Hereditary Multiple Osteochondromas: Disease Characteristics Research Report

Scope. This report synthesizes disease-level resources, human cohorts, molecular studies, clinical-trial records, and model-organism evidence. Evidence is aggregated rather than derived from an individual electronic health record, except where a cited study explicitly analyzed clinical records. Literature was prioritized through 2024; a 2025 imaging review is used only where it clarifies current diagnostic practice.

Executive summary

Hereditary multiple osteochondromas (HMO), historically called hereditary multiple exostoses (HME), is a highly penetrant autosomal-dominant skeletal tumor-predisposition disorder. Multiple cartilage-capped osteochondromas arise near growth plates during childhood, enlarge during skeletal growth, and usually become quiescent after physeal closure. Heterozygous loss-of-function variants in EXT1 or EXT2, encoding the Golgi heparan-sulfate polymerase complex, explain approximately 70–95% of clinically diagnosed cases. A local somatic second hit and severe focal heparan-sulfate deficiency are strongly supported as the initiating events in many osteochondromas. Clinical morbidity includes deformity, impaired growth, restricted motion, neurovascular compression, pain, fatigue, repeated surgery, and a small but important risk of secondary peripheral chondrosarcoma. There is no approved disease-modifying treatment; management remains surveillance, symptom-directed surgery, pain care, and rehabilitation. Recent 2024 work expanded the detectable variant spectrum, implicated mosaic structural variants and possible PTPN11 overlap, quantified major adult pain/fatigue burden, and demonstrated palovarotene activity against established lesions in mice—but not established clinical benefit in HMO. (borovikov2024themissingpiece pages 1-2, gnoli2024secondaryperipheralchondrosarcoma pages 1-2, amajjar2024painandfatigue pages 1-2, garcia2024analysisofthe pages 1-2)

The following table provides a compact knowledge-base summary.

Table: Compact evidence table summarizing the identity, genetics, natural history, clinical burden, diagnosis, management, recent findings, and experimental models of hereditary multiple osteochondromas. Quantitative findings and ontology suggestions are paired with the strongest available cited sources.

1. Disease information

Definition and identifiers

HMO is defined by multiple benign, cartilage-capped bony outgrowths with cortical and medullary continuity with the parent bone. A commonly used clinical/radiologic threshold is at least two juxta-epiphyseal osteochondromas. The lesions are developmental skeletal neoplasms rather than inflammatory “exostoses.” (mordenti2020thenaturalhistory pages 1-6, tepelenis2021osteochondromasanupdated pages 1-2, ruedadeeusebio2025hereditarymultipleexostoses pages 1-2)

• MONDO: MONDO:0005508, hereditary multiple osteochondromas.
• Orphanet: ORPHA:321, multiple osteochondromas.
• OMIM phenotypes: 133700, exostoses, multiple, type 1; 133701, type 2.
• MeSH: Multiple Hereditary Exostoses.
• ICD: ICD-10-CM commonly uses D16.9 (benign neoplasm of bone and articular cartilage, unspecified) or site-specific D16 codes; Q78.6 is used in some national modifications for multiple congenital exostoses. Coding therefore depends on jurisdiction and should not be treated as a globally uniform disease identifier.
• Synonyms: hereditary multiple exostoses, multiple hereditary exostoses, multiple osteochondromas, hereditary multiple osteochondromas, osteochondromatosis, diaphyseal aclasis, hereditary deforming dyschondroplasia, and multiple cartilaginous exostoses. (OpenTargets Search: hereditary multiple osteochondromas-EXT1,EXT2, darienzo2019hereditarymultipleexostoses pages 1-2)

A concise abstract statement is: “Hereditary multiple exostoses (HME), also called hereditary multiple osteochondromas, is a rare genetic disorder characterized by multiple osteochondromas that grow near the growth plates.” (D’Arienzo et al., published 20 December 2019; DOI: https://doi.org/10.2147/ORR.S183979). (darienzo2019hereditarymultipleexostoses pages 1-2)

2. Etiology, risk, and protective factors

Causal and genetic factors

The primary cause is a germline heterozygous loss-of-function variant in EXT1 or EXT2. Both are strongly supported disease targets in Open Targets; EXT1 is ENSG00000182197 and EXT2 is ENSG00000151348. In individual lesions, loss of the remaining functional allele or another somatic event can create an EXT-null clone. Human tumor series have documented loss of heterozygosity, although lesion heterogeneity can obscure it. (OpenTargets Search: hereditary multiple osteochondromas-EXT1,EXT2, pacifici2017hereditarymultipleexostoses pages 12-13, pacifici2017hereditarymultipleexostoses pages 10-12, pacifici2017hereditarymultipleexostoses pages 1-2)

