Multiple hereditary exostoses (MHE): elucidating the pathogenesis of a rare skeletal disorder through interdisciplinary research

Rare coexistence of multiple osteochondromas and solitary osteoid osteoma: A case report

Multiple osteochondromas (MOs) are inherited in an autosomal-dominant manner, with a penetrance of ~96 and 100% in female and male patients, respectively. Osteochondromas primarily involve the metaphyses and diaphyses of long bones, including the ribs. Osteoid osteomas account for ~3 and 11% of all bone tumors and benign bone tumors, respectively. Furthermore,1 the male-to-female ratio is 2-3:1, and they generally occur in the long bones of the lower extremities, with the femoral neck being the most frequent site. The present study describes the case of a 16-year-old male patient with a bony mass around the left knee joint and pain in the left calf. Radiography revealed MOs in the upper and lower extremities, while computed tomography showed a nidus in the cortex of the tibial shaft. The patient's family history included the presence of MOs, and the patient was diagnosed with MOs and a solitary osteoid osteoma. Surgical excision of the osteochondroma and curettage of the osteoid osteoma in the proximal tibia and tibial shaft, respectively, were performed simultaneously. Postoperative pathological examination revealed osteochondroma and osteoid osteoma. Furthermore, the pain resolved, and no recurrence was observed 7 months post-operation. To the best of our knowledge, no reports exist on coexisting MOs and osteoid osteoma; therefore, the present study describes the first case of such a condition. Marginal excision for osteochondroma and curettage for osteoid osteoma effectively improved the symptoms.

NFATC1 and NFATC2 expression patterns in human osteochondromas

Background

Our previous study in genetic mouse models found that NFATc1 and NFATc2 suppress osteochondroma formation from entheseal progenitors. However, it remains unclear whether NFAT signaling is also involved in human osteochondromagenesis. As the first step in addressing this question, the current study aimed to determine the expression patterns of NFATC1 and NFATC2 in human osteochondroma samples.

Methods

Immunohistochemistry (IHC) was used to examine and analyze NFATC1 and NFATC2 expression in human osteochondroma samples. The human periosteum was used to map the expression of NFATC1 under physiological conditions by IHC. Furthermore, human periosteal progenitors were isolated and identified from the periosteal tissues of bone fracture healing patients. The expression of NFATC1 in human periosteal progenitors was characterized by Western blotting compared to human bone marrow stromal cells (BMSC).

Results

The IHC results showed that the expression of NFATC1 was undetectable in most human osteochondromas cells, and only a small proportion of osteochondroma cells, especially clonally grown chondrocytes, showed positive staining of NFATC1. NFATC2 expression was also undetectable in most chondrocytes in human osteochondromas. The mouse and human periosteum showed a comparable ratio of NFATC1 positive cells (9.56 ± 0.80% vs 11.04 ± 2.05%, P = 0.3101). Furthermore, Western blotting analysis revealed that NFATC1 expression was highly enriched in human periosteal progenitors compared to BMSC.

Conclusions

NFATC1 and NFATC2 are undetectable in most human osteochondroma chondrocytes. The expression pattern of NFATC1 in human osteochondromas and the normal periosteum suggests that NFAT signaling could be suppressed during human osteochondromagenesis.

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.

Identification of Novel Mutations in the EXT1 and EXT2 Genes of Chinese Patients with Hereditary Multiple Osteochondromas

