Reversal of MYB-dependent suppression of MAFB expression overrides leukaemia phenotype in MLL-rearranged AML

The transcription factor MYB plays a pivotal role in haematopoietic homoeostasis and its aberrant expression is involved in the genesis and maintenance of acute myeloid leukaemia (AML). We have previously demonstrated that not all AML subtypes display the same dependency on MYB expression and that such variability is dictated by the nature of the driver mutation. However, whether this difference in MYB dependency is a general trend in AML remains to be further elucidated. Here, we investigate the role of MYB in human leukaemia by performing siRNA-mediated knock-down in cell line models of AML with different driver lesions. We show that the characteristic reduction in proliferation and the concomitant induction of myeloid differentiation that is observed in MLL-rearranged and t(8;21) leukaemias upon MYB suppression is not seen in AML cells with a complex karyotype. Transcriptome analyses revealed that MYB ablation produces consensual increase of MAFB expression in MYB-dependent cells and, interestingly, the ectopic expression of MAFB could phenocopy the effect of MYB suppression. Accordingly, in silico stratification analyses of molecular data from AML patients revealed a reciprocal relationship between MYB and MAFB expression, highlighting a novel biological interconnection between these two factors in AML and supporting new rationales of MAFB targeting in MLL-rearranged leukaemias.

Autophagy induction in atrophic muscle cells requires ULK1 activation by TRIM32 through unanchored K63-linked polyubiquitin chains

Optimal autophagic activity is crucial to maintain muscle integrity, with either reduced or excessive levels leading to specific myopathies. LGMD2H is a muscle dystrophy caused by mutations in the ubiquitin ligase TRIM32, whose function in muscles remains not fully understood. Here, we show that TRIM32 is required for the induction of muscle autophagy in atrophic conditions using both in vitro and in vivo mouse models. Trim32 inhibition results in a defective autophagy response to muscle atrophy, associated with increased ROS and MuRF1 levels. The proautophagic function of TRIM32 relies on its ability to bind the autophagy proteins AMBRA1 and ULK1 and stimulate ULK1 activity via unanchored K63-linked polyubiquitin. LGMD2H-causative mutations impair TRIM32's ability to bind ULK1 and induce autophagy. Collectively, our study revealed a role for TRIM32 in the regulation of muscle autophagy in response to atrophic stimuli, uncovering a previously unidentified mechanism by which ubiquitin ligases activate autophagy regulators.