Proteolytic cleavage by taspase1 and the regulation of the stability of MLL1

Chromosomal translocations of the Mixed-lineage leukemia 1 (MLL1) gene generate MLL-chimeras that drive pathogenesis of acute myeloid and lymphoid leukemia. The untranslocated MLL1 is a substrate for proteolytic cleavage by the endopeptidase, taspase1, however, the biological significance of MLL1 cleavage by this endopeptidase remains unclear. Here, we demonstrate that taspase1-dependent cleavage of MLL1 results in the destabilization of full-length MLL. Upon loss of taspase1, MLL1 association with chromatin is markedly increased due to the stabilization of its unprocessed version and this stabilization of the uncleaved MLL1 can result in the displacement of MLL-chimeras from chromatin in leukemic cells. Casein kinase II (CKII) phosphorylates MLL1 proximal to the taspase1 cleavage site, facilitating its cleavage, and pharmacological inhibition of CKII blocks taspase1-dependent MLL1 processing, increases MLL1 stability, and results in the displacement of the MLL-chimeras from chromatin. Furthermore, inhibition of CKII in MLL-AF9 mouse model of leukemia delayed leukemic progression in vivo. This study provides insights into the direct regulation of the stability of MLL1, which can be harnessed for targeted therapeutic approaches for the treatment of aggressive MLL leukemia. To characterize the functions of MLL1 cleavage by taspase1 using taspase1 knockout human cancer cell lines