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

Surgical Approach and Considerations for Compressive Thoracic Intraspinal Osteochondroma in Familial Hereditary Multiple Exostosis

INTRODUCTION

Hereditary multiple exostosis or hereditary multiple osteochondromas is a very rare clinical condition. Usually, these lesions tend to occur in the pediatric population, remaining silent until adulthood. Moreover, current studies show a small prevalence in the male population. The osteochondromas usually occur at sites with great bone activity and turnover, such as the diaphysis or metaphyseal plates (especially in children) of long bones. Their appearance in short bones (such as vertebrae) is very rare.

CASE PRESENTATION

We present a case of familial HME in a 53-year-old female patient with a very uncommon clinical description of the disease. The patient presented at our hospital with Frankel D-type paraparesis, with multiple osteochondromas (located at the right humerus, bilateral femurs, right tibia, and hip joints, besides the numerous ones over the spinal column) and urinary incontinence. She was suffering from bilateral coxarthrosis and gonarthrosis, which limited severely the range of her movements. An early menopause status was brought into consideration by the patient, being installed circa 15 years before, at 38 years old. She was currently in treatment with bisphosphonates for her concomitant osteoporosis.

CONCLUSIONS

Despite the relatively rare nature of the disease, it may be an important concern for the patient's quality of life. Intraspinal processes may trigger paraparesis or other neurological statuses, which may require a surgical treatment. The nature of the lesions is usually benign and do not require further radio- or chemotherapy.

Forearm deformity in multiple hereditary exostosis. Radiologic predictors of radial head dislocation

INTRODUCTION

Radial head dislocation in patients with multiple hereditary exostosis (MHE) can lead to functional deficit. We investigated whether the location of the exostosis and certain radiological criteria predict risk of radial head dislocation/subluxation.

HYPOTHESIS

We hypothesized that the radiological criteria differentiate between patients who need closer follow-up of the forearm and others for whom multiple radiographs are superfluous.

PATIENTS AND METHODS

We retrospectively reviewed the demographics of patients with MHE in our hospital, and radiographic measurements were made on forearm radiographs: radial length, ulnar length, ulnar variance, radial articular angle, and radial bowing.

RESULTS

Forty-nine forearms were analyzed in 30 patients. Mean age was 9.5 years at first evaluation and 11.8 years at last evaluation. Radial head dislocation or subluxation was found in 6 forearms (12%). Risk factors comprised isolated exostosis in the distal portion of the ulna or exostosis in the distal part of both the ulna and radius, radial or ulnar shortening>4.6cm, radial bowing>8.1%, radial articular angle>35°, and≥3 exostoses in the forearm.

DISCUSSION

In patients with MHE with risk factors for radial head dislocation, close follow- up with regular radiography is indicated and early surgery should be performed before the radial head dislocates.

LEVEL OF EVIDENCE

IV; retrospective study.

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 case report and literature review

Osteochondroma is the most common bone tumor representing 20%–50% of all benign bone tumors and 10%–15% of all bone tumors. Osteochondroma has similar radiological appearance in both solitary and multiple forms; the latter is an autosomal dominant disorder termed hereditary multiple exostoses. Associated complications of osteochondroma include deformity, fracture, neurovascular compromise, bursa formation, and malignant transformation. Measurement of the cartilage cap thickness is an important index suggesting secondary malignancy of osteochondroma. The upper limit of cap thickness after skeletal maturation is 1.5 cm which can be reliably measured on ultrasound or magnetic resonance imaging. Hereditary multiple exostoses are linked to the mutations of different exostoses genes located on chromosome 8, 11, and 19. We reported cases of two siblings presented with multiple osteochondromas managed by surgical excision. We evaluated their clinical and radiological presentation, genetic correlations and compared with the literature.

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.

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.

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.

The growth plate: a physiologic overview

The growth plate is the cartilaginous portion of long bones where the longitudinal growth of the bone takes place. Its structure comprises chondrocytes suspended in a collagen matrix that go through several stages of maturation until they finally die, and are replaced by osteoblasts, osteoclasts, and lamellar bone.The process of endochondral ossification is coordinated by chondrocytes and a variety of humoral factors including growth hormone, parathyroid hormone, oestrogen, growth factors, cytokines, and various signalling pathways.Chondrocytes progress from a resting state to enter the phases of proliferation and hypertrophy. Under the influence of oestrogen, the proliferation of chondrocytes decreases as the resting chondrocytes are consumed. During the terminal phase of differentiation, cartilage is replaced by blood vessels and organized bone tissue, and once chondrocytes have died, the longitudinal growth of the bone ceases and the growth plate closes.The highly complex regulatory signals involved in this process are genetically determined, and genetic perturbations in any of the associated genes can result in abnormalities of bone growth. Hundreds of chondrodysplasias have been described, pointing to the complexity of the humoral control systems involved in endochondral ossification.While our knowledge of the mechanisms behind the various bone growth control systems is improving, a deeper understanding of the underlying processes could aid clinicians to better understand bone health and bone growth abnormalities. This review describes the current clinical research into the physiology of the growth plate. Cite this article: EFORT Open Rev 2020;5:498-507. DOI: 10.1302/2058-5241.5.190088.

