Xeroderma pigmentosa

The importance of the repair of damaged DNA in keeping the cells alive and preventing cancer is also apparent from mutations in genes involved in nucleotide excision repair. The enzymes involved in nucleotide excision repair are other tools that cells have to repair mismatched DNA. Defects in DNA nucleotide excision repair can cause skin cancer (xeroderma pigmentosa) and colon cancer. 

Xeroderma pigmentosa is a rare, autosomal recessive disease caused by a defective UV-specific endonuclease. Patients with mutations are unable to repair DNA damage caused by sunlight and have been described as children of the night . 

The autosomal recessive hereditary human disease. Xeroderma pigmentosum, can arise from various defects in pyrimidine dimer repair. In one form of the disease, the excinuclease is deficient. The skin of homozygotes is sensitive to the ultraviolet component of sunli t, leading to atrophy of the dermis, keratosis, ulceration and skin cancer. In cultured normal human fibroblasts the half life of pyrimidine dimers is about 24 h, whereas practically no dimers are excised in 24 h in cultured fibroblasts from Xeroderma pigmentosa patients. 

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