Risk of having HMO is determined principally by carrying a pathogenic germline allele or having an affected parent. EXT1 disease is generally associated with a greater lesion burden, more severe deformity, and possibly greater malignancy risk than EXT2, but marked intra- and interfamilial variability limits individual prediction. Age and skeletal immaturity increase the probability of new lesion formation because lesions arise during active endochondral growth; these are modifiers of expression, not independent causes. (mordenti2020thenaturalhistory pages 1-6, tepelenis2021osteochondromasanupdated pages 2-3, pacifici2017hereditarymultipleexostoses pages 6-7)

Environmental, lifestyle, infectious, and protective factors

No toxin, infection, diet, smoking behavior, occupational exposure, or lifestyle factor is established as a cause of this Mendelian disorder. No validated environmental or genetic protective allele prevents HMO in a pathogenic-variant carrier. Avoiding mechanical irritation may reduce lesion-related symptoms, but it does not prevent osteochondroma initiation. Evidence for clinically important gene–environment interactions is insufficient.

A human biochemical study found a modest effect of EXT1 heterozygosity on postprandial lipid clearance (PMID: 25568062), but this is not evidence that diet causes or protects against HMO. Likewise, the approximately halved heparan-sulfate:chondroitin-sulfate ratio in blood is a consequence of EXT dysfunction, not an exposure biomarker (PMID: 23514715). (pacifici2018hereditarymultipleexostoses pages 7-9, pacifici2017hereditarymultipleexostoses pages 12-13)

3. Phenotypes

Core and associated manifestations

In the 158-child natural-history cohort, 80.4% developed new lesions, 57.6% new deformities, 23.4% new functional limitations, and 46.2% met criteria for progression; new lesions appeared in 28.5%, 39.9%, and 50% by 12, 24, and 36 months, respectively. (Mordenti et al., October 2020; DOI: https://doi.org/10.1016/j.bone.2020.115499). (mordenti2020thenaturalhistory pages 1-6)

Quality of life

The strongest recent quantitative evidence is the 2024 Dutch study of 353 adults. Its abstract reports: “Pain was reported by 87.8% … and fatigue by 90.4%.” Fatigue exceeded reference values for both healthy people and rheumatoid-arthritis patients (p<0.001). Fear-avoidance beliefs and fatigue were most strongly associated with pain; anxiety and depression were the strongest correlates in fatigue models. These data favor multidisciplinary management rather than a surgery-only model. (Amajjar et al., published 17 July 2024; DOI: https://doi.org/10.1371/journal.pone.0305640). (amajjar2024painandfatigue pages 1-2)

4. Genetic and molecular information

Genes and proteins

• EXT1: chromosome 8q24.11–q24.13; HGNC:3512; OMIM gene 608177.
• EXT2: chromosome 11p11–p12; HGNC:3513; OMIM gene 608210.

EXT1 and EXT2 form a Golgi-resident hetero-oligomeric glycosyltransferase complex that alternately adds glucuronic acid and N-acetylglucosamine to elongate heparan-sulfate chains. Functional polymerase activity is supported by PMID 11391482; Golgi complex localization/function by PMID 10639137. (pacifici2018hereditarymultipleexostoses pages 7-9, pacifici2017hereditarymultipleexostoses pages 12-13)

Pathogenic variants and diagnostic yield

The disease mechanism is loss of function/haploinsufficiency followed, in many lesions, by focal biallelic deficiency. Reported variant classes include nonsense, frameshift, canonical splice, missense with proven functional effect, exon or multiexon deletion, whole-gene deletion, and other structural rearrangements. Pathogenic missense calls require caution because loss of function is the established mechanism and some historical missense classifications may be incorrect. Combined EXT1/EXT2 detection ranges from roughly 70–95%; individual historical ranges were 28–65% for EXT1 and 21–61% for EXT2. Causal alleles are rare or absent from population databases as expected for a highly penetrant rare disorder; variant-specific gnomAD frequency must be documented rather than assigning a single disease-wide frequency. (darienzo2019hereditarymultipleexostoses pages 1-2, mordenti2020thenaturalhistory pages 1-6, beltrami2016hereditarymultipleexostoses pages 1-2, pacifici2017hereditarymultipleexostoses pages 6-7)

Important 2024 developments include:

