Mutations in the EXT1 and EXT2 genes in Spanish patients with multiple osteochondromas

Identification of novel germline mutations in FUT7 and EXT1 linked with hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder primarily linked with mutations in Exostosin-1 (EXT1) and Exostosin-2 (EXT2) genes. However, not all HME cases can be explained by these mutations, and its pathogenic mechanisms are not fully understood. Herein, utilizing whole-exome sequencing and genetic screening with a family trio design, we identify two novel rare mutations co-segregating with HME in a Chinese family, including a nonsense mutation (c.204G>A, p.Trp68*) in EXT1 and a missense mutation (c.893T>G, p.Phe298Cys) in FUT7. Functional assays reveal that the FUT7 mutation affects the cellular localization of FUT7 protein and regulates cell proliferation. Notably, the simultaneous loss of fut7 and ext1 in a zebrafish model results in severe chondrodysplasia, indicating a functional link between FUT7 and EXT1 in chondrocyte regulation. Additionally, we unveil that FUT7 p.Phe298Cys reduces EXT1 expression through IL6/STAT3/SLUG axis at the transcription level and through ubiquitination-related proteasomal degradation at the protein level. Together, our findings not only identify novel germline mutations in FUT7 and EXT1 genes, but also highlight the critical interaction between these genes, suggesting a potential 'second-hit' mechanism over EXT1 mutations in HME pathogenesis. This insight enhances our understanding of the mechanisms underlying HME and opens new avenues for potential therapeutic interventions.

A Novel Pathogenic Large Duplication in EXT1 Identified in a Family with Multiple Osteochondromas

Multiple osteochondromas (MO) is an autosomal dominant disorder and the most common genetic skeletal dysplasia, characterized by the growth of bone outgrowths capped by cartilage, called osteochondromas. Most MO cases are caused by mutations in the exostosin-1 (EXT1) and exostosin-2 (EXT2) genes. Only 5% of MO-causative variants are represented by single or multiple exon deletions; to date, no pathogenic large duplication has been described in the literature. In the present study, we describe the novel in-tandem intragenic duplication c.(1128_1202)_(1284+29_1344)dup involving exon 4 of EXT1 (NM_000127.2), detected in a three-generation family with MO. The variant has been detected by MLPA (multiplex ligation-dependent probe amplification) and then confirmed with qPCR (quantitative PCR). Our finding expands the spectrum of MO-causing variants describing a pathogenic large duplication, underlying the importance of quantitative analysis in patients with negative sequencing.

Cases report: Mosaic structural variants of the EXT1 gene in previously genetically unconfirmed multiple osteochondromas

Multiple osteochondromas (MO) is a rare autosomal dominant skeletal disorder characterized by the development of multiple benign tumors known as osteochondromas. The condition is predominantly caused by loss-of-function variants in the EXT1 or EXT2 genes, facilitating relatively precise clinical diagnosis through established diagnostic criteria. Despite this, a notable percentage of MO cases (10%-20%) remains unresolved after sequencing coding regions and copy number analysis of both genes. In our study, we identified mosaic structural variants in two patients who initially yielded negative results on standard genetic analysis for MO. Specifically, mosaic deletions affecting exons 8-11 and exons 2-11 in the EXT1 gene were detected. RNA analysis was performed in one case, while both cases underwent genome sequencing. To date, only six mosaic copy number variations have been reported in association with MO, representing a minority among known variants in both genes. Our report provides a detailed analysis of these findings, highlighting the significance of advanced genetic testing techniques in detecting mosaic variants in the EXT1/2 genes.

The Missing Piece of the Puzzle: Unveiling the Role of PTPN11 Gene in Multiple Osteochondromas in a Large Cohort Study

This study is aimed at investigating the clinical and genetic characteristics of 244 unrelated probands diagnosed with multiple osteochondromas (MO). The diagnosis of MO typically involves identifying multiple benign bone tumors known as osteochondromas (OCs) through imaging studies and physical examinations. However, cases with both OCs and enchondromas (ECs) may indicate the more rare condition metachondromatosis (MC), which is assumed to be distinct disease. Previous cohort studies of MO found heterozygous loss-of-function (LoF) variants only in the EXT1 or EXT2 genes, with DNA diagnostic yield ranging from 78 to 95%. The PTPN11 gene, which is causative for MC, was not previously investigated as a gene candidate for MO. In this study, we detected a total of 177 unique single nucleotide and copy number variants in three genes across 220 probands, consisting of 80 previously reported and 97 novel variants. Specifically, we identified five cases with OCs and no ECs as well as four cases with MC carrying LoF variants in the PTPN11 gene and two additional cases with ECs harboring variants in the EXT1/2 genes. These findings suggest a potential overlap between the MO and MC both phenotypically and genetically. These findings highlight the importance of expanding genetic testing beyond the EXT1 and EXT2 genes in MO cases, as other genes such as PTPN11 may also be causative. This can improve the accuracy of diagnosis and treatment for individuals with MO and MC. It is essential to determine whether MO and MC represent distinct diseases or if they encompass a broader clinical spectrum.

Phenotypic and Molecular Spectrum of a Turkish Cohort with Hereditary Multiple Osteochondromas

OBJECTIVE

Hereditary multiple osteochondromas is an autosomal dominant disorder caused by heterozygous pathogenic variants in EXT1 or EXT2. We aimed to evaluate the clinical and molecular findings of a Turkish cohort with hereditary multiple osteochondroma.

MATERIALS AND METHODS

Thirty-two patients aged 1.3-49.6 years from 22 families were enrolled. Genetic analyses were made by EXT1 and/or EXT2 sequencing and chromosomal microarray analyses.

