Calin-Mihai Dragoi

The switch-like G2/M transition and mitotic exit depend on a feedback loop comprising CDK1, Greatwall kinase, and the PP2A-B55α phosphatase. Monogenic disruptions of these regulators impair cellular functions, drive genomic instability, and promote cancer-associated characteristics. Yet how perturbations of this feedback loop interact, and whether their effects depend on cellular context, remain unclear. Here, we assess how combinatorial perturbations of CDK1, Greatwall, and PP2A-B55α affect cell-cycle control in non-transformed RPE-1 and tumour-derived HeLa cells. We identify PP2A-B55α as a context-dependent G2/M repressor with a critical role in cancer cells, where it cooperates with PKMYT1 and, to a lesser extent, WEE1 to prevent premature mitosis. In cancer cells, but not in non-transformed cells, reduced PP2A-B55α activity and Greatwall overexpression produce marked sensitisation to the PKMYT1-selective RP-6306 and modest sensitisation to the WEE1-selective adavosertib. This asymmetric sensitisation reflects a functional separation between WEE1 and PKMYT1 in S- and G2-phase control. Our work outlines fundamental and cell-type-specific contributions of CDK1, Greatwall and PP2A-B55α to cell-cycle regulation and proposes G2/M plasticity as a vulnerability with utility in PKMYT1-targeted therapy.

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.

Progression through the eukaryotic cell cycle is governed by a complex biochemical network controlling the activation of cyclin-dependent kinases. Dynamically, the cell cycle control network can be viewed as a multi-stable system, in which nonlinear feedback loops generate multiple stable attractors, corresponding to distinct cell cycle phases (G1, S, G2, M). Transitions between these states are typically irreversible, ensuring ordered phase progression. However, recent studies of endocycles – variants in which subsets of phases are abrogated – have prompted the development of more general regulatory models, allowing for reversible transitions or the decoupling of autonomous oscillatory modules. The current paper combines minimal ODE modelling, phase plane analysis, and bifurcation theory to demonstrate that two prior frameworks – Newton’s Cradle and Latching Gate – are alternative representations of a shared architecture based on mutually regulated oscillators. Leveraging this insight, a generalized model is introduced that captures a broader spectrum of physiological endocycles than previously acknowledged. To provide an intuitive representation of the principles identified by the new model, a ‘Wheel of Fortune’ analogy for cell cycle dynamics is introduced.