Principal Investigator: Wu, Xiaohua Project Summary Studying the mechanisms underlying the protection of common fragile sites and structure- prone DNA sequences Project Summary/Abstract Common fragile sites (CFSs) are large chromosomal regions that often exhibit gaps and breaks on metaphase chromosomes upon replication stress. Structure-prone AT-rich sequences present at CFSs (CFS-ATs) contribute to CFS instability. Besides CFS-ATs, other structure-prone DNA sequences, such as G-quadruplexes (G4s), are also abundant in the human genome and are associated with chromosomal rearrangement breakpoints in cancer. Since CFSs and many structure-prone DNA sequences, including G4s, are part of normal chromosomal structures, it is important to understand how the integrity of these structure-prone DNA sequences is maintained in mammalian cells. In this grant application, we will use EGFP-based DSB repair reporters to investigate the mechanisms underlying the protection of structure-prone DNA sequences present at CFSs and G4s. We will study the role of chromatin-remodeling in the maintenance of CFSs and address the DSB repair mechanism specifically used to repair DSBs arising at DNA secondary structures upon fork collapse. We will also explore the functional coordination of pathways involved in protecting structure-prone DNA sequences from DSB formation and in repairing DSBs generated upon fork collapse at structure-prone DNA sequences. Furthermore, we will study how mismatch repair (MMR) proteins help preserve the integrity of structure-prone DNA sequences. Our studies will yield molecular insights into the mechanisms underlying the maintenance of the integrity of CFS and other structure-prone DNA sequences. These studies will also lay the groundwork for development of new targeted cancer treatment strategies.