RESULTS

We found 17 intragenic pathogenic variants in EXT1 (13/17) and EXT2 (4/17), 12 of which are novel. Four probands had EXT1 deletions, including 2 patients with partial EXT1 microdeletions involving exons 2-11 and 5-11, and 2 patients with whole-gene deletions. In 21 variants, the frequency of truncating and missense variants was 76.1% and 23.8%, respectively. Two families had no detectable variants in EXT1 and EXT2. All patients had multiple osteochondromas at the long bones, mainly at the tibia, forearm, femur, and humerus. Bowing deformity of the forearms (9/32) and the lower extremities (2/32), and scoliosis (6/32) were observed. The clinical severity was not different between patients with EXT1 or EXT2 variants. One patient with an EXT2 variant and another with an EXT1 microdeletion had the most severe phenotype with class III disease. Four patients with no EXT1 or EXT2 variants had milder phenotypes. Intrafamilial variability in disease severity was not observed.

CONCLUSION

We report a hereditary multiple osteochondroma cohort with clinical and molecular data including 12 novel intragenic variants in EXT1 or EXT2, and 4 microdeletions involving EXT1. Taken together, our data expand the existing knowledge of the phenotype-genotype spectrum in hereditary multiple osteochondroma.

Heparan sulfate proteoglycans in cancer: Pathogenesis and therapeutic potential

The heparan sulfate proteoglycans (HSPGs) are glycoproteins that consist of a proteoglycan "core" protein and covalently attached heparan sulfate (HS) chain. HSPGs are ubiquitously expressed in mammalian cells on the cell surface and in the extracellular matrix (ECM) and secretory vesicles. Within HSPGs, the protein cores determine when and where HSPG expression takes place, and the HS chains mediate most of HSPG's biological roles through binding various protein ligands, including cytokines, chemokines, growth factors and receptors, morphogens, proteases, protease inhibitors, and ECM proteins. Through these interactions, HSPGs modulate cell proliferation, adhesion, migration, invasion, and angiogenesis to display essential functions in physiology and pathology. Under physiological conditions, the expression and localization of HSPGs are finely regulated to orchestrate their physiological functions, and this is disrupted in cancer. The HSPG dysregulation elicits multiple oncogenic signaling, including growth factor signaling, ECM and Integrin signaling, chemokine and immune signaling, cancer stem cell, cell differentiation, apoptosis, and senescence, to prompt cell transformation, proliferation, tumor invasion and metastasis, tumor angiogenesis and inflammation, and immunotolerance. These oncogenic roles make HSPGs an attractive pharmacological target for anti-cancer therapy. Several therapeutic strategies have been under development, including anti-HSPG antibodies, peptides and HS mimetics, synthetic xylosides, and heparinase inhibitors, and shown promising anti-cancer efficacy. Therefore, much progress has been made in this line of study. However, it needs to bear in mind that the roles of HSPGs in cancer can be either oncogenic or tumor-suppressive, depending on the HSPG and the cancer cell type with the underlying mechanisms that remain obscure. Further studies need to address these to fill the knowledge gap and rationalize more efficient therapeutic targeting.

Haploinsufficiency of EXT1 and Heparan Sulphate Deficiency Associated with Hereditary Multiple Exostoses in a Pakistani Family

Background and Objectives: Hereditary multiple exostoses (HME) is a disease characterized by cartilage-capped bony protuberances at the site of growth plates of long bones. Functional mutations in the exostosin genes (EXT1 and EXT2) are reported to affect the hedgehog signalling pathways leading to multiple enchondromatosis. However, the exact role of each EXT protein in the regulation of heparan sulphate (HS) chain elongation is still an enigma. In this study, a Pakistani family with HME is investigated to find out the genetic basis of the disease. Materials and Methods: Genotyping of eight members of the family by amplifying microsatellite markers, tightly linked to the EXT1 and EXT2 genes. Results: The study revealed linkage of the HME family to the EXT1 locus 8q24.1. Sanger sequencing identified a heterozygous deletion (c.247Cdel) in exon 1 of EXT1, segregating with the disease phenotype in the family. In silico analysis predicted a shift in the frame causing an early stop codon (p.R83GfsX52). The predicted dwarf protein constituting 134 amino acids was functionally aberrant with a complete loss of the catalytic domain at the C-terminus. Interestingly, an alternative open reading frame 3 (ORF3) caused by the frame shift is predicted to encode a protein sequence, identical to the wild type and containing the catalytic domain, but lacking the first 100 amino acids of the wild-type EXT1 protein. Conclusion: Consequently, haploinsufficiency could be the cause of HME in the investigated family as the mutated copy of EXT1 is ineffective for EXT-1/2 complex formation. The predicted ORF3 protein could be of great significance in understanding several aspects of HME pathogenesis.

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.

Genetic analysis of seven pateints with Hereditary Multiple Osteochondromas (HMO)

BACKGROUND

HMO (Hereditary Multiple Osteochondroma), an uncommon autosomal dominant disorder, is characterized by the development of multiple osteochondromas, which are nonmalignant cartilage-capped bone tumors growing outwards from long bone metaphyses.

METHODS

The present work retrospectively analyzed seven children with HMO who were enrolled for routine clinical diagnosis and treatment, including X-ray examination. Subsequent genetic detection was carried out using whole exome sequencing (WES). In addition, this work applied Sanger sequencing to be the validation approach. Moreover, this work also examined amino acid (AA) evolutionary conservatism under the influence of certain missense variants.

RESULTS

The clinical indications of all seven patients and their family members were thoroughly indexed. WES identified diagnostic variants in the EXT1 or EXT2 gene in these patients. In these variants, four were reported for the first time, namely EXT1: c.1285-2A>T, EXT2: c.1139delT, EXT1: c.203G>A, and EXT1: c.1645_1673del. Familial validation revealed that three of the variants were hereditary, while the other four were de novo, which was consistent with the phenotype in each case.

CONCLUSION

Our results expanded HMO variation spectrum, and laid certain foundations for the precise counseling of those affected families.

