Ali Shilatifard, Stowers Institute for Medical Research, Kansas City, USA   Website

Lessons learned from yeast about human leukaemia

The mixed lineage leukaemia gene, MLL, is involved in several translocations common in acute leukaemia. The MLL translocations are most commonly translocations seen in infant leukaemia, as well as, in acute leukaemia arising after treatment of a primary malignant disease with topoisomerase II inhibitors such as etoposide and daunorubicin. To gain a better insight as to the role of MLL, we have taken advantage of genetics and biochemical tools in S. cerevisiae to identify the molecular properties of the yeast MLL homologue, the Set1 protein. We isolated Set1 in a macromolecular complex, which we named COMPASS (COMplex of Proteins ASsociated with Set1). COMPASS was the first histone H3 lysine 4 (H3K4) methylase isolated and shown to be associated with transcribing RNA polymerase II. Using biochemical and genetic studies in yeast, we have also identified the molecular pathway required for COMPASS’s function. We have demonstrated that histone H2B monoubiquitination by Rad6/Bre1 is required for histone H3K4 trimethylation by COMPASS. Moreover, our laboratory and others have demonstrated that the human MLL complex is also found in a COMPASS-like complex capable of methylating H3K4 and that the molecular machinery required for the regulation of this histone crosstalk and H3K4 methylation is highly conserved from yeast to human. Indeed, there is only one Set1 in yeast, yet in mammalian cells there are multiple H3K4 methylases including Set1A/B forming the human COMPASS complexes, and the MLL1-4 forming the hCOMPASS-like complexes. Work from our laboratory recently demonstrated that Wdr82, which associates with chromatin in a histone H2B ubiquitination-dependent manner, is a specific component of the Set1 complexes, but not that of the MLL1-4 complexes. RNAi-mediated knockdown of Wdr82 results in a reduction in the H3K4 trimethylation levels, although, these cells still possess active MLL complexes. Comprehensive in vitro enzymatic studies with the Set1 and the MLL complexes demonstrated that the Set1 complex is a more robust H3K4 trimethylase in vitro than the MLL complexes. Given the in vivo and in vitro observations, it appears that the human Set1 complex plays a more widespread role in H3K4 trimethylation than the MLL complexes in mammalian cells. This fundamental observation indicates that MLL1 has specific functional targets in the mammalian genome and the identification of such targets could provide us with new avenues for the therapeutic intervention of MLL translocation-based leukaemia. We have indeed identified many of these MLL functional targets and plan to describe them in the meeting.

Lee J.S., et al. (2010) The language of histone crosstalk. Cell. 142: 682-5.

Eissenberg J.C. and Shilatifard A. (2010) Histone H3 lysine 4 (H3K4) methylation in development and differentiation. Dev Biol. 339: 240-9.

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