ABSTRACT
Aim
To characterize the clinical, radiological, and genetic features of genetically confirmed hereditary multiple exostoses (HME) in patients presenting with multiple exostoses and to contribute to the understanding of the phenotypic and genotypic spectrum.
Methods
This retrospective cohort study included 21 patients from 13 unrelated families referred to a pediatric genetics clinic for multiple exostoses, with an HME diagnosis confirmed by molecular genetic testing. Clinical findings and available skeletal survey radiographs were reviewed. Identified variants were classified, and segregation analysis was performed when feasible.
Results
Ages at presentation ranged from 2.27 to 59.6 years; 14 patients were female, and 7 were male. The most common reasons for referral were a palpable mass and imaging findings. Skeletal survey radiographs were available for 17 patients, all of whom showed multiple exostoses of variable severity. Exostoses most frequently involved the forearm (ulna and radius), femur, lower leg (tibia and fibula), and humerus; involvement of the hands and pelvis was observed at an intermediate frequency. The feet, ribs, scapula, and clavicle were less commonly affected, and no vertebral lesions were identified. As of the most recent follow-up, no malignant transformation has been identified among patients in our cohort (0%). In one of our patients (P21), a stable, non-growing mass was detected in the mesencephalon. Thirteen distinct variants were detected, five of which were novel. Fifteen patients were classified as EXT1-related HME type 1 and six as EXT2-related HME type 2. Variant types comprised seven frameshift variants, four nonsense variants, one missense variant, and one splice-site variant; all were classified as pathogenic/likely pathogenic. Among eight probands with segregation testing available, two variants (25%) were confirmed to be de novo.
Conclusion
HME exhibits marked clinical and genetic heterogeneity. Careful assessment of skeletal surveys supports the clinical diagnosis, while molecular confirmation is critical for accurate genetic counseling. The novel variants and the associated phenotypic and radiological findings reported here further expand the disease spectrum.
Introduction
In European populations, the reported prevalence of hereditary multiple exostoses (HME) is approximately 1 in 100,000 [1, 2]. HME is characterized by the development of osteochondromas, benign cartilage-capped bony tumors that typically arise from the metaphyseal regions of long bones and grow outward from the bone surface. Lesions are most often multiple and, in addition to long bones, may also involve the ribs, clavicle, and pelvis [1-3]. Most patients present with a palpable mass, whereas others are referred after lesions are detected incidentally on skeletal radiographs obtained for an alternative indication or during evaluation prompted by a positive family history [2]. The number of osteochondromas, the extent and distribution of skeletal involvement, and the severity of resulting deformities may vary considerably among individuals [1, 2]. On average, approximately six lesions are observed per patient; the skeletal distribution of lesions may follow an asymmetric or symmetric pattern across cases [2]. During skeletal growth, osteochondromas enlarge and progressively ossify; after skeletal maturity is reached, growth typically ceases, and the development of new osteochondromas is not expected thereafter [1, 2]. Osteochondromas in multiple hereditary exostoses may lead to a broad clinical spectrum, including pain, limb deformities, limb-length discrepancy, scoliosis, reduced skeletal growth, restricted joint range of motion, short stature, early-onset osteoarthritis, and neurological symptoms secondary to peripheral nerve compression [1, 2].
Although the primary malignancy risk in HME is well defined as an increased risk of secondary chondrosarcoma, evidence regarding an association with hematologic malignancies (e.g., leukemia) remains limited, with only a few cases reported; the potential contribution of EXT1/EXT2 through heparin sulfate biosynthesis and related pathways is currently being investigated [4]. Molecular genetic advances have substantially facilitated the genetic diagnosis of this syndrome. Using next-generation sequencing (NGS) and deletion/duplication analyses, genetic testing can identify heterozygous disease-causing pathogenic/likely pathogenic variants or copy-number variations (CNVs) in the EXT1 and EXT2 genes, thereby establishing the diagnosis of autosomal dominant HME [1, 2, 5].
The retrospective study discusses the clinical characteristics, radiological findings, and genetic results of 21 patients from 13 unrelated families with genetically confirmed HME.
