Bimodal Degradation of MLL Protein by the Cell Cycle SCFSkp2 and APCCdc20 E3 Lilgases Assures Cell Cycle Execution: A Critical Regulatory Circuit Lost in Leukemogenic MLL-Fusions.

Abstract

The MLL (mixed lineage leukemia) gene encodes a highly conserved 3,969 aa histone H3 K4 methyl transferase, of which chromosome translocations result in poor prognostic infant and therapy-related leukemias. Mysteriously, the pathognomonic MLL leukemia fusions consist of a common amino(N)-terminal ∼1400 aa of MLL fused in frame with more than 60 different partners with no shared characteristics. The most well known genetic function of MLL is to antagonize polycomb group of proteins for proper Hox gene expression. Consequently deregulated Hox genes caused by MLL translocations contribute to the MLL leukemogenesis. Besides Hox genes, it remains largely undetermined whether MLL participates in other biological processes. The 500kD MLL precursor undergoes evolutionarily conserved proteolytic maturation mediated by Taspase1 (Hsieh et al. 2003, MCB, 23, 186–194; Hsieh et al. 2003, Cell, 115, 293–303). Our recent studies on Taspase1 knockout cells established an MLL-E2F axis in orchestrating core cell cycle gene expression including Cyclins and possibly Cdk inhibitors (Takeda et al. 2006, Genes & Development , 20, 2397–2409). As MLL actively participates in the cell cycle regulation, we investigated the regulation of MLL through cell cycle transition. We uncovered a unique biphasic expression of MLL conferred by defined windows of degradation mediated by specialized cell cycle E3 ligases. Specifically, SCFSkp2 and APCCdc20 mark MLL for degradation at S phase and late M phase, respectively. Abolished peak expression of MLL incurs corresponding defects in G1/S transition and M phase progression. Conversely, over-expression of MLL blocks S phase progression. Remarkably, MLL degradation initiates at its N-terminal ∼1400 aa that is retained in all MLL leukemia fusions. We examined prevalent MLL-fusions, including MLL-AF4, MLL-AF9, MLL-ELL and MLL-ELL, and observed their increased resistance to degradation. Furthermore, the same resistance was observed with the leukemogenic MLL-lacZ but not the non-leukemogenic MLL-Myc tag fusion. Thus, non-oscillating expression of MLL-fusions through the cell cycle, resulted from impaired degradation, likely constitutes the universal mechanism underlying all MLL leukemias. Our data conclude an essential post-translational regulation of MLL by the cell cycle ubiquitin/proteasome system (UPS) to assure the temporal necessity of MLL in coordinating cell cycle progression. Future studies aim at providing a comprehensive analysis on the cell cycle consequences associated with MLL-fusions using genetically modified cells derived from mice carrying various MLL-fusion knockin alleles, including MLL-AF4, MLL-AF9, and MLL-CBP.