Knotty Problems during Mitosis: Mechanistic Insight into the Processing of Ultrafine DNA Bridges in Anaphase

Abstract

To survive and proliferate, cells have to faithfully segregate their newly replicated genomic DNA to the two daughter cells. However, the sister chromatids of mitotic chromosomes are frequently interlinked by so-called ultrafine DNA bridges (UFBs) that are visible in the anaphase of mitosis. UFBs can only be detected by the proteins bound to them and not by staining with conventional DNA dyes. These DNA bridges are presumed to represent entangled sister chromatids and hence pose a threat to faithful segregation. A failure to accurately unlink UFB DNA results in chromosome segregation errors and binucleation. This, in turn, compromises genome integrity, which is a hallmark of cancer. UFBs are actively removed during anaphase, and most known UFB-associated proteins are enzymes involved in DNA repair in interphase. However, little is known about the mitotic activities of these enzymes or the exact DNA structures present on UFBs. We focus on the biology of UFBs, with special emphasis on their underlying DNA structure and the decatenation machineries that process UFBs.

DNA Replication Stress and Chromosomal Instability: Dangerous Liaisons

“…c-UFBs and r-UFBs arise from the formation of double-stranded DNA catenanes (i.e., completely replicated intertwined DNA) driven by the repetitive nature and sequence content of centromeres and rDNA loci. On the other hand, CFS-UFBs and t-UFBs are mainly derived from under-replicated DNA or late-replication intermediates, and the newly identified HR-UFBs are derived from persisting recombination intermediates [82,83,86]. A complex composed of BLM and its binding partners, topoisomerase IIIα and RecQ-mediated genome instability protein 1 (RMI1) and RMI2 (BTRR), together with PICH (DNA translocase) and topoisomerase II, act by untangling structures underlying different types of UFBs, facilitated by additional factors such as the DNA translocase FANCM, RIF1 and TOPBP1, which are recruited to a subset of UFBs [87][88][89][90].…”

“…Similar to its role in c-UFBs, topoisomerase II is recruited by PICH to r-UFBs. It is believed that the transcription that overlaps mitotic chromosome condensation and hinders topoisomerase II activity drives r-UFBs [86,93].…”

Reconstitution of anaphase DNA bridge recognition and disjunction

“…CFSs are difficult-to-replicate regions that form detectable gaps or breaks on metaphase chromosomes 4 and are hotspots for chromosomal rearrangements in cancer cells 5 . Unresolved DNA interlinks that persist into anaphase generate DNA structures called UFBs [6][7][8] . Although present in essentially every anaphase, UFBs have remained undetected for decades as a result of being both histonefree and refractory to staining with commonly used dyes such as DAPI.…”

Identification of candidate molecular targets of the novel antineoplastic antimitotic NP-10

“…Such collisions may stall replication forks, thereby causing hyper-replication stress. Replication stress also causes mitotic aberrations such as chromatin bridges, lagging chromosomes, and ultrafine bridges 18,19 . Therefore, we hypothesized that oncogenic hyperstimulation affects the sensitivity to NP-10.…”