Abstract
Whilst not directly influencing the kinase activity of CDKs, Cks1 and Cks2 proteins are known to be important for substrate specificity – which is essential for timely cell cycle progression. Further to this, Cks1 and Cks2 have been shown to possess CDK independent functions, in concert with the SCFSkp2 or APCCdc20 E3 ligases, fine tuning the activity of CDKs and cell cycle progression. A Cks1/Cks2 axis has been proposed to be important for balancing the phosphorylation and degradation of an array of proteins during the cell cycle, controlling a variety of signalling pathways important in stem cell homeostasis and development.This study identifies a new target of the Cks1/Cks2 axis, the Mixed lineage leukaemia 1 (Mll1) protein. Both the MllN and MllC subunits of Mll1 interact with Cks1 and Cks2, and the cell cycle periodicity of Mll1 is severely altered in Cks-deficient MEFs. The gene transcription function of Mll1 is significantly altered in Cks-deficient MEFs, revealing an enrichment of altered Wnt signalling genes. The Wnt signalling pathway appeared to be severely downregulated in Cks1-/- MEFs and greatly upregulated in Cks2-/- MEFs. Altered Wnt signalling activity is partially dependent on Mll1 protein levels in Cks-deficient MEFs, as exogenous addition or removal of Mll1 can partially restore the WT phenotype.
Chromosome translocations disrupting the MLL1 gene, producing MLL-Fusion Proteins (MLL-FPs), result in poor prognostic mixed lineage leukaemias. There is currently great interest in targeting the cell cycle and Wnt signalling for leukaemia treatment, independent of MLL-FPs, to combat the most difficult to eradicate highly quiescent leukaemic stem cells.
Here, we found CKS1 and CKS2 interact with some, but not all, MLL-FPs, and both genes modulate Wnt signalling in MLL-AF9 and MLL-ENL carrying cell lines. Targeting the protein degradation function of CKS1 with a small molecule inhibitor of SKP2-CKS1 (C1), and more broadly a pan-Cullin degradation inhibitor (MLN4924), showed reduction of viability in both MLL-AF9 and MLL-ENL cell lines. Investigations with patient blasts carrying MLL-translocations reveal severe reduction in expression of CKS1 and CKS2 compared to healthy controls.
Together, this study highlights the importance of unearthing new players in the Cks1/Cks2 axis, with implications in development and disease. The potential exploitation of CKS1 and CKS2 as therapeutic targets in haematological malignancies shows great promise for individual and combinatorial therapies in the future.
| Date of Award | 2015 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ellen Solomon (Supervisor) |