Uncoupling CDK6 from p16INK4A - effects for hematopoiesis and lymphomagenesis

Cell cycle deregulation is a common feature of cancer (Cordon-Cardo 1995; Malumbres and Barbacid 2001; Deshpande, Sicinski et al. 2005; Kim and Diehl 2009). Accordingly, cell cycle kinases represent potential targets for the development of therapeutic drugs. In this study we intend to analyse the importance of the interaction between CDK6 and the cell cycle inhibitor family INK4 for hematopoietic stem cells and lymphoid leukemia formation. INK4 family members have been shown to exert a regulatory function in hematopoietic stem cell (HSC) cycling, self-renewal and repopulation. This finding led to the concept that G1-kinases have a major impact on HSC hemostasis and that the efficacy of bone marrow transplantation depends on the CDK4/6 kinase activity. A disturbed HSC hemostasis may represent an important side effect of CDK6 inhibitor treatment. As CDK6 is consistently up-regulated in lymphoid malignancies and we have shown that CDK6 regulates p16INK4a expression, we also plan to investigate the functional consequences of the CDK6/p16 INK4a interplay. The proposed studies will enhance our understanding of the molecular machinery driving lymphoid malignancies and should allow improved therapeutic strategies to make optimal use of CDK6 inhibitors that have been developed as anti-cancer drugs.

The cell cycle kinases CDK4 and CDK6 were long considered to be only functional during growth and proliferation of a cell. Inhibitors against CDK4/6 have been developed and have been declared a breakthrough in therapy by the FDA in 2013. We here show that the effects of these substances go way beyond cell cycle control and inhibiting cell proliferation. We uncovered that CDK6, but not its close homologue CDK4 has a second life and acts as a transcriptional regulator to regulate gene transcription in a cell. CDK6 can induce as well as suppress gene transcription. Thereby CDK6 inhibitors not only block cell growth but also interfere with other major functions in a cell. Red blood cells require CDK6 to stabilize their cytoskeleton, in the absence of CDK6 or upon CDK4/6 inhibition they display an enhanced fragility. CDK6 is of particular importance in the blood system where CDK6 regulates the activation status of hematopoietic stem cells or blood stem cells. Only in the presence of CDK6 a stem cell can leave its dormant state and become activated and respond to stress conditions. This effect extends to leukemic stem cells and does not depend on the kinase activity of CDK6. Of note leukemic cells drastically upregulate CDK6 upon transformation. Dependent on the hematopoietic cell type CDK6 is then involved in regulating different gene transcription which occurs in a kinase dependent and kinase independent manner. As such CDK6 regulates tumor angiogenesis in lymphoid cells, novel blood vessels are required to supply a tumor with oxygen and nutrition. In a subtype of acute myeloid leukemia (AML) CDK6 regulates expression of the Flt-ITD gene, that is critical for leukemogenesis as it regulates cell survival. In summary our work shows that CDK6 is much more than a cell cycle kinase but a broad regulator of gene transcription thereby offering many new opportunities for therapeutic interference.