be informed about the risk of malignant transformation after cessation of growth. Growing mass or new occurrence of symptoms after end of growth are suspicious and require further diagnostic examinations.

Mutational screening of EXT1 and EXT2 genes in Polish patients with hereditary multiple exostoses

Hereditary multiple exostoses (HME) also known as multiple osteochondromas represent one of the most frequent bone tumor disorder in humans. Its clinical presentation is characterized by the presence of multiple benign cartilage-capped tumors located most commonly in the juxta-epiphyseal portions of long bones. HME are usually inherited in autosomal dominant manner, however de novo mutations can also occur. In most patients, the disease is caused by alterations in the EXT1 and EXT2 genes. In this study we investigated 33 unrelated Polish probands with the clinical and radiological diagnosis of HME by means of Sanger sequencing and MLPA for all coding exons of EXT1 and EXT2. We demonstrated EXT1 and EXT2 heterozygous mutations in 18 (54.6 %) and ten (30.3 %) probands respectively, which represents a total of 28 (84.9 %) index cases. Sequencing allowed for the detection of causative changes in 26 (78.8 %) probands, whereas MLPA showed intragenic deletions in two (6.1 %) further cases (15 mutations represented novel changes). Our paper is the first report on the results of exhaustive mutational screening of both EXT1/EXT2 genes in Polish patients. The proportion of EXT1/EXT2 mutations in our group was similar to other Caucasian cohorts. However, we found that EXT1 lesions in Polish patients cluster in exons 1 and 2 (55.6 % of all EXT1 mutations). This important finding should lead to the optimization of cost-effectiveness rate of HME diagnostic testing. Therefore, the diagnostic algorithm for HME should include EXT1 sequencing (starting with exons 1–2), followed by EXT2 sequencing, and MLPA/qPCR for intragenic copy number changes.

Clinical characteristics of hereditary multiple exostoses: a retrospective study of mainland chinese cases in recent 23 years

Hereditary multiple exostoses (HME) are an autosomal dominant skeletal disease with wide variations in clinical manifestations among different ethnic groups. This study investigated the epidemiology, clinical presentations, pathogenetic features and treatment strategies of HME in mainland China. We searched and reviewed the related cases published since 1990 by searching electronic databases, namely SinoMed database, Wanfang database, CNKI, Web of Science and PubMed as well as Google search engines. A total of 1051 cases of HME (male-to-female ratio 1.5:1) were investigated and the diagnosis was made in 83% before the age of 10 years. Approximately 96% patients had a family history. Long bones, ribs, scapula and pelvis were the frequently affected sites. Most patients were asymptomatic with multiple palpable masses. Common complications included angular deformities, impingement on neighbouring tissues and impaired articular function. Chondrosarcomas transformation occurred in 2% Chinese cases. Among the cases examined, about 18% had mutations in EXT1 and 28% in EXT2. Frameshift, nonsense and missense mutations represented the majority of HME-causing mutations. Diagnosis of HME was made based on the clinical presentations and radiological documentations. Most patients needed no treatment. Surgical treatment was often directed to remove symptomatic exostoses, particularly those of suspected malignancy degeneration, and correction of skeletal deformities. This study shows some variance from current literature regarding other ethnic populations and may provide valuable baseline assessment of the natural history of HME in mainland China.

Hereditary multiple exostoses: anatomical distribution and burden of exostoses is dependent upon genotype and gender

BACKGROUND AND AIMS

We describe the novel anatomical distribution of exostoses in patients with hereditary multiple exostoses according to their gender and genotype.

METHODS AND RESULTS

A prospective database of 143 patients from 65 families with hereditary multiple exostoses was compiled. Patient demographics, genotype and number of exostoses according to anatomical site were recorded. The hand was affected by the greatest proportion of exostoses for both EXT1 (19%) and EXT2 (14%) genotypes and was the most prevalent site for exostoses in patients with an EXT1 genotype (92%). Patients with an EXT1 genotype had a significantly greater number of exostoses compared to those with an EXT2 genotype (2680 vs. 1828, p = 0.006); however, this was only significantly different for 10 of the 19 anatomical regions examined. Male patients with an EXT1 genotype had a significantly (p < 0.05) greater number of exostoses affecting their hands, distal radius, proximal humerus, scapular and ribs compared to female patients with the same genotype and males with an EXT2 genotype.

