Our overall objective is to understand how processing of damaged DNA relates to human genetic disease, cancer and aging. Having pioneered the discovery of nucleotide excision repair (NER), we are elucidating the sub-pathways of global genomic repair (GGR) and transcription-coupled repair (TCR). The TCR-deficient diseases, Cockayne syndrome (CS) and UV-sensitive syndrome (UVSS), present indistinguishable biochemical responses to UV;UVSS patients have only superficial consequences of sunburn while those with CS suffer severe neurological/developmental defects, segmental progeria and early death. Notably, no cancers of any type have been reported for patients with these syndromes. We hypothesize that the severe features of CS are due to apoptosis triggered by prolonged transcription arrest as a consequence of defective TCR of oxidative base damage or to defective transcriptional bypass of such damage generated by endogenous reactive oxygen species, while UVSS cells are normal with respect to processing base damage in expressed genes. In support of this model we find that CS cells are hypersensitive to oxidants, and that UVSS but not CS cells are proficient in host cell reactivation of plasmids containing oxidized bases. However, definitive biochemical evidence for TCR of oxidized bases is lacking. Our model for TCR postulates that an arrested RNA polymerase (RNAP) recruits repair enzymes to transcription- blocking lesions. Reported studies with an oxidized base positioned at a unique site in the DNA template strand indicate that RNAP can bypass, transiently pause or arrest at these lesions. We propose to use a novel transcription assay with multiple randomly-positioned lesions induced in the template, to let the transcription system tell us which lesions and which sequence contexts are most relevant for further analysis. After determining the types and positions of the lesions that cause arrest in vitro, we will construct single-lesion vectors for transfection into human cells to measure in vivo transcription rates upstream and downstream of the lesion;sequencing the transcripts will reveal transcriptional mutagenesis. We propose to develop the sensitive Comet-FISH approach with gene-specific probes to comparatively quantify low levels of particular oxidative lesions and their removal from transcribed or silent sequences and from the genome overall. Cells with missing or reduced base excision repair or NER activities will be employed to investigate processing of oxidative lesions. Specific enhancement of 8oxoG in DNA will be achieved by interference RNA-mediated MTH1 knockdown, to eliminate complications of other lesions and other oxidative effects. We will focus on differences between CS and UVSS as a model system to elucidate the role of processing of oxidative base damage in aging, disease and neurological degeneration, as well as the underlying cause of the cancer-resistance of these syndromes.
Free radicals from endogenous and environmental sources are a constant threat to genomic integrity. The induced damage can arrest DNA and RNA polymerases, events that can unleash irreversible apoptotic pathways or mutagenicity. We propose novel approaches for elucidation of the effects of oxidative DNA lesions on transcription, and for the analysis of repair of physiologically relevant levels of these lesions in transcriptionally active or silent genomic domains and in the genome overall, using the Comet- FISH assay. Results from the project will advance our understanding of cellular processes leading to carcinogenesis, aging, and other pathologies. They will also further the development of effective strategies for therapeutic intervention in human disease.
|Spivak, Graciela; Ganesan, Ann K (2014) The complex choreography of transcription-coupled repair. DNA Repair (Amst) 19:64-70|
|Guo, Jia; Hanawalt, Philip C; Spivak, Graciela (2013) Comet-FISH with strand-specific probes reveals transcription-coupled repair of 8-oxoGuanine in human cells. Nucleic Acids Res 41:7700-12|