1. A study of 244 unrelated probands found 177 unique single-nucleotide/copy-number variants across EXT1, EXT2, and PTPN11—80 known and 97 novel. Five patients with osteochondromas but no enchondromas carried PTPN11 loss-of-function variants, suggesting overlap with metachondromatosis rather than proving that PTPN11 is a routine third HMO gene. The abstract states that prior EXT1/EXT2 DNA diagnostic yields were “78 to 95%.” (Borovikov et al., February 2024; DOI: https://doi.org/10.1155/2024/8849348). (borovikov2024themissingpiece pages 1-2)
2. Mosaic EXT1 deletions involving exons 8–11 and 2–11 were identified by genome/RNA-based investigation after standard tests were negative, supporting low-level mosaicism as one explanation for unresolved disease.
3. A novel tandem intragenic duplication involving EXT1 exon 4, c.(1128_1202)_(1284+29_1344)dup, demonstrated why dosage testing should accompany sequence analysis. The published abstract notes: “Our finding expands the spectrum of MO-causing variants … underlying the importance of quantitative analysis in patients with negative sequencing.” (Bartolotti et al., September 2024; DOI: https://doi.org/10.3390/genes15091169).

Variants in constitutional blood are germline; lesion-specific second hits are somatic. Constitutional mosaicism can produce milder or asymmetric expression and changes recurrence counseling. Large chromosomal deletions encompassing EXT genes can produce contiguous-gene phenotypes; routine karyotyping is otherwise low yield.

Modifiers and epigenetics

No modifier gene or protective allele has sufficient replicated evidence for clinical use. EXT1 versus EXT2 genotype, sex, lesion location, and genetic background correlate imperfectly with severity. Evidence for disease-defining DNA methylation, histone modification, or chromatin signatures is insufficient; epigenomic assays are not diagnostic. (pacifici2017hereditarymultipleexostoses pages 13-14, pacifici2017hereditarymultipleexostoses pages 6-7)

5. Environmental information

HMO is not infectious, transmissible, toxicant-induced, or lifestyle-caused. No vaccination, antimicrobial therapy, dietary intervention, or exposure avoidance prevents the initiating lesion. Mechanical load and trauma may make an existing osteochondroma symptomatic or cause fracture/bursal inflammation but are not established causes. Smoking and alcohol may affect general surgical or cancer health but have no demonstrated disease-specific etiologic role.

6. Mechanism and pathophysiology

Ordered causal chain

1. A germline heterozygous pathogenic EXT1 or EXT2 variant leads to reduced EXT1–EXT2 glycosyltransferase activity and systemic heparan-sulfate insufficiency.
2. A focal somatic second hit in a growth-plate/perichondrial progenitor or chondrocyte results in biallelic EXT dysfunction and a severely heparan-sulfate-deficient clone; this step is demonstrated in subsets of human tumors and strongly supported by mouse models. (pacifici2017hereditarymultipleexostoses pages 12-13, pacifici2017hereditarymultipleexostoses pages 10-12, pacifici2017hereditarymultipleexostoses pages 1-2)
3. Reduced cell-surface/extracellular-matrix heparan sulfate leads to abnormal distribution, stability, and receptor presentation of morphogens.
4. This altered morphogen handling results in increased pro-chondrogenic BMP/hedgehog activity and reduced anti-chondrogenic FGF–MEK–ERK restraint and Noggin/Gremlin expression; detailed directionality is primarily model-organism/in-vitro evidence. WNT/β-catenin disturbance is implicated but less completely resolved. (pacifici2018hereditarymultipleexostoses pages 12-13, beltrami2016hereditarymultipleexostoses pages 1-2, garcia2021osteochondromapathogenesismouse pages 1-2)
5. Abnormal signaling at the growth-plate/perichondrial border leads to failure of normal border function, ectopic chondrocyte differentiation, and recruitment of adjacent wild-type cells into a heterogeneous cartilage cap.
6. Ectopic cartilage undergoes endochondral ossification resulting in a cortex- and marrow-continuous osteochondroma that grows while the physis is open.
7. Multiple lesions near active growth plates lead to physeal tethering, asymmetric growth, bowing, limb-length inequality, restricted motion, and short stature.
8. Branch A: Lesion mass effect results in pain, bursitis, tendon irritation, fracture, neurovascular compression, and—rarely—spinal-cord compromise.
9. Branch B: Additional neoplastic evolution in a cartilage cap can lead to secondary peripheral chondrosarcoma; this is uncommon and incompletely predictable.

