Edwin R. Smith

Physical association and coordinate function of the H3 K4 methyltransferase MLL1 and the H4 K16 acetyltransferase MOF

A stable complex containing MLL1 and MOF has been immunoaffinity purified from a human cell line that stably expresses an epitope-tagged WDR5 subunit. Stable interactions between MLL1 and MOF were confirmed by reciprocal immunoprecipitation, cosedimentation, and cotransfection analyses, and interaction sites were mapped to MLL1 C-terminal and MOF zinc finger domains. The purified complex has a robust MLL1-mediated histone methyltransferase activity that can effect mono-, di-, and trimethylation of H3 K4 and a MOF-mediated histone acetyltransferase activity that is specific for H4 K16. Importantly, both activities are required for optimal transcription activation on a chromatin template in vitro and on an endogenous MLL1 target gene, Hox a9, in vivo. These results indicate an activator-based mechanism for joint MLL1 and MOF recruitment and targeted methylation and acetylation and provide a molecular explanation for the closely correlated distribution of H3 K4 methylation and H4 K16 acetylation on active genes.

Covalent modifications of histones during development and disease pathogenesis

Covalent modifications of histones are central to the regulation of chromatin dynamics, and, therefore, many biological processes involving chromatin, such as replication, repair, transcription and genome stability, are regulated by chromatin and its modifications. In this review, we discuss the biochemical, molecular and genetic properties of the enzymatic machinery involved in four different types of histone modification: acetylation, ubiquitination, phosphorylation and methylation. We also discuss how perturbation of the activity of this enzymatic machinery can cause developmental defects and disease.

The Language of Histone Crosstalk

It has been suggested that a specific pattern of histone posttranslational modifications and their crosstalk may constitute a code that determines transcriptional outcomes. However, recent studies indicate that histone modifications have context-dependent effects, making their interplay more like a language within the chromatin signaling pathway than a code.

AFF4, a Component of the ELL/P-TEFb Elongation Complex and a Shared Subunit of MLL Chimeras, Can Link Transcription Elongation to Leukemia

Chromosomal translocations involving the MLL gene are associated with infant acute lymphoblastic and mixed lineage leukemia. There are a large number of translocation partners of MLL that share very little sequence or seemingly functional similarities; however, their translocations into MLL result in the pathogenesis of leukemia. To define the molecular reason why these translocations result in the pathogenesis of leukemia, we purified several of the commonly occurring MLL chimeras. We have identified super elongation complex (SEC) associated with all chimeras purified. SEC includes ELL, P-TEFb, AFF4, and several other factors. AFF4 is required for SEC stability and proper transcription by poised RNA polymerase II in metazoans. Knockdown of AFF4 in leukemic cells shows reduction in MLL chimera target gene expression, suggesting that AFF4/SEC could be a key regulator in the pathogenesis of leukemia through many of the MLL partners.

The Drosophila MSL Complex Acetylates Histone H4 at Lysine 16, a Chromatin Modification Linked to Dosage Compensation

ABSTRACT In Drosophila, dosage compensation—the equalization of most X-linked gene products in males and females—is achieved by a twofold enhancement of the level of transcription of the X chromosome in males relative to each X chromosome in females. A complex consisting of at least five gene products preferentially binds the X chromosome at numerous sites in males and results in a significant increase in the presence of a specific histone isoform, histone 4 acetylated at lysine 16. Recently, RNA transcripts (roX1 and roX2) encoded by two different genes have also been found associated with the X chromosome in males. We have partially purified a complex containing MSL1, -2, and -3, MOF, MLE, and roX2 RNA and demonstrated that it exclusively acetylates H4 at lysine 16 on nucleosomal substrates. These results demonstrate that the MSL complex is responsible for the specific chromatin modification characteristic of the X chromosome in Drosophila males.

Global Analysis of H3K4 Methylation Defines MLL Family Member Targets and Points to a Role for MLL1-Mediated H3K4 Methylation in the Regulation of Transcriptional Initiation by RNA Polymerase II

ABSTRACT A common landmark of activated genes is the presence of trimethylation on lysine 4 of histone H3 (H3K4) at promoter regions. Set1/COMPASS was the founding member and is the only H3K4 methylase in Saccharomyces cerevisiae; however, in mammals, at least six H3K4 methylases, Set1A and Set1B and MLL1 to MLL4, are found in COMPASS-like complexes capable of methylating H3K4. To gain further insight into the different roles and functional targets for the H3K4 methylases, we have undertaken a genome-wide analysis of H3K4 methylation patterns in wild-type Mll1+/+ and Mll1−/− mouse embryonic fibroblasts (MEFs). We found that Mll1 is required for the H3K4 trimethylation of less than 5% of promoters carrying this modification. Many of these genes, which include developmental regulators such as Hox genes, show decreased levels of RNA polymerase II recruitment and expression concomitant with the loss of H3K4 methylation. Although Mll1 is only required for the methylation of a subset of Hox genes, menin, a component of the Mll1 and Mll2 complexes, is required for the overwhelming majority of H3K4 methylation at Hox loci. However, the loss of MLL3/MLL4 and/or the Set1 complexes has little to no effect on the H3K4 methylation of Hox loci or their expression levels in these MEFs. Together these data provide insight into the redundancy and specialization of COMPASS-like complexes in mammals and provide evidence for a possible role for Mll1-mediated H3K4 methylation in the regulation of transcriptional initiation.

