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CDK4 or CDK6 deletion in adult hematopoiesis
contribute to the “control” of myeloid progenitor expan- sion. CDK4 deficiency may delocalize CDK6 from regu- latory transcriptional complexes involved in cytokine production to its essential task in the cell cycle. A re-local- ization of CDK6 in the absence of CDK4 has been observed in melanoma, where CDK6 was “withdrawn” from its angiogenesis-promoting role.47
CDK6 interferes with myeloid differentiation by differ- ent ways: CDK6 inhibits granulocyte differentiation by blocking RUNX1 from interacting with C/EBPα and DNA binding.8 CDK6, but not CDK4, inhibits myeloid differen- tiation in MLL-AF9 expressing leukemia9 and CDK6 defi- ciency ameliorates the hallmarks of JAK2V617F-driven myelo- proliferative neoplasms.18 In line, Cdk6D/D mice display an impaired myeloid differentiation resulting in less CD11b+Gr1hi cells mirroring the frequent occurrence of neutropenia in patients receiving CDK4/6 inhibitor treat- ment.
In Cdk6-/- mice T-cell maturation is impaired.7 We observed elevated numbers of CLP and CD3+ T cells in BM of Cdk6D/D but not of Cdk4D/D mice. CDK4/6 inhibitors affect T-cell subpopulations differentially: activation of effector T cells is enhanced27,48 while regulatory T cells are suppressed.49 The accumulation of lymphoid progenitors pinpoints to an involvement of CDK6 in exiting the lym- phoid progenitor state reminiscent of CDK6’s role in leav- ing the quiescence state in HCS. Mx1-Cre induced gene deletion is inconsistent in the thymus (data not shown and50) impeding any T-cell development analysis.
Total Cdk4-/- and Cdk6-/- mice do not mirror the adverse events observed in CDK4/6 inhibitor-treated patients. The models presented in this study now enable the char- acterization of the specific roles of CDK4 or CDK6 in cell cycle and beyond as artefacts that appear in total knock- out mice starting from embryogenesis are excluded. To date, only one tissue-specific study on the functions of CDK4 has been reported in adipocytes.51
CDK4- or CDK6-specific inhibitors would reduce side effects and represent a step towards precision medicine: while CDK4 inhibitors may be successful in hereditary
melanoma where activating CDK4 mutations are fre- quent,52,53 acute myeloid leukemia is associated with aber- rant CDK6 signaling.54 The inducible deletion of CDK6 represents an appropriate estimate for adverse effects of CDK6-targeting PROTACs.31-33
We here report the consequences of induced ablation of CDK4 and CDK6 in adult mice. The side-by-side analysis of induced deletion of CDK4 or CDK6 links anemia and neutropenia, both the most frequent adverse events in CDK4/6 therapy, to the inhibition of CDK6. The observed hematological side effects in patients may result from the accumulation of dormant HSC combined with defective myeloid differentiation and erythroid develop- ment after CDK6 inhibition.
Taken together, the novel Cdk4fl/fl and Cdk6fl/fl mouse models are powerful tools to unravel cell type-specific mechanisms and side effects of CDK4/6 therapy to improve therapeutic treatment regimens.
Disclosures
No conflicts of interest to disclose.
Contributions
BM, TB and MPM planned and conducted experiments and analyzed the data; SK did experiments; VS supervised the study; BM, TB, MPM and VS wrote the manuscript.
Acknowledgments
The authors would like to thank the Centre for Phenogenomics, Toronto, Canada, for generating Cdk4 and Cdk6 floxed mice. We are indebted to the animal caretaker as well as the genotyping team. Thanks to Philipp Jodl for excellent technical support.
Funding
This work has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation program grant agreement 694354 (VS) and was also supported by the Austrian Science Fund grant SFB-F6107 and F6101.
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