Pyknodysostosis is a rare disorder that is inherited as an autosomal recessive genetic trait. This disorder is characterized by short stature, increased bone density (osteosclerosis/osteopetrosis), underdevelopment of the tips of fingers with absent or small nails, fragile bones that may fracture easily, abnormal or absent collarbone (clavicle), and skull abnormalities with a protruding forehead and bulge on the back of the skull caused by delayed suture closure.
Pyknodysostosis is a rare disorder characterized by short stature and increased density of the bones. Individuals with this disorder typically have short arms and legs, an abnormal collarbone, and distinctive facial features including a large head with a small face, small chin, underdeveloped facial bones, a high forehead, high narrow palate and dental abnormalities. Bone fractures are common in individuals with pyknodysostosis. The small jaw may predispose affected individuals to periods of not breathing during sleep (obstructive sleep apnea).
The gene for PKND has been mapped to the same location as the gene for cathepsin K on chromosome 1q21. 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, “chromosome1q21” refers to band 21 on the long arm of chromosome 1. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Pyknodysostosis is a rare disorder that is inherited as an autosomal recessive genetic trait. 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.
Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.
All individuals carry 4-5 abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.
Pyknodysostosis is a very rare disorder that affects males and females in equal numbers. There have been approximately 133 cases of this disorder reported in the medical literature.
The diagnosis of pyknodysostosis is based on physical features and X-ray findings. Molecular genetic testing is available to identify abnormalities in the cathepsin K gene.
Individuals with pyknodysostosis will need orthopedic care when fractures occur. Appropriate dental care should be provided and craniofacial surgery may be warranted.
Treatment may be required to relieve the obstructive sleep apnea.
Genetic counseling may be of benefit for patients and their families. Other treatment is symptomatic and supportive.
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Gorlin RJ, Cohen MM, Levin LS. Syndromes of the head and neck, 3rd ed. Oxford: Oxford University Press, 1990:285-287.
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