On the regulation, function, and localization of the DNA-dependent ATPase PICH

PICH impacts the spindle assembly checkpoint via its DNA translocase and SUMO-interaction activities

Either inhibiting or stabilizing SUMOylation in mitosis causes defects in chromosome segregation, suggesting that dynamic mitotic SUMOylation of proteins is critical to maintain integrity of the genome. Polo-like kinase 1-interacting checkpoint helicase (PICH), a mitotic chromatin remodeling enzyme, interacts with SUMOylated chromosomal proteins via three SUMO-interacting motifs (SIMs) to control their association with chromosomes. Using cell lines with conditional PICH depletion/PICH replacement, we revealed mitotic defects associated with compromised PICH functions toward SUMOylated chromosomal proteins. Defects in either remodeling activity or SIMs of PICH delayed mitotic progression caused by activation of the spindle assembly checkpoint (SAC) indicated by extended duration of Mad1 foci at centromeres. Proteomics analysis of chromosomal SUMOylated proteins whose abundance is controlled by PICH activity identified candidate proteins to explain the SAC activation phenotype. Among the identified candidates, Bub1 kinetochore abundance is increased upon loss of PICH. Our results demonstrated a novel relationship between PICH and the SAC, where PICH directly or indirectly affects Bub1 association at the kinetochore and impacts SAC activity to control mitosis.

PICH, A protein that maintains genomic stability, can promote tumor growth

Genomic instability is regardedas a hallmark of cancer cells. It can be presented in many ways, among which chromosome instability has received attention. Ultrafine anaphase bridges are a typeof chromatin bridges, the untimely resolution of which can also lead to chromosome instability. PICH can play a role in maintaining chromosome stability by regulating chromosome morphologyand resolving ultrafine anaphase bridges. Recently, PICH has been found to be overexpressed in various cancers. Overexpression of PICH is related to the proliferation of tumors and poor prognosis. In this article, we consider that PICH can maintain genomic stability by regulating appropriate chromosome structure, ensuring proper chromosome segregation, and facilitating replication fork reversal. We summarize how PICH regulates chromosome stability, how PICH resolves Ultrafine anaphase bridges with other proteins, and how PICH promotes tumor progression.

The interplay of the translocase activity and protein recruitment function of PICH in ultrafine anaphase bridge resolution and genomic stability

Incomplete sister centromere decatenation results in centromeric ultrafine anaphase bridges (UFBs). PICH (PLK1-interacting checkpoint helicase), a DNA translocase, plays a crucial role in UFB resolution by recruiting UFB-binding proteins and stimulating topoisomerase IIα. However, the involvement of distinct PICH functions in UFB resolution remains ambiguous. Here, we demonstrate that PICH depletion in non-transformed diploid cells induces DNA damage, micronuclei formation, p53 activation, G1-phase delay and cell death. Whole-genome sequencing reveals that segregation defects induced by PICH depletion cause chromosomal rearrangements, including translocations and inversions, emphasizing its significance in preserving genomic integrity. Furthermore, a PICH mutant that impairs UFB recruitment of BLM and RIF1 partially inhibits UFB resolution while a translocase-inactive mutant (PICHK128A) fails to resolve UFBs. Notably, expression of PICHK128A inhibits single-stranded UFB formation and induces hypocondensed chromosomes. We propose that PICH's translocase activity plays a dual role in promoting UFB resolution by facilitating the generation of single-stranded UFBs and stimulating topoisomerase IIα.

RAD52 and ERCC6L/PICH have a compensatory relationship for genome stability in mitosis

Mammalian RAD52 is a DNA repair factor with strand annealing and recombination mediator activities that appear important in both interphase and mitotic cells. Nonetheless, RAD52 is dispensable for cell viability. To query RAD52 synthetic lethal relationships, we performed genome-wide CRISPR knock-out screens and identified hundreds of candidate synthetic lethal interactions. We then performed secondary screening and identified genes for which depletion causes reduced viability and elevated genome instability (increased 53BP1 nuclear foci) in RAD52-deficient cells. One such factor was ERCC6L, which marks DNA bridges during anaphase, and hence is important for genome stability in mitosis. Thus, we investigated the functional interrelationship between RAD52 and ERCC6L. We found that RAD52 deficiency increases ERCC6L-coated anaphase ultrafine bridges, and that ERCC6L depletion causes elevated RAD52 foci in prometaphase and interphase cells. These effects were enhanced with replication stress (i.e. hydroxyurea) and topoisomerase IIα inhibition (ICRF-193), where post-treatment effect timings were consistent with defects in addressing stress in mitosis. Altogether, we suggest that RAD52 and ERCC6L co-compensate to protect genome stability in mitosis.

