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Xeroderma pigmentosum, or XP, is an autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet (UV) light is deficient.〔James, William; Berger, Timothy; Elston, Dirk (2005). ''Andrews' Diseases of the Skin: Clinical Dermatology''. (10th ed.). Saunders. ISBN 0-7216-2921-0.〕 In extreme cases, all exposure to sunlight must be forbidden, no matter how small; as such, individuals with the disease are often colloquially referred to as children of the night. Multiple basal cell carcinomas (basaliomas) and other skin malignancies frequently occur at a young age in those with XP; metastatic malignant melanoma and squamous cell carcinoma are the two most common causes of death in XP victims. This disease involves both sexes and all races, with an incidence of 1:250,000 in the United States. XP is roughly six times more common in Japanese people〔 than in other groups. Normally, damage to DNA in epidermal cells occurs during exposure to UV light. The absorption of the high-energy light leads to the formation of pyrimidine dimers, namely cyclobutane-pyrimidine dimers and pyrimidine-6-4-pyrimidone photoproducts. In a healthy, normal human being, the damage is first excised by endonucleases. DNA polymerase then repairs the missing sequence, and ligase "seals" the transaction. This process is known as nucleotide excision repair. ==Genetics== One of the most frequent defects in xeroderma pigmentosum is an autosomal recessive genetic defect in which nucleotide excision repair (NER) enzymes are mutated, leading to a reduction in or elimination of NER. If left unchecked, damage caused by ultraviolet light can cause mutations in individual cell's DNA. The causes of the neurological abnormalities are poorly understood and are not connected with exposure to ultraviolet light. The most current theories suggest that oxidative DNA damage is generated during normal metabolism in the central nervous system, and that some types of this damage must be repaired by NER〔Brooks PJ DNA Repair (Amst). 2014 March 2; 7(7):1168-79.〕 Since DNA repair is under genetic control, it can easily undergo mutations. Many genetic disorders such as xeroderma pigmentosum (XP; MIM 278700) are caused by mutations in genes that repair DNA.〔 If the gene was not repaired correctly it could cause xeroderma pigmentosum in individuals. The autosomal recessive disorder xeroderma pigmentosum or XP has a frequency of 1 in every 250,000 individuals of all races and ethnic groups.〔Daya-Grosjean L, Sarasin A Mutat Res. 2014 March 2; 571(1-2):43-56〕 Those affected with the autosomal recessive disorder XP are extremely sensitive to UV light produced by the sun and even with a short exposure to it causes dry, flaking skin and pigmented spots that can develop into skin cancer. Individuals with XP are about 1,000 times more likely to develop skin cancer than individuals without the disorder. The molecular defects in XP cells result in a greatly elevated induction of mutations in sun-exposed skin of affected individuals. This increased mutation frequency probably accounts for the pigmentation changes and the skin cancers. Examination of mutations in the p53 gene in tumors from XP patients reveal p53 mutations characteristic of UV exposure in the majority of tumors〔 As with all genetic disorders, genetic counseling and psychological support is appropriate for the families, to discuss probability of occurrence in future pregnancies, feelings of isolation and concern about career prospects. Although there is no cure for xeroderma pigmentosum, the effects can be minimized by getting protection from the sunlight and if possible early removal of precancerous lesions. The most common fate for individuals with XP is early death from cancer due to the fact that they need to take outstanding measures to protect themselves from the dangers of the UV light. But if there is an absence of neurological problems and are always protected or away from the sunlight, the prognosis is good. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Xeroderma pigmentosum」の詳細全文を読む スポンサード リンク
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