Mixed lineage leukemia histone methylases in gene regulation and cell cycle.

摘要

Histone methyl-transferases (HMTs) are key enzymes that post-translationally methylate nuclear histone proteins and play critical roles in gene expression, epigenetic regulation and diseases in eukaryotic organisms. Mixed lineage leukemias (MLLs) are human HMTs that specifically methylate histone H3 at lysine-4 and regulate gene activation. MLLs are also well known to undergo rearrangement often in acute myeloid and lymphoid leukemias. Human DNA encodes several histone H3 lysine-4 (H3K4) specific methyl-transferases (HMTs) such as MLL1 (mixed lineage leukemia gene 1), MLL2, MLL3, hSet1 etc, which play critical roles in gene expression. These HMTs are present as distinct multi-protein complexes with several proteins in common. Herein, a human CpG dinucleotide binding protein (CGBP) has been affinity purified and characterized along with its interacting proteins from human cells. It was demonstrated that CGBP is co-purified with three H3K4 specific HMTs MLL1, MLL2, and hSet1. MLL1 is the primary source of H3K4 specific HMT activity present in affinity purified CGBP associated proteins. In addition, CGBP is co-localized with MLL1, MLL2 and hSet1 in vivo and binds to the promoter of MLL target gene HoxA7. These results demonstrated that CGBP interacts with MLL1, MLL2 as well as hSet1 HMTs and is likely a common interacting component of these three HMTs.;As chromatin condensation, relaxation, and differential gene expression are keys to proper cell cycle progression, the dynamic association of MLL and H3K4-trimethylation at different stages of cell cycle was analyzed. Interestingly, MLL1 that is normally associated with transcriptionally active chromatins (G1 phase) dissociated from condensed mitotic chromatin and returned back at the end of telophase when the nucleus starts to relax. In contrast, H3K4-trimethyl marks, which are also normally associated with the euchromatins (in G1), remained associated even with condensed chromatin throughout the cell cycle. The global levels of MLL1 and H3K4-trimethylation are not affected during cell cycle, while the H3Ser28 phosphorylation was only observed during mitosis. Interestingly, MLL target Hox genes (HoxA5, A7 and A10) were differentially expressed during the cell cycle and the recruitment of MLL1 and H3K4-trimethylation levels were modulated in the promoter of those Hox genes as a function of their expression. In addition, down regulation of MLL1 resulted in cell cycle arrest at G2/M phase. The fluctuation of H3K4-trimethylation marks at specific promoters but not at the global level indicates that H3K4-trimethylation marks that are present in the G1-phase may not be the same marks in other phases of cell cycles, rather old marks are removed and new marks are introduced. In conclusion, our studies demonstrated that MLL1 and H3K4-methylation have distinct dynamics during the cell cycle and play critical roles in differential expression of Hox genes associated with cell cycle regulation.