MLL associates specifically with a subset of transcriptionally active target genes

Abstract

MLL (mixed-lineage leukemia) is a histone H3 Lys-4 specific methyltransferase that is a positive regulator of Hox expression. MLL rearrangements and amplification are common in acute lymphoid and myeloid leukemias and myelodysplastic disorders and are associated with abnormal up-regulation of Hox gene expression. Although MLL is expressed throughout hematopoiesis, Hox gene expression is sharply down-regulated during differentiation, suggesting that either the activity of MLL or its association with target promoters must be regulated. Here we show that MLL associates with actively transcribed genes but does not remain bound after transcriptional down-regulation. Surprisingly, MLL is associated not only with promoter regions but also is distributed across the entire coding regions of genes. MLL interacts with RNA polymerase II (pol II) and colocalizes with RNA pol II at a subset of actively transcribed target in vivo. Loss of function Mll results in defects in RNA pol II distribution. Together the results suggest that an intimate association between MLL and RNA pol II occurs at MLL target genes in vivo that is required for normal initiation and͞or transcriptional elongation.histone methyltransferase ͉ Hox genes ͉ transcription P roper expression of the clustered HOX genes is essential for normal embryonic development. In addition, overexpression of select HOX genes such as HOXA9 and the HOX cofactor MEIS1 has been implicated in human myelodysplastic disorders as well as acute lymphoid and myeloid leukemias. HOXA9 and MEIS1 are normally expressed only in early hematopoietic lineages, but during later stages of differentiation, expression is down-regulated to undetectable levels (1). The mixed-lineage leukemia protein MLL, which is homologous to Drosophila trithorax, is one important regulator of HOXA9 expression. MLL-knockout mice show severe hematopoietic defects associated with defects in Hox gene (including Hoxa9) expression (2-5). Conversely, MLL rearrangements are commonly associated with lymphoid and myeloid leukemias (6-8). Translocations involving MLL delete the sequences most conserved with D. trithorax and replace them with an in frame fusion to 1 of Ͼ40 different translocation partners (9, 10). MLL fusion proteins enforce persistent expression of HOXA9 and MEIS1, which appears to be critical for leukemogenesis (11). By itself, overexpression of HOXA9 induces stem cell expansion and is associated with poor-prognosis acute myeloid leukemia (12, 13). However, when coexpressed with MEIS1, HOXA9 is acutely transforming (14).Recently, we and others found that the C-terminal SET domain of MLL protein is a histone methyltransferase that is specific for histone H3 Lys-4 (15, 16). MLL binds directly to Hox gene promoters and promotes transcriptional activation by methylating histone H3 on Lys-4 (15, 16) and also by recruiting MOF, a histone H4 Lys-16 specific acetyltransferase (17). Although an H3 Lys-4 demethylase has recently been identified (18), in general Lys-4 methylation is a long-lasting mark for sustained...

Crosstalk between leukemia-associated proteins MOZ and MLL regulates HOX gene expression in human cord blood CD34+ cells

“…Furthermore, overexpression of HOXA5 acts on myeloid proliferation and differentiation (Crooks et al, 1999). MLL was shown to regulate directly HOX gene expression (Yu et al, 1995;Milne et al, 2002Milne et al, , 2005Hsieh et al, 2003;Ernst et al, 2004;Dou et al, 2005). In our present study, ChIP experiments in CD34 þ cells indicate that both MLL and MOZ are recruited to HOXA5, HOXA7 and HOXA9 promoters, which correlates with the detection of activation epigenetic marks, such as H3K9K14ac, H4K16ac, H3K4me2 and H3K4me3.…”

“…Alternatively, MLL could be regulated by modulating its binding. As reported by Milne et al (2005), MLL is associated with actively transcribed target genes, but does not remain bound after transcriptional downregulation.…”

“…The genes most differentially expressed are HOXA5, HOXA7 and HOXA9, pivotal HOX genes in MLL-driven transformation (Hess, 2004;Kohlmann et al, 2005). MLL was reported to regulate directly HOX gene expression (Yu et al, 1995;Milne et al, 2002Milne et al, , 2005Hsieh et al, 2003;Ernst et al, 2004;Dou et al, 2005).…”

Two Distinct Roles of ARABIDOPSIS HOMOLOG OF TRITHORAX1 (ATX1) at Promoters and within Transcribed Regions of ATX1-Regulated Genes

“…The presence of SET1/TRX-type proteins, H3K4me3, and Ser5P in 59 regions of transcriptionally active genes has been observed in yeast, mammals, and plants (Ng et al, 2003;Milne et al, 2005;Kim et al, 2008). Although an earlier study has suggested that Ser5P was essential for transcription in yeast (Valay et al, 1995), recent evidence shows that phosphorylation of Ser5P is not required for transcription in yeast (Kanin et al, 2007).…”

Complex MLL rearrangements in t(4;11) leukemia patients with absent AF4 · MLL fusion allele

“…13,14 In several studies, direct binding of the der(11)-encoded MLL fusion protein to promoter regions of the above-mentioned genes has been demonstrated by ChIP (chromatin immunoprecipitation) experiments). 13,[15][16][17] This indicates that MLL fusion proteins encoded by the der (11) chromosome are able to influence directly the transcription of specific target genes.…”