Human biochemical evidence includes approximately 50% reduction in the blood heparan-sulfate:chondroitin-sulfate ratio (PMID 23514715). Human lesion studies found LOH in 6/8 analyzed osteochondromas in one series. Mouse work provides causal evidence from clonal/chondrocyte-specific Ext1 loss (PMIDs 20080592, 20534475) and compound Ext1/Ext2 deficiency (PMID 21310272). (pacifici2018hereditarymultipleexostoses pages 7-9, pacifici2017hereditarymultipleexostoses pages 12-13, pacifici2017hereditarymultipleexostoses pages 10-12)

Suggested GO terms: GO:0015012 heparan sulfate proteoglycan biosynthetic process; GO:0030201 heparan sulfate proteoglycan metabolic process; GO:0030509 BMP signaling; GO:0007224 Smoothened signaling; GO:0008543 FGFR signaling; GO:0016055 Wnt signaling; GO:0051216 cartilage development; GO:0001503 ossification; GO:0001958 endochondral ossification. Cell Ontology: CL:0000138 chondrocyte; CL:0000062 osteoblast; CL:0000134 mesenchymal stem cell, with growth-plate/perichondrial progenitor annotations where available.

Profiling and advanced technologies

No validated transcriptomic, proteomic, metabolomic, lipidomic, liquid-biopsy, single-cell, spatial, or epigenomic diagnostic signature currently exists. In a 2024 mouse/cell study, palovarotene activated an osteoarthritis-like transcriptional program, upregulated matrix-catabolic genes, downregulated matrix-anabolic genes, and activated STAT3 but not STAT1/2. These are mechanistic/preclinical observations, not patient biomarkers. (garcia2024analysisofthe pages 1-2)

7. Anatomical structures affected

Primary sites are metaphyseal/metadiaphyseal regions of bones formed by endochondral ossification—especially distal femur, proximal tibia/fibula, proximal humerus, forearm, pelvis, scapula, ribs, and vertebrae. Nearly half of lesions occur around the knee; approximate site estimates include distal femur 30%, proximal tibia 15–20%, and humerus 10–20%. Hand/foot lesions occur but are less prominent. Craniofacial bones formed predominantly by intramembranous ossification are generally spared, although cranial-base lesions have been reported. Lesions may be bilateral and multiple but are often asymmetric. (darienzo2019hereditarymultipleexostoses pages 1-2, tepelenis2021osteochondromasanupdated pages 1-2, tepelenis2021osteochondromasanupdated pages 2-3, pacifici2018hereditarymultipleexostoses pages 7-9)

At tissue level, the cartilage cap, perichondrium, growth plate, adjacent cortical/trabecular bone, marrow, tendon/bursa, and nearby nerves/vessels are relevant. At cellular level, chondrocytes and perichondrial/skeletogenic progenitors are central, with osteoblast-lineage cells participating in endochondral conversion. The key subcellular compartment is the Golgi apparatus/Golgi membrane, where EXT1–EXT2 synthesizes heparan sulfate; downstream abnormalities occur at the plasma membrane and extracellular matrix.

Suggested anatomy: UBERON:0000981 femur; UBERON:0000979 tibia; UBERON:0001424 humerus; UBERON:0000976 radius; UBERON:0001423 ulna; UBERON:0001270 pelvis; UBERON:0001132 parietal pleura/rib-region annotations should be refined to the exact lesion; UBERON:0002412 vertebral column. GO cellular components: GO:0005794 Golgi apparatus; GO:0000139 Golgi membrane; GO:0005576 extracellular region; GO:0005886 plasma membrane.

8. Temporal development

HMO is genetically present from conception but usually clinically silent at birth. Lesions can appear by age 2; median diagnosis is approximately 3 years, 50% have a visible lesion by age 5, and 80% by age 10. Growth is chronic and insidious, with the highest lesion-accrual risk during rapid skeletal growth. Lesions generally stop enlarging at skeletal maturity; new growth or pain thereafter warrants investigation for complications or malignant transformation. (mordenti2020thenaturalhistory pages 1-6, tepelenis2021osteochondromasanupdated pages 2-3, ruedadeeusebio2025hereditarymultipleexostoses pages 2-5)

There is no standard stage system. A practical course is: early palpable lesions; accumulating lesions and deformity during childhood; functional restriction and corrective surgery during later growth; stable residual lesions after maturity; and adult complications such as osteoarthritis, chronic pain, fatigue, or rare chondrosarcoma. Spontaneous true remission is not expected, although some lesions regress and symptoms may improve after surgery. The critical monitoring window is childhood/adolescence for growth disturbance and adulthood for new pain or lesion enlargement. (beltrami2016hereditarymultipleexostoses pages 7-8, mordenti2020thenaturalhistory pages 1-6)

