Hereditary multiple osteochondromas | |
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Other names | Hereditary multiple exostoses |
Photograph of the legs of a 26-year-old male showing multiple lumps leading to deformity. | |
Specialty | Medical genetics |
Hereditary multiple osteochondromas (HMO), also known as hereditary multiple exostoses, is a disorder characterized by the development of multiple benign osteocartilaginous masses (exostoses) in relation to the ends of long bones of the lower limbs such as the femurs and tibias and of the upper limbs such as the humeri and forearm bones. They are also known as osteochondromas. Additional sites of occurrence include on flat bones such as the pelvic bone and scapula. The distribution and number of these exostoses show a wide diversity among affected individuals. Exostoses usually present during childhood. The vast majority of affected individuals become clinically manifest by the time they reach adolescence. [1] [2] The incidence of hereditary multiple exostoses is around 1 in 50,000 individuals. [3] Hereditary multiple osteochondromas is the preferred term used by the World Health Organization. A small percentage of affected individuals are at risk for development of sarcomas as a result of malignant transformation. The risk that people with hereditary multiple osteochondromas have a 1 in 20 to 1 in 200 lifetime risk of developing sarcomas. [4]
A noticeable lump in relation to an extremity may be the first presenting symptom. Multiple deformities can arise, namely coronal plane deformities around the knees, ankles, shoulders, elbows, and wrists. For example, genu valgum (knock knees), ankle valgus, ulnar bowing and shortening, and radial head subluxation are encountered. The majority of affected individuals have clinically manifest osteochondromas around the knee. Forearm involvement in HMO is considerable. [1] [5] Furthermore, short stature may occur and is generally disproportionate. Such manifestations usually result from disruption of physeal growth especially that osteochondromas typically arise at the metaphyseal ends of long bones in close proximity to the physis. [1] [5] Intra-articular osteochondromas of the hip can induce limitation of range of motion, joint pain and acetabular dysplasia. [2] Likewise joint pain at other locations and neurovascular compression can occur. Furthermore, functional disability in regard to activities of daily living can be a presenting feature. Spinal deformity pain or neurological compromise should arouse suspicion of involvement of the vertebrae. [3]
According to self-reports, a far majority of patients experience pain, and about half experience generalized pain. Individuals who had HME-related complications were five times more likely to have pain, while those who had surgery were 3.8 times more likely to have pain. No differences were found between males and females with respect to pain, surgery, or HME-related complications. [6]
Some parents of children with HME have observed autism-like social problems in their children. To explore those observations more deeply, a 2012 study by the Sanford-Burnham Medical Research Institute used a mouse model of HME to observe cognitive function. The findings indicated that the mutant mice endorsed three autistic characteristics: social impairment, impairments in ultrasonic vocalization, and repetitive behavior. [7]
MHE stems from an inability to biosynthesize heparan sulfate, a proteoglycan. As Cueller et al. note: "[E]ncoding glycosyltransferases involved in the biosynthesis of ubiquitously expressed heparan sulphate (HS) chains, are associated with MHE." [8] [9]
HME is an autosomal dominant hereditary disorder. This means that a patient with HME has a 50% chance of transmitting this disorder to his or her children. Most individuals with HME have a parent who also has the condition, however, approximately 10% -20% of individuals with HME have the condition as a result of a spontaneous mutation and are thus the first person in their family to be affected.[ citation needed ]
HME has thus far been linked with mutations in three genes:
Mutations in these genes typically lead to the synthesis of a truncated EXT protein which does not function normally. It is known that EXT proteins are important enzymes in the synthesis of heparan sulfate proteoglycans; however, the exact mechanism by which altered synthesis of heparan sulfate that could lead to the abnormal bone growth associated with HME is unclear. It is thought that normal chondrocyte proliferation and differentiation may be affected, leading to abnormal bone growth. [13] [14] Since the HME genes are involved in the synthesis of a glycan (heparan sulfate), HME may be considered a congenital disorder of glycosylation according to the new CDG nomenclature suggested in 2009. [15]
For individuals with HME who are considering starting a family, preimplantation genetic testing and prenatal diagnosis are available to determine if their unborn child has inherited the disease. HME has a 96% penetrance, which means that if the affected gene is indeed transmitted to a child, the child will have a 96% of actually manifesting the disease, and 4% chance of having the disease but never manifesting it. The 96% penetrance figure comes from only one study. [16] Other studies have observed both incomplete and variable penetrance but without calculating the % penetrance, e.g. [17] In both the aforementioned studies the symptomless individuals carrying the faulty gene were predominantly female, leading to speculation that incomplete penetrance is more likely to be exhibited in females. Indeed, other work has shown that boys/men tend to have worse disease than females, as well as that the number of exostoses in affected members of the same family can vary greatly. [18] It is also possible for females to be severely affected. Severity of symptoms varies between individuals, even in the same family.[ citation needed ]
Symptoms are more likely to be severe if the mutation is on the ext1 gene rather than ext2 or ext3; ext1 is also the most commonly affected gene in patients of this disorder. [18]
It is characterized by the growth of cartilage-capped benign bone tumours around areas of active bone growth, particularly the metaphysis of the long bones. Typically five or six exostoses are found in upper and lower limbs. Image depicts adult regrowth after knee replacement.