A Genotype-Phenotype Study of Multiple Hereditary Exostoses in Forty-Three Patients

Multiple hereditary exostoses (MHE) is a rare autosomal dominant skeletal disorder with a variety of clinical manifestations. We aimed to evaluate the general clinical phenotypic severity of MHE using our own scoring system and analyzed the risk factors associated with severe clinical phenotypes. In this study, 43 patients from 30 families were analyzed. The mutations were identified by direct sequencing of polymerase chain reaction-amplified genomic DNA or by multiplex ligation-dependent probe amplification. According to a new scoring system devised by the authors, the severity of the phenotype was assessed as mild, moderate, or severe based on the deformity of each segment, number of exostoses, leg length discrepancy, and functional limitations. Of 43 patients from 30 families, 39 patients (90.7%) and 24 families (80%) presented with EXT1 or EXT2 mutations. Patients with EXT1 mutations had a significantly worse phenotype than that of patients with EXT2 mutations or without any detectable mutation. The mean clinical score of patients with an EXT1 mutation (5.76; range, 2.0–8.0; SD = 1.60) was higher than that of patients with an EXT2 mutation (4.06; range, 2.0–7.0; SD = 1.47) or of those without any detectable mutation (4.63; range, 3.0–6.0; SD = 1.44; p = 0.005). According to our classification system, more patients with EXT1 mutations had ‘severe disease’ than those with EXT2 mutations. Deformity scores were also higher in patients with EXT1 mutations (p = 0.018). In the multivariate analysis, the deformity score was found to be associated with the ‘severe’ class (p = 0.031). In conclusion, 90.7% of patients with MHE showed EXT mutations. Our scoring system showed reliable results. We suggest that the extent of deformity is an important factor in determining the phenotype of MHE and close monitoring for the development of severe disease is recommended in patients with high deformity scores.

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.

Natural History and Characteristics of Hand Exostoses in Multiple Hereditary Exostoses

PURPOSE

Multiple hereditary exostoses (MHEs) comprise a rare skeletal disorder. This study aimed to elucidate the natural history and characteristics of hand exostoses by focusing on their progression or regression and their association with shortening and angular deformation of the finger bones.

METHODS

Of 60 MHE patients who presented to our hospital between 2005 and 2019, 32 patients (62 hands) who underwent hand x-ray examinations were included in a study of initial presentation. Among them, 15 patients (30 hands) who underwent consecutive x-ray examinations before epiphyseal closure were included in a subsequent follow-up study (follow-up period, 6.9 years). We investigated the incidence and common location of hand involvement by exostoses during the initial examination study. We further investigated the progression or regression of hand exostoses and the influence of hand exostoses on longitudinal bone growth and the angular deformation of finger bones during the follow-up study.

RESULTS

In the initial study, we observed exostoses in 30 (60 hands) of 32 (62 hands) patients. The average number of exostoses per hand was 5.2. Exostoses developed more frequently in the middle and ring proximal phalanges and the fifth metacarpal. In the follow-up study, 3.1 exostoses per hand spontaneously regressed and 2.9 exostoses per hand occurred de novo with growth. Progression of exostoses was mainly observed by approximately age 15 years; thereafter, spontaneous regression became dominant. Regression of exostoses was frequently observed in metacarpals, whereas de novo exostoses frequently occurred in phalangeal bones, particularly in distal phalanges. Growth plate involvement by exostoses did not influence the longitudinal growth of finger bones, but it increased their angulation.

CONCLUSIONS

The hand is a common location of exostoses development for MHE patients. Although some exostoses in the hands regress with skeletal maturity, hand involvement by exostoses can result in angular deformity.

TYPE OF STUDY/LEVEL OF EVIDENCE

Diagnostic IV.

Correlation between mutated genes and forearm deformity in patients with multiple osteochondroma

BACKGROUNDS

Exostosin-1 (EXT1) and exostosin-2 (EXT2) cause multiple osteochondromas (MO). In this study, we investigated the correlation between forearm deformity and mutant EXTs in Japanese families with MO.

METHODS

We evaluated 112 patients in 71 families with MO. Genomic DNA was isolated from peripheral blood leucocytes. Of these, 28 patients were selected and underwent radiography for their forearms since they had gross forearm deformities. We measured the radial articular angle (RAA), ulna variance (UV), carpal slip (CS), and percentage of radial bowing (%RB) to compare between patients with mutant EXT1 or EXT2 and those with missense or other mutations using Student's t-test.

RESULTS

Twenty-two (78.6%) and 6 (11.4%) out of 28 patients had mutations in EXT1 and EXT2, respectively. Nine (32.1%) and 19 (67.9%) of the 28 patients had missense and other mutations, respectively. The mean age of patients with EXT1 and EXT2 were 25.9 ± 20.3 and 33.5 ± 25.4 years, respectively and those with missense mutation and other mutations were 28.7 ± 27.0 and 24.6 ± 17.0 years, respectively. There were no significant differences in RAA, UV, and RB between patients harbouring mutant EXT1 or EXT2 (RAA, 40.1 ± 8.7 and 31.5 ± 13.9°; UV, -2.7 ± 5.7 and -3.1 ± 3.7 mm; %RB, 8.6 ± 1.5 and 8.3 ± 2.0%). CS was significantly greater in patients with mutant EXT1 than that in those with mutant EXT2 (EXT1, 44.1 ± 16.8%; EXT2, 18.6 ± 14.0%). There were no significant differences in RAA, UV, CS and %RB between patients with missense and other mutations.

CONCLUSIONS

Patients with mutant EXT1 displayed greater CS than patients with mutant EXT2, indicating that patients with MO harbouring EXT1 mutations sustain more severe ulnar drift deformities than those with EXT2 mutations.

Mutation spectrum of EXT1 and EXT2 in the Saudi patients with hereditary multiple exostoses

BACKGROUND

Hereditary Multiple Exostoses (HME), also known as Multiple Osteochondromas (MO) is a rare genetic disorder characterized by multiple benign cartilaginous bone tumors, which are caused by mutations in the genes for exostosin glycosyltransferase 1 (EXT1) and exostosin glycosyltransferase 2 (EXT2). The genetic defects have not been studied in the Saudi patients.

AIM OF STUDY

We investigated mutation spectrum of EXT1 and EXT2 in 22 patients from 17 unrelated families.