9. Inheritance and population

Inheritance is autosomal dominant. An affected heterozygous individual has a 50% transmission probability per pregnancy. Penetrance is near complete and has historically been estimated at approximately 100% in males and 96% in females, although ascertainment and modern imaging may alter this difference. Expressivity is highly variable, including within families. Anticipation is not established. De novo disease and constitutional/germline mosaicism occur; therefore, an apparently negative family history does not exclude HMO. Consanguinity has no special etiologic role in this dominant disorder. (darienzo2019hereditarymultipleexostoses pages 1-2, mangualgarcia2015multipleskeletaldeformities pages 1-2)

Reported prevalence is approximately 1:50,000 in Western populations; incidence estimates extend to 1:100,000 in some sources. Historical male:female ratios of about 1.5:1 probably reflect reduced clinical ascertainment in mildly affected females rather than true sex-linked inheritance. No consistently high-risk ethnicity is established, although founder variants may occur in individual families or isolates. Population carrier frequency cannot be reliably inferred from gnomAD because pathogenic alleles are heterogeneous, very rare, and incompletely captured. (darienzo2019hereditarymultipleexostoses pages 1-2, ruedadeeusebio2025hereditarymultipleexostoses pages 1-2)

10. Diagnostics

Clinical and imaging diagnosis

Diagnosis is based on physical examination, family history, and imaging showing at least two osteochondromas with cortical and medullary continuity. Plain radiography is first line. CT is valuable for complex pelvic, scapular, costal, or spinal anatomy. MRI best depicts the hyaline-cartilage cap, marrow, soft tissue, neurovascular structures, and suspected spinal or malignant complications. Ultrasound can measure a superficial cartilage cap. (tepelenis2021osteochondromasanupdated pages 1-2, ruedadeeusebio2025hereditarymultipleexostoses pages 1-2, tepelenis2021osteochondromasanupdated pages 7-8)

Red flags for secondary chondrosarcoma include new or increasing pain, growth after skeletal maturity, an irregular/lobulated surface, internal lysis or irregular calcification, erosion of adjacent bone, a soft-tissue mass, and a thick cartilage cap. Common thresholds are >2 cm in adults and >3 cm in children, although some HMO literature uses 1.5–2 cm; thresholds must be interpreted with age and imaging quality. Histology shows a perichondrial/fibrous covering, hyaline-cartilage cap, and endochondral bone. Suspected malignancy requires expert musculoskeletal radiology and pathology, with biopsy planned by the treating sarcoma team. (gnoli2024secondaryperipheralchondrosarcoma pages 1-2, tepelenis2021osteochondromasanupdated pages 7-8)

Genetic testing algorithm

1. Sequence EXT1 and EXT2 using a validated panel with full coding and splice-region coverage.
2. Perform exon-level deletion/duplication analysis—such as MLPA—if not already integrated.
3. Reassess phenotype and variant classification under ACMG/AMP criteria; do not assume every rare missense allele is causal.
4. If unresolved, consider high-depth genome sequencing, structural-variant analysis, and RNA studies; test another tissue when low-level mosaicism is suspected.
5. Add PTPN11 when enchondromas, lesions directed toward joints, spontaneous regression, or other features suggest metachondromatosis/overlap.
6. Use parental testing for segregation, de novo confirmation, and recurrence counseling.

WES can identify coding variants but may miss mosaic or structural lesions; WGS is more useful for noncoding and structural variants. CMA is appropriate for a syndromic phenotype suggesting a large deletion; karyotype/FISH are not routine first-line tests. Mitochondrial and repeat-expansion assays are not relevant. No omics-based clinical diagnostic is validated. (borovikov2024themissingpiece pages 1-2, pacifici2017hereditarymultipleexostoses pages 10-12, pacifici2017hereditarymultipleexostoses pages 6-7)

Differential diagnosis and screening

Differentials include solitary osteochondroma; metachondromatosis (PTPN11, osteochondromas plus enchondromas, often joint-directed); Ollier disease and Maffucci syndrome (enchondromas, with vascular anomalies in Maffucci); dysplasia epiphysealis hemimelica/Trevor disease (epiphyseal osteochondral overgrowth); Langer–Giedion syndrome/TRPS II (contiguous deletion including EXT1); and secondary peripheral chondrosarcoma. (borovikov2024themissingpiece pages 13-13, borovikov2024themissingpiece pages 1-2)

There is no newborn biochemical screen. Cascade clinical and molecular testing is appropriate for relatives. Prenatal or preimplantation genetic testing is possible after the familial variant is known. Routine indiscriminate repeated whole-body ionizing imaging is undesirable; surveillance should be individualized.

