Cryo-EM Structure of the Fork Protection Complex Bound to CMG at a Replication Fork.

The eukaryotic replisome, organized around the Cdc45-MCM-GINS (CMG) helicase, orchestrates chromosome replication. Multiple factors associate directly with CMG, including Ctf4 and the heterotrimeric fork protection complex (Csm3/Tof1 and Mrc1), which has important roles including aiding normal replication rates and stabilizing stalled forks. How these proteins interface with CMG to execute these functions is poorly understood. Here we present 3 to 3.5 Å resolution electron cryomicroscopy (cryo-EM) structures comprising CMG, Ctf4, and the fork protection complex at a replication fork. The structures provide high-resolution views of CMG-DNA interactions, revealing a mechanism for strand separation, and show Csm3/Tof1 "grip" duplex DNA ahead of CMG via a network of interactions important for efficient replication fork pausing. Although Mrc1 was not resolved in our structures, we determine its topology in the replisome by cross-linking mass spectrometry. Collectively, our work reveals how four highly conserved replisome components collaborate with CMG to facilitate replisome progression and maintain genome stability.

TIMELESS-TIPIN and UBXN-3 promote replisome disassembly during DNA replication termination in Caenorhabditis elegans.

The eukaryotic replisome is rapidly disassembled during DNA replication termination. In metazoa, the cullin-RING ubiquitin ligase CUL-2LRR-1 drives ubiquitylation of the CMG helicase, leading to replisome disassembly by the p97/CDC-48 "unfoldase". Here, we combine in vitro reconstitution with in vivo studies in Caenorhabditis elegans embryos, to show that the replisome-associated TIMELESS-TIPIN complex is required for CUL-2LRR-1 recruitment and efficient CMG helicase ubiquitylation. Aided by TIMELESS-TIPIN, CUL-2LRR-1 directs a suite of ubiquitylation enzymes to ubiquitylate the MCM-7 subunit of CMG. Subsequently, the UBXN-3 adaptor protein directly stimulates the disassembly of ubiquitylated CMG by CDC-48_UFD-1_NPL-4. We show that UBXN-3 is important in vivo for replisome disassembly in the absence of TIMELESS-TIPIN. Correspondingly, co-depletion of UBXN-3 and TIMELESS causes profound synthetic lethality. Since the human orthologue of UBXN-3, FAF1, is a candidate tumour suppressor, these findings suggest that manipulation of CMG disassembly might be applicable to future strategies for treating human cancer.

Manual Microcatheter-tip Modification: A bailout option for difficult "tip-in" technique during chronic total occlusion percutaneous coronary intervention.

The retrograde approach in chronic total occlusion (CTO) interventions often encounters significant challenges, particularly, when aligning the retrograde microcatheter (MC) with the antegrade system is difficult, complicating or even preventing standard externalization. To address these issues, techniques like the "tip-in" have proven to be effective backup strategies. We introduce the "Manual Microcatheter-tip Modification" (MMM) technique as an alternative when the "tip-in" method faces complications. We present a case of a left anterior descending CTO where MMM was successfully employed for the first time, enabling successful revascularization by manually modifying the MC tip to engage the retrograde guidewire. We explore the technical details within the framework of contemporary CTO PCI. This new technique could enhance the management of CTO interventions, offering innovative solutions when traditional externalization methods are problematic.

The Fork Protection Complex: A Regulatory Hub at the Head of the Replisome.

As well as accurately duplicating DNA, the eukaryotic replisome performs a variety of other crucial tasks to maintain genomic stability. For example, organizational elements, like cohesin, must be transferred from the front of the fork to the new strands, and when there is replication stress, forks need to be protected and checkpoint signalling activated. The Tof1-Csm3 (or Timeless-Tipin in humans) Fork Protection Complex (FPC) ensures efficient replisome progression and is required for a range of replication-associated activities. Recent studies have begun to reveal the structure of this complex, and how it functions within the replisome to perform its diverse roles. The core of the FPC acts as a DNA grip on the front of the replisome to regulate fork progression. Other flexibly linked domains and motifs mediate interactions with proteins and specific DNA structures, enabling the FPC to act as a hub at the head of the replication fork.

Tip-in endoscopic mucosal resection for large colorectal sessile polyps.

