MOZ and MLL, encoding a histone acetyltransferase (HAT) and a histone methyltransferase, respectively, are targets for recurrent chromosomal translocations found in acute myeloblastic or lymphoblastic leukemia. In MOZ (MOnocytic leukemia Zinc-finger protein)/CBP- or mixed lineage leukemia (MLL)-rearranged leukemias, abnormal levels of HOX transcription factors have been found to be critical for leukemogenesis. We show that MOZ and MLL cooperate to regulate these key genes in human cord blood CD34+ cells. These chromatin-modifying enzymes interact, colocalize and functionally cooperate, and both are recruited to multiple HOX promoters. We also found that WDR5, an adaptor protein essential for lysine 4 trimethylation of histone H3 (H3K4me3) by MLL, colocalizes and interacts with MOZ. We detected the binding of the HAT MOZ to H3K4me3, thus linking histone methylation to acetylation. In CD34+ cells, depletion of MLL causes release of MOZ from HOX promoters, which is correlated to defective histone activation marks, leading to repression of HOX gene expression and alteration of commitment of CD34+ cells into myeloid progenitors. Thus, our results unveil the role of the interaction between MOZ and MLL in CD34+ cells in which both proteins have a critical role in hematopoietic cell-fate decision, suggesting a new molecular mechanism by which MOZ or MLL deregulation leads to leukemogenesis.
MOZ (MOnocytic leukemia Zinc-finger protein) (also called MYST3 or KAT6A) is a histone acetyltransferase (HAT) of the MYST (MOZ/YBF2/SAS2/TIP60 homology domain) family (Borrow et al., 1996b; Yang, 2004). In acute myeloblastic leukemia (AML), a chromosomal translocation fuses MOZ to a partner gene that can be CBP, p300, TIF2, NCOA3, or an unidentified one (Yang, 2004; Esteyries et al., 2008). MOZ is a coactivator of various transcription factors particularly with hematopoietic specificity, such as RUNX1 (AML1) or Spi-1/PU.1 (Kitabayashi et al., 2001; Katsumoto et al., 2006). Analysis of Moz knockout mice suggested its important role in the maintenance of hematopoietic stem cells and differentiation of myeloid cells (Katsumoto et al., 2006; Thomas et al., 2006; Perez-Campo et al., 2009).
Mixed lineage leukemia (MLL) (called KMT2A) is a histone methyltransferase (Hess, 2004). After post-translational cleavage, the resulting MLL-N and MLL-C parts become incorporated into a high-molecular-weight protein complex that activates transcription. Rearrangements of the MLL gene occur in several human leukemias, including acute lymphoblastic leukemia and AML (Hess, 2004).
MOZ acetylates in vitro lysine 16 of histone H4 (H4K16ac) and histone H3 similar to other HATs of the MYST family (Champagne et al., 2001; Kitabayashi et al., 2001; Carrozza et al., 2003). MLL catalyzes di- (H3K4me2) and trimethylation of lysine 4 of histone H3 (H3K4me3) (Hess, 2004). These histone modifications as well as acetylation of histone H3 (H3K9K14ac) and H4K16ac are associated with transcriptionally active regions (Eberharter and Becker, 2002; Santos-Rosa et al., 2002; Dion et al., 2005). Conversely, trimethylation of lysine 9 of histone H3 (H3K9me3) and trimethylation of lysine 27 of histone H3 (H3K27me3) are repressive marks. These modifications regulate one another, providing regulatory crosstalks. For instance, MLL cooperates with the H4K16 acetyltransferase MOF (Dou et al., 2005) to activate transcription. Moreover, H3K4 methylation catalyzed by MLL prevents repression of transcription by antagonizing H3R2 methylation mediated by the histone arginine methyltransferase PRMT6 (Guccione et al., 2007).
Abnormal activity of epigenetic modifying enzymes can contribute to cell transformation (Hake et al., 2004). Leukemic cells that express MLL gene fusions harbor a gene expression profile that distinguishes them from acute lymphoblastic or myeloblastic leukemia without MLL gene rearrangements. 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., 2002, 2005; Hsieh et al., 2003; Ernst et al., 2004; Dou et al., 2005).