11. Outcome and prognosis

Life expectancy is usually near normal unless malignant or severe neurovascular complications occur. Disease burden is chiefly chronic morbidity rather than mortality. Pain, fatigue, mobility restriction, deformity, early osteoarthritis, work/sport limitations, and repeated operations can substantially impair quality of life. (amajjar2024painandfatigue pages 1-2, mordenti2020thenaturalhistory pages 1-6)

Malignant transformation estimates vary from approximately 0.5–5% in population-oriented literature, with higher figures in referral centers reflecting ascertainment bias. In the 2024 Rizzoli series of 105 HMO-associated secondary peripheral chondrosarcomas, age at diagnosis ranged from 13–63 years (median 34); pelvis was the commonest site (46/105, 44%), followed by lower limbs (35). Of 103 graded tumors, 59 were grade 1, 40 grade 2, and 4 grade 3. Grade 2/3 histology and partial resection predicted worse disease-free survival. The abstract concludes that malignant transformation “can occur also in younger patient[s]” and therefore age alone should not negate suspicious findings. (Gnoli et al., February 2024; DOI: https://doi.org/10.1186/s13023-023-03006-8). (gnoli2024secondaryperipheralchondrosarcoma pages 1-2)

For secondary peripheral chondrosarcoma, complete surgical margins and grade are major prognostic factors. Historical estimates include approximately 90% five-year survival overall and ten-year survival of 83% for grade I versus 29% for grade III disease, but these figures are not HMO-specific in every source and should not be used as an individualized calculator. (beltrami2016hereditarymultipleexostoses pages 7-8, tepelenis2021osteochondromasanupdated pages 7-8)

12. Treatment

Current standard care

No pharmacologic therapy prevents or reverses HMO. Asymptomatic lesions are observed. Indications for surgery include persistent pain, functional restriction, tendon or joint impingement, progressive deformity, fracture, neurovascular compromise, spinal-cord compression, and suspected malignancy. Procedures include complete marginal excision with removal of cartilage cap/perichondrium, corrective osteotomy, hemiepiphysiodesis, limb or ulnar lengthening, and sarcoma-wide resection. Incomplete cap/perichondrium removal increases recurrence. Reported recurrence after complete excision is below 2%, while postoperative complication estimates are approximately 11.6–12.5%. (beltrami2016hereditarymultipleexostoses pages 7-8, tepelenis2021osteochondromasanupdated pages 7-8)

Forearm surgery illustrates the evidence limitations: among 315 forearms in a systematic review, 66.7% underwent ulnar lengthening with or without associated procedures and 20.6% isolated excision, but evidence that reconstruction improves long-term function or quality of life was poor. Treatment must therefore be individualized rather than triggered by radiographic appearance alone.

Supportive care includes age-appropriate analgesia, physical and occupational therapy, maintenance of strength and safe range of motion, gait or orthotic assessment, workplace/school accommodation, and psychological treatment for fear avoidance, anxiety, or depression. Refractory pain occasionally prompts neuromodulation or intrathecal analgesia, but evidence is limited to case reports. (amajjar2024painandfatigue pages 1-2)

Suggested NCIT intervention terms: NCIT:C15329 Surgical Procedure; NCIT:C52003 Excision; NCIT:C51932 Osteotomy; NCIT:C15214 Rehabilitation; NCIT:C16809 MRI; NCIT:C101293 Genetic Testing.

Experimental and targeted therapy

Palovarotene is a retinoic-acid receptor-γ agonist (CHEBI:177944) investigated in phase 2 pediatric HMO trial NCT03442985. The study enrolled genetically confirmed, symptomatic children aged 2–14 but was terminated/held amid concern about premature growth-plate closure, a particularly serious risk in a pediatric skeletal-growth disorder. No approved HMO indication or established response rate exists. (NCT03442985 chunk 2, garcia2024analysisofthe pages 1-2)

In a 2024 mouse study, escalating systemic palovarotene held pre-existing tumor volume near baseline—0.31±0.049 mm³ versus 0.27±0.031 initially (p=0.66)—while controls grew to 1.03±0.23 mm³ (p=0.023 versus treated). Local nanoparticle delivery also reduced growth (0.26±0.10 versus 0.52±0.11 mm³; p=0.008). The abstract states: “These findings suggest that palovarotene treatment is effective against pre-existing osteochondromas and that the Stat3 pathway is involved.” This is preclinical evidence and does not overcome human safety/efficacy uncertainty. (Garcia et al., published 11 July 2024; DOI: https://doi.org/10.3390/ijms25147610). (garcia2024analysisofthe pages 1-2)

