Human cells compromised for p53 function exhibit defective global and transcription-coupled nucleotide excision repair, whereas cells compromised forpRb function are defective only in global repair

After exposure to DNA-damaging agents, the p53 tumor suppressor protects ag ainst neoplastic transformation by inducing growth arrest and apoptosis, A series of investigations has also demonstrated that, in UV-exposed cells, p 53 regulates the removal of DNA photoproducts from the genome overall (glob al nucleotide excision repair), but does not participate in an overlapping pathway that removes damage specifically from the transcribed strand of act ive genes (transcription-coupled nucleotide excision repair). Here, the hig hly sensitive ligation-mediated PCR was employed to quantify, at nucleotide resolution, the repair of UVB-induced cyclobutane pyrimidine dimers (CPDs) in genetically p53-deficient Li-Fraumeni skin fibroblasts, as well as in h uman lung fibroblasts expressing the human papillomavirus (HPV) E6 oncoprot ein that functionally inactivates p53. Lung fibroblasts expressing the HPV E7 gene product, which similarly inactivates the retinoblastoma tumor-suppr essor protein (pRb), were also investigated. pRb acts downstream of p53 to mediate G(1) arrest, but has no demonstrated role in DNA repair. Relative t o normal cells, HPV EG-expressing lung fibroblasts and Li-Fraumeni skin fib roblasts each manifested defective CPD repair along both the transcribed an d nontranscribed strands of the p53 and/or c-jun loci. HPV E7-expressing lu ng fibroblasts also exhibited reduced CPD removal, but only along the nontr anscribed strand. Our results provide striking evidence that transcription- coupled repair, in addition to global repair, are p53-dependent in UV-expos ed human fibroblasts. Moreover, the observed DNA-repair defect in HPV E7-ex pressing cells reveals a function for this oncoprotein in HPV-mediated carc inogenesis, and may suggest a role for pRb in global nucleotide excision re pair.