Elucidating the role of DNA double-strand breaks in aging

Abstract

Aging has long been associated with genome instability. DNA double-strand breaks (DSBs) are highly toxic lesions that arise frequently in mammalian cells, due to endogenous or exogenous agents, and are considered a main contributor to genomic instability. Mammals have evolved well-organized machinery, known as the DNA damage response (DDR), which is fine-tuned to detect and repair damage such as DSBs. The DDR has three potentially adverse cellular outcomes after sensing a DSB: (1) self-elimination, called apoptosis; (2) irreversible cell cycle arrest, known as senescence; and (3) aberrant repair of the break, leading to mutations. Apoptosis and senescence could explain aging phenotypes, such as atrophy and stem cell decline while mutations are generally considered the cause of cancer, a major age-related disease, or transcriptional dysregulation. Evidence has shown that clastogenic agents known to induce DSBs are linked to aging. Furthermore, mice deficient in proteins involved in DSB repair have also been shown to suffer from premature aging phenotypes. However, no study has directly shown that DSBs, in the context of normal DDR mechanisms, can accelerate aging. Here, we have directly tested whether DSBs alone can drive an aging phenotype using a unique restriction enzyme-based system to induce clean breaks within young mouse livers. Results unequivocally show that DSBs are capable of causing a distinct subset of canonical aging pathologies, markers of senescence, and alterations in transcriptional profiles. We further explored these data to understand the transcriptional landscape that underlies normal aging. Here, we were able to show that aging is characterized by increased transcriptional variation as well as broad expression alterations in every component of the transcriptome, and we were able to construct large interaction networks of the altered non-coding RNAs and protein- coding transcripts to reveal dominant aging phenotypes in mouse liver. Finally, we asked whether genes implicated in longevity determination are capable of modulating the effects of DNA damage, a potential mechanism to confer resistance to genomic instability over time. Here, we found that FOXO3A, a transcription factor involved in a diverse range of cellular functions, such as metabolic and stress regulation, is capable of suppressing mutation accumulation in response to DNA damage induced by bleomycin. Furthermore, we show that FOXO3A is involved in mitigating the cell cycle and accelerates DNA damage foci clearance. Together, our findings presented within this thesis highlight the role that DNA DSBs play in driving mammalian aging, while also providing a comprehensive transcriptional profile of aging mouse liver, and propose a mechanism whereby FOXO3A, a longevity-associated gene, is capable of promoting genome maintenance in response to genome instability.