Methods
Between February 2022 and December 2025, we enrolled 21 patients from 13 unrelated families who were referred to the Pediatric Genetics Department of Universty of Health Sciences Türkiye, Ankara Etlik City Hospital. The study cohort comprised patients in whom multiple exostoses were identified on radiographs and/or a disease-causing variant was detected in either the EXT1 or EXT2 gene by genetic testing. This retrospective study was approved by the Clinical Research Ethics Committee of the University of Health Sciences, Ankara Etlik City Hospital (approval no: AEŞH-BADEK1-2025-349, date: 02.09.2025). Written informed consent for genetic testing was obtained from the patients and their legal guardians prior to testing. Clinical and anthropometric data included medical and family histories, dysmorphic features, age at presentation, sex, weight, height, and head circumference. These measurements were reported as standard deviation scores (SDS) using nationally validated reference standards for children in Türkiye [6]. For participants older than 18 years, height, weight, and head circumference SDS values were calculated using the reference standards for age 18 years [6]. To ensure analytical accuracy, patients who could not be evaluated for specific variables (denoted as “NA: not available” throughout the tables and text) were excluded from percentage calculations for those variables within the cohort. With the exception of four patients (P2, P7, P10, and P18), systematic skeletal survey radiographs were obtained for the remaining 17 patients. The routine skeletal survey protocol included skull radiographs in two projections, anteroposterior radiographs of the upper and lower extremities, hand and foot radiographs, spine radiographs in two projections, a pelvic radiograph, and a posteroanterior chest radiograph. The survey was used to assess the location of skeletal abnormalities (e.g., exostoses) and evaluate radiographic features of spondylar, epiphyseal, and metaphyseal dysplasia. In addition, anthropometric measurements and skeletal survey radiographs of parents with identified exostoses were reviewed.
Diagnostic evaluation was performed using NGS-based clinical exome sequencing (CES) and Sanger sequencing. Genomic deoxyribonucleic acid was extracted from the patient’s peripheral blood following standard procedures. The sequencing library was prepared using the Clinical Exome Solution v3 capture kit (SOPHiA Genetics SA, Switzerland), and sequenced on the MiSeq platform (Illumina Inc., CA, USA). The generated data were interpreted using current databases (PubMed, OMIM, DGV, ClinVar, DECIPHER, and ClinGen), and variant pathogenicity was classified according to the American College of Medical Genetics and Genomics criteria [7]. Variants were reported with reference to the NCBI RefSeq transcripts NM_000127.3 (EXT1) and NM_207122 (EXT2), respectively. For candidate variants with the potential to explain the observed phenotype, Sanger sequencing was performed for validation in the proband and for segregation analysis in affected parents and siblings. Sanger sequencing was carried out using the Applied Biosystems 3500 Genetic Analyzer (Thermo Scientific, USA).
In this study, genetic diagnoses were established by CES in 12 patients (P1, P3, P6, P9, P11, P13-P17, P19, and P20). Variant confirmation was performed by Sanger sequencing in 19 patients (all except P16 and P19). In some cases, both CES and Sanger sequencing were performed.
Statistical Analysis
No formal statistical analysis was performed in this study. Accordingly, no statistical software was used; the results are presented descriptively.
Results
Of the 21 patients included in the study, 14 (66.7%) were female and 7 (33.3%) were male. The reasons for presentation included family screening, a history of exostoses, a palpable, hard mass in the extremities, joint pain, detection of exostoses on skeletal survey radiographs obtained for screening purposes, short stature, limb bowing, hypoglycemia, and onset of puberty. Age at presentation ranged from 2.27 to 59.6 years. Among patients with available information, no prenatal problems were identified; with respect to birth history, only one patient (P6) was small for gestational age. Only one of the 13 families (P21) was consanguineous.
Based on anthropometric evaluation, two patients (P4, P16) had a body weight ≤-2 SDS, and one patient (P14) had a head circumference <-2 SDS. Five patients (P3-P5, P7, P17) were classified as having short stature (height ≤-2 SDS). Three patients (P5, P11, P17) exhibited variable dysmorphic facial features. None of the patients reported hearing- or vision-related complaints, and no developmental or cognitive impairments were observed.
Cranial imaging was performed in selected patients; brain magnetic resonance imaging (MRI) findings were normal in P1 and P6, the latter of whom was followed for complex febrile seizures. Chiari type 1 malformation was identified in P14, and a benign-appearing, non-growing mass in the mesencephalon was observed in P21. Abdominal ultrasonography was performed in five patients (P1, P3, P15, P17, P21), and findings were normal in all patients. On echocardiography, minimal mitral regurgitation was detected in P14 (who was subsequently diagnosed with acute rheumatic fever) and in two other patients (P15 and P21). A secundum atrial septal defect was identified in P7. Echocardiography was normal in P3 and P17. In addition, epileptiform activity was detected on EEG in P6. One patient (P21) was receiving treatment for anorexia nervosa and an anxiety disorder.
