Xeroderma pigmentosum: A genetic disease characterized by such extraordinary sensitivity to sunlight that it results in the development of skin cancer at a very early age. Children with xeroderma pigmentosum (XP) can only play outdoors safely after nightfall. They have been called midnight children, the children of the dark, the children of the night and, perjoratively, vampire children.
XP is due to defective repair of damage done to DNA (the genetic material), damage caused by ultraviolet (UV) light. Whereas normal persons can repair UV-induced damage by inserting new bases into the DNA, XP patients lack the normal capacity to repair the DNA damage inflicted by UV light.
A person with XP develops severe sunburn and eye irritation within minutes of exposure to sunlight. Other signs of XP include the development of ultradry skin (the word "xeroderma" means extreme dryness of the skin) plus blisters, heavy freckling and dark spots on the skin (the word "pigmentosum" refers to these pigmented areas of skin).
Damage to DNA is cumulative; it is additive and cannot be reversed. Recurrent exposure of a XP person to UV light can cause the rapid development of cancerous and non-cancerous growths on both the skin and eyes. Even children with XP can develop skin cancer. About one in every five XP patients also develops one or more of the following problems: blindness, deafness, mental retardation, poor coordination, spasticity, or retarded physical growth.
The life span of XP patients varies. Those with undiagnosed XP who regularly expose themselves to sunlight may die young of skin cancer. Those with a diagnosis of XP who protect themselves from sunlight may live a long life. The life expectancy of most patients falls between these extremes.
Diagnosis requires tests on skin or blood samples. Although the disease itself is incurable, patients can maintain their health by:
protecting themselves completely from ultraviolet light,
getting frequent skin and eye examinations, and
having cancerous growths removed without delay.
XP is inherited as an autosomal recessive trait (meaning that the gene for XP is on a nonsex chromosome [an autosome] and that a person must possess two doses of that gene to manifest the syndrome). In actuality, XP is not one disease. A number of diseases clinically paint the XP picture. Genes for XP reside in diverse locations including chromosomes 3p25, 9q22.3, 11p12-p11, and 19q13.2-q13.3.
Source : Medical Dictionary