Abstract:
Chromatin bridges are strings of chromatin connecting the anaphase poles or daughter nuclei and have been linked to tumourigenesis. Chromatin bridges can arise from segregation of interlinked chromosomes after improper resolution of double strand DNA catenates, or from dicentric chromosomes generated by end-to-end chromosome fusions. In response to chromatin bridges, the abscission checkpoint delays completion of cytokinesis (abscission) to prevent chromosome breakage or tetraploidization; however, how chromatin bridges are detected by the abscission checkpoint has not been previously reported. Here, we show that spontaneous or replication stress-induced chromatin bridges exhibit “knots” of catenated and overtwisted DNA next to the midbody. Topoisomerase IIα (Top2α), an enzyme that can relax DNA supercoils and untangle catenated DNA molecules by catalyzing passage of one double-stranded DNA molecule through a Top2-linked double-stranded break in another DNA molecule, forms abortive Top2-DNA cleavage complexes (Top2ccs) on DNA knots. Furthermore, impaired Top2α-DNA cleavage activity correlates with chromatin bridge breakage in cytokinesis. Proteasomal degradation of Top2ccs is required for localization of the DNA damage sensor protein Rad17 to Top2α-generated double strand DNA ends on DNA knots. In turn, Rad17 promotes local recruitment of the MRN (Mre11-Rad50-Nbs1) protein complex and downstream ATM-Chk2-INCENP signaling to delay abscission and prevent chromatin breakage. In contrast, dicentric chromosomes that do not exhibit knotted DNA fail to recruit Top2α next to the midbody and to activate the abscission checkpoint in human cells. These findings are the first to describe a mechanism by which the abscission checkpoint senses chromatin bridges, through generation of abortive Top2ccs on DNA knots, to preserve genome integrity.
Audience Take Away Notes:
- Chromatin bridges
- Abscission checkpoint signaling
- Mechanisms that maintain genome integrity in cytokinesis