Aim: To detect mutations in the EXT1 and EXT2 genes in four Chinese families with hereditary multiple osteochondromas (HMO). HMO is an autosomal dominant disorder characterized by the overgrowth of multiple cartilage-capped bones in the metaphysis of long bones and flat bones. Methods: Polymerase chain reaction-based amplification followed by DNA sequencing of the complete coding sequences of EXT1 and EXT2 was performed for four Chinese families with HMO. Results: The mutant allele was found in six patients: three mutations were found in EXT1 and two in EXT2. A novel frameshift mutation, which generates a premature stop codon at codon 586 and causes partial loss of the glycosyltransferase domain, was detected in exon 9 of EXT1 (F579Yfs*8). We hypothesize that F579Yfs*8 is a pathogenic mutation. Two novel missense mutations (G339S and V545D) were found in EXT1. The variant c.1634T>A (V545D) is apparently benign. In addition we found a novel deletion mutation in EXT2, c.856_864 del TTCCTCCTG, which results in the deletion of 286Phe, 287Leu, and 288Leu, that is likely pathogenic. Finally, we identified a likely benign variant in exon 13 of EXT2. c.2035-41T>C (rs3740878). Conclusions: We found three novel, potentially pathogenic mutations in EXT1 and EXT2, including a novel frameshift mutation. More importantly, our study results have expanded the spectrum of EXT mutations conducive to the genetic diagnosis and counseling of patients with HMO.

A Novel Intronic Splicing Mutation in the EXT2 Gene of a Chinese Family with Multiple Osteochondroma

Background: Multiple osteochondroma (MO), an autosomal dominant genetic disease, is caused by heterozygous mutations in the EXT1 and EXT2 genes. Approximately 80% of pathogenic mutations are nonsense/missense mutations, small indels, and splicing mutations. Splicing mutations, particularly at the 3' and 5' splice sites, disrupt normal mRNA processing and cause exon skipping or aberrant splicing, ultimately resulting in protein truncation and loss of function. Methods: Polymerase chain reaction (PCR) and Sanger sequencing were applied to detect subtle mutations in a Chinese family with MO, the pathogenicity of a splicing variant was predicted by bioinformatics and further verified using a minigene splicing assay. Results: A novel and heterozygous splicing mutation, c.626 + 2_626 + 5delTAGG, was identified in the EXT2 gene of the proband and the father by PCR and Sanger sequencing, whereas the unaffected mother and brother had wild-type alleles at the same site. Bioinformatics predicted that the 5' splicing site of exon 3 in the EXT2 gene was destroyed due to this mutation. A hybrid minigene splicing assay (HMSA) indicated that the mutation disturbed the normal splicing of the EXT2 gene mRNA and led to a deletion of 79 bp at the 5' end of exon 3, which resulted in aberrant splicing of exon 3 and introduced an earlier stop codon in the EXT2 gene. Conclusion: A novel splicing mutation was identified that produced the MO phenotype through aberrant splicing in a Chinese family. This observation, expands our knowledge of the spectrum of molecular pathogenic mechanisms leading to aberrant mRNA splicing.

An unusual diagnosis for an usual test

Background

Hereditary multiple osteochondromas (HMO) is a genetic condition characterized by the presence of multiple osteochondromas, usually at the lateral side of the most active growth plate of a long bone. These lesions may persist, be asymptomatic during childhood, and may increase in number and size until growth plates close. Therefore, diagnosis of HMO in children and young people can be challenging; while short stature can be more evident at the onset of puberty, asymptomatic ostheocondromas can progress into different degrees of orthopedic deformity. Moreover, multiple complications may arise due to the presence of osteochondromas, including tendon and compression muscle pain, neurovascular disorders, obstetric problems, scoliosis and malignant transformation into secondary peripheral chondrosarcoma in adulthood.

Case presentation

We report the case of a girl admitted to our Institute for growth delay. While laboratory tests, including growth hormone stimulation test, were normal, left hand X-ray revealed multiple osteochondromas, suggestive for HMO. The genetic test for EXT1 and EXT2 genes confirmed the radiological diagnosis, with a mutation inherited from the mother who displayed the same radiological abnormalities along with recurrent limb pain episodes.

Conclusions

HMO is a genetic condition whose diagnosis can be challenging, especially in females. Every pediatricians should consider a skeletal dysplasia in case of unexplained growth delay and a skeletal survey might be fundamental in reaching a diagnosis.