Genetic testing identified a point mutation in either EXT1 or EXT2 in all patients. No CNV-related etiology was detected. Overall, 13 distinct pathogenic or likely pathogenic variants were identified in 15 (71.4%) patients with EXT1 and in six (28.6%) patients with EXT2. Of these variants, seven were frameshift, four were nonsense, one was missense, and one affected a splice site. Eight variants had been previously reported in ClinVar, whereas the remaining five were novel. Family segregation analysis was performed on eight probands; only two (25%) were found to harbor de novo variants. For P14, P16, and P19, parental inheritance was suspected; however, segregation analysis could not be performed. In P15, segregation analysis in the mother was negative, whereas paternal testing could not be undertaken because the father was deceased. The medical history indicated that the father did not report similar symptoms. The patients’ clinical and anthropometric characteristics are summarized in Table 1. Skeletal survey radiographs from 17 patients were evaluated for exostoses, and the findings are presented in Table 2 and Figures 1-3. The genetic analysis results are compared in Table 3.
Discussion
HME is a syndrome that is predominantly caused by pathogenic/likely pathogenic variants in the EXT1 or EXT2 genes and typically shows an inherited pattern, with sporadic cases occurring less frequently [8-11]. Whereas the diagnosis was historically based on clinical findings, the widespread use of molecular testing now facilitates diagnostic confirmation and enables provision of appropriate genetic counseling, including discussion of preimplantation genetic testing options for families [1]. Hematologic, renal, central nervous system, and cardiac findings may rarely accompany the condition. These manifestations are often not intrinsic features of the disease; rather, they may result from compressive mass effects due to lesions in uncommon locations or may reflect incidental/secondary conditions [1, 3, 12-14]. In our cohort, exostoses were most frequently identified in the forearm (ulna and radius), the femur, the lower leg (tibia and fibula), and the humerus. They were observed with moderate frequency in the hand and pelvis. More rarely, exostoses were detected in the foot, ribs, scapula, and clavicle, but no vertebral exostoses were observed in any patient. When interpreting radiographs, priority should be given to the extremities; however, it should be kept in mind that exostoses may also occur in other anatomical regions.
Scoliosis has been reported in approximately 30% of individuals with HME, with some studies describing even higher rates [2, 3, 15]. In our series, the prevalence of scoliosis was 17.6% (3/17), lower than rates reported in the literature. On the other hand, the detection of heparan sulfate (HS) expression in human fetal vertebrae and intervertebral discs suggests that HS may influence axial skeletal development [16]. Moreover, the literature has also demonstrated that HME is a risk factor in the etiology of scoliosis [15]. The reported prevalence of scoliosis may vary depending on several factors, including the number and size of exostoses, patient age at assessment, and the Cobb angle threshold used to define scoliosis. In addition to axial deformities, cases of compressive myelopathy due to intraspinal exostoses—although less frequent—have also been reported [17]. Therefore, for patients with HME who present with symptoms suggestive of neurological deficits, we recommend advanced imaging modalities. In the long bones, osteochondromas are typically asymptomatic and most commonly present as cosmetic concerns or palpable, firm masses. When symptomatic, they may lead to complications related to mechanical compression of adjacent anatomical structures, including bursitis, tendinitis, neuropathy, and arterial or venous thrombosis, as well as pseudoaneurysm formation; in rare cases, chronic compartment syndrome has also been reported [18, 19]. Exostoses that impair joint range of motion may increase shear forces across the joint during movement. In addition, cartilaginous hypertrophy secondary to HS deficiency is considered a potential risk factor for early-onset osteoarthritis [20]. HME can affect long-bone growth, leading to short stature, limb-length discrepancies, and angular deformities of the extremities. The most common deformities include coxa valga, genu valgum, and ankle valgus. Radial and ulnar deviation and progressive deformity may result in elbow dislocation. Additional deformity-related manifestations include shortening and angular deformities of the metacarpal and metatarsal bones, as well as hip-related pathology such as acetabular dysplasia, femoroacetabular impingement, hip subluxation or dislocation, and patellar dislocation, reflecting a broad spectrum of orthopedic conditions associated with HME [21-23].
Three of our patients (3/21, 14.3%) underwent surgery for osteochondromas. Patient P3 underwent bilateral hemiepiphysiodesis of the distal femur, proximal tibia, and distal tibia to address bilateral knee and ankle valgus deformities. In Patient P5, MRI was performed because of pain in the left iliac wing and right distal medial femur and demonstrated a cartilage cap thickness of 3 mm; excision was subsequently performed for pain palliation. Patient P21 underwent excision of painful osteochondromas located at the right distal medial femur and the lateral aspect of the proximal tibia.
Malignant transformation is one of the most concerning complications of osteochondromas. The association between HME and skeletal malignancies is well established, and approximately 5-10% of patients may develop low-grade chondrosarcoma with limited metastatic potential [24, 25]. The most common sites of malignant transformation include the proximal femur, proximal humerus, scapula, and pelvis [26]. Less frequently, osteosarcoma, fibrosarcoma, and malignant fibrous histiocytoma have also been reported [27]. In addition, hematologic malignancies, cerebellar astrocytoma, atypical teratoid/rhabdoid tumor, and cancer of the lung, thyroid, and colon have been described in these patients [27-32]. As of the most recent follow-up, no malignant transformation has been identified in any patient in our cohort (0%). In one of our patients (P21), a non-growing, stable mass was detected in the mesencephalon.