RAD52 and ERCC6L/PICH have a compensatory relationship for genome stability in mitosis

The mammalian RAD52 protein is a DNA repair factor that has both strand annealing and recombination mediator activities, yet is dispensable for cell viability. To characterize genetic contexts that reveal dependence on RAD52 to sustain cell viability (i.e., synthetic lethal relationships), we performed genome-wide CRISPR knock-out screens. Subsequent secondary screening found that depletion of ERCC6L in RAD52-deficient cells causes reduced viability and elevated genome instability, measured as accumulation of 53BP1 into nuclear foci. Furthermore, loss of RAD52 causes elevated levels of anaphase ultrafine bridges marked by ERCC6L, and conversely depletion of ERCC6L causes elevated RAD52 foci both in prometaphase and interphase cells. These effects were enhanced with combination treatments using hydroxyurea and the topoisomerase IIα inhibitor ICRF-193, and the timing of these treatments are consistent with defects in addressing such stress in mitosis. Thus, loss of RAD52 appears to cause an increased reliance on ERCC6L in mitosis, and vice versa. Consistent with this notion, combined depletion of ERCC6L and disrupting G2/M progression via CDK1 inhibition causes a marked loss of viability in RAD52-deficient cells. We suggest that RAD52 and ERCC6L play compensatory roles in protecting genome stability in mitosis.

RIF1 suppresses the formation of single-stranded ultrafine anaphase bridges via protein phosphatase 1

Resolution of ultrafine anaphase bridges (UFBs) must be completed before cytokinesis to ensure sister-chromatid disjunction. RIF1 is involved in UFB resolution by a mechanism that is not yet clear. Here, we show that RIF1 functions in mitosis to inhibit the formation of 53BP1 nuclear bodies and micronuclei. Meanwhile, RIF1 localizes on PICH-coated double-stranded UFBs but not on RPA-coated single-stranded UFBs. Depletion of RIF1 leads to an elevated level of RPA-coated UFBs, in a BLM-dependent manner. RIF1 interacts with all three isoforms of protein phosphatase 1 (PP1) at its CI domain in anaphase when CDK1 activity declines. CDK1 negatively regulates RIF1-PP1 interaction via the CIII domain of RIF1. Importantly, depletion of PP1 phenocopies RIF1 depletion, and phosphorylation-resistant mutant of PICH shows reduced interaction with the BTR complex and bypasses the need of RIF1 in preventing the formation of single-stranded UFBs. Overall, our data show that PP1 is the effector of RIF1 in UFB resolution.

PICH acts as a force-dependent nucleosome remodeler

In anaphase, any unresolved DNA entanglements between the segregating sister chromatids can give rise to chromatin bridges. To prevent genome instability, chromatin bridges must be resolved prior to cytokinesis. The SNF2 protein PICH has been proposed to play a direct role in this process through the remodeling of nucleosomes. However, direct evidence of nucleosome remodeling by PICH has remained elusive. Here, we present an in vitro single-molecule assay that mimics chromatin under tension, as is found in anaphase chromatin bridges. Applying a combination of dual-trap optical tweezers and fluorescence imaging of PICH and histones bound to a nucleosome-array construct, we show that PICH is a tension- and ATP-dependent nucleosome remodeler that facilitates nucleosome unwrapping and then subsequently slides remaining histones along the DNA. This work elucidates the role of PICH in chromatin-bridge dissolution, and might provide molecular insights into the mechanisms of related SNF2 proteins.

PICH Supports Embryonic Hematopoiesis by Suppressing a cGAS-STING-Mediated Interferon Response

The Plk1-interacting checkpoint helicase (PICH) protein localizes to ultrafine anaphase DNA bridges in mitosis along with a complex of DNA repair proteins. Previous studies show PICH deficiency-induced embryonic lethality in mice. However, the function of PICH that is required to suppress embryonic lethality in PICH-deficient mammals remains to be determined. Previous clinical studies suggest a link between PICH deficiency and the onset of acquired aplastic anemia. Here, using Pich knock-out (KO) mouse models, the authors provide evidence for a mechanistic link between PICH deficiency and defective hematopoiesis. Fetal livers from Pich-KO embryos exhibit a significantly elevated number of hematopoietic stem cells (HSCs); however, these HSCs display a higher level of apoptosis and a much-reduced ability to reconstitute a functional hematopoietic system when transplanted into lethally irradiated recipients. Moreover, these HSCs show an elevated cytoplasmic dsDNA expression and an activation of the cGAS-STING pathway, resulting in excessive production of type I interferons (IFN). Importantly, deletion of Ifnar1 or cGAS reverses the defective hematopoiesis. The authors conclude that loss of PICH results in defective hematopoiesis via cGAS-STING-mediated type I IFN production.