BMP antagonism, heparanase inhibition, hedgehog/IHH inhibition, and restoration of FGF restraint have reduced chondrogenesis or lesion formation in models. They remain experimental, with no validated HMO gene therapy, CRISPR therapy, RNA therapy, cell therapy, immunotherapy, or pharmacogenomic treatment algorithm. (pacifici2018hereditarymultipleexostoses pages 12-13, beltrami2016hereditarymultipleexostoses pages 8-8, pacifici2017hereditarymultipleexostoses pages 10-12, pacifici2018hereditarymultipleexostoses pages 7-9)

13. Prevention

Primary prevention through lifestyle change or immunization is not possible. Reproductive options after molecular confirmation include genetic counseling, prenatal diagnosis, and IVF with preimplantation genetic testing. Counseling should address 50% transmission risk, variable expressivity, mosaicism, and the inability to predict severity precisely from genotype.

Secondary prevention consists of cascade testing and early clinical recognition in at-risk children, allowing monitoring of growth, limb alignment, motion, spinal/neurologic signs, and symptomatic lesions. Tertiary prevention includes timely correction of progressive deformity, complete excision when indicated, rehabilitation, pain management, and prompt MRI/sarcoma referral for adult lesion growth or new persistent pain. There is no accepted population screening program or prophylactic medication. Suggested follow-up intervals in older reviews—6–12 months in growing children and 12–24 months in adults—are expert-practice recommendations rather than high-level trial evidence and should be individualized. (beltrami2016hereditarymultipleexostoses pages 7-8)

14. Other species and natural disease

A well-characterized, naturally occurring veterinary counterpart attributable to orthologous EXT1/EXT2 variants is not established strongly enough in the retrieved evidence for a curated natural-disease assertion. Osteochondromas occur in domestic animals, but sporadic veterinary lesions should not automatically be labeled orthologous HMO. There is no zoonotic potential or cross-species transmission.

Relevant taxa for experimental comparative biology are Mus musculus (NCBI Taxon:10090) and Danio rerio (Taxon:7955). EXT heparan-sulfate biosynthesis and endochondral ossification are evolutionarily conserved, explaining why engineered models reproduce key mechanisms.

15. Model organisms

Mouse

Ext1- or Ext2-single-heterozygous mice are often largely normal, limiting their utility as simple haploinsufficiency models. Compound Ext1+/−;Ext2+/− mice, chondrocyte-specific conditional Ext1-null mice, and mosaic/clonal Ext1 inactivation models develop multiple osteochondroma-like lesions. These models strongly support severe focal heparan-sulfate deficiency and a second-hit mechanism and are used to test BMP antagonists, heparanase inhibitors, and RARγ agonists. Key studies include PMIDs 16236767, 20080592, 20534475, and 21310272. Limitations include engineered timing/cell specificity, more uniform genetic backgrounds, and incomplete reproduction of variable human pain, deformity, and malignancy. (beltrami2016hereditarymultipleexostoses pages 8-8, pacifici2017hereditarymultipleexostoses pages 12-13, pacifici2018hereditarymultipleexostoses pages 7-9, pacifici2017hereditarymultipleexostoses pages 1-2)

Zebrafish

The dackel/ext2 mutant reproduces abnormal cartilage and bone patterning. Homozygous ext2-null fish show failed terminal chondrocyte differentiation, impaired osteoblast differentiation, altered bone–fat lineage balance, misshapen bones, and muscle abnormalities. These models are powerful for developmental imaging and pathway screening but do not anatomically reproduce every human osteochondroma. Dental abnormalities observed in ext2 mutants generated a hypothesis of underrecognized dental manifestations in patients, illustrating both comparative utility and the need for human validation.

Cellular systems

Primary chondrocytes, lesion-derived cells, and cultured skeletal progenitors permit assays of heparan-sulfate production, BMP/FGF/hedgehog/WNT signaling, matrix synthesis, and retinoid response. No validated human HMO organoid, iPSC assay, or CRISPR screen currently functions as a clinical diagnostic or treatment-selection platform.

