TRAIP resolves DNA replication-transcription conflicts during the S-phase of unperturbed cells.

Cell division is the basis for the propagation of life and requires accurate duplication of all genetic information. DNA damage created during replication (replication stress) is a major cause of cancer, premature aging and a spectrum of other human disorders. Over the years, TRAIP E3 ubiquitin ligase has been shown to play a role in various cellular processes that govern genome integrity and faultless segregation. TRAIP is essential for cell viability, and mutations in TRAIP ubiquitin ligase activity lead to primordial dwarfism in patients. Here, we have determined the mechanism of inhibition of cell proliferation in TRAIP-depleted cells. We have taken advantage of the auxin induced degron system to rapidly degrade TRAIP within cells and to dissect the importance of various functions of TRAIP in different stages of the cell cycle. We conclude that upon rapid TRAIP degradation, specifically in S-phase, cells cease to proliferate, arrest in G2 stage of the cell cycle and undergo senescence. Our findings reveal that TRAIP works in S-phase to prevent DNA damage at transcription start sites, caused by replication-transcription conflicts.

Pathogenic variants in SLF2 and SMC5 cause segmented chromosomes and mosaic variegated hyperploidy.

Embryonic development is dictated by tight regulation of DNA replication, cell division and differentiation. Mutations in DNA repair and replication genes disrupt this equilibrium, giving rise to neurodevelopmental disease characterized by microcephaly, short stature and chromosomal breakage. Here, we identify biallelic variants in two components of the RAD18-SLF1/2-SMC5/6 genome stability pathway, SLF2 and SMC5, in 11 patients with microcephaly, short stature, cardiac abnormalities and anemia. Patient-derived cells exhibit a unique chromosomal instability phenotype consisting of segmented and dicentric chromosomes with mosaic variegated hyperploidy. To signify the importance of these segmented chromosomes, we have named this disorder Atelís (meaning - incomplete) Syndrome. Analysis of Atelís Syndrome cells reveals elevated levels of replication stress, partly due to a reduced ability to replicate through G-quadruplex DNA structures, and also loss of sister chromatid cohesion. Together, these data strengthen the functional link between SLF2 and the SMC5/6 complex, highlighting a distinct role for this pathway in maintaining genome stability.

Global landscape of replicative DNA polymerase usage in the human genome.

The division of labour among DNA polymerase underlies the accuracy and efficiency of replication. However, the roles of replicative polymerases have not been directly established in human cells. We developed polymerase usage sequencing (Pu-seq) in HCT116 cells and mapped Polε and Polα usage genome wide. The polymerase usage profiles show Polε synthesises the leading strand and Polα contributes mainly to lagging strand synthesis. Combining the Polε and Polα profiles, we accurately predict the genome-wide pattern of fork directionality plus zones of replication initiation and termination. We confirm that transcriptional activity contributes to the pattern of initiation and termination and, by separately analysing the effect of transcription on co-directional and converging forks, demonstrate that coupled DNA synthesis of leading and lagging strands is compromised by transcription in both co-directional and convergent forks. Polymerase uncoupling is particularly evident in the vicinity of large genes, including the two most unstable common fragile sites, FRA3B and FRA3D, thus linking transcription-induced polymerase uncoupling to chromosomal instability. Together, our result demonstrated that Pu-seq in human cells provides a powerful and straightforward methodology to explore DNA polymerase usage and replication fork dynamics.

AMBRA1 p.Gln30Arg Mutation, Identified in a Cowden Syndrome Family, Exhibits Hyperproliferative Potential in hTERT-RPE1 Cells.

Cowden syndrome (CS) is a rare autosomal dominant disorder associated with multiple hamartomatous and neoplastic lesions in various organs. Most CS patients have been found to have germline mutations in the PTEN tumor suppressor. In the present study, we investigated the causative gene of CS in a family of PTEN (phosphatase and tensin homolog deleted on chromosome 10) -negative CS patients. Whole exome sequencing analysis revealed AMBRA1 (Autophagy and Beclin 1 Regulator 1) as a novel candidate gene harboring two germline variants: p.Gln30Arg (Q30R) and p.Arg1195Ser (R1195S). AMBRA1 is a key regulator of the autophagy signaling network and a tumor suppressor. To functionally validate the role of AMBRA1 in the clinical manifestations of CS, we generated AMBRA1 depletion and Q30R mutation in hTERT-RPE1 (humanTelomerase Reverse Transcriptase-immortalized Retinal Pigmented Epithelial cells) using the CRISPR-Cas9 gene editing system. We observed that both AMBRA1-depleted and mutant cells showed accumulation in the S phase, leading to hyperproliferation, which is a characteristic of hamartomatous lesions. Specifically, the AMBRA1 Q30R mutation disturbed the G1/S transition of cells, leading to continuous mitotic entry of mutant cells, irrespective of the extracellular condition. From our analysis of primary ciliogenesis in these cells, we speculated that the mitotic entry of AMBRA1 Q30R mutants could be due to non-functional primary cilia that lead to impaired processing of extracellular sensory signals. Additionally, we observed a situs inversus phenotype in ambra1-depleted zebrafish, a developmental abnormality resulting from dysregulated primary ciliogenesis. Taken together, we established that the AMBRA1 Q30R mutation that we observed in CS patients might play an important role in inducing the hyperproliferative potential of cells through regulating primary ciliogenesis.

