Genetic interference with Taspase1 heterodimerization will not aid to target t(4;11) leukemias

The chromosomal translocation t(4;11)(q21;q23) is associated with high-risk acute lymphoblastic leukemia of infants. The resulting AF4•MLL oncoprotein becomes activated by Taspase1 hydrolysis, leading to oncogenic transcriptional activation. Hence, Taspase1’s proteolytic activity is a critical step in AF4•MLL pathophysiology. The Taspase1 proenzyme is autoproteolytically processed in its a- and b-subunits and is assumed to assemble into an abba-heterodimer, the active protease. However, studies addressing Taspase1’s multimerization in vivo were missing. Hence, we probed Taspase1 multimerization in living cancer cells and attempted to interfere with its function by blocking the formation of active Taspase1 multimers through enforced expression of inactive Taspase1 mutants.

The consequences of overexpressing the catalytically dead Taspase1 mutant, Taspase1^T234V, or the highly attenuated variant Taspase1^D233A, on Taspase1’s processing of AF4•MLL and of other bona fide Taspase1 targets was analyzed in living cancer cells employing an optimized cell-based assay. Notably, even a nine-fold overexpression of the respective Taspase1 mutants neither inhibited Taspase1’s cis- nor trans-cleavage activity in vivo, arguing against heterocomplex formation required for its (patho)biological activity. Likewise, enforced expression of the inactive a- or b-subunits showed no trans-dominant effect against the ectopically or endogenously expressed enzyme. Probing Taspase1 multimerization in living cells further demonstrated that neither wild type Taspase1 nor inactive variants assemble into stable (hetero)complexes. Hence, inefficient heterodimerization appears to be the mechanism why inactive Taspase1 variants fail to inhibit Taspase1’s activity in trans.

Collectively, our results support a model in which Taspase1 is active already as an ab-monomer. Consequently, therapeutic strategies targeting Taspase1 multimerization by genetic or chemical decoys seem unlikely to interfere with the AF4•MLL oncoprotein and disease.