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Xeroderma pigmentosa, 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. This disorder leads to multiple basaliomas and other skin malignancies at a young age. In severe cases, it is necessary to avoid sunlight completely. The two most common causes of death for XP victims are metastatic malignant melanoma and squamous cell carcinoma[1]. XP is about six times more common in Japanese people[1] than in other groups.
Fictional characters with severe cases of XP sometimes play prominent roles in books and movies. Some of the most notable characters include Christopher Snow, the protagonist and narrator of Dean Koontz’ Moonlight Bay Trilogy of novels, and the children of the protagonist of Alejandro Amenábar’s 2001 film, The Others.
The most common defect in xeroderma pigmentosum is an autosomal recessive genetic defect whereby nucleotide excision repair (NER) enzymes are mutated, leading to a reduction in or elimination of NER.[2] Unrepaired damage can lead to mutations, altering the information of the DNA in individual cells. If mutations affect important genes, like tumour suppressor genes (e.g. p53) or proto oncogenes then this disorder may lead to cancer. Patients exhibit elevated risk of developing cancer, such as basal cell carcinoma.
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 CPD’s (cyclobutane-pyrimidine-dimers) and 6-4PP’s (pyrimidine-6-4-pyrimidone photoproducts). The normal repair process entails nucleotide excision. The damage is excised by endonucleases, then the gap is filled by a DNA polymerase and “sealed” by a ligase.
There are 7 complementation groups, plus one variant form :
Some of the most common symptoms of XP include:
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