Inhibitors of Rho kinases (ROCK) induce multiple mitotic defects and synthetic lethality in BRCA2-deficient cells
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Curated by eLife
eLife assessment
The authors have uncovered a new target that may be exploited to selectively kill BRCA2 mutant cancer cells. Strengths of the study include the novel pathway uncovered (ROCK kinases) and the strong data in support of the findings. Weaknesses include limited detail regarding the mechanism of BRCA2-specific cell death by ROCK kinase inhibitors, limited information on why some ROCK kinase inhibitors are not effective, as well as whether the cell killing in BRCA2 wild-type cells by ROCK kinase inhibitors is the same mechanism but just attenuated. The work will be of interest to cancer biologists and colleagues studying kinases.
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Abstract
The trapping of Poly-ADP-ribose polymerase (PARP) on DNA caused by PARP inhibitors (PARPi) triggers acute DNA replication stress and synthetic lethality (SL) in BRCA2-deficient cells. Hence, DNA damage is accepted as a prerequisite for SL in BRCA2-deficient cells. In contrast, here we show that inhibiting ROCK in BRCA2-deficient cells triggers SL independently from acute replication stress. Such SL is preceded by polyploidy and binucleation resulting from cytokinesis failure. Such initial mitosis abnormalities are followed by other M phase defects, including anaphase bridges and abnormal mitotic figures associated with multipolar spindles, supernumerary centrosomes and multinucleation. SL was also triggered by inhibiting Citron Rho-interacting kinase, another enzyme that, similarly to ROCK, regulates cytokinesis. Together, these observations demonstrate that cytokinesis failure triggers mitotic abnormalities and SL in BRCA2-deficient cells. Furthermore, the prevention of mitotic entry by depletion of Early mitotic inhibitor 1 (EMI1) augmented the survival of BRCA2-deficient cells treated with ROCK inhibitors, thus reinforcing the association between M phase and cell death in BRCA2-deficient cells. This novel SL differs from the one triggered by PARPi and uncovers mitosis as an Achilles heel of BRCA2-deficient cells.
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eLife assessment
The authors have uncovered a new target that may be exploited to selectively kill BRCA2 mutant cancer cells. Strengths of the study include the novel pathway uncovered (ROCK kinases) and the strong data in support of the findings. Weaknesses include limited detail regarding the mechanism of BRCA2-specific cell death by ROCK kinase inhibitors, limited information on why some ROCK kinase inhibitors are not effective, as well as whether the cell killing in BRCA2 wild-type cells by ROCK kinase inhibitors is the same mechanism but just attenuated. The work will be of interest to cancer biologists and colleagues studying kinases.
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Reviewer #1 (Public Review):
In this report, the authors use what they describe as a novel phenotypic survival screening method to uncover ATP-dependent kinases that may show synthetic lethality (when inhibited) with BRCA2 loss. Interestingly, they find that inhibiting ROCK kinases in BRCA2 deficient cells (but not BRCA1 deficient cells), triggers synthetic lethality. They further show that the synthetic lethality is independent of acute replication stress and is preceded by enhanced M-phase defects (anaphase bridges and abnormal mitotic figures). These data, therefore, suggest a new pathway (ROCK kinases) that may be targeted to induce synthetic lethality in BRCA2 deficient cells.
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Reviewer #2 (Public Review):
This paper describes a novel synthetic lethal interaction between BRCA2 loss and the cytokinesis regulators, including ROCK. The SL effects are restricted to short-term in vitro assays, and the underlying mechanisms remain largely elusive. The impact of the work in its current form is limited.
Strengths:
- A novel synthetic lethal (SL) interaction, which appears independent from the -BRCA2 SL interaction that depends on replication fork stalling and DNA damage induction.
- The SL interaction is validated in a panel of genetic models of BRCA2 deficiency.
- The SL interaction can be induced using clinically approved agents.Weaknesses
- The evidence that this SL interaction is independent of replication defects is not solid.
- The SL interaction is based on chemical inhibitors only, with 6 out of 9 ROCK …Reviewer #2 (Public Review):
This paper describes a novel synthetic lethal interaction between BRCA2 loss and the cytokinesis regulators, including ROCK. The SL effects are restricted to short-term in vitro assays, and the underlying mechanisms remain largely elusive. The impact of the work in its current form is limited.
Strengths:
- A novel synthetic lethal (SL) interaction, which appears independent from the -BRCA2 SL interaction that depends on replication fork stalling and DNA damage induction.
- The SL interaction is validated in a panel of genetic models of BRCA2 deficiency.
- The SL interaction can be induced using clinically approved agents.Weaknesses
- The evidence that this SL interaction is independent of replication defects is not solid.
- The SL interaction is based on chemical inhibitors only, with 6 out of 9 ROCK inhibitors not demonstrating the SL interaction.
- The mechanisms by which ROCKi specifically affects BRCA2-defective cells are elusive.
- It remains unclear what the cause of the multiple mitotic defects is.Combined, it remains unclear if the identified SL interaction is therapeutically meaningful. Clinical stage inhibitors are available, and various BRCA2-deficient cancer models have been described, allowing the authors to address this in long-term in vitro assays and in vivo assays. Also, the authors describe multiple phenotypic consequences, but the order of events and the reason why the effects are specific to BRCA2 remain largely unclear. Furthermore, the notion that the observed effects are independent of replication defects requires further substantiation.
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Reviewer #3 (Public Review):
Martino et al. demonstrated that BRCA2-deficient cells (but not BRCA1-deficient cells) bear additional vulnerabilities (i.e., cytokinesis failure) outside S phase that could represent new synthetic lethality targets. Strengths of the study include the ability of the authors to recapitulate the cell death by ROCK inhibition by inhibiting another key cytokinesis enzyme, CITK. The claims are well supported by the data, and because the study indicates HR failure/replication stress is not the only possible way to achieve synthetic lethality in BRACA mutant cancers the study will be of broad interest to potential readers.
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