Effects of mixed lineage leukaemia, the human homologue of Trithorax, and its leukaemic fusion proteins in Drosophila melanogaster

Mixed Lineage Leukaemia (MLL) refers to the protein implicated in translocating with over 30 partner genes to form fusion proteins that lead to the development of acute leukaemia. MLL mediated leukaemia predominantly afflicts children under 5 years of age or younger, and is associated with very poor survival rates. Although MLL shares limited homology with the Drosophila Trithorax (TRX) protein, both MLL and TRX are speculated to alter gene expression profiles by modulating higher order chromatin structures. The work presented here introduces Drosophila melanogaster as a supplementary model system for understanding how MLL leukaemic fusion proteins can contribute to leukemogenesis. The expression in the Drosophila system of the two most widely generated fusion proteins in leukaemic patients, MLL-AF9 and MLL-AF4, has defined pathway(s) through which the proteins may interfere with to induce fly lethality. Both fusion proteins were found to affect higher order chromatin structure by modifying the méthylation pattern of histone H3 at lysine 9. Modified higher order chromatin modulates pathways targeted by specific MLL fusion protein, suggesting that the C-terminal portion of the fusion proteins triggers this step in cell cycle deregulation. Finally, the finding that late trx lethals are also associated with similar modified higher order chromatin structures displayed by MLL fusion proteins uncovered an additional role for TRX in the late development of Drosophila. The results provide guidance in understanding why so many PcG and trxG members cause cancer when mutated, and suggest the existence of additional and distinct functions by PcG and trxG proteins at late development. The expression of MLL fusion proteins in Drosophila unveils relevant and concrete pathways through which particular MLL fusion proteins may act to prevent proper cell cycle progression. Put simply, this study links the targeting by MLL fusion proteins to TRX functions required for completion of late Drosophila development

DNA Hypermethylation in Drosophila melanogaster Causes Irregular Chromosome Condensation and Dysregulation of Epigenetic Histone Modifications

The level of genomic DNA methylation plays an important role in development and disease. In order to establish an experimental system for the functional analysis of genome-wide hypermethylation, we overexpressed the mouse de novo methyltransferase Dnmt3a in Drosophila melanogaster. These flies showed severe developmental defects that could be linked to reduced rates of cell cycle progression and irregular chromosome condensation. In addition, hypermethylated chromosomes revealed elevated rates of histone H3-K9 methylation and a more restricted pattern of H3-S10 phosphorylation. The developmental and chromosomal defects induced by DNA hypermethylation could be rescued by mutant alleles of the histone H3-K9 methyltransferase gene Su(var)3-9. This mutation also resulted in a significantly decreased level of genomic DNA methylation. Our results thus uncover the molecular consequences of genomic hypermethylation and demonstrate a mutual interaction between DNA methylation and histone methylation