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X-linked spondyloepiphyseal dysplasia tarda is a condition that impairs bone growth and occurs almost exclusively in males. The name of the condition indicates that it affects the bones of the spine (spondylo-) and the ends (epiphyses) of long bones in the arms and legs. "Tarda" indicates that signs and symptoms of this condition are not present at birth, but appear later in childhood, typically between ages 6 and 10.
Males with X-linked spondyloepiphyseal dysplasia tarda have skeletal abnormalities and short stature. Affected boys grow steadily until late childhood, when their growth slows. Male adult height ranges from 4 feet 10 inches to 5 feet 6 inches. Individuals with X-linked spondyloepiphyseal dysplasia tarda have a short trunk and neck, and their arms appear disproportionately long. Impaired growth of the spinal bones (vertebrae) causes the short stature seen in this disorder. The spinal abnormalities include flattened vertebrae (platyspondyly) with hump-shaped bulges, progressive thinning of the discs between vertebrae, and an abnormal curvature of the spine (scoliosis or kyphosis). Other skeletal features of X-linked spondyloepiphyseal dysplasia tarda include an abnormality of the hip joint that causes the upper leg bones to turn inward (coxa vara); a broad, barrel-shaped chest; and decreased mobility of the elbow and hip joints. Arthritis often develops in early adulthood, typically affecting the hip joints and spine.
The prevalence of X-linked spondyloepiphyseal dysplasia tarda is estimated to be 1 in 150,000 to 200,000 people worldwide.
Mutations in the TRAPPC2 gene (often called the SEDL gene) cause X-linked spondyloepiphyseal dysplasia tarda. The TRAPPC2 gene provides instructions for producing the protein sedlin. The function of sedlin is unclear. Researchers believe that sedlin is part of a large molecule called the trafficking protein particle (TRAPP) complex, which plays a role in the transport of proteins between various cell compartments (organelles). Because sedlin is active (expressed) in cells throughout the body; it is unclear why mutations in the TRAPPC2 gene affect only bone growth.
Changes in this gene are associated with X-linked spondyloepiphyseal dysplasia tarda.
X-linked spondyloepiphyseal dysplasia tarda is inherited in an X-linked recessive pattern. The TRAPPC2 gene is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
In X-linked recessive inheritance, a female with one mutated copy of the gene in each cell is called a carrier. She can pass on the altered gene, but usually does not experience signs and symptoms of the disorder. In rare cases, however, females who carry a TRAPPC2 mutation may develop arthritis in early adulthood.
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