MYC assembles and stimulates topoisomerases 1 and 2 in a "topoisome".

High-intensity transcription and replication supercoil DNA to levels that can impede or halt these processes. As a potent transcription amplifier and replication accelerator, the proto-oncogene MYC must manage this interfering torsional stress. By comparing gene expression with the recruitment of topoisomerases and MYC to promoters, we surmised a direct association of MYC with topoisomerase 1 (TOP1) and TOP2 that was confirmed in vitro and in cells. Beyond recruiting topoisomerases, MYC directly stimulates their activities. We identify a MYC-nucleated "topoisome" complex that unites TOP1 and TOP2 and increases their levels and activities at promoters, gene bodies, and enhancers. Whether TOP2A or TOP2B is included in the topoisome is dictated by the presence of MYC versus MYCN, respectively. Thus, in vitro and in cells, MYC assembles tools that simplify DNA topology and promote genome function under high output conditions.

Topoisomerase 1 activity during mitotic transcription favors the transition from mitosis to G1.

As cells enter mitosis, chromatin compacts to facilitate chromosome segregation yet remains transcribed. Transcription supercoils DNA to levels that can impede further progression of RNA polymerase II (RNAPII) unless it is removed by DNA topoisomerase 1 (TOP1). Using ChIP-seq on mitotic cells, we found that TOP1 is required for RNAPII translocation along genes. The stimulation of TOP1 activity by RNAPII during elongation allowed RNAPII clearance from genes in prometaphase and enabled chromosomal segregation. Disruption of the TOP1-RNAPII interaction impaired RNAPII spiking at promoters and triggered defects in the post-mitotic transcription program. This program includes factors necessary for cell growth, and cells with impaired TOP1-RNAPII interaction are more sensitive to inhibitors of mTOR signaling. We conclude that TOP1 is necessary for assisting transcription during mitosis with consequences for growth and gene expression long after mitosis is completed. In this sense, TOP1 ensures that cellular memory is preserved in subsequent generations.

Chromosome oscillation promotes Aurora A-dependent Hec1 phosphorylation and mitotic fidelity.

Most cancer cells show chromosomal instability, a condition where chromosome missegregation occurs frequently. We found that chromosome oscillation, an iterative chromosome motion during metaphase, is attenuated in cancer cell lines. We also found that metaphase phosphorylation of Hec1 at serine 55, which is mainly dependent on Aurora A on the spindle, is reduced in cancer cell lines. The Aurora A-dependent Hec1-S55 phosphorylation level was regulated by the chromosome oscillation amplitude and vice versa: Hec1-S55 and -S69 phosphorylation by Aurora A is required for efficient chromosome oscillation. Furthermore, enhancement of chromosome oscillation reduced the number of erroneous kinetochore-microtubule attachments and chromosome missegregation, whereas inhibition of Aurora A during metaphase increased such errors. We propose that Aurora A-mediated metaphase Hec1-S55 phosphorylation through chromosome oscillation, together with Hec1-S69 phosphorylation, ensures mitotic fidelity by eliminating erroneous kinetochore-microtubule attachments. Attenuated chromosome oscillation and the resulting reduced Hec1-S55 phosphorylation may be a cause of CIN in cancer cell lines.

Replication timing maintains the global epigenetic state in human cells.

The temporal order of DNA replication [replication timing (RT)] is correlated with chromatin modifications and three-dimensional genome architecture; however, causal links have not been established, largely because of an inability to manipulate the global RT program. We show that loss of RIF1 causes near-complete elimination of the RT program by increasing heterogeneity between individual cells. RT changes are coupled with widespread alterations in chromatin modifications and genome compartmentalization. Conditional depletion of RIF1 causes replication-dependent disruption of histone modifications and alterations in genome architecture. These effects were magnified with successive cycles of altered RT. These results support models in which the timing of chromatin replication and thus assembly plays a key role in maintaining the global epigenetic state.

