Although targeted therapies often elicit profound initial patient responses, these effects are transient due to residual disease leading to acquired resistance. How tumors transition between drug responsiveness, tolerance and resistance, especially in the absence of pre-existing subclones, remains unclear. In EGFR-mutant lung adenocarcinoma cells, we demonstrate that residual disease and acquired resistance in response to EGFR inhibitors requires AURKA activity. Non-genetic resistance through the activation of AURKA by its co-activator TPX2 emerges in response to chronic EGFR inhibition where it mitigates drug-induced apoptosis. Aurora kinase inhibitors suppress this adaptive survival program, increasing the magnitude and duration of EGFR inhibitor response in pre-clinical models. Treatment induced activation of AURKA was associated with resistance to EGFR inhibitors in-vitro, in-vivo and in individuals with EGFR-mutant lung adenocarcinoma. These findings delineate a path whereby drug resistance emerges from drug-tolerant cells and unveils a synthetic lethal strategy for enhancing responses to EGFR inhibitors by suppressing AURKA driven residual disease and acquired resistance.

The spindle assembly checkpoint (SAC) delays anaphase until kinetochores are properly attached to the spindle. Kuhn and Dumont demonstrate that the mammalian SAC monitors geometry-specific molecular cues or force on specific kinetochore linkages that end-on but not lateral attachments generating persistent tension can provide.

Cyclins associate with cyclin-dependent serine/threonine kinase 1 (CDK1) to generate the M phase-promoting factor (MPF) activity essential for progression through mitosis and meiosis. Although cyclin B1 (CCNB1) is required for embryo development, previous studies concluded that CCNB2 is dispensable for cell cycle progression. Given previous findings of high Ccnb2 mRNA translation rates in prophasearrested oocytes, we re-evaluated the role of this cyclin during meiosis. Ccnb2 −/− oocytes underwent delayed germinal vesicle breakdown and showed defects during the metaphase-to-anaphase transition. This defective maturation was associated with compromised Ccnb1 and Moloney sarcoma oncogene (Mos) mRNA translation, delayed spindle assembly and increased errors in chromosome segregation. Given these defects, a significant percentage of oocytes failed to complete meiosis I because the spindle assembly checkpoint remained active and anaphase-promoting complex/cyclosome function was inhibited. In vivo, CCNB2 depletion caused ovulation of immature oocytes, premature ovarian failure, and compromised female fecundity. These findings demonstrate that CCNB2 is required to assemble sufficient pre-MPF for timely meiosis re-entry and progression. Although endogenous cyclins cannot compensate, overexpression of CCNB1/ 2 rescues the meiotic phenotypes, indicating similar molecular properties but divergent modes of regulation of these cyclins.

The spindle assembly checkpoint (SAC) prevents anaphase until all kinetochores attach to the spindle. Each mammalian kinetochore binds many microtubules, but how many attached microtubules are required to turn off the checkpoint, and how the kinetochore monitors microtubule numbers, are not known and are central to understanding SAC mechanisms and function. To address these questions, here we systematically tune and fix the fraction of Hec1 molecules capable of microtubule binding. We show that Hec1 molecules independently bind microtubules within single kinetochores, but that the kinetochore does not independently process attachment information from different molecules. Few attached microtubules (20% occupancy) can trigger complete Mad1 loss, and Mad1 loss is slower in this case. Finally, we show using laser ablation that individual kinetochores detect changes in microtubule binding, not in spindle forces that accompany attachment. Thus, the mammalian kinetochore responds specifically to the binding of each microtubule and counts microtubules as a single unit in a sensitive and switch-like manner. This may allow kinetochores to rapidly react to early attachments and maintain a robust SAC response despite dynamic microtubule numbers.

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