Hereditary Multiple Osteochondromas

National Organization for Rare Disorders, Inc.

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It is possible that the main title of the report Hereditary Multiple Osteochondromas is not the name you expected. Please check the synonyms listing to find the alternate name(s) and disorder subdivision(s) covered by this report.


  • EXT
  • hereditary multiple exostoses
  • multiple osteochondromas
  • diaphyseal aclasis
  • external chondromatosis syndrome
  • multiple cartilaginous exostoses
  • multiple exostoses
  • multiple exostoses syndrome
  • multiple osteochondromatosis

Disorder Subdivisions

  • None

General Discussion


Hereditary multiple osteochondromas (HMO) is a rare genetic disorder characterized by multiple benign (noncancerous) bone tumors that are covered by cartilage (osteochondromas), often on the growing end (metaphysis) of the long bones of the legs, arms, and digits. These osteochondromas usually continue to grow until shortly after puberty and may lead to bone deformities, skeletal abnormalities, short stature, nerve compression and reduced range of motion. Hereditary multiple osteochondromas is inherited as an autosomal dominant genetic condition and is associated with abnormalities (mutations) in the EXT1or EXT2 gene.


Hereditary multiple osteochondromas was formerly called hereditary multiple exostoses.


Hereditary multiple osteochondromas is a rare disorder that affects bone growth. Bony tumors (exostoses or osteochondromas), covered with cartilage, typically appear in the growth zones (metaphyses) of the long bones adjacent to the areas where tendon and muscles attach to the bone. These growths vary in size and number among affected individuals, even within the same family. Some individuals will present with a few large "lumps" while others will show several small growths. The median age of diagnosis is three years and almost all affected individuals are diagnosed by 12 years of age.

In many cases, no treatment is required. If the exostoses are small, they may have little or no effect on the patient. However, in more severe cases, the growths may cause deformities of the forearm, knees, ankles, spine and/or pelvis. They may impose upon nerves, tendons and/or blood vessels, and interfere with movement or circulation, causing substantial pain as a result of pinched nerves or compressed tendons.

Bones that develop exostoses most often are the upper arm (humerus), forearm, knee and shoulder blades (scapulae). Bowing of the forearm and ankle are the problems that most often require surgical correction.

Approximately 40 percent of affected individuals have mild short stature as a result of shortened and/or bowed legs. If the vertebrae are affected, spinal cord compression may result, causing numbness and/ or paralysis. Urinary obstruction has been observed due to exostoses of the pelvic area.

The bony growths that characterize this disorder continue to grow until shortly after puberty at which time normally new growth no longer develops. The risk for development of malignant (cancerous) tumors, mostly chondrosarcomas, is approximately 1 to 5%.


Hereditary multiple osteochondromas is inherited as an autosomal dominant genetic condition. Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary to cause a particular disease. The abnormal gene can be inherited from either parent or can be the result of a new mutation in the affected individual. Approximately 10% of cases of HMO are thought to be the result of new mutations. At present two genes, EXT1 and EXT2, are known to show mutations in HMO patients and it is thought that these genes function as tumor suppressors. For some affected individuals no mutation in either gene is detected. In almost all these cases, the "mutation negative" patients do not have a familial history for exostoses. Most likely, they have an EXT1 or EXT2 mutation in only part of their body cells and the mutation is absent or undetectable in blood cells, which are usually used for DNA analysis. Data indicates that individuals with EXT1 mutations may have more severe effects than those with EXT2 mutations. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy. The risk is the same for males and females.

Affected Populations

The prevalence of HMO has been estimated to be about 1 of 50,000 live births. A high prevalence of this disorder has been reported in some isolated communities. Hereditary multiple osteochondromas is a disorder that affects males and females in equal numbers but in general males tend to be more severely affected.

Standard Therapies

The diagnosis of HMO is based on clinical features, X-ray findings and family history. Molecular genetic testing of the EXT1 and EXT2 genes is available to confirm the diagnosis.


Surgery may be required to relieve pain, improve movement, restore normal circulation, or for cosmetic reasons. Malignant degeneration of a tumor is treated surgically, possibly in combination with chemotherapy and radiation therapy.

Most of the malignant degenerations to cancers are to cartilage tumors or chondrosarcomas, which are slow growing and generally insensitive to chemotherapy.

Monitoring the size of affected bones by annual scans to screen for malignant degeneration is sometimes recommended. Rapid growth and increased pain are signs of a possible malignant change.

Genetic counseling is recommended for affected individuals and their families. Other treatment is symptomatic and supportive.

Investigational Therapies

Information on current clinical trials is posted on the Internet at All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the National Institutes of Health (NIH) in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222

TTY: (866) 411-1010


For information about clinical trials sponsored by private sources, contact:

Contact for additional information about hereditary multiple osteochondromas:

Wim Wuyts, PhD

Department of Medical Genetics

University and University Hospital of Antwerp

Prins Boudewijnlaan 43

2650 Edegem


Phone: 32-3-275.97.06

Fax: 32-3-275.97.23



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Jones KB, Piombo V, Searby C et al. A mouse model of osteochondromagenesis from clonal inactivation of Ext1 in chondrocytes. Proc Natl Acad Sci U S A. 2010;107(5):2054-2059.

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Ropero S, Setien F, Espada J, et al. Epigenetic loss of the familiar tumor-suppressor gene exostosin-1 (EXT1) disrupts heparan sulfate synthesis in cancer cells. Hum Mol Gent. 2004;13:2753-2765.

Noonan KJ, Feinberg JR, Levanda A, et al. Natural history of multiple hereditary osteochondromatosis of the lower extremity and ankle. J Pediatr Orthop. 2002;22 :120-4.

Bernard MA, Hall CE, Hogue DA, et al. Diminished levels of the putative tumor suppressor proteins EXT1 and EXT2 in exostoses chondrocytes. Cell Motil Cytoskeleton. 2001;48:149-162.

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Schmale GA, Wuyts W, Chansky HA, Raskind WH. (Updated September 5, 2008). Hereditary Multiple Osteochondromas. In: GeneReviews at GeneTests: Medical Genetics Information Resource (database online). Copyright, University of Washington, Seattle. 1997-2012. Available at Accessed September 19, 2012.

Online Mendelian Inheritance in Man (OMIM). The Johns Hopkins University. Exostoses, Multiple, Type I. Entry No: 133700. Last Edited June 1, 2011. Available at: Accessed September 19, 2012.

Online Mendelian Inheritance in Man (OMIM). The Johns Hopkins University. Exostoses, Multiple, Type II. Entry No: 133701. Last Edited December 8, 2009. Available at: Accessed September 19, 2012.


NIH/National Institute of Arthritis and Musculoskeletal and Skin Diseases

Information Clearinghouse

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Bethesda, MD 20892-3675


Tel: (301)495-4484

Fax: (301)718-6366

Tel: (877)226-4267

TDD: (301)565-2966



MHE Coalition

6783 York Road, Apt. #104

Parma Heights, OH 44130-4596


Tel: (440)842-8817



MHE and Me- A Support Group for Kids with Multiple Hereditary Exostoses

PO Box 651

Pine Island, NY 10969-0651


Tel: (845)258-6058

Fax: (845)258-6058



Genetic and Rare Diseases (GARD) Information Center

PO Box 8126

Gaithersburg, MD 20898-8126

Tel: (301)251-4925

Fax: (301)251-4911

Tel: (888)205-2311

TDD: (888)205-3223


MHE Research Foundation

79-77 Street

Brooklyn, NY 11209

Tel: (718)569-0479

Fax: (201)786-1758

Tel: (877)486-1758



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