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.

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 de novo mutation in the EXT2 gene associated with osteochondromatosis in a litter of American Staffordshire Terriers

Background

We aimed to identify mutations associated with osteochondromatosis in a litter of American Staffordshire Terrier puppies.

Hypothesis

We hypothesized that the associated mutation would be located in a gene that causes osteochondromatosis in humans.

Animals

A litter of 9 American Staffordshire puppies, their sire and dam, 3 of 4 grandparents, 26 healthy unrelated American Staffordshire Terriers, and 154 dogs of 27 different breeds.

Methods

Whole genome sequencing was performed on the proband, and variants were compared against polymorphisms derived from 154 additional dogs across 27 breeds, as well as single nucleotide polymorphism database 146. One variant was selected for follow‐up sequencing. Parentage and genetic mosaicism were evaluated across the litter.

Results

We found 56,301 genetic variants unique to the proband. Eleven variants were located in or near the gene exostosin 2 (EXT2), which is strongly associated with osteochondromatosis in humans. One heterozygous variant (c.969C > A) is predicted to result in a stop codon in exon 5 of the gene. Sanger sequencing identified the identical mutation in all affected offspring. The mutation was absent in the unaffected offspring, both parents, all available grandparents, and 26 healthy unrelated American Staffordshire Terriers.

Conclusions and Clinical Importance

These findings represent the first reported mutation associated with osteochondromatosis in dogs. Because this mutation arose de novo, the identical mutation is unlikely to be the cause of osteochondromatosis in other dogs. However, de novo mutations in EXT2 are common in humans with osteochondromatosis, and by extension, it is possible that dogs with osteochondromatosis could be identified by sequencing the entire EXT2 gene.

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: Clinical, Molecular and Radiologic Survey in 9 Families

Hereditary multiple exostoses (HME) represents a heterogeneous group of diseases often associated with progressive skeletal deformities. Most frequently, mutations in EXT1 and EXT2 genes with autosomal dominant inheritance are responsible for HME. In our group of 9 families with HME we evaluated the clinical course of the disease and analysed molecular background using Sanger sequencing and MLPA in EXT1 and EXT2 genes. The mean age in our group of patients, when the first exostosis was recognised was 4.5 years (range 2-10 years) and the number of exostoses per one patient documented on X-ray ranged from 2 to 54. Most of the exostoses developed before the growth was completed and they were dominantly localised in the distal femurs, proximal tibia, proximal humerus and distal radius. In all patients, at least one to 8 surgeries were necessary due to complaints and local complications, but neither patient developed malignant transformation. In half of the patients, the disease resulted in short stature. DNA analyses were positive in 7 families. In five probands, different EXT1 gene mutations resulting in premature stop-codon (p.Gly124Argfs*65, p.Leu191*, p.Trp364Lysfs*11, p.Val371Glyfs*10, p.Leu490Profs*31) were found. In two probands, nonsense mutations were found in EXT2 gene (p.Val187Profs*115, p.Cys319fs*46). Five mutations have been novel and two mutations have occurred de novo in probands. Although the risk for malignant transformation is usually low, especially in patients with low number of exostoses, early diagnostics and longitudinal follow up of patients is of a big importance, because early surgery can prevent progression of secondary bone deformities.

Identification of mutations in EXT1 and EXT2 genes in six Chinese families with multiple osteochondromas

The aim of the present study was to identify mutations of major causative genes in six unrelated Chinese families with multiple osteochondromas (MO). Radiographic examinations and genetic analyses were performed in 8 patients exhibiting typical features of MO. Analysis was also performed on unaffected members of the six families and 250 healthy volunteers. Radiographies of the patients revealed multiple exostoses in the cartilage of long bones. A total of five different mutations were identified, one in exostosin‑1 (EXT1) and four in exostosin‑2 (EXT2). Two novel mutations were detected in EXT2: A missense mutation, c.1385G>A, in exon 8, resulting in p.Trp462X; and a splice site mutation, c.725+1G>C, which consisted of a heterozygous guanine‑to‑cytosine transition at nucleotide 725+1 in intron 3. Three common EXT mutations were also detected: c.1036C>T in exon 5 of EXT2 resulting in p.Gln346X; c.1299C>A in exon 8 of EXT2 resulting in p.Phe433Leu; and c.1038A>T in exon 2 of EXT1 resulting in p.Arg346Ser. In conclusion, the present study identified a novel missense mutation (c.1385G>A) in exon 8 and a splicing mutation (c.725+1G>C) in intron 3 of the EXT2 gene, which are responsible for MO in certain Chinese patients. The findings are useful for expanding the database of known EXT2 mutations and understanding the genetic basis of MO in Chinese patients, which may improve genetic counseling and the prenatal diagnosis of MO.

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.

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.