Most common locations are: [19]
HME can lead to the shortening and bowing of bones; affected individuals often have a short stature. Depending on their location the exostoses can cause problems including: pain or numbness from nerve compression, vascular compromise, inequality of limb length, irritation of tendon and muscle, Madelung's deformity [20] as well as a limited range of motion at the joints upon which they encroach. A person with HME has an increased risk of developing a rare form of bone cancer called chondrosarcoma as an adult. [20] Problems may be had in later life and these could include weak bones and nerve damage. [21] [22] [23] The reported rate of transformation ranges from as low as 0.57% [16] to as high as 8.3% of people with HME. [24] Some authors have described an association between HME and the presence of popliteal pseudoaneurysms. [25]
The diagnosis of HMO is based upon establishing an accurate correlation between the above-mentioned clinical features and the characteristic radiographic features. Family history can provide an important clue to the diagnosis. This is supplemented by testing for the two genes in which pathogenic variants are known to cause HMO namely EXT1 and EXT2. A combination of sequence analysis and deletion analysis of the entire coding regions of both EXT1 and EXT2 detects pathogenic variants in 70–95% of affected individuals. [3] [5] The hallmark of radiographic diagnosis is the presence of osteochondromas at the metaphyseal ends of long bones in which the cortex and medulla of the osteochondroma represent a continuous extension of the host bone. This is readily demonstrable in radiographs of the knees. [3] [1]
The indications for surgical intervention in individuals with HMO remain unclear and vary greatly across the medical literature. In general surgical treatment of HMO includes one or more of the following procedures: ostechondroma excision, gradual or acute bone lengthening such as the ulna lengthening, corrective osteotomies, temporary hemiepiphysiodesis to correct angular joint deformities such as distal radius hemiepiphysiodesis and medial distal tibial hemiepiphysiodesis. [1] [3] Nevertheless, there is little evidence to support the ongoing pediatric orthopedic practice in hereditary multiple osteochondromas. Recent systematic reviews found insufficient evidence to prove that the ongoing surgical treatment of HMO improves function considerably or to prove that it impacts the quality of life of affected children. [1] [2] To enhance the amount of evidence in the medical literature certain recommendations have been put forward. The construction of well-designed prospective studies that can provide a more clear relationship between surgical procedures, patient characteristics and outcomes is on high demand. Otherwise, following the current study designs will continue to raise more questions than answers. [1] [2] Total hip arthroplasty has been used to remedy severe and painful HMO of the hip joint. Total hip arthroplasty in individuals with HMO is challenging because of distortion of anatomy and repeated surgeries performed to address complaints related to exostosis. [26]
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders are the most common, the term is mostly used when discussing disorders with a single genetic cause, either in a gene or chromosome. The mutation responsible can occur spontaneously before embryonic development, or it can be inherited from two parents who are carriers of a faulty gene or from a parent with the disorder. When the genetic disorder is inherited from one or both parents, it is also classified as a hereditary disease. Some disorders are caused by a mutation on the X chromosome and have X-linked inheritance. Very few disorders are inherited on the Y chromosome or mitochondrial DNA.
Langer–Giedion syndrome (LGS) is a very uncommon autosomal dominant genetic disorder caused by a deletion of a small section of material on chromosome 8. It is named after the two doctors who undertook the main research into the condition in the 1960s. Diagnosis is usually made at birth or in early childhood.
Sanfilippo syndrome, also known as mucopolysaccharidosis type III (MPS III), is a rare lifelong genetic disease that mainly affects the brain and spinal cord. It is caused by a problem with how the body breaks down certain large sugar molecules called glycosaminoglycans (also known as GAGs or mucopolysaccharides). In children with this condition, these sugar molecules build up in the body and eventually lead to damage of the central nervous system and other organ systems.
An exostosis, also known as a bone spur, is the formation of new bone on the surface of a bone. Exostoses can cause chronic pain ranging from mild to debilitatingly severe, depending on the shape, size, and location of the lesion. It is most commonly found in places like the ribs, where small bone growths form, but sometimes larger growths can grow on places like the ankles, knees, shoulders, elbows and hips. Very rarely are they on the skull.
Osteochondromas are the most common benign tumors of the bones. The tumors take the form of cartilage-capped bony projections or outgrowth on the surface of bones (exostoses). It is characterized as a type of overgrowth that can occur in any bone where cartilage forms bone. Tumors most commonly affect long bones about the knee and in the forearm. Additionally, flat bones such as the pelvis and scapula may be affected.
