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

Abstract

Repair of damaged DNA is a complex process, particularly when it is compacted into nucleosomes. There are a number of genetic disorders in which there are deficiencies in DNA repair. Knowledge of the genes and proteins involved in the DNA repair defects in these disorders is of major importance in developing an understanding of the molecular mechanisms utilized by proteins playing a role in these 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 is acting as a processivity factor enabling endonucleases involved in NER to localize sites of damage using a processive mechanism of action, which is also critical for endonucleolytic incision of damaged nucleosomal DNA. In XP-A cells, the XPA protein is defective in ability to act as a processivity factor and the endonucleases localize damage sites by a distributive mechanism and are defective in ability to incise damaged nucleosomal DNA. This defect can be corrected by expression of a recombinant normal human XPA protein in XP-A cells. 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 is found in two XP-A patients with very severe XP. These studies, as well as others, 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 therapeutic approaches for this disorder.