METHODS

Genomic DNA was extracted from peripheral leucocytes. The coding regions and intron-exon boundaries of both EXT1 and EXT2 genes were screened for mutations by PCR-sequencing analysis. Gross deletions were analyzed by MLPA analysis.

RESULTS

EXT1 mutations were detected in 6 families (35%) and 3 were novel mutations: c.739G > T (p. E247*), c.1319delG (p.R440Lfs*4), and c.1786delA (p.S596Afs*25). EXT2 mutations were detected in 7 families (41%) and 3 were novel mutations: c.541delG (p.D181Ifs*89), c.583delG (p.G195Vfs*75), and a gross deletion of approximately 10 kb including promoter and exon 1. Five patients from different families had no family history and carried de novo mutations (29%, 5/17). No EXT1 and EXT2 mutations were found in the remaining four families. In total, EXT1 and EXT2 mutations were found in 77% (13/17) of Saudi HME patients.

CONCLUSION

EXT1 and EXT2 mutations contribute significantly to the pathogenesis of HME in the Saudi population. In contrast to high mutation rate in EXT 1 (65%) and low mutation rate in EXT2 (25%) in other populations, the frequency of EXT2 mutations are much higher (41%) and comparable to that of EXT1 among Saudi patients. De novo mutations are also common and the six novel EXT1/EXT2 mutations further expands the mutation spectrum of HME.

Hereditary multiple osteochondromas in Jordanian patients: Mutational and immunohistochemical analysis of EXT1 and EXT2 genes

The aim of the present study was to investigate the molecular characteristics of hereditary multiple osteochondromas (HMO) in a subset of Jordanian patients with a focus on the genetic variants of exostosin (EXT1)/(EXT2) and their protein expression. Patients with HMO and their family members were included. Recorded clinical characteristics included age, sex, tumors number and location, joint deformities and associated functional limitations. Mutational analysis of EXT1 and EXT2 exonic regions was performed. Immunohistochemical staining for EXT1 and EXT2 was performed manually using two different commercially available rabbit anti-human EXT1 and EXT2 antibodies. A total of 16 patients with HMO from nine unrelated families were included, with a mean age of 13.9 years. A total of 75% (12/16) of the patients were male and (69%) (11/16) had a mild disease (class I). EXT mutation analysis revealed only EXT1 gene mutations in 13 patients. Seven variants were detected, among which three were novel: c.1019G>A, p. (Arg340His), c.962+1G>A and c.1469del, p. (Leu490Argfs*9). Of the 16 patients, 3 did not harbor any mutations for either EXT1 or EXT2. Immunohistochemical examination revealed decreased expression of EXT1 protein in all patients with EXT1 mutation. Surprisingly, EXT2 protein was not detected in these patients, although none had EXT2 mutations. The majority of Jordanian patients with HMO, who may represent an ethnic group that is infrequently investigated, were males and had a mild clinical disease course; whereas most patients with EXT1 gene mutations were not necessarily associated with a severe clinical disease course. The role of EXT2 gene remains a subject of debate, since patients with EXT1 mutations alone did not express the non-mutated EXT2 gene.

An analysis of osteoporosis in patients with hereditary multiple exostoses

We analyzed osteoporosis in 20 HME patients. According to the T-score of BMD, 30% and 67.5% of the patients fell in the range of osteopenia in the lumbar spine and femoral neck. Our results indicate HME patients have low bone mass. They do not have abnormal bone metabolism.

INTRODUCTION

There are few reports of osteoporosis in hereditary multiple exostoses (HME) patients. Therefore, the purpose of this study was to analyze osteoporosis in HME patients.

METHODS

This retrospective cohort study included 20 patients diagnosed with HME. Patients underwent bone mineral density (BMD) measurement of the lumbar spine (n = 20) and femoral neck (n = 40). Bone metabolic parameters, including serum osteocalcin and urinary cross-linked N-telopeptide of type 1 collagen (NTx), were analyzed in all subjects. EXT1 and EXT2 genes were sequenced using genomic DNA. We also examined the correlation between genotype and BMD Z-score and T-score.

RESULTS

The mean BMD values of the lumbar spine were 1.085 ± 0.116 g/cm² (n = 11) in male and 1.108 ± 0.088 g/cm² (n = 9) in female. The mean BMD values of the femoral neck area were 0.759 ± 0.125 g/cm² (n = 22) in male and 0.749 ± 0.115 g/cm² (n = 18) in female. Z-score of most HME patients show < 0, indicating that these patients tend to have low bone mass compared with the age-matched population. According to the T-score of BMD, 30% (6 of 20) and 67.5% (27 of 40) of the patients fell in the range of osteopenia in the lumbar spine and femoral neck areas, respectively. Serum osteocalcin and urinary NTx were in the normal range in most patients. There was no significant correlation between genotypes and Z-score.

CONCLUSION

HME patients have low bone mass, especially in the femoral neck area. They do not have abnormal bone metabolism, and there was no correlation between genotypes and Z-score.

A mutation in SLC20A2 (c.C1849T) promotes proliferation while inhibiting hypertrophic differentiation in ATDC5 chondrocytes

AIMS

This study aimed to investigate the effect of solute carrier family 20 member 2 (SLC20A2) gene mutation (identified from a hereditary multiple exostoses family) on chondrocyte proliferation and differentiation.

METHODS

ATDC5 chondrocytes were cultured in insulin-transferrin-selenium medium to induce differentiation. Cells were transfected with pcDNA3.0 plasmids with either a wild-type (WT) or mutated (MUT) SLC20A2 gene. The inorganic phosphate (Pi) concentration in the medium of cells was determined. The expression of markers of chondrocyte proliferation and differentiation, the Indian hedgehog (Ihh), and parathyroid hormone-related protein (PTHrP) pathway were evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting.