MCM10 compensates for Myc-induced DNA replication stress in breast cancer stem-like cells.

Cancer stem-like cells (CSCs) induce drug resistance and recurrence of tumors when they experience DNA replication stress. However, the mechanisms underlying DNA replication stress in CSCs and its compensation remain unclear. Here, we demonstrate that upregulated c-Myc expression induces stronger DNA replication stress in patient-derived breast CSCs than in differentiated cancer cells. Our results suggest critical roles for mini-chromosome maintenance protein 10 (MCM10), a firing (activating) factor of DNA replication origins, to compensate for DNA replication stress in CSCs. MCM10 expression is upregulated in CSCs and is maintained by c-Myc. c-Myc-dependent collisions between RNA transcription and DNA replication machinery may occur in nuclei, thereby causing DNA replication stress. MCM10 may activate dormant replication origins close to these collisions to ensure the progression of replication. Moreover, patient-derived breast CSCs were found to be dependent on MCM10 for their maintenance, even after enrichment for CSCs that were resistant to paclitaxel, the standard chemotherapeutic agent. Further, MCM10 depletion decreased the growth of cancer cells, but not of normal cells. Therefore, MCM10 may robustly compensate for DNA replication stress and facilitate genome duplication in cancer cells in the S-phase, which is more pronounced in CSCs. Overall, we provide a preclinical rationale to target the c-Myc-MCM10 axis for preventing drug resistance and recurrence of tumors.

The RIF1-long splice variant promotes G1 phase 53BP1 nuclear bodies to protect against replication stress.

Human cells lacking RIF1 are highly sensitive to replication inhibitors, but the reasons for this sensitivity have been enigmatic. Here, we show that RIF1 must be present both during replication stress and in the ensuing recovery period to promote cell survival. Of two isoforms produced by alternative splicing, we find that RIF1-Long alone can protect cells against replication inhibition, but RIF1-Short is incapable of mediating protection. Consistent with this isoform-specific role, RIF1-Long is required to promote the formation of the 53BP1 nuclear bodies that protect unrepaired damage sites in the G1 phase following replication stress. Overall, our observations show that RIF1 is needed at several cell cycle stages after replication insult, with the RIF1-Long isoform playing a specific role during the ensuing G1 phase in damage site protection.

Inducible Degradation of the Human SMC5/6 Complex Reveals an Essential Role Only during Interphase.

The cohesin- and condensin-related SMC5/6 complex has largely been studied in the context of DNA repair. Nevertheless, SMC5/6 has an undefined essential function even in the absence of cellular stress. Through the use of an auxin-inducible degradation system for rapidly depleting subunits of the SMC5/6 complex, we show that SMC5/6 is essential for viability in cancer-derived and normal human cells. Impairment of SMC5/6 function is associated with spontaneous induction of DNA damage, p53 activation, cell-cycle arrest, and senescence, as well as an increased frequency of various mitotic chromosome segregation abnormalities. However, we show that this chromosome missegregation is apparent only when SMC5/6 function is impaired during the preceding S and G2 phases. In contrast, degradation of SMC5/6 immediately prior to mitotic entry has little or no impact on the fidelity of chromosome segregation, highlighting the importance of the complex during interphase in order to ensure faithful sister chromatid disjunction.

HsSAS-6-dependent cartwheel assembly ensures stabilization of centriole intermediates.

At the onset of procentriole formation, a structure called the cartwheel is formed adjacent to the pre-existing centriole. SAS-6 proteins are thought to constitute the hub of the cartwheel structure. However, the exact function of the cartwheel in the process of centriole formation has not been well characterized. In this study, we focused on the functions of human SAS-6 (HsSAS-6, also known as SASS6). By using an in vitro reconstitution system with recombinant HsSAS-6, we first observed its conserved molecular property of forming the central part of the cartwheel structure. Furthermore, we uncovered critical functions of HsSAS-6 by using a combination of an auxin-inducible HsSAS-6-degron (AID) system and super-resolution microscopy in human cells. Our results demonstrate that the HsSAS-6 is required not only for the initiation of centriole formation, but also for the stabilization of centriole intermediates. Moreover, after procentriole formation, HsSAS-6 is necessary for limiting Plk4 accumulation at the centrioles and thereby suppressing the formation of initiation sites that would otherwise promote the development of extra procentrioles. Overall, these findings illustrate the conserved and fundamental functions of the cartwheel in centriole duplication.

Generation of conditional auxin-inducible degron (AID) cells and tight control of degron-fused proteins using the degradation inhibitor auxinole.