HOX genes, which encode transcription factors, were first identified for their role in embryogenesis and morphogenesis (McGinnis and Krumlauf, 1992). Later on, it was demonstrated that HOX genes exerted a crucial function in normal hematopoiesis (van Oostveen et al., 1999). The highly dynamic pattern of their expression characterizes blood lineage specificity and maturation stages (Shen et al., 1989; Sauvageau et al., 1994). Hoxa9 is normally expressed in primitive hematopoietic cells, becomes downregulated as cells differentiate and promotes hematopoietic stem cell self-renewal (Argiropoulos and Humphries, 2007). In mice, expression of MLL chimeric proteins such as MLL/ENL or MLL/CBP leads to overexpression of Hoxa7, Hoxa9 and Meis1 genes essential for leukemic transformation (Ayton and Cleary, 2003).
MOZ/CBP-associated leukemias are also associated with abnormal levels of Hox transcripts (Camos et al., 2006). Altogether, these observations prompted us to investigate a putative crosstalk of MOZ with MLL through their potential association on specific HOXA5, HOXA7 and HOXA9 promoters to act on their transcriptional regulation in human hematopoietic multipotent cells.
In this study, we demonstrate that MOZ cooperates with MLL to regulate HOX gene expression in human cord blood CD34+ cells. Both proteins physically interact, colocalize in the nucleus, cooperate functionally to regulate HOX gene expression and are recruited on HOX promoters. Furthermore, MOZ associates with the adaptor protein WDR5, which is essential for recognizing trimethylation of H3K4, the specific epigenetic mark of MLL. Interestingly, MOZ also binds to trimethylated lysine 4 of histone H3 (H3H4me3), thus coupling histone acetylation to histone methylation. In CD34+ cells, downregulation of MLL causes release of MOZ from HOX promoters, which is correlated to defective histone activation marks and a decrease of HOX gene expression. On the other hand, myeloid colony formation is altered in small interfering RNA (siRNA)-MLL or -MOZ targeted CD34+ cells. Therefore, this study unravels a crucial partnership between MOZ and MLL in the epigenetic control of HOX promoter genes and in HOX expression, in human multipotent CD34+ cells. Alteration of the MOZ/MLL crosstalk involving a deregulation of hematopoietic transcription factors could affect the differentiation process and increase the capacity to create leukemogenic abnormalities in hematopoietic cells.
MOZ colocalizes and interacts with MLL in vivo
MOZ has been previously described as a nuclear protein in hematopoietic and nonhematopoietic cell lines (Kindle et al., 2005). This cellular localization was confirmed by indirect immunofluorescence microscopy (Figures 1a–c) in the human embryonic kidney cell line HEK293T, the myeloid leukemia cell line K562 and the human cord blood CD34+ cells. The cellular localization of MLL was tested simultaneously and the nuclei were counterstained with 4,6-diamidino-2-phenylindole (DAPI) in HEK293T (Figure 1a), K562 (Figure 1b) and CD34+ cells (Figure 1c). MOZ displayed a predominantly nuclear diffuse localization pattern and was also localized into discrete subnuclear speckles (Figures 1a–c). Likewise, MLL appeared to be localized in the nucleus (Figures 1a–c) and the overlay suggested colocalization of MOZ and MLL in this compartment as a mixed (yellow) color was observed (Figures 1a–c).
To establish whether this colocalization was corroborated with a biochemical in vivo interaction, we performed endogenous coimmunoprecipitation assays. We generated a monoclonal antibody against residues 856–870 of MOZ. This antibody was used to immunoprecipitate the protein from HEK293T or hematopoietic K562 cell lysates (Figure 1d). In the MOZ immunoprecipitate, a mouse monoclonal antibody specific to MLL-C identified a single band. In addition, MOZ was detected after immunoprecipitation with the MLL-C antibody (Figure 1e), indicating that endogenous MLL and MOZ proteins interact with each other. We confirmed this interaction with exogenous epitope-tagged proteins. The anti-MLL-C antibody coimmunoprecipitated the Myc-tagged MOZ from extracts of HEK293T cells cotransfected with the corresponding expression vectors (Figure 1f). Likewise, anti-Myc or anti-FLAG antibodies coimmunoprecipitated exogenous HA-MLL-C corresponding to the same band as the one detected with the anti-MLL-C antibody (Figures 1g and i). In the human cord blood CD34+ cells, MLL-C was detected after immunoprecipitation with the MOZ antibody (Figure 1h), indicating that both endogenous MLL and MOZ interact in hematopoietic stem/progenitor cells. We also studied the putative interaction of MOZ with MLL-N. In HEK293T cells, MLL-N was coimmunoprecipitated by an anti-MOZ (Supplementary Figure 1a).