Regulation of mitotic chromosome architecture and resolution of ultrafine anaphase bridges by PICH

To ensure genome stability, chromosomes need to undergo proper condensation into two linked sister chromatids from prophase to prometaphase, followed by equal segregation at anaphase. Emerging evidence has shown that persistent DNA entanglements connecting the sister chromatids lead to the formation of ultrafine anaphase bridges (UFBs). If UFBs are not resolved soon after anaphase, they can induce chromosome missegregation. PICH (PLK1-interacting checkpoint helicase) is a DNA translocase that localizes on chromosome arms, centromeres and UFBs. It plays multiple essential roles in mitotic chromosome organization and segregation. PICH also recruits other associated proteins to UFBs, and together they mediate UFB resolution. Here, the proposed mechanism behind PICH's functions in chromosome organization and UFB resolution will be discussed. We summarize the regulation of PICH action at chromosome arms and centromeres, how PICH recognizes UFBs and recruits other UFB-associated factors, and finally how PICH promotes UFB resolution together with other DNA processing enzymes.

CENP-A chromatin prevents replication stress at centromeres to avoid structural aneuploidy

Chromosome segregation relies on centromeres, yet their repetitive DNA is often prone to aberrant rearrangements under pathological conditions. Factors that maintain centromere integrity to prevent centromere-associated chromosome translocations are unknown. Here, we demonstrate the importance of the centromere-specific histone H3 variant CENP-A in safeguarding DNA replication of alpha-satellite repeats to prevent structural aneuploidy. Rapid removal of CENP-A in S phase, but not other cell-cycle stages, caused accumulation of R loops with increased centromeric transcripts, and interfered with replication fork progression. Replication without CENP-A causes recombination at alpha-satellites in an R loop-dependent manner, unfinished replication, and anaphase bridges. In turn, chromosome breakage and translocations arise specifically at centromeric regions. Our findings provide insights into how specialized centromeric chromatin maintains the integrity of transcribed noncoding repetitive DNA during S phase.

Anaphase Bridges: Not All Natural Fibers Are Healthy

At each round of cell division, the DNA must be correctly duplicated and distributed between the two daughter cells to maintain genome identity. In order to achieve proper chromosome replication and segregation, sister chromatids must be recognized as such and kept together until their separation. This process of cohesion is mainly achieved through proteinaceous linkages of cohesin complexes, which are loaded on the sister chromatids as they are generated during S phase. Cohesion between sister chromatids must be fully removed at anaphase to allow chromosome segregation. Other (non-proteinaceous) sources of cohesion between sister chromatids consist of DNA linkages or sister chromatid intertwines. DNA linkages are a natural consequence of DNA replication, but must be timely resolved before chromosome segregation to avoid the arising of DNA lesions and genome instability, a hallmark of cancer development. As complete resolution of sister chromatid intertwines only occurs during chromosome segregation, it is not clear whether DNA linkages that persist in mitosis are simply an unwanted leftover or whether they have a functional role. In this review, we provide an overview of DNA linkages between sister chromatids, from their origin to their resolution, and we discuss the consequences of a failure in their detection and processing and speculate on their potential role.

Plk1 protects kinetochore-centromere architecture against microtubule pulling forces

During mitosis, sister chromatids attach to microtubules which generate ~ 700 pN pulling force focused on the centromere. We report that chromatin-localized signals generated by Polo-like kinase 1 (Plk1) maintain the integrity of the kinetochore and centromere against this force. Without sufficient Plk1 activity, chromosomes become misaligned after normal condensation and congression. These chromosomes are silent to the mitotic checkpoint, and many lag and mis-segregate in anaphase. Their centromeres and kinetochores lack CENP-A, CENP-C, CENP-T, Hec1, Nuf2, and Knl1; however, CENP-B is retained. CENP-A loss occurs coincident with secondary misalignment and anaphase onset. This disruption occurs asymmetrically prior to anaphase and requires tension generated by microtubules. Mechanistically, centromeres highly recruit PICH DNA helicase and PICH depletion restores kinetochore disruption in pre-anaphase cells. Furthermore, anaphase defects are significantly reduced by tethering Plk1 to chromatin, including H2B, and INCENP, but not to CENP-A. Taken as a whole, this demonstrates that Plk1 signals are crucial for stabilizing centromeric architecture against tension.

