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Xeroderma pigmentosum



Xeroderma pigmentosum
Classification & external resources
ICD-10 Q82.1
ICD-9 757.33
DiseasesDB 14198
eMedicine derm/462  neuro/399
MeSH D014983

Xeroderma pigmentosum, or XP, is an autosomal recessive genetic disorder of DNA repair in which the body's normal ability to repair damage caused by ultraviolet (UV) light is deficient. This leads to multiple basaliomas and other skin malignancies at a young age. In severe cases, it is necessary to avoid sunlight completely.

Additional recommended knowledge

Contents

Causes

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 CPDs (Cyclobutane-Pyrimidine-Dimers) and 6-4PP (pyrimidine-6-4-pyrimidone photoproducts). The normal repair process is called nucleotide excision repair. The damage is excised by endonucleases, then the gap is filled by a DNA polymerase and sealed by a ligase. The most common defect in xeroderma pigmentosum is a genetic defect whereby nucleotide excision repair (NER) enzymes are mutated, leading to a reduction in or elimination of NER.

Unrepaired damage can lead to mutations, altering the information of the DNA. If mutations affect important genes, like tumour suppressor genes (e.g. p53) or proto oncogenes then this may lead to cancer. Since in XP patients the frequencies of mutations is much elevated, these patients have a predisposition for cancer.

Types

There are 7 complementation groups, plus one variant form:

Type Diseases Database OMIM Gene Locus Also known as/Description
Type A, I, XPA29877 278700 XPA 9q22.3 Xeroderma pigmentosum group A. Classical form of XP.
Type B, II, XPB 29878 133510 XPB 2q21 Xeroderma pigmentosum group B.
Type C, III, XPC 29879 278720 XPC 3p25 Xeroderma pigmentosum group C.
Type D, IV, XPD 29880 278730 278800 XPD ERCC6 19q13.2-q13.3 , 10q11 Xeroderma pigmentosum group D or De Sanctis-Cacchione syndrome. De Sanctis-Cacchione syndrome can be considered a subtype of XPD.
Type E, V, XPE 29881 278740 DDB2 11p12-p11 Xeroderma pigmentosum group E.
Type F, VI, XPF 29882 278760 ERCC4 16p13.3-p13.13 Xeroderma pigmentosum group F.
Type G, VII, XPG 29883 278780 133530 RAD2 ERCC5 13q33 Xeroderma pigmentosum group G.
Type V, XPV 278750 POLH 6p21.1-p12 Xeroderma pigmentosum variant. XPV patients suffer from mutation in a gene that codes for a specialized DNA polymerase called polymerase-η (eta). Polymerase-η can replicate over the damage and is needed when cells enter S-phase in the presence of a DNA-damage.

Symptoms

Some of the most common symptoms of XP are:

  • An unusually severe sunburn after a short sun exposure. The sunburn may last for several weeks. The sunburn usually occurs during a child’s first sun exposure.
  • Development of many freckles at an early age.
  • Irregular dark spots on the skin.
  • Thin skin.
  • Excessive dryness of skin.
  • 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.
  • Premature aging of skin, lips, eyes, mouth and tongue.

NO

Treatment The most important part of managing the condition is reducing exposure to the sun. The number of keratoses can be reduced with Isotretinoin ([1]) (though there are significant side-effects.) Existing keratoses can be treated using cryotherapy or fluorouracil.[1]

References

     
    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Xeroderma_pigmentosum". A list of authors is available in Wikipedia.
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