Chromosomal Single-Strand Break Repair: Mechanisms and Degenerative Disease

Breaks in the genetic material (DNA) can lead to a variety of hereditary and age-related diseases, including cancer and neurodegeneration. Whilst the causal link between DNA breaks and cancer is quite well understood, the causal link between DNA breaks and degeneration of the nervous system is not. The aim of the proposed programme is to address critical unanswered questions and new hypotheses concerning the mechanism/s by which breaks in a single strand of DNA (DNA single-strand breaks) are repaired, and the link between this process and degenerative disease. Understanding how DNA single-strand breaks can cause neurodegeneration is important not only for understanding hereditary diseases associated with neurological dysfunction (and hopefully in future for treating and managing such diseases), but most likely also for understanding degenerative diseases associated with normal ageing. This is because DNA single-strand breaks are the commonest lesions arising in cells and are a product of oxidative stress, which in turn is a major etiological factor in pathologies associated with ageing. Indeed, our recent experiments reveal that loss of the critical DNA single-strand break repair protein XRCC1 results in premature onset of molecular and pathological hallmarks of ageing and, moreover, that un-repaired DNA damage in the brain can trigger these hallmarks, systemically (i.e. in undamaged tissues). In addition, we have identified molecular links between the DNA damage response and the hereditary neurodegenerative disease ataxia oculomotor apraxia-2, which we propose identifies a new component of the cellular control of gene expression in the presence of DNA lesions and can explain the basis of this disease. We will now pursue our recent novel findings and hypotheses in this research Programme. In particular, we will address critical questions concerning the organisation and importance of DNA single-strand break repair in vivo, focussing on a novel unidentified role for XRCC1 and on the mechanism by which this protein is recruited at single-strand breaks, and on how DNA single-strand break repair impacts on hereditary and ageing-related degenerative disease. Ultimately, we envisage this work will identify new avenues for treatment & management of degenerative diseases that are associated, wholly or partly, with DNA breakage.

The aim of the proposed programme is to address critical unanswered questions and new hypotheses concerning mechanism/s of DNA single-strand break repair and the link between this process and human degenerative disease. For example, we will employ molecular and cellular approaches to pursue the new molecular links we have uncovered between the response to DNA strand breaks and the hereditary neurodegenerative disease, ataxia oculomotor apraxia-2. This work will involve the use of siRNA and gene targeting technology in human, DT40, and murine cells, in conjunction with biochemical assays that measure DNA strand break repair and related processes in vitro. The proposed programme is relevant not only to hereditary neurodegenerative disease, however, but also to degenerative pathologies associated with normal human ageing. This is because we have found that loss of the critical DNA single-strand break repair scaffold protein XRCC1 in brain results in premature onset of molecular/pathological hallmarks of ageing, systemically. We will thus also pursue these findings in the new Programme, using not only the cellular model systems described above but also physiological approaches. Finally, we will address key unanswered questions concerning the organisation and importance of DNA single-strand break repair in vivo, focussing on a critical but unidentified role for XRCC1, and on how XRCC1 function is modulated within the context of chromatin by the proteins APLF and PARP-1. To do this, we will again employ a combination of biochemical, cellular, and physiological approaches. Together, the experiments proposed in this research Programme will enable us to address both critical unanswered questions concerning cellular mechanisms of DNA single-strand break repair and also the pathophysiological consequences of not repairing such breaks.

Beyond academia and related research fields, the work in this Programme has the potential to impact, in the long-term, on the health sector and 'quality of life'/'Lifelong Health and Wellbeing'. This is because the Programme will address the possible impact of DNA single-strand breaks on the maintenance of normal neurological function and on molecular hallmarks/pathologies associated with normal ageing. For example, the programme will investigate the link between un-repaired DNA strand breaks and progressive, debilitating, degeneration of the nervous system in the hereditary genetic disease, ataxia oculomotor apraxia-2. In addition, the programme will investigate the link between un-repaired single-strand breaks in the brain and consequent down-regulation of growth hormone signalling in the liver; a molecular hallmark of normal ageing that can influence pathologies such as osteoporosis. A thorough understanding of the relationship between un-repaired SSBs and pathologies associated with hereditary neurodegenerative disease and with age could thus help inform the health sector in respect to etiological factors that promote degenerative disease and 'unhealthy' ageing (e.g. environmental factors that induce excessive SSBs). This research could thus, in the long-term, inform on environmental and life-style issues relating to 'quality of life' and 'Lifelong Health and Wellbeing'.