To identify the region(s) of MOZ essential for interaction with MLL-C, extracts from HEK293T cells transiently transfected with both HA-MLL-C and FLAG-MOZ (or the three FLAG-tagged MOZ deletion mutants N352, N760 and C1409 (Supplementary Figure 1b) (Ullah et al., 2008) were immunoprecipitated with anti-FLAG antibody. The presence of MOZ proteins or MLL-C in the immunoprecipitates was assessed by immunoblotting with anti-FLAG or anti-HA antibodies, respectively (Figure 1i). MLL-C coimmunoprecipitated with mutant N760 (containing the N-terminal 760 residues) (Ullah et al., 2008). By contrast, mutants N352 (containing the N-terminal 352 residues) and C1409 (containing the C-terminal 595 residues) interacted very weakly with MLL-C, although the amount of proteins immunoprecipitated was much higher than that observed for mutant N760 (Figure 1i). Altogether, these results indicate that MOZ interacts with MLL.
MOZ and MLL synergistically stimulate the HoxA7 promoter activity
HOXA5, HOXA7 and HOXA9 are downstream targets of MLL (Hess, 2004). To examine the potential cooperative effect of MOZ and MLL on HOX gene activation, we asked first whether exogenous MOZ would enhance the activity of the murine HoxA7 promoter (cloned directly in front of a luciferase gene without the presence of a minimal promoter), which is conserved between human and mouse (Knittel et al., 1995). The luciferase vector harboring the HoxA7 promoter was co-transfected with various amounts of MOZ expression plasmid or control empty vector into human HEK293T cells before measuring luciferase activity. MOZ demonstrated a dose-dependent enhancement of HoxA7 promoter activity (Figure 2a). The ability of MOZ to increase Hox gene transcription was further established by studying Hoxa5, Hoxa7 and Hoxa9 mRNA after transfection of HEK293T cells with a Myc-tagged MOZ (approximately twofold increase, data not shown). MLL also transactivated the HoxA7 promoter in a dose-dependent manner (Figure 2b). Interestingly, when MOZ was co-transfected with MLL in the monoblastic cell line U-937, the expression of the reporter gene luciferase was synergistically increased in comparison with MOZ and MLL tested separately (Figure 2c). The synergistic effect of MOZ and MLL was confirmed by using the MIP1-α gene promoter, a previously described target of MOZ (Figure 2d) (Bristow and Shore, 2003).
Both MOZ and MLL are recruited to the same HOX loci in vivo
To determine whether MOZ and MLL were recruited to the same region of HOX genes promoters (Figure 3a), we performed chromatin immunoprecipitation (ChIP) experiments. Chromatin was prepared from HEK293T cells (Figure 3b), then immunoprecipitated with an anti-MLL or an anti-MOZ (lanes 3 and 4) antibody. Serial dilutions of the chromatin from HEK293T or CD34+ cells were used to demonstrate the linearity of PCR (Supplementary Figure 2). These experiments demonstrate that MOZ and MLL are recruited to the same HOXA7 promoter sequence, which correlates with the MOZ- and MLL-dependent transactivation of HOXA7. MLL and MOZ are also recruited to HOXA5 and to HOXA9 promoters in HEK293T cells (Figure 3b). Epigenetic activating marks corresponding to acetylated and methylated forms of histones H3 (H3K9K14ac) and H4 (H4K16ac) (Figure 3b) as well as the di- and trimethylated forms of H3K4 were detected (Figure 3b).
The recruitment of MOZ and MLL on HOX genes promoters was then explored in human cord blood CD34+ cells as HOX genes exert crucial functions in these cells (van Oostveen et al., 1999). ChIP assays performed in lysates from CD34+ cells indicated that MLL and MOZ were both recruited to HOXA5, HOXA7 and HOXA9 promoters, which correlated with the presence of H3K9K14ac, H4K16ac, H3K4me2 and H3K4me3 as in tested cell lines (Figure 3c). These results suggest that MOZ and MLL are recruited to the same HOX loci in vivo, and this recruitment is associated with activating epigenetic marks.
