Replication Timing and Transcription Identifies a Novel Fragility Signature Under Replication Stress

Common fragile sties (CFSs) are regions susceptible to replication stress and are hotspots for chromosomal instability in cancer. Several features characterizing CFSs have been associated with their instability, however, these features are prevalent across the genome and do not account for all known CFSs. Therefore, the molecular mechanism underlying CFS instability remains unclear. Here, we explored the transcriptional profile and temporal order of DNA replication (replication timing, RT) of cells under replication stress conditions. We show that the RT of only a small portion of the genome is affected by replication stress, and that CFSs are enriched for delayed RT. We identified a signature for chromosomal fragility, comprised of replication stress-induced delay in RT of early/mid S-phase replicating regions within actively transcribed large genes. This fragility signature enabled precise mapping of the core fragility region. Furthermore, the signature enabled the identification of novel fragile sites that were not detected cytogenetically, highlighting the improved sensitivity of our approach for identifying fragile sites. Altogether, this study reveals a link between altered DNA replication and transcription of large genes underlying the mechanism of CFS expression. Thus, investigating the RT and transcriptional changes in cancer may contribute to the understanding of mechanisms promoting genomic instability in cancer.