A new class of ultrafine anaphase bridges generated by homologous recombination

Abstract

Ultrafine anaphase bridges (UFBs) are a potential source of genome instability that is a hallmark of cancer. UFBs can arise from DNA catenanes at centromeres/rDNA loci, late replication intermediates induced by replication stress, and DNA linkages at telomeres. Recently, it was reported that DNA intertwinements generated by homologous recombination give rise to a new class of UFBs, which have been termed homologous recombination ultrafine bridges (HR-UFBs). HR-UFBs are decorated with PICH and BLM in anaphase, and are subsequently converted to RPA-coated, single-stranded DNA bridges. Breakage of these sister chromatid entanglements leads to DNA damage that can be repaired by non-homologous end joining in the next cell cycle, but the potential consequences include DNA rearrangements, chromosome translocations and fusions. Visualisation of these HR-UFBs, and knowledge of how they arise, provides a molecular basis to explain how upregulation of homologous recombination or failure to resolve recombination intermediates leads to the development of chromosomal instability observed in certain cancers.

DNA Replication Stress and Chromosomal Instability: Dangerous Liaisons

“…Indeed, FANCD2 was localized to APH-induced gaps and breaks on mitotic chromosomes, where it colocalized with γ-H2AX, a marker of DBSs [76,77]. In addition, by cooperating with the helicase BLM, FANCD2 was shown to promote the resolution of ultrafine bridges (UFBs) [77], thin DAPI-negative DNA threads that evolve from joint DNA molecules, arising from replication or recombination intermediates that persist until mitosis and are resolved during anaphase and telophase [81][82][83][84].…”

“…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].…”

“…In late anaphase, HR-UFBs are converted by PICH and BLM into RPA-bound ssDNA that is susceptible to breaks [96]. Camptothecin, a topoisomerase I inhibitor, increases HR-UFB levels, indicating that DSBs are required for the formation of the recombination intermediates that induce HR-UFBs [37,83].…”

Cytokinesis-Block Micronucleus Cytome Assay Evolution into a More Comprehensive Method to Measure Chromosomal Instability

“…Reports have recently emerged indicating that mutations in genes that affect cohesin, condensin, and separase functions, which control chromatid separation during mitosis, cause multiple anaphase bridges similar to those that may result in formation of FUS, CIR, and HS nuclei [ 45 , 46 , 47 , 48 ]. Furthermore, a proportion of the multiple narrow bridges observed between the nuclei of FUS cells may also be ultra-fine bridges (UFBs), which can arise from DNA catenanes at centromeres/rDNA loci, late replication intermediates induced by replication stress, and DNA linkages at telomeres [ 49 ]. However, this has not been previously tested and would require the detection of proteins that specifically bind UFBs, including Bloom’s syndrome helicase (BLM), PLK1-interacting checkpoint helicase (PICH) and replication protein A (RPA), to verify their involvement [ 49 ].…”

“…Furthermore, a proportion of the multiple narrow bridges observed between the nuclei of FUS cells may also be ultra-fine bridges (UFBs), which can arise from DNA catenanes at centromeres/rDNA loci, late replication intermediates induced by replication stress, and DNA linkages at telomeres [ 49 ]. However, this has not been previously tested and would require the detection of proteins that specifically bind UFBs, including Bloom’s syndrome helicase (BLM), PLK1-interacting checkpoint helicase (PICH) and replication protein A (RPA), to verify their involvement [ 49 ].…”