RESULTS

The expression of SLC20A2 in MUT group was similar to WT group. The Pi concentration in the medium of cells in MUT group was significantly higher than WT group, which meant the SLC20A2 mutation inhibited Pi uptake in ATDC5 chondrocytes. The proliferation rate of ATDC5 chondrocytes in MUT group was greater than WT group. The expression of aggrecan (Acan), α-1 chain of type II collagen (COL2A1), and SRY-box transcription factor 9 (SOX9) were higher in MUT group than WT group. However, the expression of Runt-related transcription factor 2 (Runx2), α-1 chain of type X collagen (COL10A1), and matrix metallopeptidase 13 (MMP13) was significantly decreased in the MUT group. Similar results were obtained by Alcian blue and Alizarin red staining. The expression of Ihh and PTHrP in MUT group was higher than WT group. An inhibitor (cyclopamine) of Ihh/PTHrP signalling pathway inhibited the proliferation and restored the differentiation of chondrocytes in MUT group.

CONCLUSION

A mutation in SLC20A2 (c.C1948T) decreases Pi uptake in ATDC5 chondrocytes. SLC20A2 mutation promotes chondrocyte proliferation while inhibiting chondrocyte differentiation. The Ihh/PTHrP signalling pathway may play an important role in this process. Cite this article: Bone Joint Res 2020;9(11):751-760.

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.

Novel deletion and 2397 G>T mutations of the EXT1 gene identified in two Chinese pedigrees with hereditary multiple exostoses using exon sequencing

BACKGROUND

Hereditary multiple exostoses (HME), a rare genetic pediatric disorder, has a peculiar pathogenic mechanism. The results of previous studies have shown that HME is associated with mutations of the EXT1 and EXT2 genes at a molecular genetics level. In our study, two families who received therapy in the Department of Orthopedics of Shanghai Children's Hospital between June, 2017 and November, 2018 were recruited, and a mutational analysis of the EXT1 genes was conducted to further elucidating the relationship between HME and EXT1.

METHODS

Venous blood samples were collected from individuals with HME and their families. Exon sequencing and RT-PCR were performed to comprehensively analyze 11 exons of the EXT1 gene.

RESULTS

The deletion of exon 7 and the 2397 G>T mutation in exon 7 caused deletion mutation and nonsense mutation only in the HME patients. The mutations in exon 7 were tested and verified by Sanger sequencing. RT-PCR showed that the mRNA expression of EXT1 was significantly decreased in the mutation samples compared with the normal samples, which exerted a great influence on the function of EXT1.

CONCLUSIONS

This study identified new mutation sites for the pathogenesis of HME and further clarified the relationship between HME and EXT1.

Whole‑exome sequencing identifies a novel mutation of SLC20A2 (c.C1849T) as a possible cause of hereditary multiple exostoses in a Chinese family

Although the main causative genes for hereditary multiple exostoses (HME) are exostosin (EXT)‑1 and EXT‑2, there are numerous patients with HME without EXT‑1 and EXT‑2 mutations. The present study aimed to identify novel candidate genes for the development of HME in patients without EXT‑1 and EXT‑2 mutations. Whole‑exome sequencing was performed in a Chinese family with HME and without EXT‑1 and EXT‑2 mutations, followed by a combined bioinformatics pipeline including annotation and filtering processes to identify candidate variants. Candidate variants were then validated using Sanger sequencing. A total of 1,830 original variants were revealed to be heterozygous mutations in three patients with HME which were not present in healthy controls. Two mutations [c.C1849T in solute carrier family 20 member 2 (SLC20A2) and c.G506A in leucine zipper and EF‑hand containing transmembrane protein 1 (LETM1)] were identified as possible causative variants for HME through a bioinformatics filtering procedure and harmful prediction. Sanger sequencing results confirmed these two mutations in all patients with HME. A mutation in SLC20A2 (c.C1849T) led to a change in an amino acid (p.R617C), which may be involved in the development of HME by inducing metabolic disorders of phosphate and abnormal proliferation and differentiation in chondrocytes. In conclusion, the present study revealed two mutations [SLC20A2 (c.C1849T) and LETM1 (c.G506A) in a Chinese family with HME. The mutation in SLC20A2 (c.C1849T)] was more likely to be involved in the development of HME.

Bone development and remodeling in metabolic disorders

There are many metabolic disorders that present with bone phenotypes. In some cases, the pathological bone symptoms are the main features of the disease whereas in others they are a secondary characteristic. In general, the generation of the bone problems in these disorders is not well understood and the therapeutic options for them are scarce. Bone development occurs in the early stages of embryonic development where the bone formation, or osteogenesis, takes place. This osteogenesis can be produced through the direct transformation of the pre-existing mesenchymal cells into bone tissue (intramembranous ossification) or by the replacement of the cartilage by bone (endochondral ossification). In contrast, bone remodeling takes place during the bone's growth, after the bone development, and continues throughout the whole life. The remodeling involves the removal of mineralized bone by osteoclasts followed by the formation of bone matrix by the osteoblasts, which subsequently becomes mineralized. In some metabolic diseases, bone pathological features are associated with bone development problems but in others they are associated with bone remodeling. Here, we describe three examples of impaired bone development or remodeling in metabolic diseases, including work by others and the results from our research. In particular, we will focus on hereditary multiple exostosis (or osteochondromatosis), Gaucher disease, and the susceptibility to atypical femoral fracture in patients treated with bisphosphonates for several years.

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 tailored approach to fusion transcript identification increases diagnosis of rare inherited disease

BACKGROUND

RNA sequencing has been proposed as a means of increasing diagnostic rates in studies of undiagnosed rare inherited disease. Recent studies have reported diagnostic improvements in the range of 7.5-35% by profiling splicing, gene expression quantification and allele specific expression. To-date however, no study has systematically assessed the presence of gene-fusion transcripts in cases of germline disease. Fusion transcripts are routinely identified in cancer studies and are increasingly recognized as having diagnostic, prognostic or therapeutic relevance. Isolated reports exist of fusion transcripts being detected in cases of developmental and neurological phenotypes, and thus, systematic application of fusion detection to germline conditions may further increase diagnostic rates. However, current fusion detection methods are unsuited to the investigation of germline disease due to performance biases arising from their development using tumor, cell-line or in-silico data.