The EXT gene family, comprising EXT1 and EXT2, functions as a tumor suppressor and encodes critical glycosyltransferases involved in HS biosynthesis. HS proteoglycans formed by the attachment of HS chains to core proteins, play an important role in regulating bone and cartilage development [25, 33, 34]. Heterozygous variants in EXT1/EXT2 reduce HS levels by approximately 50%; however, this reduction alone is not sufficient for osteochondroma formation [35]. The development of a tumorigenic cell is consistent with Knudson’s “two-hit” hypothesis, requiring an additional somatic event affecting the second allele (e.g., loss of heterozygosity, or a second pathogenic mutation) [1, 8]. Loss-of-function variants in these genes provide the biological basis for osteochondroma development [1, 2]. Variants in EXT1, compared with those in EXT2, have been reported to be associated with a more severe phenotype, a higher number of osteochondromas, and an increased risk of malignant transformation [1, 36]. In this genetically heterogeneous syndrome, no underlying genetic etiology is identified in approximately 10-13% of cases, whereas variants are reported in EXT1 in 65-70% and in EXT2 in 30-35% of cases [1]. In our study, consistent with the literature, variants were most frequently detected in EXT1 (15/21; 71.4%) and less commonly in EXT2 (6/21; 28.6%). Although variants can be distributed throughout the genes, some reports indicate that exons 1 and 6 in EXT1 and the first eight exons in EXT2 constitute mutational “hot spot” regions [1]. Consistent with the literature, 80% of patients carrying an EXT1 variant had variants in these two exons (60% in exon 1 and 20% in exon 6), whereas all patients with EXT2 variants harbored variants within the first eight exons. De novo disease has been reported in the literature at a rate of approximately 10%; in our study, a higher frequency of 25% was observed [1]. This discrepancy may be related to the inability to perform genetic testing on the parents of all probands and the limited sample size.
Multilocus genomic variation is defined as the concurrent identification of pathogenic variants at more than one independent locus in patients presenting with complex clinical phenotypes [37]. In this context, in addition to the EXT1 variant, other genomic variant were identified in our patients P1 and P3: a paternally inherited heterozygous variant of uncertain significance in COL11A1 in P1, and a maternally inherited heterozygous, likely pathogenic variant in NOG in P3. These cases were published by Kablan et al. [37].
Follow-up of patients with HME should include documentation of growth parameters at each visit and a particularly thorough assessment of the musculoskeletal system. Given the variable clinical manifestations and potential complications, the management of HME requires a multidisciplinary approach. Optimal care, coordinated through a multidisciplinary team that includes specialists in pediatric oncology, orthopedic surgery, pediatric genetics and medical genetics, pediatric neurology, pediatric radiology, and physiotherapy/rehabilitation, encompasses comprehensive clinical evaluation, the appropriate use of imaging modalities, an expanded differential diagnosis, and the integration of advanced molecular testing. This comprehensive collaboration facilitates early recognition of mild or atypical clinical features and complications and supports accurate genetic diagnosis. Diagnostic certainty can be enhanced through testing approaches such as EXT1/EXT2-focused NGS and deletion/duplication analyses. Management and follow-up are individualized according to parameters such as the number and location of lesions, the severity of the deformity, the symptom burden, and the age. Regarding the risk of secondary chondrosarcoma, warning signs such as new-onset or increasing pain, rapid growth, and thickening of the osteochondroma cartilage cap should be assessed meticulously. This approach enables effective management of HME-specific complications and improves quality of life; it also supports more accurate genetic counseling for families regarding prognosis and reproductive options in the context of autosomal dominant inheritance, penetrance, and variable expressivity.
Study Limitations
The main limitations of this study include its single-center design, limited sample size, and the inability to perform segregation analyses in some families. Nevertheless, our findings are expected to provide meaningful contributions to future research and patient management. Multicenter studies with larger cohorts will help clarify the pathogenesis of this syndrome more precisely and further expand its clinical spectrum.
Conclusion
HME is a clinically heterogeneous entity with variable expressivity. In the presence of a family history, patients should be monitored closely, given the high penetrance and risk of malignant transformation. Careful follow-up is warranted for restricted joint range of motion, cosmetic concerns, and associated skeletal abnormalities. In patients with these features, HME should routinely be considered in the differential diagnosis and comprehensive molecular testing should be prioritized to elucidate the underlying genetic etiology.