BACKGROUND: Tip-in endoscopic mucosal resection (EMR) is a modified EMR technique using which en bloc resection of large colorectal sessile polyps can be performed; however, its usefulness for colorectal sessile polyps of > 20 mm has not been reported. This study examined treatment outcomes of tip-in and conventional EMR for large colorectal sessile polyps of ≥ 20 mm. METHODS: This was a retrospective case-control study conducted at a single tertiary center in Japan. Subjects included those with large colorectal sessile polyps of ≥ 20 mm, excluding pedunculated-type polyps, who underwent endoscopic resection between January 2010 and January 2019. The primary outcome was endoscopic treatment outcomes when using tip-in and conventional EMR, and the secondary outcome was the local recurrence rate after endoscopic treatment. RESULTS: Forty-three colorectal lesions were treated using tip-in EMR and 83 using conventional EMR. Tip-in EMR had a significantly higher en bloc resection rate (90.7% vs. 69.8.%), and significantly shorter treatment duration (6.64 ± 0.64 min vs. 10.47 ± 0.81 min) than conventional EMR. However, for lesions > 30 mm, en bloc resection rate was 50.0% and 52.6% for tip-in and conventional EMR, respectively, indicating no significant difference. Perforation rates with tip-in and conventional EMR were 4.6% and 3.6%, respectively, indicating no significant difference. Local recurrence was examined in 80 cases who were followed up for > 6 months after endoscopic resection; recurrence rate was 0% and 7.0% in tip-in and conventional EMR cases, respectively, without significance difference. CONCLUSIONS: Tip-in EMR showed high en-block resection rate, particularly in polyps of < 30 mm, and no residual tumor was found. This technique is a potential endoscopic treatment alternative for large colorectal sessile polyps of ≥ 20 mm.

Tipin functions in the protection against topoisomerase I inhibitor.

The replication fork temporarily stalls when encountering an obstacle on the DNA, and replication resumes after the barrier is removed. Simultaneously, activation of the replication checkpoint delays the progression of S phase and inhibits late origin firing. Camptothecin (CPT), a topoisomerase I (Top1) inhibitor, acts as a DNA replication barrier by inducing the covalent retention of Top1 on DNA. The Timeless-Tipin complex, a component of the replication fork machinery, plays a role in replication checkpoint activation and stabilization of the replication fork. However, the role of the Timeless-Tipin complex in overcoming the CPT-induced replication block remains elusive. Here, we generated viable TIPIN gene knock-out (KO) DT40 cells showing delayed S phase progression and increased cell death. TIPIN KO cells were hypersensitive to CPT. However, homologous recombination and replication checkpoint were activated normally, whereas DNA synthesis activity was markedly decreased in CPT-treated TIPIN KO cells. Proteasome-dependent degradation of chromatin-bound Top1 was induced in TIPIN KO cells upon CPT treatment, and pretreatment with aphidicolin, a DNA polymerase inhibitor, suppressed both CPT sensitivity and Top1 degradation. Taken together, our data indicate that replication forks formed without Tipin may collide at a high rate with Top1 retained on DNA by CPT treatment, leading to CPT hypersensitivity and Top1 degradation in TIPIN KO cells.

The human Tim-Tipin complex interacts directly with DNA polymerase epsilon and stimulates its synthetic activity.

The Tim-Tipin complex plays an important role in the S phase checkpoint and replication fork stability in metazoans, but the molecular mechanism underlying its biological function is poorly understood. Here, we present evidence that the recombinant human Tim-Tipin complex (and Tim alone) markedly enhances the synthetic activity of DNA polymerase ε. In contrast, no significant effect on the synthetic ability of human DNA polymerase α and δ by Tim-Tipin was observed. Surface plasmon resonance measurements and co-immunoprecipitation experiments revealed that recombinant DNA polymerase ε directly interacts with either Tim or Tipin. In addition, the results of DNA band shift assays suggest that the Tim-Tipin complex (or Tim alone) is able to associate with DNA polymerase ε bound to a 40-/80-mer DNA ligand. Our results are discussed in view of the molecular dynamics at the human DNA replication fork.

Timeless-Tipin interactions with MCM and RPA mediate DNA replication stress response.

