XPA confers ability of endonucleases to act processively and to incise damaged nucleosomal DNA

Repair of damaged DNA is a complex process, particularly when it is compacted into nucleosomes. There are a number of genetic disorders with deficiencies in DNA repair. Knowledge of the genes and proteins involved in these repair deficiencies is critical in developing an understanding of the molecular mechanisms utilized by proteins in the DNA repair pathways. One of these genetic disorders is xeroderma pigmentosum (XP), which is defective in nucleotide excision repair (NER). Patients in XP complementation group A (XP-A) are among the most severely affected with the lowest levels of DNA repair. The XPA protein, which is defective in these patients, plays a number of roles in the DNA repair process. One particularly important role proposed is acting as a processivity factor enabling endonucleases (XPF and XPG) and the XPB/TFIIH translocase to localize to damage sites using a processive mechanism of action. Another proposed role is in interacting with chromatin-remodeling proteins so as to enhance accessibility of lesions in nucleosomal DNA to endonucleolytic incision and other DNA repair activities. In XP-A cells, the XPA protein is proposed to be defective in ability to act as a processivity factor; endonucleases localize damage sites by a distributive mechanism and are also defective in incision of damaged nucleosomal DNA. This defect is corrected by recombinant normal human XPA. Mutations in exons 3 and 5 in the DNA binding domain of the XPA gene lead to loss of ability of XPA to act as a processivity factor. The mutation in exon 5 was found in two XP-A patients with severe XP. These studies emphasize the importance of correlating specific mutations in an XP gene and the resulting defect in a particular repair protein with the clinical severity of XP and could lead to development of novel therapeutic approaches for this disorder.