METHODS

We describe a tailored approach to fusion candidate identification and prioritization in a cohort of 47 undiagnosed, suspected inherited disease patients. We modify an existing fusion transcript detection algorithm by eliminating its cell line-derived filtering steps, and instead, prioritize candidates using a custom workflow that integrates genomic and transcriptomic sequence alignment, biological and technical annotations, customized categorization logic, and phenotypic prioritization.

RESULTS

We demonstrate that our approach to fusion transcript identification and prioritization detects genuine fusion events excluded by standard analyses and efficiently removes phenotypically unimportant candidates and false positive events, resulting in a reduced candidate list enriched for events with potential phenotypic relevance. We describe the successful genetic resolution of two previously undiagnosed disease cases through the detection of pathogenic fusion transcripts. Furthermore, we report the experimental validation of five additional cases of fusion transcripts with potential phenotypic relevance.

CONCLUSIONS

The approach we describe can be implemented to enable the detection of phenotypically relevant fusion transcripts in studies of rare inherited disease. Fusion transcript detection has the potential to increase diagnostic rates in rare inherited disease and should be included in RNA-based analytical pipelines aimed at genetic diagnosis.

Identification of pathogenic mutations in 6 Chinese families with multiple exostoses by whole-exome sequencing and multiplex ligation-dependent probe amplification: Case series

RATIONALE

Hereditary multiple exostoses (HMEs) is an autosomal dominant skeletal disorder.

PATIENT CONCERNS

Six probands of the 6 unrelated Han Chinese families were identified as having HME. These patients had exostoses at multiple sites and significantly affected joints malformation and movement.

DIAGNOSES

Hereditary multiple exostoses.

INTERVENTIONS

To detect the genetic mechanism of HME in 6 unrelated Chinese families, whole-exome sequencing (WES) and multiplex ligation-dependent probe amplification (MLPA) were used after genomic DNA was isolated from peripheral blood leucocytes. Point mutations identified by these methods were verified by Sanger sequencing after PCR amplification.

OUTCOMES

Six mutations in the EXT1 and EXT2 genes were identified, including a heterozygous deletion mutation from exon 2 to exon 8 (Family 1), a c.448C>T, p.(Gln150X) heterozygous nonsense mutation (Family 4), a c.1057-2A>T heterozygous splicing substitution (Family 5), and a c.1468dupC, p.(Leu490fs519X) (Family 6) heterozygous duplication mutation in the EXT1 gene in addition to a heterozygous deletion mutation from exon 2 to exon 3 (Family 2) and a c.1197C>G, p.(Tyr399X) heterozygous nonsense mutation (Family 3) in the EXT2 gene.

LESSONS

Overall, we identified 5 novel mutations and 1 recurrent mutation in the EXT1 and EXT2 genes in 6 Chinese families with HME. Our findings expand the mutational spectrum of the EXT1 and EXT2 genes and are useful for genetic counseling and prenatal diagnosis.

RNA-Seq detects a SAMD12-EXT1 fusion transcript and leads to the discovery of an EXT1 deletion in a child with multiple osteochondromas

Background

We describe a patient presenting with pachygyria, epilepsy, developmental delay, short stature, failure to thrive, facial dysmorphisms, and multiple osteochondromas.

Methods

The patient underwent extensive genetic testing and analysis in an attempt to diagnose the cause of his condition. Clinical testing included metaphase karyotyping, array comparative genomic hybridization, direct sequencing and multiplex ligation‐dependent probe amplification and trio‐based exome sequencing. Subsequently, research‐based whole transcriptome sequencing was conducted to determine whether it might shed light on the undiagnosed phenotype.

Results

Clinical exome sequencing of patient and parent samples revealed a maternally inherited splice‐site variant in the doublecortin (DCX) gene that was classified as likely pathogenic and diagnostic of the patient's neurological phenotype. Clinical array comparative genome hybridization analysis revealed a 16p13.3 deletion that could not be linked to the patient phenotype based on affected genes. Further clinical testing to determine the cause of the patient's multiple osteochondromas was unrevealing despite extensive profiling of the most likely causative genes, EXT1 and EXT2, including mutation screening by direct sequence analysis and multiplex ligation‐dependent probe amplification. Whole transcriptome sequencing identified a SAMD12‐EXT1 fusion transcript that could have resulted from a chromosomal deletion, leading to the loss of EXT1 function. Re‐review of the clinical array comparative genomic hybridization results indicated a possible unreported mosaic deletion affecting the SAMD12 and EXT1 genes that corresponded precisely to the introns predicted to be affected by a fusion‐causing deletion. The existence of the mosaic deletion was subsequently confirmed clinically by an increased density copy number array and orthogonal methodologies

Conclusions

While mosaic mutations and deletions of EXT1 and EXT2 have been reported in the context of multiple osteochondromas, to our knowledge, this is the first time that transcriptomics technologies have been used to diagnose a patient via fusion transcript analysis in the congenital disease setting.

The role of EXT1 gene mutation and its high expression of calcitonin gene-related peptide in the development of multiple exostosis

OBJECTIVE

Screening and identifying the gene mutation of EXT1, EXT2 and EXT3 associated with multiple exostosis (ME) and the expression in tumor tissues.

METHODS

Nine patients with multiple exostosis were collected and genomic DNA was extracted. Polymerase chain reaction (PCR) amplification and direct sequencing techniques were used to screen all exons, 5' and 3' ends of the EXT1, EXT2 and EXT3 related causative genes. EXT1, EXT2 and EXT3 gene were screened and quantified by RNA-SEQ and RT-qPCR. The concentration of calcitonin gene-related peptide (CGRP) in peripheral blood of tumor patients and normal controls was detected by ELISA.