CONCLUSION

The anatomical distribution of exostoses varies according to genotype and gender; however, the reason for this difference is not clear and may relate to different biochemical pathways.

Heparan sulfate in skeletal development, growth, and pathology: the case of hereditary multiple exostoses

Heparan sulfate (HS) is an essential component of cell surface and matrix-associated proteoglycans. Due to their sulfation patterns, the HS chains interact with numerous signaling proteins and regulate their distribution and activity on target cells. Many of these proteins, including bone morphogenetic protein family members, are expressed in the growth plate of developing skeletal elements, and several skeletal phenotypes are caused by mutations in those proteins as well as in HS-synthesizing and modifying enzymes. The disease we discuss here is hereditary multiple exostoses (HME), a disorder caused by mutations in HS synthesizing enzymes EXT1 and EXT2, leading to HS deficiency. The exostoses are benign cartilaginous-bony outgrowths, form next to growth plates, can cause growth retardation and deformities, chronic pain and impaired motion, and progress to malignancy in 2-5% of patients. We describe recent advancements on HME pathogenesis and exostosis formation deriving from studies that have determined distribution, activities and roles of signaling proteins in wild-type and HS-deficient cells and tissues. Aberrant distribution of signaling factors combined with aberrant responsiveness of target cells to those same factors appear to be a major culprit in exostosis formation. Insights from these studies suggest plausible and cogent ideas about how HME could be treated in the future.

Cell biology of osteochondromas: bone morphogenic protein signalling and heparan sulphates

Frequent benign outgrowths from bone known as osteochondromas, exhibiting typical endochondral ossification, are reported from single to multiple lesions. Characterised by a high incidence of osteochondromas and skeletal deformities, multiple hereditary exostoses (MHE) is the most common inherited musculoskeletal condition. While factors for severity remain unknown, mutations in exostosin 1 and exostosin 2 genes, encoding glycosyltransferases involved in the biosynthesis of ubiquitously expressed heparan sulphate (HS) chains, are associated with MHE. HS-binding bone morphogenetic proteins (BMPs) are multifunctional proteins involved in the morphogenesis of bone and cartilage. HS and HS proteoglycans are involved in BMP-mediated morphogenesis by regulating their gradient formation and activity. Mutations in exostosin genes disturb HS biosynthesis, subsequently affecting its functional role in the regulation of signalling pathways. As BMPs are the primordial morphogens for bone development, we propose the hypothesis that BMP signalling may be critical in osteochondromas. For this reason, the outcomes of exostosin mutations on HS biosynthesis and interactions within osteochondromas and MHE are reviewed. Since BMPs are HS binding proteins, the interactions of HS with the BMP signalling pathway are discussed. The impact of mouse models in the quest to better understand the cell biology of osteochondromas is discussed. Several challenges and questions still remain and further investigations are needed to explore new approaches for better understanding of the pathogenesis of osteochondromas.

Toward an understanding of the short bone phenotype associated with multiple osteochondromas

Individuals with multiple osteochondromas (MO) demonstrate shortened long bones. Ext1 or Ext2 haploinsufficiency cannot recapitulate the phenotype in mice. Loss of heterozygosity for Ext1 may induce shortening by steal of longitudinal growth into osteochondromas or by a general derangement of physeal signaling. We induced osteochondromagenesis at different time points during skeletal growth in a mouse genetic model, then analyzed femora and tibiae at 12 weeks using micro-CT and a point-distribution-based shape analysis. Bone lengths and volumes were compared. Metaphyseal volume deviations from normal, as a measure of phenotypic widening, were tested for correlation with length deviations. Mice with osteochondromas had shorter femora and tibiae than controls, more consistently when osteochondromagenesis was induced earlier during skeletal growth. Volumetric metaphyseal widening did not correlate with longitudinal shortening, although some of the most severe shortening was in bones with abundant osteochondromas. Loss of heterozygosity for Ext1 was sufficient to drive bone shortening in a mouse model of MO, but shortening did not correlate with osteochondroma volumetric growth. While a steal phenomenon seems apparent in individual cases, some other mechanism must also be capable of contributing to the short bone phenotype, independent of osteochondroma formation. Clones of chondrocytes lacking functional heparan sulfate must blunt physeal signaling generally, rather than stealing growth potential focally.