MOZ interacts with WDR5 and recognizes trimethyl lysine 4 of histone H3
Several proteins have been reported recently to selectively bind methylated K4 of histone H3, such as CHD1 and JMJD2A (Sims et al., 2005; Huang et al., 2006), as well as the WD40-repeat protein WDR5 (Wysocka et al., 2005). WDR5 is a major H3K4-methyl-associated protein and a common component of mammalian H3K4 methyltransferase complexes (Wysocka et al., 2005). This protein is not essential for the recruitment of MLL complexes to chromatin (Huang et al., 2006), but its interaction with histone H3 may stabilize or enhance the assembly of MLL complexes at promoters, thereby facilitating trimethylation of K4 and stimulating transcriptional activation at specific gene loci. Indeed, we confirmed the interaction of MLL with WDR5 in HEK293T (Figure 4b) or K562 cells (Figure 4c). With regard to the localization of MOZ with WDR5, we identified in K562 cells that WDR5 partially colocalized with MOZ into the nuclear compartment (Figure 4a). In addition, coimmunoprecipitation experiments identified an interaction between endogenous MOZ and WDR5 in both HEK293T (Figure 4b) and K562 cells (Figure 4c).
To identify the region(s) of MOZ essential for interaction with WDR5, extracts from HEK293T cells transiently transfected with FLAG-MOZ (or the three FLAG-tagged MOZ deletion mutants N352, N760 and C1409) (Ullah et al., 2008) were immunoprecipitated with anti-WDR5 antibody. The presence of WDR5 in the immunoprecipitates was assessed by immunoblotting with anti-WDR5 antibody (Figure 4d). WDR5 coimmunoprecipitated with mutant N352 as well as mutant N760. In contrast, mutant C1409 did not interact (Figure 1d). These results indicate that the WDR5-interaction domain is located within the N-terminal domain of MOZ. Hence, MOZ interacts with MLL as well as with WDR5.
As MOZ harbors PHD fingers similar to those that are able to bind methyl-lysine-containing motifs, we examined whether MOZ could selectively bind to methylated H3K4. To this end, we performed histone tail peptide assays. Lysates from HEK293T cells transfected with a Myc-tagged-MOZ construct were incubated with biotinylated synthetic histone H3 peptides (from the tail of H3) that were either unmodified or methylated on K4. These lysates were then mixed with neutravidin-coated agarose beads and, after washing, the proteins that remained bound to the peptides were dissolved in SDS loading buffer, resolved by SDS– polyacrylamide gel electrophoresis and immunoblotted with an anti-Myc antibody. As expected, WDR5 was recruited to the trimethyl K4 of histone H3 (Figure 4g) (Wysocka et al., 2005). Interestingly, MOZ is also able to bind this trimethyl K4 of histone H3 (Figure 4e). To determine a direct interaction, we carried out in vitro histone tail peptide assays by using in vitro-translated Myc-MOZ, and observed that synthesized Myc-MOZ binds to trimethyl K4 of histone H3 (Figure 4f). Therefore, our results identify the ability for this HAT to bind directly to the trimethyl K4 of histone H3.
MOZ or MLL knockdown alters HOX promoter recruitment, epigenetic modifications and HOX gene expression
Knockdown of MLL (Supplementary Figure 3a) or MOZ (Supplementary Figure 3b) in CD34+ cells using siRNA caused a strong and specific decrease in the corresponding RNA and protein levels. MLL or MOZ siRNA did not affect the expression of MOZ or MLL, respectively (Supplementary Figures 3a and b). ChIP experiments followed by quantitative PCR on HOXA5, HOXA7 and HOXA9 regions (Figure 5a) indicated that binding of MOZ to the HOX promoters was decreased after MOZ or MLL siRNA knockdown, suggesting that MOZ and MLL cooperate on HOX promoters in vivo. Semiquantitative PCR also suggested that MLL was less recruited onto the HOX regions when CD34+ cells were co-transfected with MOZ siRNA (Supplementary Figure 4a). In addition, it has been recently reported that the absence of Moz in mice results in reduced association of Mll1 with Hox gene loci and reduced Hox gene expression, suggesting that Moz may recruit Mll1 to Hox gene loci (Voss et al., 2009).
