Cellular accumulation of DNA double strand breaks (DSBs) has been widely suggested to be a hallmark step leading to premature aging, cellular senescence, and aging. However, how DSB can accumulate in the cells that have intact DSB repair genes remains unknown. Our recent unexpected novel findings on the possible involvement of xeroderma pigmentosum group A (XPA) in the laminopathy-based premature aging, typically Hutchinson-Gilford progeria syndrome (HGPS), open an opportunity to address this challenging question. Interestingly, the same lamin A-related molecular mechanism responsible for HGPS was recently found to be active in human aging. Although XPA, a protein exclusively involved in nucleotide excision repair (NER), has no role in DSB repair, our preliminary data showed that XPA dysfunctionally localizes to DSB sites in laminopathy-based premature aging cells, implying the possible blockage of the recruitment of DSB repair proteins to the damage sites for repair. Strikingly, siRNA knockdown of XPA in the progeroid cells partially restored DSB repair. The goal of this project is to delineate the molecular basis of DSB accumulation and the role of XPA in laminopathy-related premature aging, particularly HGPS, and aging, and to gain a potential major advance in understanding the basic mechanisms of premature aging and aging. We will test the main hypothesis that XPA plays an important role in development of accelerated aging. The investigation will be carried out in five Specific Aims: (1) To elucidate the molecular basis of XPA-DSB mislocalization in progeria cells;and (2) To determine the effects of XPA-DSB mislocalization on cellular DNA damage responses. Due to its novelty and potential impact, this project may involve considerable risk but represents a highly innovative effort with the potential to lead to a breakthrough in understanding the mechanism of premature aging and aging, which fits well into the theme of pilot or feasibility studies of the Small Grant Program. Public Health Relevance: Aging which is a normal, but puzzling biological process, not only governs human lifespan, but also significantly reduces the quality of life due to the development of gerontological diseases. On the other hand, abnormal aging such as premature aging is a typical phenotype of devastating progeria diseases. While it is believed that one of the major causes to premature aging and aging is the accumulation of DNA damage in cells, the underlying molecular mechanism remains elusive. This project represents an initial or pilot effort with a long-term goal aiming to delineate the mechanisms of human premature aging and aging, and to provide a molecular basis for future development of novel strategies for better treatment of progeria and gerontological diseases.