Current research infrastructure and evidence gaps

The Italian Registry of Multiple Osteochondromas, NCT04133285, is a recruiting retrospective/prospective cohort designed for up to 10,000 participants through 2032. It integrates clinical, genetic, genealogic, imaging, surgery, quality-of-life, tissue, and longitudinal malignancy data. Such registries are essential because rarity, variable expressivity, and referral bias undermine precise prevalence, malignancy-risk, and treatment-effect estimates. (NCT04133285 chunk 1)

Major gaps are: validated age- and genotype-specific surveillance schedules; prospective surgical comparative studies; robust malignant-transformation biomarkers; replicated modifier genes; longitudinal patient-reported outcomes; human single-cell/spatial maps of lesion initiation; and a safe disease-modifying therapy. The current expert interpretation is therefore conservative: molecular diagnosis and multidisciplinary longitudinal care are clinically actionable, whereas pathway-targeted treatment remains investigational.

References

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13. (pacifici2018hereditarymultipleexostoses pages 12-13): Maurizio Pacifici. Hereditary multiple exostoses: are there new plausible treatment strategies? Expert Opinion on Orphan Drugs, 6:385-391, Jun 2018. URL: https://doi.org/10.1080/21678707.2018.1483232, doi:10.1080/21678707.2018.1483232. This article has 18 citations.

14. (tepelenis2021osteochondromasanupdated pages 7-8): KOSTAS TEPELENIS, GEORGIOS PAPATHANAKOS, AIKATERINI KITSOULI, THEODOROS TROUPIS, ALEXANDRA BARBOUTI, KONSTANTINOS VLACHOS, PANAGIOTIS KANAVAROS, and PANAGIOTIS KITSOULIS. Osteochondromas: an updated review of epidemiology, pathogenesis, clinical presentation, radiological features and treatment options. In Vivo, 35:681-691, Jan 2021. URL: https://doi.org/10.21873/invivo.12308, doi:10.21873/invivo.12308. This article has 247 citations and is from a peer-reviewed journal.

15. (beltrami2016hereditarymultipleexostoses pages 7-8): G. Beltrami, G. Ristori, G. Scoccianti, A. Tamburini, and R. Capanna. Hereditary multiple exostoses: a review of clinical appearance and metabolic pattern. Clinical cases in mineral and bone metabolism : the official journal of the Italian Society of Osteoporosis, Mineral Metabolism, and Skeletal Diseases, 13 2:110-118, May 2016. URL: https://doi.org/10.11138/ccmbm/2016.13.2.110, doi:10.11138/ccmbm/2016.13.2.110. This article has 120 citations.

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26. (borovikov2024themissingpiece pages 13-13): Artem Borovikov, Nailya Galeeva, Andrey Marakhonov, Aysylu Murtazina, Varvara Kadnikova, Kseniya Davydenko, Anna Orlova, Peter Sparber, Tatiana Markova, Maria Orlova, Darya Osipova, Tatyana Nagornova, Natalia Semenova, Olga Levchenko, Alexandra Filatova, Margarita Sharova, Peter Vasiluev, Ilya Kanivets, Denis Pyankov, Artem Sharkov, Vasilisa Udalova, Vladimir Kenis, Natalia Nikitina, Maria Sumina, Konstantin Zherdev, Aleksandr Petel'guzov, Oleg Chelpachenko, Pavel Zubkov, Ivan Dan, Andrey Snetkov, Alexandra Akinshina, Yury Buklemishev, Oxana Ryzhkova, Vyacheslav Tabakov, Ekaterina Zakharova, Sergey Korostelev, Rena Zinchenko, Mikhail Skoblov, Alexander Polyakov, Elena Dadali, Sergey Kutsev, and Olga Shchagina. The missing piece of the puzzle: unveiling the role of ptpn11 gene in multiple osteochondromas in a large cohort study. Human Mutation, 2024:1-13, Feb 2024. URL: https://doi.org/10.1155/2024/8849348, doi:10.1155/2024/8849348. This article has 6 citations and is from a domain leading peer-reviewed journal.

27. (beltrami2016hereditarymultipleexostoses pages 8-8): G. Beltrami, G. Ristori, G. Scoccianti, A. Tamburini, and R. Capanna. Hereditary multiple exostoses: a review of clinical appearance and metabolic pattern. Clinical cases in mineral and bone metabolism : the official journal of the Italian Society of Osteoporosis, Mineral Metabolism, and Skeletal Diseases, 13 2:110-118, May 2016. URL: https://doi.org/10.11138/ccmbm/2016.13.2.110, doi:10.11138/ccmbm/2016.13.2.110. This article has 120 citations.

Artifacts

• Edison artifact artifact-00

Reference Validation

Checked with linkml-reference-validator 0.2.1.

All extracted references resolved successfully.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

• NCIT:C121924 (Enostosis) (1 mention)

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: Orphanet, OMIM, PMIDs, Taxon, ORPHA.

62 of 73 terms resolved to a current term; the rest could not be looked up either way.