Alexis R. Barr

WEE1 phosphorylates and inhibits the activity of cyclin-dependent kinases to prevent mitotic entry and establish a G2 DNA damage checkpoint ^1–3 . Consequently, WEE1 inhibitors have been developed to be synthetically lethal in TP53- mutated tumours that become reliant on this WEE1-mediated checkpoint, and such inhibitors are undergoing clinical trials. However, WEE1 inhibitors also have efficacy in TP53- wild-type tumours, and many trials have been terminated owing to high levels of toxic side effects, suggesting that WEE1 has unknown functions. Here we show that WEE1 promotes cell cycle re-entry from quiescence (G0) by opposing the activity of the E3 ubiquitin ligase, anaphase-promoting complex–cyclosome bound to CDH1 (APC/C ^CDH1 ). WEE1 phosphorylates CDH1 ( FZR1 ) at key residues that mediate the interaction between CDH1 and APC/C. Cells with loss of function of WEE1 during G0/G1 have delayed S phase entry, impaired G1/S transition, abnormal S phase accumulation of cyclin-dependent kinase inhibitor p21 ^Cip1/Kip1 and a p21-dependent G2 arrest. Reduced expression of APC/C ^CDH1 or p21 renders cells more sensitive to acute WEE1 inhibition, and both pathways are downregulated in cells with acquired WEE1 inhibitor resistance. These results reveal a mechanism of cell cycle control that has implications for how WEE1 inhibitors should be used in the clinic.

Background: Cancer progression involves not only uncontrolled proliferation but also the strategic entry of tumour cells into reversible (quiescent) or irreversible (senescent) states of cell cycle arrest (G0). These states can give rise to rare persister-like cancer cells that survive hostile tumour microenvironment conditions, facilitating drug resistance, metastasis and disease relapse. Despite their importance, identifying and understanding the mechanisms regulating these cell populations remains challenging. Methods: We leveraged single-cell and spatially profiled primary breast tumours to quantify G0 arrest and proliferation decisions in cancer cells, revealing molecular and spatial features associated with proliferation-G0 dynamics. Results: We uncovered a G0 persister-like state with reduced copy number alteration burden and hallmarks of dormancy, characterised by transcriptional reprogramming of stress response pathways and increased epithelial-mesenchymal plasticity. Spatial analyses revealed distinct ecological niches: G0 cells inhabited protective niches with complement pathway activity proximal to CXCL10 + macrophages and myofibroblastic cancer-associated fibroblasts (CAFs), whereas proliferative zones were associated with CLEC9A + dendritic cells and PERK signalling, with distinct drug sensitivities. Conclusions: Our findings highlight key principles underpinning G0-proliferation dynamics and niche specialisation in breast cancer, offering novel insights into the spatial drivers of tumour heterogeneity and evolution.

During each cell cycle, cells must decide whether to continue to proliferate or to exit the cell cycle into a reversible arrest state, known as quiescence, or G0. This decision must be highly regulated to ensure proper tissue homeostasis. Studies on kinase-driven signalling pathways that regulate this decision point have dominated the field, and the role of phosphatases remains comparatively underexplored, yet the role of phosphatases is vitally important in signal transduction. In the present review, we examine how phosphatases contribute to the regulation of quiescence in mammalian cells across three stages: entry into quiescence, maintenance of the quiescent state, and quiescence exit into the cell cycle. We discuss how phosphatases counteract mitogenic signalling pathways, including MAPK/ERK and PI3K-AKT-mTOR, and maintain low cyclin-dependent kinase (CDK) activity through dephosphorylation of key cell cycle regulators, such as the retinoblastoma family proteins and CDK inhibitors. Finally, we highlight emerging evidence that dynamic regulation of phosphatase activity shapes the transition from quiescence back into proliferation. Understanding how phosphatases regulate the reversible nature of cell cycle arrest is important for understanding how tissues maintain homeostasis and how dysregulation of quiescence contributes to disease, including cancer.