PLK1 facilitates chromosome biorientation by suppressing centromere disintegration driven by BLM-mediated unwinding and spindle pulling

Centromeres provide a pivotal function for faithful chromosome segregation. They serve as a foundation for the assembly of the kinetochore complex and spindle connection, which is essential for chromosome biorientation. Cells lacking Polo-like kinase 1 (PLK1) activity suffer severe chromosome alignment defects, which is believed primarily due to unstable kinetochore-microtubule attachment. Here, we reveal a previously undescribed mechanism named 'centromere disintegration' that drives chromosome misalignment in PLK1-inactivated cells. We find that PLK1 inhibition does not necessarily compromise metaphase establishment, but instead its maintenance. We demonstrate that this is caused by unlawful unwinding of DNA by BLM helicase at a specific centromere domain underneath kinetochores. Under bipolar spindle pulling, the distorted centromeres are promptly decompacted into DNA threadlike molecules, leading to centromere rupture and whole-chromosome arm splitting. Consequently, chromosome alignment collapses. Our study unveils an unexpected role of PLK1 as a chromosome guardian to maintain centromere integrity for chromosome biorientation.

Loss of PICH promotes chromosome instability and cell death in triple-negative breast cancer

Triple-negative breast cancer (TNBC), defined by the lack of expression of estrogen, progesterone, and ERBB2 receptors, has the worst prognosis of all breast cancers. It is difficult to treat owing to a lack of effective molecular targets. Here, we report that the growth of TNBC cells is exceptionally dependent on PICH, a DNA-dependent ATPase. Clinical samples analysis showed that PICH is highly expressed in TNBC compared to other breast cancer subtypes. Importantly, its high expression correlates with higher risk of distal metastasis and worse clinical outcomes. Further analysis revealed that PICH depletion selectively impairs the proliferation of TNBC cells, but not that of luminal breast cancer cells, in vitro and in vivo. In addition, knockdown of PICH in TNBC cells induces the formation of chromatin bridges and lagging chromosomes in anaphase, frequently resulting in micronucleation or binucleation, finally leading to mitotic catastrophe and apoptosis. Collectively, our findings show the dependency of TNBC cells on PICH for faithful chromosome segregation and the clinical potential of PICH inhibition to improve treatment of patients with high-risk TNBC.

ERCC6L promotes cell growth and invasion in human colorectal cancer

Excision repair cross-complementation group 6 like (ERCC6L), a recently discovered DNA helicase, has been demonstrated to be highly expressed in a variety of human cancer types. However, the precise role of ERCC6L in colorectal cancer (CRC) remains unclear. The current study aimed to investigate the potential role of ERCC6L in the development and progression of CRC. Reverse transcription-quantitative polymerase chain reaction, western blot analysis and immunohistochemistry were used to detect the expression level of ERCC6L in 30 matched pairs of CRC and adjacent noncancerous tissues. The function of ERCC6L in cell proliferation, cycle, apoptosis, invasion and colony formation was examined in CRC cell lines. ERCC6L was revealed to be highly expressed in CRC tissues and cell lines compared with normal controls (P<0.05). The expression level of ERCC6L was significantly associated with tumor size (P<0.05), but not with other clinical features, including age, gender, differentiation and clinical stage. It was identified that reducing ERCC6L expression using small interfering RNA significantly inhibited the proliferation and colony-forming ability of CRC cell lines. Flow cytometric analysis demonstrated that ERCC6L knockdown in CRC cells inhibited cell cycle progression and increased the number of cells in the G0/G1 phase without affecting apoptosis. Furthermore, ERCC6L knockdown markedly decreased the number of invading CRC cells compared with control cells. These results suggest that ERCC6L promotes the growth and invasion of CRC cells, and ERCC6L may be a potential new target for cancer therapy.

A journey with common fragile sites: From S phase to telophase

Some regions of the genome, notably common fragile sites (CFSs), are hypersensitive to replication stress and often involved in the generation of gross chromosome rearrangements in cancer cells. CFSs nest within very large genes and display cell-type-dependent instability. Fragile or not, large genes tend to replicate late in S-phase. A number of data now show that transcription perturbs replication completion across the body of large genes, particularly upon replication stress. However, the molecular mechanisms by which transcription elicits such under-replication and subsequent instability remain unclear. We present here our view of the mechanisms responsible for CFS under-replication and those allowing the cells to cope with this problem in G2 and mitosis. We notably focus on the major role played by the FANC proteins in the protection of CFSs from S phase up to late mitosis. We finally discuss a possible rationale for the conservation of large genes across vertebrate evolution.

