MLL protects CpG clusters from methylation within the Hoxa9 gene, maintaining transcript expression

Abstract

Homeobox (HOX) genes play a definitive role in determination of cell fate during embryogenesis and hematopoiesis. MLL-related leukemia is coincident with increased expression of a subset of HOX genes, including HOXA9. MLL functions to maintain, rather than initiate, expression of its target genes. However, the mechanism of MLL maintenance of target gene expression is not understood. Here, we demonstrate that Mll binds to specific clusters of CpG residues within the Hoxa9 locus and regulates expression of multiple transcripts. The presence of Mll at these clusters provides protection from DNA methylation. shRNA knock-down of Mll reverses the methylation protection status at the previously protected CpG clusters; methylation at these CpG residues is similar to that observed in Mll null cells. Furthermore, reconstituting MLL expression in Mll null cells can reverse DNA methylation of the same CpG residues, demonstrating a dominant effect of MLL in protecting this specific region from DNA methylation. Intriguingly, an oncogenic MLL-AF4 fusion can also reverse DNA methylation, but only for a subset of these CpGs. This method of transcriptional regulation suggests a mechanism that explains the role of Mll in transcriptional maintenance, but it may extend to other CpG DNA binding proteins. Protection from methylation may be an important mechanism of epigenetic inheritance by regulating the function of both de novo and maintenance DNA methyltransferases.homeodomain ͉ leukemia ͉ maintenance

Epigenetic regulations in hematopoietic Hox code

“…Loss of the SET domain is associated with a dramatic reduction in histone H3 lysine 4 monomethylation and DNA methylation defects at the same Hox loci (Terranova et al, 2006). Although its DNMT1 domain binds in vitro to unmethylated CpG-rich DNA (Fuks et al, 2000), Mll binds to specific clusters of CpG residues within the Hoxa9 locus and protects the CpG clusters from DNA methylation in murine embryonic fibroblast cells (Erfurth et al, 2008), providing another mechanism to maintain Hox gene activation.…”

Methylation-mediated repression of microRNA-143 enhances MLL–AF4 oncogene expression

“…Loss of the carboxy-terminal regions of MLL in chimeric oncoproteins would be predicted to result in abrogation of transactivation and histone methyl-transferase functions. In contrast, most transforming MLL fusion proteins function as transcriptional regulators and induce aberrant expression of downstream MLL targets (Metzler et al, 2006;Erfurth et al, 2008). Therefore, we predict that the different prognosis of patients with various MLL rearrangements is likely dependent on the partner proteins to which MLL is fused.…”

“…The t(4;11)(q21;q23) is the second most popular chromosomal translocation in adults with ALL. However, the detailed role of this fusion protein in leukemogenesis is not well understood (Erfurth et al, 2008;Liu et al, 2009). All MLL fusion proteins retain the amino-terminal portion containing AT hooks and the MLL CxxC domain, thus preserving DNA-binding activity.…”

MicroRNAs: new players in acute myeloid leukaemia

“…Leukaemias related with MLL are in most of the cases characterised by an over-expression of a subset of HOX genes including HOXA9 (Rozovskaia et al, 2001). MLL binds to specific clusters of CpG islands of HOXA9, adjacent to miR-196b, and maintains its expression by protecting these clusters from DNA methylation (Erfurth et al, 2008). Recently, Popovic et al (2009) showed that MLL regulates miR-196b expression similarly to the surrounding HOX genes.…”