Understanding the Role of Ulp1 as a clock for heterochromatin repair

PROJECT SUMMARY/ABSTRACT: Maintenance of DNA integrity and information is essential for cell viability and genome stability. Various extrinsic and intrinsic sources of DNA damage induce an especially detrimental form of lesion to chromosomes known as double-strand breaks (DSBs). Repairing these breaks in pericentromeric heterochromatin is uniquely challenging. Heterochromatin is mostly composed of repeated DNA sequences, and the availability of up to millions of potential donor sequences associated with different chromosomes can trigger abnormal recombination during homologous recombination (HR) repair. Some of the most important components required for heterochromatin repair are commonly deregulated in cancer and other genome instability disorders, suggesting heterochromatin repair defects as major contributors for these diseases. Understanding the molecular mechanisms of heterochromatin repair and genome stability is essential for understanding how environmental exposure to DNA damaging agents induce cancer and why individual sensitivity varies. We will work with the D. melanogaster cell line model system, where the organization of heterochromatin in a distinct domain and established approaches greatly facilitate the study of the molecular mechanisms involved. We will also extend our studies to mouse and human cells, to establish conserved pathways. We previously identified a unique pathway enabling ‘safe’ HR repair of heterochromatic DSBs, where repair starts inside the heterochromatin domain, but it continues only after relocalization of repair sites to the nuclear periphery. A critical regulator of this pathway is the SUMO protease Ulp1, which is required for restarting repair at the nuclear periphery through unknown targets. We propose to gain insights into this function, by: i) establishing the importance of Ulp1 compartmentalization to the nuclear periphery in the spatial and temporal regulation of heterochromatin repair; ii) identifying functional Ulp1 partners responsible for HR restart at the nuclear periphery; and iii) identifying Ulp1 targets for heterochromatin repair. This work will provide a deeper understanding of the fundamental mechanisms protecting repeated DNAs from massive aberrant recombination and chromosomal rearrangements and illuminate a missing link between HR progression and the stability of repeated DNA sequences. I expect this research will provide a better understanding of the mechanisms through which environmental exposures result in genomic instability and cancer, and to enable the development of better strategies for prevention and treatment.