MLL5, a trithorax homolog, indirectly regulates H3K4 methylation, represses cyclin A2 expression, and promotes myogenic differentiation

Abstract

Most cells in adult tissues are nondividing. In skeletal muscle, differentiated myofibers have exited the cell cycle permanently, whereas satellite stem cells withdraw transiently, returning to active proliferation to repair damaged myofibers. We have examined the epigenetic mechanisms operating in conditional quiescence by analyzing the function of a predicted chromatin regulator mixed lineage leukemia 5 (MLL5) in a culture model of reversible arrest. MLL5 is induced in quiescent myoblasts and regulates both the cell cycle and differentiation via a hierarchy of chromatin and transcriptional regulators. Knocking down MLL5 delays entry of quiescent myoblasts into S phase, but hastens S-phase completion. Cyclin A2 (CycA) mRNA is no longer restricted to S phase, but is induced throughout G 0/G1, with activation of the cell cycle regulated element (CCRE) in the CycA promoter. Overexpressed MLL5 physically associates with the CCRE and impairs its activity. MLL5 also regulates CycA indirectly: Cux, an activator of CycA promoter and S phase is induced in RNAi cells, and Brm/Brg1, CCRE-binding repressors that promote differentiation are repressed. In knockdown cells, H3K4 methylation at the CCRE is reduced, reflecting quantitative global changes in methylation. MLL5 appears to lack intrinsic histone methyl transferase activity, but regulates expression of histone-modifying enzymes LSD1 and SET7/9, suggesting an indirect mechanism. Finally, expression of muscle regulators Pax7, Myf5, and myogenin is impaired in MLL5 knockdown cells, which are profoundly differentiation defective. Collectively, our results suggest that MLL5 plays an integral role in novel chromatin regulatory mechanisms that suppress inappropriate expression of S-phase-promoting genes and maintain expression of determination genes in quiescent cells.Brm ͉ CCRE ͉ Pax7 ͉ quiescence ͉ reversible arrest

Genomic instability and myelodysplasia with monosomy 7 consequent to EVI1 activation after gene therapy for chronic granulomatous disease

“…In this context, it is noteworthy that the myeloid/lymphoid or mixed-lineage leukemia 5 gene (MLL5), a trithorax homolog with a crucial role in adult hematopoiesis and a postulated role in chromatin remodeling and in the control of cellular growth, was recently mapped to 7q22, a region that is frequently deleted in AML [54][55][56][57] .…”

Camptothecin-induced downregulation of MLL5 contributes to the activation of tumor suppressor p53

“…However, such downregulation might not be directly related to the inhibition of Top1, as b-lapachone, another class of Top1 inhibitor with a mode of action distinct from CPT, was unable to degrade MLL5 or activate p53 through Ser392 phosphorylation (Supplementary Figure S9). RNAi-mediated knockdown of MLL5 was previously shown to delay or arrest the G 1 /S transition (Cheng et al, 2008;Sebastian et al, 2009). Therefore, it is plausible that the elimination of MLL5 is required for delaying the cell cycle progression, allowing deployment of damage repair mechanism.…”

“…Ectopically overexpressed MLL5 caused cell cycle arrest at G 1 /S phase (Deng et al, 2004), whereas siRNA-mediated knockdown of MLL5 arrested cell cycle at both G 1 /S and G 2 /M phases (Cheng et al, 2008). Upon MLL5 knockdown, the entry of quiescent myoblasts into S-phase was delayed, but the completion of S-phase progression was hastened (Sebastian et al, 2009). Recently, we showed that phosphorylation of MLL5 by mitotic kinase Cdc2 is required for mitotic entry (Liu et al, 2010b).…”

“…In contrast, MLL5 lacks the cysteine-rich post-SET domain found in the rest of family members, which is believed to be required for histone methyltransferase activity (Kouzarides, 2002). No intrinsic histone methyltransferase activity was found on MLL5, although it physically associates with cyclin A2 promoter and indirectly regulates H3K4 methylation through histone-modifying enzymes LSD1 and SET7/9 (Sebastian et al, 2009). However, a short N-terminal isoform of MLL5, containing both PHD and SET domain, once glycosylated was discovered to act as a mono-and di-methyltransferase of H3K4 in promyelocytic leukemia cells (Fujiki et al, 2009).…”

Phosphorylation of Mixed Lineage Leukemia 5 by Cdc2 Affects Its Cellular Distribution and Is Required for Mitotic Entry

“…Whether MLL5 possesses the histone lysine methyltransferase activity is still highly debatable. Several reports suggested that MLL5 lacks such intrinsic methyltransferase activity (14,25). However, a short N-terminal isoform of MLL5 that contains both the PHD and SET domain was recently identified in a multisubunit complex in association with nuclear retinoic acid receptor-␣ (26), in which the isoform acts as a mono-and dimethyltransferase to H3K4.…”