The accuracy of replication is one of the most important mechanisms ensuring the stability of the genome. The fork protection complex prevents premature replisome stalling and/or premature disassembly upon stress. Here, we characterize the Timeless-Tipin complex, a component of the fork protection complex. We used microscopy approaches, including colocalization analysis and proximity ligation assay, to investigate the spatial localization of the complex during ongoing replication in human cells. Taking advantage of the replication stress induction and the ensuing polymerase-helicase uncoupling, we characterized the Timeless-Tipin localization within the replisome. Replication stress was induced using hydroxyurea (HU) and aphidicolin (APH). While HU depletes the substrate for DNA synthesis, APH binds directly inside the catalytic pocket of DNA polymerase and inhibits its activity. Our data revealed that the Timeless-Tipin complex, independent of the stress, remains bound on chromatin upon stress induction and progresses together with the replicative helicase. This is accompanied by the spatial dissociation of the complex from the blocked replication machinery. Additionally, after stress induction, Timeless interaction with RPA, which continuously accumulates on ssDNA, was increased. Taken together, the Timeless-Tipin complex acts as a universal guardian of the mammalian replisome in an unperturbed S-phase progression as well as during replication stress.

Efficiency of the Guide Extension Catheter-Facilitated Tip-in Technique in the Recanalization of Coronary Chronic Total Occlusion.

Background: The tip-in technique, which involves advancing an antegrade microcatheter cross the lesion over a retrograde guidewire, is an elaborated maneuver in the recanalization of coronary chronic total occlusion (CTO). We seek to assess the efficiency of a guide extension catheter-facilitated tip-in technique in comparison to the traditional retrograde approach, which is accomplished by an externalization wire. Methods: Thirty-three CTO patients successfully revascularized using guide extension catheter-facilitated "tip-in" were included and matched with another 33 patients by J-CTO score and operators, whose CTO was recanalized using an externalized wire. The manipulation time from the first retrograde wire entering the antegrade guide to the first antegrade balloon inflation in the occlusion was calculated. Results: Compared with the wire-externalization group, the manipulation time in the tip-in group was significantly shortened [389s; interquartile range (IQR), 272-478 vs 706s; IQR, 560-914; p < 0.001]. There was a trend in decreasing total operation time and radiation dose, but it did not reach statistical significance. Conclusion: Guide extension catheter-facilitated tip-in is an efficient method to achieve the recanalization of CTO in a retrograde way, which would be pivotal when the retrograde microcatheter could not be advanced into the antegrade guide catheter.

"Double Tip-in" Technique to Facilitate Retrograde Chronic Total Occlusion Approach.

"Tip-in" technique used in chronic total occlusion revascularization can sometimes be challenging. Herein, we describe a novel method to facilitate "tip-in". After retrograde lesion crossing, the retrograde wire is advanced in a stepwise fashion into the antegrade guide catheter, the guide extension catheter and finally into the antegrade microcatheter. The use of a small lumen guide extension catheter to facilitate "tip-in" works by decreasing the area of operation, hence maximizing the chances of the wire and microcatheter meeting in the same plane. Overall, this newly described "double tip-in" technique can increase procedural success and decrease procedural time.

Knock-down of the TIM/TIPIN complex promotes apoptosis in melanoma cells.

The Timeless (TIM) and it's interacting partner TIPIN protein complex is well known for its role in replication checkpoints and normal DNA replication processes. Recent studies revealed the involvement of TIM and TIPIN in human malignancies; however, no evidence is available regarding the expression of the TIM/TIPIN protein complex or its potential role in melanoma. Therefore, we investigated the role of this complex in melanoma. To assess the role of the TIM/TIPIN complex in melanoma, we analyzed TIM/TIPIN expression data from the publicly accessible TCGA online database, Western blot analysis, and RT-qPCR in a panel of melanoma cell lines. Lentivirus-mediated TIM/TIPIN knockdown in A375 melanoma cells was used to examine proliferation, colony formation, and apoptosis. A xenograft tumor formation assay was also performed. The TIM/TIPIN complex is frequently overexpressed in melanoma cells compared to normal melanocytes. We also discovered that the overexpression of TIM and TIPIN was significantly associated with poorer prognosis of melanoma patients. Furthermore, we observed that shRNA-mediated knockdown of TIM and TIPIN reduced cell viability and proliferation due to the induction of apoptosis and increased levels of γH2AX, a marker of DNA damage. In a xenograft tumor nude mouse model, shRNA-knockdown of TIM/TIPIN significantly reduced tumor growth. Our results suggest that the TIM/TIPIN complex plays an important role in tumorigenesis of melanoma, which might reveal novel approaches for the development of new melanoma therapies. Our studies also provide a beginning structural basis for understanding the assembly of the TIM/TIPIN complex. Further mechanistic investigations are needed to determine the complex's potential as a biomarker of melanoma susceptibility. Targeting TIM/TIPIN might be a potential therapeutic strategy against melanoma.