The Unresolved Problem of DNA Bridging

Accurate duplication and transmission of identical genetic information into offspring cells lies at the heart of a cell division cycle. During the last stage of cellular division, namely mitosis, the fully replicated DNA molecules are condensed into X-shaped chromosomes, followed by a chromosome separation process called sister chromatid disjunction. This process allows for the equal partition of genetic material into two newly born daughter cells. However, emerging evidence has shown that faithful chromosome segregation is challenged by the presence of persistent DNA intertwining structures generated during DNA replication and repair, which manifest as so-called ultra-fine DNA bridges (UFBs) during anaphase. Undoubtedly, failure to disentangle DNA linkages poses a severe threat to mitosis and genome integrity. This review will summarize the possible causes of DNA bridges, particularly sister DNA inter-linkage structures, in an attempt to explain how they may be processed and how they influence faithful chromosome segregation and the maintenance of genome stability.

Imaging of DNA Ultrafine Bridges in Budding Yeast

DNA ultrafine bridges (UFBs) are a type of chromatin-free DNA bridges that connect sister chromatids in anaphase and pose a threat to genome stability. However, little is known about the origin of these structures, and how they are sensed and resolved by the cell. In this chapter, we review tools and methods for studying UFBs by fluorescence microscopy including chemical and genetic approaches to induce UFBs in the model organism Saccharomyces cerevisiae.

Detection of Ultrafine Anaphase Bridges

Ultrafine anaphase bridges (UFBs) are thin DNA threads linking the separating sister chromatids in the anaphase of mitosis. UFBs are thought to form when topological DNA entanglements between two chromatids are not resolved prior to anaphase onset. In contrast to other markers of defective chromosome segregation, UFBs cannot be detected by direct staining of the DNA, but instead can be detected using immunofluorescence-based approaches. Due to the fact that they are short-lived and fragile in nature, UFBs can be challenging to detect. In this chapter, we describe methods that have been optimized for successful detection of UFBs. We also provide guidelines for the optimization of UFBs detection depending on the antibody and the cell line to be used.

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

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.

A novel TPR-BEN domain interaction mediates PICH-BEND3 association

PICH is a DNA translocase required for the maintenance of chromosome stability in human cells. Recent data indicate that PICH co-operates with topoisomerase IIα to suppress pathological chromosome missegregation through promoting the resolution of ultra-fine anaphase bridges (UFBs). Here, we identify the BEN domain-containing protein 3 (BEND3) as an interaction partner of PICH in human cells in mitosis. We have purified full length PICH and BEND3 and shown that they exhibit a functional biochemical interaction in vitro. We demonstrate that the PICH–BEND3 interaction occurs via a novel interface between a TPR domain in PICH and a BEN domain in BEND3, and have determined the crystal structure of this TPR–BEN complex at 2.2 Å resolution. Based on the structure, we identified amino acids important for the TPR–BEN domain interaction, and for the functional interaction of the full-length proteins. Our data reveal a proposed new function for BEND3 in association with PICH, and the first example of a specific protein–protein interaction mediated by a BEN domain.

Rescue from replication stress during mitosis

Genomic instability is a hallmark of cancer and a common feature of human disorders, characterized by growth defects, neurodegeneration, cancer predisposition, and aging. Recent evidence has shown that DNA replication stress is a major driver of genomic instability and tumorigenesis. Cells can undergo mitosis with under-replicated DNA or unresolved DNA structures, and specific pathways are dedicated to resolving these structures during mitosis, suggesting that mitotic rescue from replication stress (MRRS) is a key process influencing genome stability and cellular homeostasis. Deregulation of MRRS following oncogene activation or loss-of-function of caretaker genes may be the cause of chromosomal aberrations that promote cancer initiation and progression. In this review, we discuss the causes and consequences of replication stress, focusing on its persistence in mitosis as well as the mechanisms and factors involved in its resolution, and the potential impact of incomplete replication or aberrant MRRS on tumorigenesis, aging and disease.

