Combination Therapy Targeting CKS1-Dependent Protein Degradation Reduces AML Burden Whilst Protecting Normal Haematopoietic Stem Cellsfrom Cytarabine Toxicity

Human chromosome translocations at 11q23, disrupting the MLL1 gene, result in poor prognostic mixed lineage leukaemias. Current chemotherapy treatment protocols produce an unsatisfactory outcome. Indeed, the average five-year event free survival rate is 44% in paediatric cases, and adult cases have been estimated as low as 15% for two-year survival rates, indicating there is an unmet critical need for more effective therapies. In recent years, there has been great interest in targeting the epigenetic factors involved in MLL-rearranged (MLL-r) leukaemic transformation and maintenance; however, epigenetic plasticity, the potential role of the remaining MLL1 allele and the elusive leukaemic stem cells present in acute myeloid leukaemia (AML), provide many routes to chemoresistance. There is currently great interest in targeting the cell cycle and key intracellular signalling pathways (e.g. Wnt signalling), independent of specific aberrant lesions in AML (e.g. MLL-fusion proteins, DNMT3a mutants), to combat highly quiescent leukaemic stem cells, which are the most difficult to eradicate. In addition, protection of the resident normal haematopoietic stem cells (HSCs), during aggressive induction chemotherapy protocols, provides another route to reduce the competitive advantage of AML cells in vivo.

We previously identified two new genes, involved in the regulation of MLL1, Wnt signalling and the cell cycle: the CDK subunits CKS1 and CKS2 (Grey et al. 2017). Here, we investigated the roles of CKS1 and CKS2 during normal and malignant haematopoiesis in vivo, revealing differences in key signalling pathways involved in haematopoiesis and leukaemogenesis, implicating the CKS1/CKS2 axis as a valid therapeutic target. We demonstrate that primary AML patient samples, engrafted in immune deficient mice, are sensitive to inhibition of CKS1-dependent protein degradation, with reduced tumour burden after treatment and significant improvement in survival times. In addition, patient samples showed CKS1-sensitivity irrespective of inherent resistance to Cytarabine.

Current chemotherapy protocols, using Cytarabine and Doxorubicin, can be significantly deleterious to resident normal HSCs in vivo. Transient inhibition of CKS1-dependent protein degradation, in vivo, provides a protective function to human CD34⁺ HSPCs when treated with Cytarabine/Doxorubicin (5+3 dosing protocol), resulting in reduced apoptosis and increased stem cell potential post-therapy. Importantly, combination treatment of CKS1 inhibition with Cytarabine/Doxorubicin significantly reduces AML tumour burden and improves overall survival, by selectively killing AML cells and preserving normal resident HSCs.

Altogether, these results open a promising alternative approach for modulating protein phosphorylation and degradation to selectively target leukaemic cells, with the great advantage to protect normal resident HSCs from cytotoxic effects of induction chemotherapy.