Transcription-Associated Breaks in Xeroderma Pigmentosum Group D Cells from Patients with Combined Features of Xeroderma Pigmentosum and Cockayne Syndrome

Abstract

Defects in the XPD gene can result in several clinical phenotypes, including xeroderma pigmentosum (XP), trichothiodystrophy, and, less frequently, the combined phenotype of XP and Cockayne syndrome (XP-D/CS). We previously showed that in cells from two XP-D/CS patients, breaks were introduced into cellular DNA on exposure to UV damage, but these breaks were not at the sites of the damage. In the present work, we show that three further XP-D/CS patients show the same peculiar breakage phenomenon. We show that these breaks can be visualized inside the cells by immunofluorescence using antibodies to either γ-H2AX or poly-ADP-ribose and that they can be generated by the introduction of plasmids harboring methylation or oxidative damage as well as by UV photoproducts. Inhibition of RNA polymerase II transcription by four different inhibitors dramatically reduced the number of UV-induced breaks. Furthermore, the breaks were dependent on the nucleotide excision repair (NER) machinery. These data are consistent with our hypothesis that the NER machinery introduces the breaks at sites of transcription initiation. During transcription in UV-irradiated XP-D/CS cells, phosphorylation of the carboxy-terminal domain of RNA polymerase II occurred normally, but the elongating form of the polymerase remained blocked at lesions and was eventually degraded.

ACKNOWLEDGMENTS

We are grateful to E. Seeberg for the Tag and Mag1 enzymes, to M. Berneburg for help with early experiments, and to R. Wood for helpful suggestions.

This work was supported by EC contracts QLG1-CT-1999-00181 and MRTN-CT-2003-503618 and an MRC programme grant to A.R.L. and by grants from the Associazione Italiana per la Ricerca sul Cancro (AIRC), the Italian Ministry of Health (Ricerca Finalizzata), and the MIUR (FIRB grant RBNE01RNN7) to M.S.