PICH promotes mitotic chromosome segregation: Identification of a novel role in rDNA disjunction

PICH is an SNF2-family DNA translocase that appears to play a role specifically in mitosis. Characterization of PICH in human cells led to the initial discovery of "ultra-fine DNA bridges" (UFBs) that connect the 2 segregating DNA masses in the anaphase of mitosis. These bridge structures, which arise from specific regions of the genome, are a normal feature of anaphase but had escaped detection previously because they do not stain with commonly used DNA dyes. Nevertheless, UFBs are important for genome maintenance because defects in UFB resolution can lead to cytokinesis failure. We reported recently that PICH stimulates the unlinking (decatenation) of entangled DNA by Topoisomerase IIα (Topo IIα), and is important for the resolution of UFBs. We also demonstrated that PICH and Topo IIα co-localize at the rDNA (rDNA). In this Extra View article, we discuss the mitotic roles of PICH and explore further the role of PICH in the timely segregation of the rDNA locus.

Characterization of the NTPR and BD1 interacting domains of the human PICH-BEND3 complex

Chromosome integrity depends on DNA structure-specific processing complexes that resolve DNA entanglement between sister chromatids. If left unresolved, these entanglements can generate either chromatin bridging or ultrafine DNA bridging in the anaphase of mitosis. These bridge structures are defined by the presence of the PICH protein, which interacts with the BEND3 protein in mitosis. To obtain structural insights into PICH-BEND3 complex formation at the atomic level, their respective NTPR and BD1 domains were cloned, overexpressed and crystallized using 1.56 M ammonium sulfate as a precipitant at pH 7.0. The protein complex readily formed large hexagonal crystals belonging to space group P6122, with unit-cell parameters a = b = 47.28, c = 431.58 Å and with one heterodimer in the asymmetric unit. A complete multiwavelength anomalous dispersion (MAD) data set extending to 2.2 Å resolution was collected from a selenomethionine-labelled crystal at the Swiss Light Source.

SUMO-interacting motifs (SIMs) in Polo-like kinase 1-interacting checkpoint helicase (PICH) ensure proper chromosome segregation during mitosis

Polo-like kinase 1 (Plk1)-interacting checkpoint helicase (PICH) localizes at the centromere and is critical for proper chromosome segregation during mitosis. However, the precise molecular mechanism of PICH's centromeric localization and function at the centromere is not yet fully understood. Recently, using Xenopus egg extract assays, we showed that PICH is a promiscuous SUMO binding protein. To further determine the molecular consequence of PICH/SUMO interaction on PICH function, we identified 3 SUMO-interacting motifs (SIMs) on PICH and generated a SIM-deficient PICH mutant. Using the conditional expression of PICH in cells, we found distinct roles of PICH SIMs during mitosis. Although all SIMs are dispensable for PICH's localization on ultrafine anaphase DNA bridges, only SIM3 (third SIM, close to the C-terminus end of PICH) is critical for its centromeric localization. Intriguingly, the other 2 SIMs function in chromatin bridge prevention. With these results, we propose a novel SUMO-dependent regulation of PICH's function on mitotic centromeres.

PICH promotes sister chromatid disjunction and co-operates with topoisomerase II in mitosis

PICH is a SNF2 family DNA translocase that binds to ultra-fine DNA bridges (UFBs) in mitosis. Numerous roles for PICH have been proposed from protein depletion experiments, but a consensus has failed to emerge. Here, we report that deletion of PICH in avian cells causes chromosome structural abnormalities, and hypersensitivity to an inhibitor of Topoisomerase II (Topo II), ICRF-193. ICRF-193-treated PICH^−/− cells undergo sister chromatid non-disjunction in anaphase, and frequently abort cytokinesis. PICH co-localizes with Topo IIα on UFBs and at the ribosomal DNA locus, and the timely resolution of both structures depends on the ATPase activity of PICH. Purified PICH protein strongly stimulates the catalytic activity of Topo II in vitro. Consistent with this, a human PICH^−/− cell line exhibits chromosome instability and chromosome condensation and decatenation defects similar to those of ICRF-193-treated cells. We propose that PICH and Topo II cooperate to prevent chromosome missegregation events in mitosis.

During mitosis the translocase PICH binds to ultrafine bridges formed from DNA catenanes that are unresolved by topoisomerase II. In this study, the authors show that PICH stimulates toposiomerase II activity and that they cooperate to resolve these structures.