Metachondromatosis is an autosomal dominant, incompletely penetrant genetic disease affecting the growth of bones, leading to exostoses primarily in the hands and feet as well as enchondromas of long bone metaphyses and iliac crests. This syndrome affects mainly tubular bones, though it can also involve the vertebrae, small joints, and flat bones. The disease is thought to affect exon 4 of the PTPN11 gene. Metachondromatosis is believed to be caused by an 11 base pair deletion resulting in a frameshift and nonsense mutation. The disease was discovered and named in 1971 by Pierre Maroteaux, a French physician, when he observed two families with skeletal radiologic features with exostoses and Ollier disease. The observation of one family with five affected people led to the identification of the disease as autosomal dominant. There have been less than 40 cases of the disease reported to date.
Heparan sulfate (HS) is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan in which two or three HS chains are attached in close proximity to cell surface or extracellular matrix proteins. In this form, HS binds to a variety of protein ligands, including Wnt, and regulates a wide range of biological activities, including developmental processes, angiogenesis, blood coagulation, abolishing detachment activity by GrB, and tumour metastasis. HS has also been shown to serve as cellular receptor for a number of viruses, including the respiratory syncytial virus. One study suggests that cellular heparan sulfate has a role in SARS-CoV-2 Infection, particularly when the virus attaches with ACE2.
Nail–patella syndrome is a genetic disorder that results in small, poorly developed nails and kneecaps, but can also affect many other areas of the body, such as the elbows, chest, and hips. The name "nail–patella" can be very misleading because the syndrome often affects many other areas of the body, including even the production of certain proteins. The severity of these effects varies depending on the individual. It is also referred to as iliac horn syndrome, hereditary onychoosteodysplasia, Fong disease or Turner–Kieser syndrome.
The epiphyseal plate, epiphysial plate, physis, or growth plate is a hyaline cartilage plate in the metaphysis at each end of a long bone. It is the part of a long bone where new bone growth takes place; that is, the whole bone is alive, with maintenance remodeling throughout its existing bone tissue, but the growth plate is the place where the long bone grows longer.
Ollier disease is a rare sporadic nonhereditary skeletal disorder in which typically benign cartilaginous tumors (enchondromas) develop near the growth plate cartilage. This is caused by cartilage rests that grow and reside within the metaphysis or diaphysis and eventually mineralize over time to form multiple enchondromas. Key signs of the disorder include asymmetry and shortening of the limb as well as an increased thickness of the bone margin. These symptoms are typically first visible during early childhood with the mean age of diagnosis being 13 years of age. Many patients with Ollier disease are prone to develop other malignancies including bone sarcomas that necessitate treatment and the removal of malignant bone neoplasm. Cases in patients with Ollier disease has shown a link to IDH1, IDH2, and PTH1R gene mutations. Currently, there are no forms of treatment for the underlying condition of Ollier disease but complications such as fractures, deformities, malignancies that arise from it can be treated through surgical procedures. The prevalence of this condition is estimated at around 1 in 100,000. It is unclear whether the men or women are more affected by this disorder due to conflicting case studies.
Multiple epiphyseal dysplasia (MED), also known as Fairbank's disease, is a rare genetic disorder that affects the growing ends of bones. Long bones normally elongate by expansion of cartilage in the growth plate near their ends. As it expands outward from the growth plate, the cartilage mineralizes and hardens to become bone (ossification). In MED, this process is defective.
Exostosin-1 is a protein that in humans is encoded by the EXT1 gene.
Exostosin glycosyltransferase-2 is a protein that in humans is encoded by the EXT2 gene.
Exostosin-like 3 is a protein that in humans is encoded by the EXTL3 gene.
Exostosin-like 1 is a protein that in humans is encoded by the EXTL1 gene.
Exostosin-like 2 is a protein that in humans is encoded by the EXTL2 gene. EXTL2 Glycosyltransferase is required for the biosynthesis of heparan-sulfate and responsible for the alternating addition of beta-1-4-linked glucuronic acid (GlcA) and alpha-1-4-linked N-acetylglucosamine (GlcNAc) units to nascent heparan sulfate chains.
Hereditary sensory and autonomic neuropathy (HSAN) or hereditary sensory neuropathy (HSN) is a condition used to describe any of the types of this disease which inhibit sensation.
Trevor disease, also known as dysplasia epiphysealis hemimelica and Trevor's disease, is a congenital bone developmental disorder. There is 1 case per million population. The condition is three times more common in males than in females.
Potocki–Shaffer syndrome (PSS), also known as DEFECT11 syndrome or chromosome 11p11.2 deletion syndrome, is a rare contiguous gene syndrome that results from the microdeletion of section 11.2 on the short arm of chromosome 11 (11p11.2). The syndrome has its name from Dr. Lorraine (Lori) Potocki and Dr. Lisa Shaffer who discovered the deletion on the 11th chromosome and studied the impacts.
Glucuronosyl-N-acetylglucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase is an enzyme with systematic name UDP-N-acetyl-D-glucosamine:beta-D-glucuronosyl-(1->4)-N-acetyl-alpha-D-glucosaminyl-proteoglycan 4-alpha-N-acetylglucosaminyltransferase. This enzyme catalyses the following chemical reaction