Chromatin determinants of the inner-centromere rely on replication factors with functions that impart cohesion.

Replication fork-associated factors promote genome integrity and protect against cancer. Mutations in the DDX11 helicase and the ESCO2 acetyltransferase also cause related developmental disorders classified as cohesinopathies. Here we generated vertebrate model cell lines of these disorders and cohesinopathies-related genes. We found that vertebrate DDX11 and Tim-Tipin are individually needed to compensate for ESCO2 loss in chromosome segregation, with DDX11 also playing complementary roles with ESCO2 in centromeric cohesion. Our study reveals that overt centromeric cohesion loss does not necessarily precede chromosome missegregation, while both these problems correlate with, and possibly originate from, inner-centromere defects involving reduced phosphorylation of histone H3T3 (pH3T3) in the region. Interestingly, the mitotic pH3T3 mark was defective in all analyzed replication-related mutants with functions in cohesion. The results pinpoint mitotic pH3T3 as a postreplicative chromatin mark that is sensitive to replication stress and conducts with different kinetics to robust centromeric cohesion and correct chromosome segregation.

TIPIN depletion leads to apoptosis in breast cancer cells.

Triple-negative breast cancer (TNBC) is the breast cancer subgroup with the most aggressive clinical behavior. Alternatives to conventional chemotherapy are required to improve the survival of TNBC patients. Gene-expression analyses for different breast cancer subtypes revealed significant overexpression of the Timeless-interacting protein (TIPIN), which is involved in the stability of DNA replication forks, in the highly proliferative associated TNBC samples. Immunohistochemistry analysis showed higher expression of TIPIN in the most proliferative and aggressive breast cancer subtypes including TNBC, and no TIPIN expression in healthy breast tissues. The depletion of TIPIN by RNA interference impairs the proliferation of both human breast cancer and non-tumorigenic cell lines. However, this effect may be specifically associated with apoptosis in breast cancer cells. TIPIN silencing results in higher levels of single-stranded DNA (ssDNA), indicative of replicative stress (RS), in TNBC compared to non-tumorigenic cells. Upon TIPIN depletion, the speed of DNA replication fork was significantly decreased in all BC cells. However, TIPIN-depleted TNBC cells are unable to fire additional replication origins in response to RS and therefore undergo apoptosis. TIPIN knockdown in TNBC cells decreases tumorigenicity in vitro and delays tumor growth in vivo. Our findings suggest that TIPIN is important for the maintenance of DNA replication and represents a potential treatment target for the worst prognosis associated breast cancers, such as TNBC.

Separation of intra-S checkpoint protein contributions to DNA replication fork protection and genomic stability in normal human fibroblasts.

The ATR-dependent intra-S checkpoint protects DNA replication forks undergoing replication stress. The checkpoint is enforced by ATR-dependent phosphorylation of CHK1, which are mediated by the TIMELESS-TIPIN complex and CLASPIN. Although loss of checkpoint proteins is associated with spontaneous chromosomal instability, few studies have examined the contribution of these proteins to unchallenged DNA metabolism in human cells that have not undergone carcinogenesis or crisis. Furthermore, the TIMELESS-TIPIN complex and CLASPIN may promote replication fork protection independently of CHK1 activation. Normal human fibroblasts (NHF) were depleted of ATR, CHK1, TIMELESS, TIPIN or CLASPIN and chromosomal aberrations, DNA synthesis, activation of the DNA damage response (DDR) and clonogenic survival were evaluated. This work demonstrates in NHF lines from two individuals that ATR and CHK1 promote chromosomal stability by different mechanisms that depletion of CHK1 produces phenotypes that resemble more closely the depletion of TIPIN or CLASPIN than the depletion of ATR, and that TIMELESS has a distinct contribution to suppression of chromosomal instability that is independent of its heterodimeric partner, TIPIN. Therefore, ATR, CHK1, TIMELESS-TIPIN and CLASPIN have functions for preservation of intrinsic chromosomal stability that is separate from their cooperation for activation of the intra-S checkpoint response to experimentally induced replication stress. These data reveal a complex and coordinated program of genome maintenance enforced by proteins known for their intra-S checkpoint function.