SUMOylation regulates polo-like kinase 1-interacting checkpoint helicase (PICH) during mitosis

Mitotic SUMOylation has an essential role in faithful chromosome segregation in eukaryotes, although its molecular consequences are not yet fully understood. In Xenopus egg extract assays, we showed that poly(ADP-ribose) polymerase 1 (PARP1) is modified by SUMO2/3 at mitotic centromeres and that its enzymatic activity could be regulated by SUMOylation. To determine the molecular consequence of mitotic SUMOylation, we analyzed SUMOylated PARP1-specific binding proteins. We identified Polo-like kinase 1-interacting checkpoint helicase (PICH) as an interaction partner of SUMOylated PARP1 in Xenopus egg extract. Interestingly, PICH also bound to SUMOylated topoisomerase IIα (TopoIIα), a major centromeric small ubiquitin-like modifier (SUMO) substrate. Purified recombinant human PICH interacted with SUMOylated substrates, indicating that PICH directly interacts with SUMO, and this interaction is conserved among species. Further analysis of mitotic chromosomes revealed that PICH localized to the centromere independent of mitotic SUMOylation. Additionally, we found that PICH is modified by SUMO2/3 on mitotic chromosomes and in vitro. PICH SUMOylation is highly dependent on protein inhibitor of activated STAT, PIASy, consistent with other mitotic chromosomal SUMO substrates. Finally, the SUMOylation of PICH significantly reduced its DNA binding capability, indicating that SUMOylation might regulate its DNA-dependent ATPase activity. Collectively, our findings suggest a novel SUMO-mediated regulation of the function of PICH at mitotic centromeres.

Cohesin removal precedes topoisomerase IIα-dependent decatenation at centromeres in male mammalian meiosis II

Sister chromatid cohesion is regulated by cohesin complexes and topoisomerase IIα. Although relevant studies have shed some light on the relationship between these two mechanisms of cohesion during mammalian mitosis, their interplay during mammalian meiosis remains unknown. In the present study, we have studied the dynamics of topoisomerase IIα in relation to that of the cohesin subunits RAD21 and REC8, the shugoshin-like 2 (Schizosaccharomyces pombe) (SGOL2) and the polo-like kinase 1-interacting checkpoint helicase (PICH), during both male mouse meiotic divisions. Our results strikingly show that topoisomerase IIα appears at stretched strands connecting the sister kinetochores of segregating early anaphase II chromatids, once the cohesin complexes have been removed from the centromeres. Moreover, the number and length of these topoisomerase IIα-connecting strands increase between lagging chromatids at anaphase II after the chemical inhibition of the enzymatic activity of topoisomerase IIα by etoposide. Our results also show that the etoposide-induced inhibition of topoisomerase IIα is not able to rescue the loss of centromere cohesion promoted by the absence of the shugoshin SGOL2 during anaphase I. Taking into account our results, we propose a two-step model for the sequential release of centromeric cohesion during male mammalian meiosis II. We suggest that the cohesin removal is a prerequisite for the posterior topoisomerase IIα-mediated resolution of persisting catenations between segregating chromatids during anaphase II.

Cytokinesis-block micronucleus assay adapted for analyzing genomic instability of human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are multipotent cells used in cell therapy research. One of the problems involving hMSCs is the possibility of genetic instability during in vitro expansion required to obtain a suitable number of cells for clinical applications. The cytokinesis-block micronucleus (CBMN) assay measures genetic instability by analyzing the presence of micronucleus (MN), nucleoplasmic bridges (NPBs), and nuclear buds (NBUDs) in binucleated cells. The present study describes modifications in the CBMN assay methodology to analyze genetic instability in hMSCs isolated from the umbilical vein and in vitro expanded. The best protocol to achieve binucleated hMSCs with preserved cytoplasm was as follows: cytochalasin B concentration (4.0 μg/mL), use of hypotonic treatment (3 min), and the fixative solution (9 methanol:1 acetic acid). These adaptations were reproduced in three hMSC primary cell cultures and also in XP4PA and A549 cell lines. The frequency of hMSCs treated with mitomycin-C presenting MN was lower than that with other nuclear alterations, indicating that the hMSCs contain mechanisms to avoid a high level of chromosomal breaks. However, a high frequency of cells with NPBs was detected and spontaneous anaphase bridges under normal hMSC in vitro culture were observed. Considering that anaphase bridges are characteristic alterations in tumor cells, the CBMN assay is indicated as an important tool associated with other genetic analyses in order to ensure the safe clinical use of hMSCs in cell therapy.

Sending sisters their separate ways

Repair protein helps resolve entanglements between sister chromatids.

TopBP1/Dpb11 binds DNA anaphase bridges to prevent genome instability

TopBP1/Dpb11 prevents accumulation of anaphase chromatin bridges by stimulating the Mec1/ATR kinase and suppressing homologous recombination.

