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Xeroderma pigmentosum

XP

Overview

Xeroderma pigmentosum (XP) is a rare autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet (UV) light is deficient. 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[4] 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.

Symptoms - Xeroderma pigmentosum

Symptoms include:

  • Severe sunburn when exposed to only small amounts of sunlight. These often occur during a child's first exposure to sunlight.
  • Development of many freckles at an early age
  • Rough-surfaced growths (solar keratoses), and skin cancers
  • Eyes that are painfully sensitive to the sun and may easily become irritated, bloodshot and clouded
  • Blistering or freckling on minimum sun exposure
  • Spider Veins
  • Limited growth of hair on chest and legs
  • Scaly skin
  • Dry skin
  • Irregular dark spots on the skin
  • Corneal ulcerations

Causes - Xeroderma pigmentosum

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.

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. 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.

Prevention - Xeroderma pigmentosum

Experts recommend genetic counseling for persons with a family history of xeroderma pigmentosum who wish to have children.

Diagnosis - Xeroderma pigmentosum

The doctor will perform a physical exam and ask if you have a family history of xeroderma pigmentosum.

An eye exam may show:

  • Clouding of the cornea
  • Keratitis
  • Lid Tumors
  • Blepharitis

The following tests can help diagnose the condition in a baby before the birth:

  • Amniocentesis
  • Chorionic villous sampling
  • Culture of amniotic cells

The following tests can help diagnose the disorder after the birth of the child:

  • Culture of skin fibroblasts
  • Skin biopsy

Prognosis - Xeroderma pigmentosum

Fewer than 40% of individuals with the disease survive beyond the age of 20. Some XP victims with less severe cases do manage to live well into their 40s.

Treatment - Xeroderma pigmentosum

The most obvious, and often important part of treatment, is avoiding exposure to sunlight. Keratoses can also be treated using cryotherapy or fluorouracil.

Resources - Xeroderma pigmentosum

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