NORD gratefully acknowledges Miia Melkoniemi, MD, University of Helsinki, Finland, for assistance in the preparation of this report.
Heterozygous OSMED (oto-spondyl-megaepiphyseal dysplasia) is a rare genetic disorder characterized by skeletal malformations resulting in shortening of the upper limbs and thighs and short stature (rhizomelic dwarfism). Additional symptoms include distinctive facial features and delays in psychomotor development. After the initial period of growth deficiency, affected individuals experience gradual improvement in bone growth that leads to normal physical development by early childhood. Mental and motor development is also normal by early childhood. In some cases, affected individuals develop hearing loss. Heterozygous OSMED occurs because of disruptions or changes (mutations) to the COL11A2 gene.
A group of collagen disorders (i.e., OSMED, Weissenbacher-Zweymuller syndrome and non-ocular Stickler syndrome or Stickler syndrome type III) are all caused by mutations to the COL11A2 gene (allelic disorders). Some researchers consider these three disorders separate entities; others believe that they are the same disorder or different expresses of one disorder. Recently, some researchers have suggested that the name OSMED be used as a general heading to consist of "heterozygous OSMED," which encompasses Weissenbacher-Zweymuller syndrome and Stickler syndrome type III and is inherited as an autosomal dominant trait and "homozygous OSMED," which encompasses autosomal recessive cases of oto-spondylo-megaepiphyseal dysplasia.
Heterozygous OSMED is characterized by skeletal malformations, distinct facial features and delayed psychomotor development. The specific symptoms affecting each child vary from case to case.
Affected children have abnormally short bones of the upper arms and thighs (rhizomelia) resulting in short stature during infancy and early childhood (rhizomelic dwarfism). The long bones of the upper arm (humeri) and thigh (femora) are short with broad heads (dumbbell-shaped). Affected individuals may also have clefts that resemble fractures in certain bones of the vertebrae (vertebral coronal clefts).
Affected infants may also exhibit a delay in the acquisition of skills requiring coordination of muscular and mental activity (psychomotor delays). As affected individuals age, they experience an increase in growth rate eventually reaching normal height by 5 or 6 years of age. Mental and motor development also becomes normal by this age.
Distinctive facial features associated with heterozygous OSMED include an abnormally small jaw (micrognathia), widely spaced eyes (hypertelorism), depressed nasal bridge, a small upturned nose, and underdevelopment of the bones of the middle portion of the face (midface hypoplasia) giving the face a flat appearance. Affected individuals may also have Pierre-Robin sequence, an assortment of abnormalities that may occur as a distinct syndrome or as part of another underlying disorder. Pierre-Robin sequence is characterized by an unusually small jaw (micrognathia), downward displacement or retraction of the tongue (glossoptosis), and incomplete closure of the roof of the mouth (cleft palate). Cleft palate may also occur as an isolated finding.
Some individuals develop hearing loss because of an impaired ability of the auditory nerves to transmit sensory input to the brain (sensorineural hearing loss). Such hearing loss may become progressively more pronounced.
Heterozygous OSMED is inherited as an autosomal dominant trait. Some cases occur randomly as the result of a spontaneous genetic change (i.e., new mutation). Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.
Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.
Investigators have determined that some cases of heterozygous OSMED occur due to changes or disruptions (mutations) of the collagen XI, apha-2 polypeptide (COL11A2) gene located on the short arm (p) of chromosome 6 (6p21.3). Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 6p21.3” refers to band 21.3 on the short arm of chromosome 6. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
The COL11A2 gene is involved in the formation (synthesis) of collagen, specifically type XI collagen. Collagen is the body’s major structural protein forming an essential part of connective tissues and is the main component of ligaments, tendons and cartilage. Collagen is also found in bone. Type XI collagen is usually found in cartilage, the specialized tissue that serves as a buffer or cushion for bones at joints. The COL11A2 gene encodes for proteins that are essential to the development and function of type XI collagen. Mutations to this gene result in abnormalities in the production of collagen XI, which in turn affects the proper formation and development cartilage and bone.
Heterozygous OSMED affects males and females in equal numbers. Both heterozygous and homozygous OSMED are extremely rare; approximately 30 cases have been reported in the medical literature. The exact incidence of this disorder is unknown. These disorders may be underdiagnosed making it difficult to determine their true frequency in the general population.
Heterozygous OSMED may be referred to as a type XI collagen disorder (collagenopathy). Type XI collagenopathies are disorders that involve abnormalities affecting type XI collagen and include homozygous OSMED and Stickler syndrome type II.
A diagnosis of heterozygous OSMED is made based upon a thorough clinical evaluation, a detailed patient history, identification of characteristic symptoms, and a variety of specialized tests including x-rays. X-ray studies reveal characteristic skeletal malformations associated with heterozygous OSMED.
The treatment of heterozygous OSMED is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, physicians who diagnose and treat abnormalities of the skeleton, joints, muscles, and related tissues (orthopedists), orthopedic surgeons, specialists who asses and treat hearing problems (audiologists), and other healthcare professionals may need to systematically and comprehensively plan an affect child's treatment.
Hearing aids may be used to treat hearing loss. Surgery may be necessary to correct certain skeletal malformations and abnormalities such as cleft palate. Genetic counseling may be of benefit for affected individuals and their families. Other treatment is symptomatic and supportive.
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