Use of reverse stemless shoulder arthroplasty in a patient with multiple hereditary exostosis

INTRODUCTION

Multiple hereditary exostosis (MHE) is the formation of benign, cartilage-capped bony outgrowths predominantly extending from the metaphysis of long bones that presents with reduction in growth, deformity, restricted motion, short stature and premature osteoarthritis.

AIM

To review the clinical and radiological results of a stemless reverse shoulder arthroplasty in a case of MHE.

CASE

81-year-old, right hand dominant short-statured retired male engineer with long standing shoulder pain and restricted movements on the background of an old proximal humeral fracture managed conservatively.The radiographs revealed osteoarthritis and a mal-united proximal humerus fracture on the background of MHE with 3 plane bone deformity and a lack of medullary canal.

RESULTS

The patient underwent a stemless reverse shoulder arthroplasty. At 6 months post operatively the patient had recovered well with a range of movement including: forward flexion 110°, external rotation 20° and abduction of 80°. Internal rotation remained limited to buttocks. Improved ADLIER, Subjective shoulder value and Constant Murley score compared to pre-operative figures.

CONCLUSION

The stemless humeral component relies on metaphyseal impaction for stability. When the humeral canal is malformed or in presence of malunited distal fractures, it circumvents the need of navigating a deformed diaphysis with encouraging postoperative results.

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.

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.

Translating genomic risk into an early detection strategy for sarcoma

Sarcomas have a strong genetic etiology, and the study of families affected by sarcomas has informed much of what we now understand of modern cancer biology. The recent emergence of powerful genetic technologies has led to astonishing reductions in costs and increased throughput. In the clinic, these technologies are revealing a previously unappreciated and rich landscape of genetic cancer risk. In addition to both known and new cancer risk mutations, genomic tools are cataloguing complex and polygenic risk patterns, collectively explaining between 15-25% of apparently sporadic sarcoma cases. The impact on clinical management is exemplified by Li-Fraumeni Syndrome, the most penetrant sarcoma syndrome. Whole body magnetic resonance imaging can identify surgically resectable cancers in up to one in ten individuals with Li-Fraumeni Syndrome. Taken together, parallel developments in genomics, therapeutics and imaging technologies will drive closer engagement between genetics and multidisciplinary care of the sarcoma patient in the 21st century.

A novel EXT2 frameshift mutation identified in a family with multiple osteochondromas

Multiple osteochondromas (MO) is an autosomal inherited disease that is characterized by benign bone tumors. However, the underlying mechanism of MO at a molecular level requires further investigation. The majority of mutations associated with MO occur in the exostosin glycosyltransferase genes (EXT)1 or EXT2. In the present study, the genetic causes of the disease were investigated. Polymerase chain reaction amplification, followed by DNA sequencing of the complete EXT1 and EXT2 coding regions, were conducted in a family with MO (n=5). A novel frameshift mutation in exon 3 of EXT2 (c.660delG) was detected. The production of a defective EXT2 protein, lacking 450 C-terminal amino acid residues is predicted to be caused by the c.660delG mutation, located within the exostosin domain of EXT2. The missing residues contain the exostosin and glycosyltransferase family 64 domains, which are critical for the function of EXT2. The novel c.660delG frameshift mutation in the EXT2 gene extends the etiological understanding of MO and may provide an effective reference for genetic counseling and prenatal diagnosis in this family.

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

Background

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

Methods

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

Results

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

Conclusion

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

Epigenetic Regulation of the Biosynthesis & Enzymatic Modification of Heparan Sulfate Proteoglycans: Implications for Tumorigenesis and Cancer Biomarkers