DNA anaphase bridges are a potential source of genome instability that may lead to chromosome breakage or nondisjunction during mitosis. Two classes of anaphase bridges can be distinguished: DAPI-positive chromatin bridges and DAPI-negative ultrafine DNA bridges (UFBs). Here, we establish budding yeast Saccharomyces cerevisiae and the avian DT40 cell line as model systems for studying DNA anaphase bridges and show that TopBP1/Dpb11 plays an evolutionarily conserved role in their metabolism. Together with the single-stranded DNA binding protein RPA, TopBP1/Dpb11 binds to UFBs, and depletion of TopBP1/Dpb11 led to an accumulation of chromatin bridges. Importantly, the NoCut checkpoint that delays progression from anaphase to abscission in yeast was activated by both UFBs and chromatin bridges independently of Dpb11, and disruption of the NoCut checkpoint in Dpb11-depleted cells led to genome instability. In conclusion, we propose that TopBP1/Dpb11 prevents accumulation of anaphase bridges via stimulation of the Mec1/ATR kinase and suppression of homologous recombination.

Structure-specific endonucleases: guardians of fragile site stability

Fragile sites are conserved loci predisposed to form breaks in metaphase chromosomes. The inherent instability of these loci is associated with chromosomal rearrangements in cancers and is a feature of cells from patients with chromosomal instability syndromes. One class of fragile sites, the common fragile sites (CFSs), have previously been shown to recruit several DNA repair proteins after the completion of bulk DNA synthesis in the cell, probably indicative of their inability to complete timely DNA replication. CFS loci are also prone to trigger mitotic non-disjunction of sister chromatids, leading to the formation of ultra-fine anaphase bridges (UFBs) and micronuclei. We discuss recent developments in the CFS field; in particular, the role of DNA structure-specific endonucleases in promoting cleavage at CFSs.

WAPL-mediated removal of cohesin protects against segregation errors and aneuploidy

The classical X shape of mitotic human chromosomes is the consequence of two distinct waves of cohesin removal. First, during prophase and prometaphase, the bulk of cohesin is driven from chromosome arms by the cohesin antagonist WAPL. This arm-specific cohesin removal is referred to as the prophase pathway [1-4]. The subsequent cleavage of the remaining centromeric cohesin by Separase is known to be the trigger for anaphase onset [5-7]. Remarkably the biological purpose of the prophase pathway is unknown. We find that this pathway is essential for two key mitotic processes. First, it is important to focus Aurora B at centromeres to allow efficient correction of erroneous microtubule-kinetochore attachments. In addition, it is required to facilitate the timely decatenation of sister chromatids. As a consequence, WAPL-depleted cells undergo anaphase with segregation errors, including both lagging chromosomes and catenanes, resulting in micronuclei and DNA damage. Stable WAPL depletion arrests cells in a p53-dependent manner but causes p53-deficient cells to become highly aneuploid. Our data show that the WAPL-dependent prophase pathway is essential for proper chromosome segregation and is crucial to maintain genomic integrity.

How unfinished business from S-phase affects mitosis and beyond

The eukaryotic cell cycle is conventionally viewed as comprising several discrete steps, each of which must be completed before the next one is initiated. However, emerging evidence suggests that incompletely replicated, or unresolved, chromosomes from S-phase can persist into mitosis, where they present a potential threat to the faithful segregation of sister chromatids. In this review, we provide an overview of the different classes of loci where this 'unfinished S-phase business' can lead to a variety of cytogenetically distinct DNA structures throughout the various steps of mitosis. Furthermore, we discuss the potential ways in which cells might not only tolerate this inevitable aspect of chromosome biology, but also exploit it to assist in the maintenance of genome stability.

PICH: a DNA translocase specially adapted for processing anaphase bridge DNA

The Plk1-interacting checkpoint helicase (PICH) protein localizes to ultrafine anaphase bridges (UFBs) in mitosis alongside a complex of DNA repair proteins, including the Bloom's syndrome protein (BLM). However, very little is known about the function of PICH or how it is recruited to UFBs. Using a combination of microfluidics, fluorescence microscopy, and optical tweezers, we have defined the properties of PICH in an in vitro model of an anaphase bridge. We show that PICH binds with a remarkably high affinity to duplex DNA, resulting in ATP-dependent protein translocation and extension of the DNA. Most strikingly, the affinity of PICH for binding DNA increases with tension-induced DNA stretching, which mimics the effect of the mitotic spindle on a UFB. PICH binding also appears to diminish force-induced DNA melting. We propose a model in which PICH recognizes and stabilizes DNA under tension during anaphase, thereby facilitating the resolution of entangled sister chromatids.