Spondyloepiphyseal dysplasia tarda (SEDT; SEDL) is a rare, hereditary skeletal disorder that only affects males. Physical characteristics include moderate short stature (dwarfism), moderate-to-severe spinal deformities, barrel-shaped chest, disproportionately short trunk, and premature osteoarthritis.
Symptoms of SEDT are usually not apparent until six to eight years of age, hence the distinction “tarda” or “late”. This is the most obvious distinction from the congenital form (SED congenita, or SEDC), which is usually obvious at birth. Unlike SEDC, cleft palate and retinal detachment are not associated with SEDT.
At about 6 to 8 or 10 years, spinal growth appears to decline and then stops. Limb growth continues, resulting in a disproportionately short trunk. Arm span may exceed height by 4 to 8 inches. The shoulders may assume a hunched appearance, the neck appears short and the chest broadens (barrel chest). During adolescence, various skeletal abnormalities may cause pain in the back, hips, shoulders, knees and ankles. Some individuals with SEDT may have a flat appearance to the face, but the shape of the head is usually normal. As adults, people with SEDT have mild dwarfism, with a short trunk, large chest cage and relatively normal limb length. Hands, head and feet appear to be normal size, and final adult height usually ranges from 4’10” to 5’6″.
SEDT is inherited as an X-linked recessive genetic trait. The locus has been mapped to the short arm of the X chromosome at Xp22.2-p22.1. The gene, known as SEDL or TRAPPC2, is widely expressed in tissues throughout the body, but when mutated or deleted appears to only affect cartilage. The protein product, sedlin, is thought to function in intracellular protein trafficking [Venditti et al, 2012] or gene regulation [Jeyabalan et al, 2010]. There are no other disorders linked to TRAPPC2.
Chromosomes, which are present in the nucleus of all cells, carry the genetic information for each individual. Human 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 Xp22.2-p22.1” refers to a region between bands 22.1 and 22.2 on the short arm of the X chromosome. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
X-linked recessive genetic disorders are conditions caused by an abnormal gene on the X chromosome. Females have two X chromosomes, but one of the X chromosomes is normally “turned off” early in fetal development, and most of the genes on that chromosome are inactivated. Females who have a disease gene present on one of their X chromosomes are carriers for that disorder, but rarely display any of its manifestations.
Males have one X chromosome and, if they inherit an X chromosome that contains a disease gene, they will develop the disease. Males with X-linked disorders pass the disease gene to all of their daughters, who will be carriers. Males cannot pass an X-linked gene to their sons because fathers pass their Y chromosome to their male offspring. Female carriers of an X-linked disorder have a 25% chance with each pregnancy to have a carrier daughter like themselves, a 25% chance to have a non-carrier daughter, a 25% chance to have a son affected with the disease, and a 25% chance to have an unaffected son.
SEDT does not exhibit any ethnic predisposition. Affected individuals have been described in European, American, Asian, and Australian populations (but not in African-Americans to date). One estimate suggests that the incidence is 2 persons per million.
The diagnosis of SEDT is usually made on the basis of characteristic radiological (X-ray) findings. These appear in late childhood but usually before puberty.[[no paragraph]] Some of these signs are: flattened vertebral bodies (platyspondyly), typical distortions of the upper and lower vertebral surfaces ("humping"), short necks of the thigh bones (femurs), an angular deformity of the neck of the femur (coxa vara), and signs of early arthritis, especially in the hip joints.
Molecular testing for mutations in the SEDL (TRAPPC2) gene is commercially available.
Treatment is supportive and symptomatic. Hip replacement may be necessary by the fourth to fifth decade of life.
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Contact for additional information about spondyloepiphyseal dysplasia tarda:
George E Tiller, MD, PhD
Regional Chief, Department of Genetics
4900 Sunset Blvd
Los Angeles, CA 90027 USA
Tel 323-783-1916 (voicemail)
Tel 323-783-5612 (secretary)
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