RESULTS

Between the two patients with ME, the EXT1 gene was found in one patient to have c.79 T>A mutation, which caused the change of p.M27T, the non polar methionine was replaced by the high frequency mutation of polar threonine, and the rest of patients was found the splicing mutation c.1284 + 8 delAT of the heterozygosity of the EXT1 gene. The serum CGRP concentration of ME patients (623 + 49 pg/ml) was significantly higher than that of normal controls (196 + 68 pg/ml), and EXT1 mutation patients were also higher than non mutation patients.

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.

Analysis of mutations in EXT1 and EXT2 in Brazilian patients with multiple osteochondromas

Background

Multiple osteochondromas is a dysplasia characterized by growth of two or more osteochondromas. It is genetically heterogeneous, caused by pathogenic variants in EXT1 or EXT2 genes in 70%–90% of patients. The EXT1 is more often mutated than EXT2 gene, with a variable prevalence between populations. There are no data about EXT1 and EXT2 pathogenic variants in patients with multiple osteochondromas in Brazilian population. The aim of this survey is to characterize these to determine the genotype profile of this population.

Methods

DNA sequencing (Sanger Method) and MLPA analysis were performed to identify point mutations and deletions/duplications in the sample of 153 patients in 114 families.

Results

Germline variants were identified in 83% of families in which EXT2 variants were detected in 46% and EXT1 in 37% of cases. No variants were detected in 17% of them. We identified 50 different variants, 33 (13 frameshift, 11 nonsense, 5 missense, 2 splice site mutation, and 2 large deletions) in EXT1 and 17 (6 frameshift, 6 splice site mutation, 3 nonsense, 1 missense, and 1 large deletion) in EXT2. Of all 50 variants, 31 (62%) were novel, including 20 out of 33 (60,6%) EXT1 and 11 out of 17 (64.7%) EXT2 alleles. The vast majority of variants (88%) were “loss‐of‐function” and two novel hotspots in EXT2 gene were observed in our study.

Conclusion

The prevalence of variants detected in the EXT2 gene differs from other researches from Latin America, European, and Asian population. This uncommon prevalence could be related with the newly characterized variant hotspot sites detected in EXT2 gene (p.Ala409Profs*26 and p.Ser290*). A high number of novel variants were also identified indicating that Brazilian population has a unique genetic profile. Characterizing this population and establishing its genotype is essential to understand the molecular pathogenesis of this disease in Brazil.

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.

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.

Differences in molecular regulation between osteochondroma and bizarre parosteal osteochondromatous proliferation

The differences in molecular mechanisms between osteochondroma and bizarre parosteal osteochondromatous proliferation (BPOP) remain to be fully elucidated. In the present study, the differentially expressed genes between BPOP and osteochondroma were obtained from the Gene Expression Omnibus online database, and the associations among these genes were analyzed using the Database for Annotation, Visualization, and Integrated Discovery (DAVID) online bioinformatics software. The results revealed several differentially expressed genes between human BPOP and osteochondroma. These differentially expressed genes were also enriched in different subgroups based on the analysis using DAVID online software, including ‘transforming growth factor β receptor signaling pathway’, ‘BMP signaling pathway’, ‘Wnt receptor signaling pathway’, ‘response to chemical stimulus’, ‘regulation of inflammatory response’, ‘response to stress’, ‘glycosaminoglycan binding’, ‘polysaccharide binding’, ‘extracellular matrix structural constituent’ and ‘growth factors binding’. Taken together, these findings led to the conclusion that different gene regulatory mechanisms exist between BPOP and osteochondroma. Environmental stimulation and inflammation may contribute to BPOP or osteochondroma, and differences in extracellular matrix may contribute to differences in biological characteristics between BPOP and osteochondroma.

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.

Identification of a novel mutation in the EXT1 gene from a patient with multiple osteochondromas by exome sequencing

Multiple osteochondromas (MO) is an autosomal skeletal disease with an elusive molecular mechanism. To further elucidate the genetic mechanism of the disease a three-generation Chinese family with MO was observed and researched, and a novel frameshift mutation (c.335_336insA) in the exotosin 1 (EXT1) gene of one patient with MO was observed through exome sequencing. This was further validated by Sanger sequencing and comparison with 200 unrelated healthy controls. Immunohistochemistry and multiple sequence alignment were performed to determine the pathogenicity of the candidate mutation. Multiple sequence alignment suggested that codon 335 and 336 in the EXT1 gene were highly conserved regions in vertebrates. Immunohistochemistry revealed that EXT1 protein expression levels were decreased in a patient with MO and this mutation compared with a patient with MO who had no EXT1 mutation. Owing to the appearance of c.335_336insA in exon 1 of EXT1, a premature stop codon was introduced, resulting in truncated EXT1. As a result integrated and functional EXT1 was reduced. EXT1 is involved in the biosynthesis of heparan sulfate (HS), an essential molecule, and its dysfunction may lead to MO. The novel mutation of c.335_336insA in the EXT1 gene reported in the present study has enlarged the causal mutation spectrum of MO, and may assist genetic counseling and prenatal diagnosis of MO.

Large-scale mutational analysis in the EXT1 and EXT2 genes for Japanese patients with multiple osteochondromas

BACKGROUND

Multiple osteochondroma (MO) is an autosomal dominant skeletal disorder characterized by the formation of multiple osteochondromas, and exostosin-1 (EXT1) and exostosin-2 (EXT2) are major causative genes in MO. In this study, we evaluated the genetic backgrounds and mutational patterns in Japanese families with MO.

RESULTS

We evaluated 112 patients in 71 families with MO. Genomic DNA was isolated from peripheral blood leucocytes. The exons and exon/intron junctions of EXT1 and EXT2 were directly sequenced after PCR amplification. Fifty-two mutations in 47 families with MO in either EXT1 or EXT2, and 42.3% (22/52) of mutations were novel mutations. Twenty-nine families (40.8%) had mutations in EXT1, and 15 families (21.1%) had mutations in EXT2. Interestingly, three families (4.2%) had mutations in both EXT1 and EXT2. Twenty-four families (33.8%) did not exhibit mutations in either EXT1 or EXT2. With regard to the types of mutations identified, 59.6% of mutations were inactivating mutations, and 38.5% of mutations were missense mutations.