To examine whether MLL and MOZ control histone post-translational modifications, CD34+ cells were transfected either with MLL siRNA or MOZ siRNA followed by ChIP analysis of the HOX promoters (Figure 5b). As expected, MLL knockdown caused a decrease in H3K4me2 and H3K4me3 levels. H3K4me2 and -me3 levels were also affected negatively by MOZ knockdown, suggesting a functional cooperation between MOZ and MLL at the level of the studied promoters. Moreover, both MLL and MOZ knockdowns provoked a partial decrease of H3K9K14ac and H4K16ac levels indicating the involvement of MOZ in these epigenetic modifications in vivo. As a control, we also carried out a PCR analysis on GAPDH promoter, after ChIP experiments in siRNA-transfected CD34+ cells. We did not observe any binding of either MOZ or MLL in control and siRNA-treated CD34+ cells and no variations with regard to the post-translational histone modifications (Supplementary Figure 4b). Hox gene expression decreased to ∼60% in MLL siRNA-treated CD34+ cells and ∼40% in MOZ siRNA-treated CD34+ cells (Figure 5c). Hence, MOZ and MLL may be associated in vivo to regulate HOX expression in these cells.
Knockdown of MOZ or MLL inhibits myeloid colony formation from CD34+ cells
To investigate whether reduction in MOZ or MLL expression in CD34+ cells could affect their ability to form myeloid colonies, we transfected MOZ or MLL siRNA duplex in these cells, and performed myeloid colony assays. We observed that a decrease in MOZ expression affected the number of myeloid colony, that is, provoked a decrease in the number of monocyte colony-forming units and granulocyte colony-forming units after 14 days of culture (Figure 6). A similar effect was identified in MLL siRNA-treated CD34+ cells. Therefore, depleted MOZ and MLL CD34+ cells are impaired in their ability to differentiate into myeloid progenitors, suggesting a role of both modifying enzymes in myelopoiesis.
This study demonstrates that the MYST family HAT MOZ and the histone methyltransferase MLL interact and cooperate with each other in epigenetic modifications of HOX gene promoters, particularly in human cord blood CD34+ cells. These two associated proteins act on H3K4 methylation and H4K16 acetylation, respectively, interact potentially with DNA-binding transcription factors and stimulate transcription of MLL target genes, that is, expression of several HOX genes.
The role of histone modifications in regulating gene activation and repression was described several years ago (Jenuwein and Allis, 2001). Histone acetylation and histone H3K4 methylation are predominant modifications associated with gene activation, whereas histone H3K9 methylation is correlated with gene repression. The H3K4 methyltransferase MLL was found to interact with the HAT CBP (Ernst et al., 2001), as well as the SWI/SNF chromatin-remodeling complex (Rozenblatt-Rosen et al., 1998), to activate the transcription of targets genes. An MLL supercomplex appears to function at the level of promoters. This evolutionarily conserved histone post-translational modification complex could recruit other proteins that are either activators or repressors of transcription (Milne et al., 2002; Nakamura et al., 2002).
MOZ and MLL colocalize and interact in vivo. Both proteins cooperate to enhance the expression of MLL-regulated HOX genes, which may affect hematopoietic progenitor cell formation. MLL is able to modify histones at the promoters of certain HOX genes, maintaining gene transcription through its SET domain as it harbors histone H3K4 methyltransferase activity (Milne et al., 2002; Nakamura et al., 2002). One important function of MLL is the maintenance of HOX genes expression during embryonic development.
Homeodomain genes (HOX) encode transcription factors regulating pattern formation, differentiation and proliferation. There is considerable evidence correlating deregulated HOX expression with human acute leukemia, including translocations targeting specific HOX genes such as HOXA9 (Borrow et al., 1996a) or MLL, which normally acts as a direct regulator of HOX expressions (Hanson et al., 1999). Abnormal expression of murine Hoxa7 and Hoxa9 cause AML (Nakamura et al., 1996). HOXA9 is the single most informative gene discriminating AML from acute lymphoblastic leukemia (Golub et al., 1999). 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., 2002, 2005; Hsieh et al., 2003; Ernst et al., 2004; Dou et al., 2005). In our present study, ChIP experiments in CD34+ 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. Knockdown of MLL or MOZ inhibits MOZ recruitment and the epigenetic marks from HOX promoters due to the enzymatic activities of MOZ or MLL. Furthermore, it was recently shown a possible functional link between Moz and Mll1 in mouse embryos (Voss et al., 2009). In mice, Moz seems to be required for normal Hox gene expression and body segment identity specification, like TrxG proteins, but Moz is also required for the association of Mll1 with Hox gene loci in vivo. These results suggest that MLL and MOZ functionally cooperate to modulate HOX gene expression through specific epigenetic modifications, likely also in human CD34+ cells.