Emerging evidence suggests that the enzymes in the biosynthetic pathway for the synthesis of heparan sulfate moieties of heparan sulfate proteoglycans (HSPGs) are epigenetically regulated at many levels. As the exact composition of the heparan sulfate portion of the resulting HSPG molecules is critical to the broad spectrum of biological processes involved in oncogenesis, the epigenetic regulation of heparan sulfate biosynthesis has far-reaching effects on many cellular activities related to cancer progression. Given the current focus on developing new anti-cancer therapeutics focused on epigenetic targets, it is important to understand the effects that these emerging therapeutics may have on the synthesis of HSPGs as alterations in HSPG composition may have profound and unanticipated effects. As an introduction, this review will briefly summarize the variety of important roles which HSPGs play in a wide-spectrum of cancer-related cellular and physiological functions and then describe the biosynthesis of the heparan sulfate chains of HSPGs, including how alterations observed in cancer cells serve as potential biomarkers. This review will then focus on detailing the multiple levels of epigenetic regulation of the enzymes in the heparan sulfate synthesis pathway with a particular focus on regulation by miRNA and effects of epigenetic therapies on HSPGs. We will also explore the use of lectins to detect differences in heparan sulfate composition and preview their potential diagnostic and prognostic use in the clinic.

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.

Hereditary Multiple Exostoses: a review of clinical appearance and metabolic pattern

Hereditary multiple exostoses (HME) is an inherited genetic condition characterized by the presence of multiple exostoses (osteochondromas). MHE is a relatively rare autosomal dominant disorder, mainly caused by loss of function mutations in two genes: exostosin-1 (EXT1) and exostosin-2 (EXT2). These genes are linked to heparan sulfate (HS) synthesis, but the specific molecular mechanism leading to the disruption of the cartilage structure and the consequent exostoses formation is still not resolved. The aim of this paper is to encounter the main aspects of HME reviewing the literature, in order to improve clinical features and evolution, and the metabolic-pathogenetic mechanisms underlying. Although MHE may be asymptomatic, a wide spectrum of clinical manifestations is found in paediatric patients with this disorder. Pain is experienced by the majority of patients, even restricted motion of the joint is often encountered. Sometimes exostoses can interfere with normal development of the growth plate, giving rise to limb deformities, low stature and scoliosis. Other many neurovascular and associated disorders can lead to surgery. The most feared complication is the malignant transformation of an existing osteochondroma into a secondary peripheral chondrosarcoma, during adulthood. The therapeutic approach to HME is substantially surgical, whereas the medical one is still at an experimental level. In conclusion, HME is a complex disease where the paediatrician, the geneticist and the orthopaedic surgeon play an interchangeable role in diagnosis, research and therapy. We are waiting for new studies able to explain better the role of HS in signal transduction, because it plays a role in other bone and cartilage diseases (in particular malignant degeneration) as well as in skeletal embryology.

NFAT restricts osteochondroma formation from entheseal progenitors

Osteochondromas are common benign osteocartilaginous tumors in children and adolescents characterized by cartilage-capped bony projections on the surface of bones. These tumors often cause pain, deformity, fracture, and musculoskeletal dysfunction, and they occasionally undergo malignant transformation. The pathogenesis of osteochondromas remains poorly understood. Here, we demonstrate that nuclear factor of activated T cells c1 and c2 (NFATc1 and NFATc2) suppress osteochondromagenesis through individual and combinatorial mechanisms. In mice, conditional deletion of NFATc1 in mesenchymal limb progenitors, Scleraxis-expressing (Scx-expressing) tendoligamentous cells, or postnatally in Aggrecan-expressing cells resulted in osteochondroma formation at entheses, the insertion sites of ligaments and tendons onto bone. Combinatorial deletion of NFATc1 and NFATc2 gave rise to larger and more numerous osteochondromas in inverse proportion to gene dosage. A population of entheseal NFATc1- and Aggrecan-expressing cells was identified as the osteochondroma precursor, previously believed to be growth plate derived or perichondrium derived. Mechanistically, we show that NFATc1 restricts the proliferation and chondrogenesis of osteochondroma precursors. In contrast, NFATc2 preferentially inhibits chondrocyte hypertrophy and osteogenesis. Together, our findings identify and characterize a mechanism of osteochondroma formation and suggest that regulating NFAT activity is a new therapeutic approach for skeletal diseases characterized by defective or exaggerated osteochondral growth.

