Common fragile sites are characterised by faulty condensin loading after replication stress

Cells coordinate interphase to mitosis transition but recurrent cytogenetic lesions appear at common fragile sites (CFSs) in a tissue-specific manner following replication stress, marking regions of instability in cancer. Despite such a distinct defect no model fully explains their molecular configuration. We show that CFSs are characterised by impaired chromatin folding manifested as disrupted mitotic structures visible using molecular FISH probes in the presence and absence of replication stress. Chromosome condensation assays reveal that compaction-resistant chromatin lesions persist at CFSs throughout the cell cycle and mitosis. Subsequently cytogenetic and molecular lesions arise due to faulty condensin loading at CFSs, through a defect in condensin I mediated compaction and are coincident with mitotic DNA synthesis (MIDAS). This model suggests that in conditions of exogenous replication stress, aberrant condensin loading leads to molecular defects and CFS formation, concomitantly providing an environment for MIDAS, which, if not resolved, result in chromosome instability.