For all drugs, effective target engagement requires sufficient intracellular concentrations of drug to be reached, but whether tumour heterogeneity impacts drug distribution and efficacy is poorly studied. Poly (ADP-ribose) polymerase (PARP) inhibitors have transformed treatment opportunities for women with high-grade serous ovarian carcinoma, but resistance remains a clinical hurdle in this highly heterogeneous tumour type. Here, we present a patient-derived explant multi-modal imaging pipeline, which demonstrates that cell-intrinsic PARP inhibitor accumulation is highly variable, both between patients and within tumours. Spatial transcriptomics reveals enrichment of apoptotic and lysosomal signatures in high-drug regions. Rucaparib, an intrinsically fluorescent PARP inhibitor, accumulates heterogeneously at the single-cell level, with rucaparib-high cells demonstrating increased drug response relative to rucaparib-low. Mechanistically, lysosomal sequestration creates a rucaparib reservoir that determines drug levels in the nucleus. Perturbation of lysosomal content alters intracellular levels of weak base PARP inhibitors rucaparib and niraparib, but not olaparib. Together these data suggest that lysosomes act as a reservoir for a subset of PARP inhibitor drugs to improve drug response.

DNA replication is tightly regulated to occur once per cell cycle, with the MCM2-7 helicase loaded onto replication origins only during G1-phase. In higher eukaryotes, geminin negatively regulates this process during S-, G2- and M-phases by binding the essential licensing factor CDT1. Although geminin’s function is crucial for genomic stability, its inhibitory mechanism remains elusive. Here, we utilise a fully reconstituted human DNA replication licensing assay to dissect geminin’s role. AlphaFold modelling provides structural insights into an N-terminal CDT1-binding helix of geminin, which proves essential for inhibition. Structural docking of the CDT1-geminin complex into the ORC-CDC6-CDT1-MCM2-7 (OCCM) assembly shows that geminin’s long coiled-coil domain sterically clashes with the MCM2 C-terminus, rather than directly blocking CDT1 binding to ORC-CDC6-MCM2-7. Shortening the coiled-coil preserves geminin dimerisation and CDT1 binding but abolishes inhibition, confirming its mechanistic role. Surprisingly, geminin is not able to fully inhibit DNA licensing. However, CDK1/2-cyclin A can partially inhibit DNA licensing and, in conjunction with geminin, result in a complete block. These findings uncover geminin’s steric inhibitory mechanism and suggest that a dual CDK-geminin axis controls human DNA replication.

Wee1 phosphorylates and inhibits CDK activity to inhibit mitotic entry and establish a G2 DNA damage checkpoint. Consequently, Wee1 inhibitors are in clinical trials, developed to be synthetically lethal in TP53 mutant tumours that become reliant on a Wee1-mediated DNA damage checkpoint. However, Wee1 inhibitors have efficacy in TP53 wild-type tumours and many trials have been terminated due to high levels of toxic side-effects, suggesting that Wee1 has unknown functions. Here, we show that Wee1 promotes cell cycle re-entry from quiescence (G0) by opposing the activity of the E3 ubiquitin ligase, APC/C ^{Cdh 1} . Wee1 phosphorylates Cdh1 ( FZR1 ) at key residues that mediate the interaction between Cdh1 and APC/C. Cells with loss-of-function of Wee1 during G0/G1 have delayed S-phase entry, an impaired G1/S transition, abnormal S-phase accumulation of the CDK inhibitor p21 and enter a p21-dependent G2 arrest. Reduced expression of APC/C ^{Cdh 1} or p21 renders cells more sensitive to acute Wee1 inhibition and both pathways are downregulated in acquired Wee1 inhibitor resistance. Our study reveals a new cell cycle control mechanism that has implications for how Wee1 inhibitors should be used in the clinic.

For all drugs, effective target engagement requires sufficient intracellular concentrations of drug to be reached, but whether tumour heterogeneity impacts drug distribution and efficacy is poorly studied. PARP inhibitors have transformed treatment of high-grade serous ovarian carcinoma (HGSOC), but resistance remains a clinical hurdle in this highly heterogeneous tumour type. We developed a patient-derived explant multi-modal imaging pipeline, which demonstrated that cell-intrinsic PARP inhibitor accumulation is highly variable, both between patients and within tumours. Spatial transcriptomics revealed enrichment of apoptotic and lysosomal signatures in ‘high-drug’ regions. Rucaparib, an intrinsically fluorescent PARP inhibitor, accumulates heterogeneously at the single-cell level, with ‘rucaparib-high’ cells demonstrating increased drug response relative to ‘rucaparib low’. Mechanistically, lysosomal sequestration creates a rucaparib reservoir that determines drug levels in the nucleus. Perturbation of lysosomal content altered intracellular levels of weak base PARP inhibitors rucaparib and niraparib, but not olaparib. Together these data suggest that lysosomes act as a reservoir for a subset of PARP inhibitor drugs to improve drug response.