Diagnosis and Management of Hereditary Sarcoma

Sarcomas are rare and heterogeneous diseases that affect a younger population than most epithelial cancers. Epidemiologic studies suggest a strong genetic component to sarcomas, and many familial cancer syndromes have been described, in which sarcomas are a feature. The best known of these are the Li-Fraumeni and retinoblastoma syndromes, study of which has been pivotal to elucidating the molecular basis for the cell response to DNA damage and the cell division. Although much has been learnt about cancer biology from the study of sarcoma families, in general clinical management of increased sarcoma risk has lagged behind other cancer predisposition syndromes. With the advent of genomic tools for genetic testing, it is likely that a substantial fraction of sarcoma patients will be identified as carriers of known risk alleles. The translation of this knowledge into effective risk management programs and cancer treatments will be essential to changes in routine clinical practice.

FGFR3 Deficiency Causes Multiple Chondroma-like Lesions by Upregulating Hedgehog Signaling

Most cartilaginous tumors are formed during skeletal development in locations adjacent to growth plates, suggesting that they arise from disordered endochondral bone growth. Fibroblast growth factor receptor (FGFR)3 signaling plays essential roles in this process; however, the role of FGFR3 in cartilaginous tumorigenesis is not known. In this study, we found that postnatal chondrocyte-specific Fgfr3 deletion induced multiple chondroma-like lesions, including enchondromas and osteochondromas, adjacent to disordered growth plates. The lesions showed decreased extracellular signal-regulated kinase (ERK) activity and increased Indian hedgehog (IHH) expression. The same was observed in Fgfr3-deficient primary chondrocytes, in which treatment with a mitogen-activated protein kinase (MEK) inhibitor increased Ihh expression. Importantly, treatment with an inhibitor of IHH signaling reduced the occurrence of chondroma-like lesions in Fgfr3-deficient mice. This is the first study reporting that the loss of Fgfr3 function leads to the formation of chondroma-like lesions via downregulation of MEK/ERK signaling and upregulation of IHH, suggesting that FGFR3 has a tumor suppressor-like function in chondrogenesis.

Interactions of signaling proteins, growth factors and other proteins with heparan sulfate: mechanisms and mysteries

Heparan sulfate (HS) is a component of cell surface and matrix-associated proteoglycans (HSPGs) that, collectively, play crucial roles in many physiologic processes including cell differentiation, organ morphogenesis and cancer. A key function of HS is to bind and interact with signaling proteins, growth factors, plasma proteins, immune-modulators and other factors. In doing so, the HS chains and HSPGs are able to regulate protein distribution, bio-availability and action on target cells and can also serve as cell surface co-receptors, facilitating ligand-receptor interactions. These proteins contain an HS/heparin-binding domain (HBD) that mediates their association and contacts with HS. HBDs are highly diverse in sequence and predicted structure, contain clusters of basic amino acids (Lys and Arg) and possess an overall net positive charge, most often within a consensus Cardin-Weintraub (CW) motif. Interestingly, other domains and residues are now known to influence protein-HS interactions, as well as interactions with other glycosaminoglycans, such as chondroitin sulfate. In this review, we provide a description and analysis of HBDs in proteins including amphiregulin, fibroblast growth factor family members, heparanase, sclerostin and hedgehog protein family members. We discuss HBD structural and functional features and important roles carried out by other protein domains, and also provide novel conformational insights into the diversity of CW motifs present in Sonic, Indian and Desert hedgehogs. Finally, we review progress in understanding the pathogenesis of a rare pediatric skeletal disorder, Hereditary Multiple Exostoses (HME), characterized by HS deficiency and cartilage tumor formation. Advances in understanding protein-HS interactions will have broad implications for basic biology and translational medicine as well as for the development of HS-based therapeutics.