A Human Protein Complex Homologous to the Drosophila MSL Complex Is Responsible for the Majority of Histone H4 Acetylation at Lysine 16

ABSTRACT We describe a stable, multisubunit human histone acetyltransferase complex (hMSL) that contains homologs of the Drosophila dosage compensation proteins MOF, MSL1, MSL2, and MSL3. This complex shows strong specificity for histone H4 lysine 16 in chromatin in vitro, and RNA interference-mediated knockdown experiments reveal that it is responsible for the majority of H4 acetylation at lysine 16 in the cell. We also find that hMOF is a component of additional complexes, forming associations with host cell factor 1 and a protein distantly related to MSL1 (hMSL1v1). We find two versions of hMSL3 in the hMSL complex that differ by the presence of the chromodomain. Lastly, we find that reduction in the levels of hMSLs and acetylation of H4 at lysine 16 are correlated with reduced transcription of some genes and with a G2/M cell cycle arrest. This is of particular interest given the recent correlation of global loss of acetylation of lysine 16 in histone H4 with tumorigenesis.

Human Mediator Subunit MED26 Functions as a Docking Site for Transcription Elongation Factors

Promoter-proximal pausing by initiated RNA polymerase II (Pol II) and regulated release of paused polymerase into productive elongation has emerged as a major mechanism of transcription activation. Reactivation of paused Pol II correlates with recruitment of super-elongation complexes (SECs) containing ELL/EAF family members, P-TEFb, and other proteins, but the mechanism of their recruitment is an unanswered question. Here, we present evidence for a role of human Mediator subunit MED26 in this process. We identify in the conserved N-terminal domain of MED26 overlapping docking sites for SEC and a second ELL/EAF-containing complex, as well as general initiation factor TFIID. In addition, we present evidence consistent with the model that MED26 can function as a molecular switch that interacts first with TFIID in the Pol II initiation complex and then exchanges TFIID for complexes containing ELL/EAF and P-TEFb to facilitate transition of Pol II into the elongation stage of transcription.

The super elongation complex (SEC) and MLL in development and disease

Transcriptional regulation at the level of elongation is vital for the control of gene expression and metazoan development. The mixed lineage leukemia (MLL) protein and its Drosophila homolog, Trithorax, which exist within COMPASS (complex of proteins associated with Set1)-like complexes, are master regulators of development. They are required for proper homeotic gene expression, in part through methylation of histone H3 on Lys 4. In humans, the MLL gene is involved in a large number of chromosomal translocations that create chimeric proteins, fusing the N terminus of MLL to several proteins that share little sequence similarity. Several frequent translocation partners of MLL were found recently to coexist in a super elongation complex (SEC) that includes known transcription elongation factors such as eleven-nineteen lysine-rich leukemia (ELL) and P-TEFb. Importantly, the SEC is required for HOX gene expression in leukemic cells, suggesting that chromosomal translocations involving MLL could lead to the overexpression of HOX and other genes through the involvement of the SEC. Here, we review the normal developmental roles of MLL and the SEC, and how MLL fusion proteins can mediate leukemogenesis.

Involvement of human MOF in ATM function

ABSTRACT We have determined that hMOF, the human ortholog of the Drosophila MOF gene (males absent on the first), encoding a protein with histone acetyltransferase activity, interacts with the ATM (ataxia-telangiectasia-mutated) protein. Cellular exposure to ionizing radiation (IR) enhances hMOF-dependent acetylation of its target substrate, lysine 16 (K16) of histone H4 independently of ATM function. Blocking the IR-induced increase in acetylation of histone H4 at K16, either by the expression of a dominant negative mutant ΔhMOF or by RNA interference-mediated hMOF knockdown, resulted in decreased ATM autophosphorylation, ATM kinase activity, and the phosphorylation of downstream effectors of ATM and DNA repair while increasing cell killing. In addition, decreased hMOF activity was associated with loss of the cell cycle checkpoint response to DNA double-strand breaks. The overexpression of wild-type hMOF yielded the opposite results, i.e., a modest increase in cell survival and enhanced DNA repair after IR exposure. These results suggest that hMOF influences the function of ATM.