Visualization of chromatin events during DNA excision repair in XP cells: deficiency in localized but not generalized chromatin events.

Cells derived from patients with xeroderma pigmentosum (XP) are known to be compromised in excision repair after u.v. irradiation, but the exact site of the molecular defect in the repair pathway is not known. The purpose of this study was to examine the ability of XP cells to alter their chromatin in preparation for excision repair. Treatment of quiescent normal human fibroblasts with u.v. light results in two types of chromatin changes that can be visualized in G1 phase prematurely condensed chromosomes (PCC): a generalized elongation of the G1 PCC, and regions of localized elongation or gaps. Quiescent XP cells (complementation groups A, C, D and G) were treated with u.v. light, and their G1 PCC were examined for chromatin changes in response to damage. All cell lines tested showed a normal generalized chromatin elongation response to u.v. treatment. However, the XP cell lines were found to be deficient in the generation of localized regions of decondensation, even when incubated after u.v. treatment in the presence of cytosine arabinoside and hydroxyurea. Since the regions of localized decondensation in the G1 PCC have been previously shown to be the sites of unscheduled DNA synthesis, these results suggest that while XP cells do exhibit a generalized chromatin response to u.v. irradiation, they are defective in their ability to alter the chromatin in a localized fashion in preparation for excision repair after u.v. irradiation.