Controlling protein expression using a degron is advantageous because the protein of interest can be rapidly depleted in a reversible manner. We pioneered the development of the auxin-inducible degron (AID) technology by transplanting a plant-specific degradation pathway to non-plant cells. In human cells expressing an E3 ligase component, OsTIR1, it is possible to degrade a degron-fused protein with a half-life of 15-45 min in the presence of the phytohormone auxin. We reported previously the generation of human HCT116 mutants in which the C terminus of endogenous proteins was fused with the degron by CRISPR-Cas9-based knock-in. Here, we show new plasmids for N-terminal tagging and describe a detailed protocol for the generation of AID mutants of human HCT116 and DLD1 cells. Moreover, we report the use of an OsTIR1 inhibitor, auxinole, to suppress leaky degradation of degron-fused proteins. The addition of auxinole is also useful for rapid re-expression after depletion of degron-fused proteins. These improvements enhance the utility of AID technology for studying protein function in living human cells.

Inhibiting the MCM8-9 complex selectively sensitizes cancer cells to cisplatin and olaparib.

MCM8 and MCM9 are paralogues of the MCM2-7 eukaryotic DNA replication helicase proteins and play a crucial role in a homologous recombination-mediated repair process to resolve replication stress by fork stalling. Thus, deficiency of MCM8-9 sensitizes cells to replication stress caused, for example, by platinum compounds that induce interstrand cross-links. It is suggested that cancer cells undergo more replication stress than normal cells due to hyperstimulation of growth. Therefore, it is possible that inhibiting MCM8-9 selectively hypersensitizes cancer cells to platinum compounds and poly(ADP-ribose) polymerase inhibitors, both of which hamper replication fork progression. Here, we inhibited MCM8-9 in transformed and nontransformed cells and examined their sensitivity to cisplatin and olaparib. We found that knockout of MCM9 or knockdown of MCM8 selectively hypersensitized transformed cells to cisplatin and olaparib. In agreement with reported findings, RAS- and human papilloma virus type 16 E7-mediated transformation of human fibroblasts increased replication stress, as indicated by induction of multiple DNA damage responses (including formation of Rad51 foci). Such replication stress induced by oncogenes was further increased by knockdown of MCM8, providing a rationale for cancer-specific hypersensitization to cisplatin and olaparib. Finally, we showed that knocking out MCM9 increased the sensitivity of HCT116 xenograft tumors to cisplatin. Taken together, the data suggest that conceptual MCM8-9 inhibitors will be powerful cancer-specific chemosensitizers for platinum compounds and poly(ADP-ribose) polymerase inhibitors, thereby opening new avenues to the design of novel cancer chemotherapeutic strategies.

Chk1-mediated Cdc25A degradation as a critical mechanism for normal cell cycle progression.

Chk1 (encoded by CHEK1 in mammals) is an evolutionarily conserved protein kinase that transduces checkpoint signals from ATR to Cdc25A during the DNA damage response (DDR). In mammals, Chk1 also controls cellular proliferation even in the absence of exogenous DNA damage. However, little is known about how Chk1 regulates unperturbed cell cycle progression, and how this effect under physiological conditions differs from its regulatory role in DDR. Here, we have established near-diploid HCT116 cell lines containing endogenous Chk1 protein tagged with a minimum auxin-inducible degron (mAID) through CRISPR/Cas9-based gene editing. Establishment of these cells enabled us to induce specific and rapid depletion of the endogenous Chk1 protein, which resulted in aberrant accumulation of DNA damage factors that induced cell cycle arrest at S or G2 phase. Cdc25A was stabilized upon Chk1 depletion before the accumulation of DNA damage factors. Simultaneous depletion of Chk1 and Cdc25A partially suppressed the defects caused by Chk1 single depletion. These results indicate that, similar to its function in DDR, Chk1 controls normal cell cycle progression mainly by inducing Cdc25A degradation.

Dynein-Dynactin-NuMA clusters generate cortical spindle-pulling forces as a multi-arm ensemble.

To position the mitotic spindle within the cell, dynamic plus ends of astral microtubules are pulled by membrane-associated cortical force-generating machinery. However, in contrast to the chromosome-bound kinetochore structure, how the diffusion-prone cortical machinery is organized to generate large spindle-pulling forces remains poorly understood. Here, we develop a light-induced reconstitution system in human cells. We find that induced cortical targeting of NuMA, but not dynein, is sufficient for spindle pulling. This spindle-pulling activity requires dynein-dynactin recruitment by NuMA's N-terminal long arm, dynein-based astral microtubule gliding, and NuMA's direct microtubule-binding activities. Importantly, we demonstrate that cortical NuMA assembles specialized focal structures that cluster multiple force-generating modules to generate cooperative spindle-pulling forces. This clustering activity of NuMA is required for spindle positioning, but not for spindle-pole focusing. We propose that cortical Dynein-Dynactin-NuMA (DDN) clusters act as the core force-generating machinery that organizes a multi-arm ensemble reminiscent of the kinetochore.

