DOE Low Dose Radiation Research Program Workshop I Abstracts November 10-12, 1999, Washington, D.C.

18. Mechanisms of DNA Damage Responses to Low Dose Ionizing Radiation: Molecular, Biochemical, and Cellular Studies

P. K. Cooper, D. Chen, M. Meuth, and B. Rydberg
Lawrence Berkeley National Laboratory, Life Sciences Division, Department of Cell and Molecular Biology, Building 74, One Cyclotron Road, Berkeley, CA 94720
pkcooper@lbl.gov

Summary: Identification and characterization of mechanisms and gene products involved in three different crucial pathways for repair of DNA double-strand breaks and other oxidative lesions.

Abstract: Although the importance of genomic surveillance pathways for avoidance of carcinogenesis is clear, the detailed nature and regulation of DNA repair processes responsive to low level ionizing radiation (LLIR) is still largely unknown. The possibility that repair pathways are induced by LLIR is of particular significance for assessment of risk from environmentally relevant doses. For example, failure of low doses to induce repair systems might cause disproportionate genetic damage, whereas continuous exposure to LLIR might be protective relative to risk estimated from linearity. It is therefore essential to critically examine the detailed mechanisms and the regulation of repair systems for LLIR-induced damage. In particular, DNA double-strand breaks (DSBs) produced by IR are presumed to account for its clastogenic effects as well as its high lethality. Thus efficient correct rejoining of DSBs is essential for maintenance of genomic integrity, and cells deficient in DSB rejoining are hypersensitive to genetic damage by LLIR. In addition, a transcription-coupled base excision repair (BER) pathway is critical for mutation avoidance and is implicated in LLIR-inducible BER at oxidative lesions. These two pathways may be interconnected or jointly regulated in response to LLIR. This research program is designed to identify and characterize the proteins and pathways involved in repair of DSBs and base damage induced by LLIR, their associated damage sensing pathways, and the protein- protein interactions critical for their function through three specific aims: (1) to identify and characterize the components of multi-protein complexes for DNA damage sensing and double-strand break repair (DSBR) after LLIR; (2) to investigate cellular parameters affecting the efficiency and fidelity of DSBR at very low levels of damage, including possible inducibility and overlap with transcription-coupled repair (TCR); and (3) to characterize LLIR-inducible base excision repair (BER) and its relationship to the adaptive response, the bystander effect, and both spontaneous and LLIR-induced mutagenesis. Our approach thus includes closely coordinated genetic, molecular and biochemical analyses and in addition employs ultra-sensitive assays for damage detection. These studies will also create the foundation for structure-function analyses of critical repair proteins.