CONCLUSIONS

We found that the prevalence of EXT1 mutations was greater than that of EXT2 mutations in Japanese MO families. Additionally, we identified 22 novel EXT1 and EXT2 mutations in this Japanese MO cohort. This study represents the variety of genotype in MO.

Integrated analysis of gene expression and genomic aberration data in osteosarcoma (OS)

Cytogenetic analyses have revealed that complex karyotypes with numerous and highly variable genomic aberrations including single-nucleotide polymorphisms (SNPs) and copy number variants (CNVs), are observed in most of the conventional osteosarcomas (OSs). Several genome-wide studies have reported that the dysregulated expression of many genes is correlated with genomic aberrations in OS. We first compared OS gene expression in Gene Expression Omnibus (GEO) data sets and genomic aberrations in International Cancer Genome Consortium (ICGC) database to identify differentially expressed genes (DEGs) associated with SNPs or CNVs in OS. Then the function annotation of SNP- or CNV-associated DEGs was performed in terms of gene ontology analysis, pathway analysis and protein-protein interactions (PPIs). Finally, the expression of genes correlated with both SNPs and CNVs were confirmed by quantitative reverse-transcription PCR. Eight publicly available GEO data sets were obtained, and a set of 979 DEGs were identified (472 upregulated and 507 downregulated DEGs). Moreover, we obtained 1039 SNPs mapped in 938 genes, and 583 CNV sites mapped in 2915 genes. Comparing genomic aberrations and DGEs, we found 41 SNP-associated DEGs and 124 CNV-associated DEGs, in which 7 DGEs were associated with both SNPs and CNVs, including WWP1, EXT1, LDHB, C8orf59, PLEKHA5, CCT3 and VWF. The result of function annotation showed that ossification, bone development and skeletal system development were the significantly enriched terms of biological processes for DEGs. PPI network analysis showed that CCT3, COPS3 and WWP1 were the significant hub proteins. We conclude that these genes, including CCT3, COPS3 and WWP1 are candidate driver genes of importance in OS tumorigenesis.

A patient with novel mutations causing MEN1 and hereditary multiple osteochondroma

We report of a male patient aged 32 years who presented with primary hyperparathyroidism. Three parathyroid glands were resected. At the age of 46 years, nervus facialis irritation was noted, and an MRI scan incidentally revealed a non-functioning pituitary adenoma with affection of the chiasma opticum. The patient underwent transsphenoidal operation resulting in pituitary insufficiency postoperatively. At the same time, primary hyperparathyroidism reoccurred and a parathyroid adenoma located at the thymus was resected. The mother of the patient died early due to multiple tumors. The patient was suspected to have multiple endocrine neoplasia type 1 (MEN1) and genetic analysis was performed. In addition, on clinical examination, multiple exostoses were noticed and an additional genetic analysis was performed. His father was reported to have multiple osteochondromas too. MEN1 was diagnosed in the patient showing a novel heterozygote mutation c.2T>A in exon 2, codon 1 (start codon ATG>AAG;p.Met1?) of the MEN1 gene. In genetic mutational analysis of the EXT1 gene, another not yet known mutation c.1418-2A>C was found in intron 5 of the EXT1 gene (heterozygotic). In conclusion, we report novel mutations of the EXT1 and the MEN1 genes causing hereditary multiple osteochondromas and MEN1 in one patient.

LEARNING POINTS

It is important to ask for the patient's family history in detail.Patients with MEN1 are characterized by the occurrence of tumors in multiple endocrine tissues and nonendocrine tissues, most frequently parathyroid (95%), enteropancreatic neuroendocrine (50%), and anterior pituitary (40%) tissues.Familiar MEN1 has a high degree of penetrance (80-95%) by the age over 50; however, combinations of the tumors may be different in members of the same family.Patients with EXT1 gene mutations should be monitored for possible transformation of bone lesions into osteochondrosarcoma.

Mutant EXT1 in Taiwanese Patients with Multiple Hereditary Exostoses

BACKGROUND

Multiple hereditary exostoses (MHE) is characterized by multiple benign projections of bone capped by cartilage, most numerous in metaphyses of long bones. HME are usually inherited in autosomal dominant mode, chief genes EXT1 and EXT2.

METHODS

Two MHE patients were identified from clinic and enrolled in genetic study, complete coding regions of EXT1 and EXT2, including intron/exon boundaries, sequenced via DNA samples drawn from participants.

RESULTS

DNA sequencing revealed mutant EXT1 gene in both cases, within which frame-shift mutation c.447delC (p.Ser149fsX156) in exon1 and nonsense mutation c.2034T>G (p.Tyr678X) in exon10, emerged. Neither mutation was detected in control group.

CONCLUSIONS

Our results extended the spectrum of EXT1 mutations, revealing similar incidence of EXT1 and EXT2 in Taiwanese MHE patients.

A broad spectrum of genomic changes in latinamerican patients with EXT1/EXT2-CDG

Multiple osteochondromatosis (MO), or EXT1/EXT2-CDG, is an autosomal dominant O-linked glycosylation disorder characterized by the formation of multiple cartilage-capped tumors (osteochondromas). In contrast, solitary osteochondroma (SO) is a non-hereditary condition. EXT1 and EXT2, are tumor suppressor genes that encode glycosyltransferases involved in heparan sulfate elongation. We present the clinical and molecular analysis of 33 unrelated Latin American patients (27 MO and 6 SO). Sixty-three percent of all MO cases presented severe phenotype and two malignant transformations to chondrosarcoma (7%). We found the mutant allele in 78% of MO patients. Ten mutations were novel. The disease-causing mutations remained unknown in 22% of the MO patients and in all SO patients. No second mutational hit was detected in the DNA of the secondary chondrosarcoma from a patient who carried a nonsense EXT1 mutation. Neither EXT1 nor EXT2 protein could be detected in this sample. This is the first Latin American research program on EXT1/EXT2-CDG.

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.