Ki-67 and condensins support the integrity of mitotic chromosomes through distinct mechanisms.

Although condensins play essential roles in mitotic chromosome assembly, Ki-67 (also known as MKI67), a protein localizing to the periphery of mitotic chromosomes, had also been shown to make a contribution to the process. To examine their respective roles, we generated a set of HCT116-based cell lines expressing Ki-67 and/or condensin subunits that were fused with an auxin-inducible degron for their conditional degradation. Both the localization and the dynamic behavior of Ki-67 on mitotic chromosomes were not largely affected upon depletion of condensin subunits, and vice versa. When both Ki-67 and SMC2 (a core subunit of condensins) were depleted, ball-like chromosome clusters with no sign of discernible thread-like structures were observed. This severe defective phenotype was distinct from that observed in cells depleted of either Ki-67 or SMC2 alone. Our results show that Ki-67 and condensins, which localize to the external surface and the central axis of mitotic chromosomes, respectively, have independent yet cooperative functions in supporting the structural integrity of mitotic chromosomes.

Xrn2 accelerates termination by RNA polymerase II, which is underpinned by CPSF73 activity.

Termination is a ubiquitous phase in every transcription cycle but is incompletely understood and a subject of debate. We used gene editing as a new approach to address its mechanism through engineered conditional depletion of the 5' → 3' exonuclease Xrn2 or the polyadenylation signal (PAS) endonuclease CPSF73 (cleavage and polyadenylation specificity factor 73). The ability to rapidly control Xrn2 reveals a clear and general role for it in cotranscriptional degradation of 3' flanking region RNA and transcriptional termination. This defect is characterized genome-wide at high resolution using mammalian native elongating transcript sequencing (mNET-seq). An Xrn2 effect on termination requires prior RNA cleavage, and we provide evidence for this by showing that catalytically inactive CPSF73 cannot restore termination to cells lacking functional CPSF73. Notably, Xrn2 plays no significant role in either Histone or small nuclear RNA (snRNA) gene termination even though both RNA classes undergo 3' end cleavage. In sum, efficient termination on most protein-coding genes involves CPSF73-mediated RNA cleavage and cotranscriptional degradation of polymerase-associated RNA by Xrn2. However, as CPSF73 loss caused more extensive readthrough transcription than Xrn2 elimination, it likely plays a more underpinning role in termination.

Conditional Degrons for Controlling Protein Expression at the Protein Level.

The conditional depletion of a protein of interest (POI) is useful not only for loss-of-function studies, but also for the modulation of biological pathways. Technologies that work at the level of DNA, mRNA, and protein are available for temporal protein depletion. Compared with technologies targeting the pretranslation steps, direct protein depletion (or protein knockdown approaches) is advantageous in terms of specificity, reversibility, and time required for depletion, which can be achieved by fusing a POI with a protein domain called a degron that induces rapid proteolysis of the fusion protein. Conditional degrons can be activated or inhibited by temperature, small molecules, light, or the expression of another protein. The conditional degron-based technologies currently available are described and discussed.

Acute inactivation of the replicative helicase in human cells triggers MCM8-9-dependent DNA synthesis.

DNA replication fork progression can be disrupted at difficult to replicate loci in the human genome, which has the potential to challenge chromosome integrity. This replication fork disruption can lead to the dissociation of the replisome and the formation of DNA damage. To model the events stemming from replisome dissociation during DNA replication perturbation, we used a degron-based system for inducible proteolysis of a subunit of the replicative helicase. We show that MCM2-depleted cells activate a DNA damage response pathway and generate replication-associated DNA double-strand breaks (DSBs). Remarkably, these cells maintain some DNA synthesis in the absence of MCM2, and this requires the MCM8-9 complex, a paralog of the MCM2-7 replicative helicase. We show that MCM8-9 functions in a homologous recombination-based pathway downstream from RAD51, which is promoted by DSB induction. This RAD51/MCM8-9 axis is distinct from the recently described RAD52-dependent DNA synthesis pathway that operates in early mitosis at common fragile sites. We propose that stalled replication forks can be restarted in S phase via homologous recombination using MCM8-9 as an alternative replicative helicase.