P1407: CDK6 IN HEMATOPOIETIC STEM CELLS: MORE THAN A CELL CYCLE KINASE

Abstract

Background: Cell cycle kinase CDK6 acts as a transcriptional regulator and links cell cycle progression to differentiation and cellular homeostasis. CDK6 has been implicated in the initiation and progression of lymphoid and myeloid malignancies and regulates the balance of long-term hematopoietic stem cell (LT-HSC) dormancy and activation. CDK6 acts in a kinase-dependent and kinase-independent way to regulate haematopoiesis. Aims: As CDK4/6 inhibitors have entered the clinics, we aim at understanding and dissecting kinase-dependent and independent-functions of CDK6 in adult HSCs with application of different stress conditions. Methods: To investigate kinase-independent effects of CDK6 in HSCs, we made use of a mouse model harbouring a kinase inactivated mutant of CDK6 (Cdk6KM/KM). We performed in vivo serial transplantation and homing assays to study HSC re-population capacity. Low-input RNA-Seq of LT-HSCs after the 2nd serial transplantation round was used to understand transcriptional effects. Stressed haematopoiesis was further induced by chemical treatment with polyI:C. Results:Cdk6-/- HSCs are impaired in their ability to re-populate. This was not the case for Cdk6KM/KM hematopoietic cells evidenced by re-plating assays in vitro and serial transplantation settings in vivo. Cdk6KM/KM derived LT-HSCs maintain a stem-cell signature and express genes indicative for HSC dormancy at high levels compared to wild type LT-HSCs. Transcriptional analysis pointed to an altered and improved homing of Cdk6KM/KM LT-HSCs, which was confirmed in homing assays in vivo. This indicates a repressive role of CDK6 in stem cell homing. Upon polyI:C treatment, Cdk6KM/KM mice reacted with decreased stimulation as more Lin-Sca-1+c-kit+ (LSK) cells were residing in the bone marrow. Summary/Conclusion: Our data uncovers kinase-dependent and kinase-independent functions of CDK6 for HSC self-renewal and homing. Whereas serial transplantation settings and homing depend on kinase-inactive CDK6, poly-I:C treatment relies on kinase-activity.