Secreted proteins regulate the balance between cellular proliferation and G0 arrest and therefore play important roles in tumour dormancy. Tumour dormancy presents a significant clinical challenge for breast cancer patients, where non-proliferating, G0-arrested cancer cells remain at metastatic sites, below the level of clinical detection, some of which can re-enter proliferation and drive tumour relapse. Knowing which secreted proteins can regulate entry into and exit from G0 allows us to manipulate their signalling to prevent tumour relapse. To identify novel secreted proteins that can promote breast cancer G0 arrest, we performed a secretome-wide, image-based screen for proteins that increase the fraction of cells in G0 arrest. From a secretome library of 1282 purified proteins, we identified 29 candidates that promote G0 arrest in non-transformed and transformed breast epithelial cells. The assay we have developed can be adapted for use in other perturbation screens in other cell types. All datasets have been made available for re-analysis and our candidate proteins are presented for alternative bioinformatic refinement or further experimental follow up.

Systemic therapy options for urothelial carcinoma have expanded in recent years, with both immunotherapy and cytotoxic chemotherapy being widely available. However, we lack biomarkers to select which drug is likely to work best in individual patients. A new article in this journal by Jin, Xu, Su, et al reports that disruptive versus non‐disruptive TP53 mutations may guide these personalised therapy choices. Intriguingly, patients with disruptive TP53 tumour mutations had poor overall survival versus those with non‐disruptive TP53 mutations or wild type TP53 but responded particularly well to immunotherapy. Of relevance, an increased tumour mutational burden and increased effector CD8⁺ T‐cell infiltration was seen in tumours with disruptive mutations. The impact of different TP53 mutations on prognosis and therapy choices appears to be tumour‐ and therapy‐type specific, with no clear consensus on overall tumour phenotype according to type of mutation. Nonetheless, profiling of specific types of TP53 mutation is increasingly clinically feasible with targeted sequencing or immunohistochemistry. There is an urgent need for additional studies in urothelial cancer clarifying how the type of TP53 mutation present within a tumour can best be used as a predictive biomarker. Further important remaining questions include the impact of TP53 mutations on other clinically important aspects of the tumour microenvironment, including cancer‐associated fibroblasts. Furthermore, the impact of gain‐of‐function mutations in TP53 and other related genes signalling upstream or downstream of TP53 is of wide interest. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

The mammalian cell cycle alternates between two phases – S-G2-M with high levels of A- and B-type cyclins (CycA and CycB, respectively) bound to cyclin-dependent kinases (CDKs), and G1 with persistent degradation of CycA and CycB by an activated anaphase promoting complex/cyclosome (APC/C) bound to Cdh1 (also known as FZR1 in mammals; denoted APC/C:Cdh1). Because CDKs phosphorylate and inactivate Cdh1, these two phases are mutually exclusive. This ‘toggle switch’ is flipped from G1 to S by cyclin-E bound to a CDK (CycE:CDK), which is not degraded by APC/C:Cdh1, and from M to G1 by Cdc20-bound APC/C (APC/C:Cdc20), which is not inactivated by CycA:CDK or CycB:CDK. After flipping the switch, cyclin E is degraded and APC/C:Cdc20 is inactivated. Combining mathematical modelling with single-cell timelapse imaging, we show that dysregulation of CycB:CDK disrupts strict alternation of the G1-S and M-G1 switches. Inhibition of CycB:CDK results in Cdc20-independent Cdh1 ‘endocycles’, and sustained activity of CycB:CDK drives Cdh1-independent Cdc20 endocycles. Our model provides a mechanistic explanation for how whole-genome doubling can arise, a common event in tumorigenesis that can drive tumour evolution.