However, we have to take into account that MLL or MOZ does not function as single enzymes. Indeed, it has emerged as a common theme that noncatalytic subunits regulate the acetyltransferase activity and substrate specificity of the MYST family of HATs. Furthermore, similar to most histone-modifying enzymes, the MLL-family methyltransferases exist in multiprotein complexes. As MOZ is the catalytic subunit of a tetrameric complex (Ullah et al., 2008) and MLL is a methyltransferase belonging to different multisubunit complexes (Ruthenburg et al., 2007), it will be interesting to investigate how the noncatalytic subunits of the complexes are involved in the cooperation between MOZ and MLL to act on HOX gene expression.
Association of MOZ with hematopoietic transcription factors such as AML1 or Spi-1/PU.1 (Kitabayashi et al., 2001; Katsumoto et al., 2006) could take place within an MLL complex. Indeed, it was recently shown that interactions between AML1/RUNX1 and MLL provide epigenetic regulation of gene expression in normal hematopoiesis and in leukemia (Huang G, personal communication) (Huang et al., 2008). MOZ is also associated with other transcription factors such as RUNX2, C/EBPα, C/EBPβ, c-Jun, NF-κB, Nrf2/MafK and p53 (Pelletier et al., 2002; Ohta et al., 2005, 2007; Katsumoto et al., 2006; Chan et al., 2007; Rokudai et al., 2009). In addition, it was reported that MLL is a protein partner of p53 (Dou et al., 2005) and HCF-1 in a cell cycle-dependent manner (Tyagi et al., 2007).
Our results also demonstrate that MOZ is a partner of both MLL and WDR5. The tandem PHD fingers of MOZ, which are similar to those identified in Requiem and homologs (Borrow et al., 1996b; Nabirochkina et al., 2002), could recognize methyl-lysine-containing motifs (Kouzarides, 2007). WDR5 was shown to cooperate with the nucleosome-remodeling factor NURF (Wysocka et al., 2006). Furthermore, the MLL–WDR5 complex was proposed to be essential in regulating the establishment and spreading of K4 methylation, in conjunction with H4 K16 acetylation, in transcriptionally active chromatin (Dou et al., 2005). In this study, we show that MOZ interacts directly with the lysine 4 of histone H3. MLL, WDR5 and MOZ may be associated to trigger trimethylation of H3K4. Hence, MOZ may couple histone acetylation to histone methylation throughout its recruitment on lysine methylated by MLL.
MLL was also found to be associated with MOF, a histone H4 K16 specific HAT (Dou et al., 2005). Although MLL is expressed throughout hematopoiesis, the MLL target gene HOXA9 is downregulated. One possibility may correspond to the fact that MLL could be continually associated with target genes, but the recruitment of coregulators such as MOZ, MOF (Dou et al., 2005), CBP (Ernst et al., 2001) or the SWI/SNF complex (Rozenblatt-Rosen et al., 1998) is inhibited. 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.
MLL and MOZ genes can be rearranged as a consequence of a chromosomal translocation in human acute leukemias (Krivtsov and Armstrong, 2007; Yang and Ullah, 2007). We mapped the MLL-binding domain also to the MYST domain of MOZ. This domain remains intact in fusion proteins from chromosome translocations in related leukemia (Yang and Ullah, 2007), suggesting that MLL may also interact with the leukemic fusion proteins. In addition, depletion of MOZ as well as depletion of MLL affect CD34+ cells commitment to myeloid progenitors, suggesting a role of each of these proteins in myelopoiesis and thus corroborating the observations made with Moz knockout mice (Katsumoto et al., 2006; Thomas et al., 2006; Perez-Campo et al., 2009). Additional studies will be needed to determine whether the cooperation between MOZ and MLL may explain the partially common leukemogenic pathway observed in MOZ- and MLL-rearranged leukemias (Camos et al., 2006). This pathway may possibly involve altered Hox genes expression and the associated deficiencies observed in hematopoietic stem cells from Mll or Moz knockout cells (Katsumoto et al., 2006; Terranova et al., 2006; Thomas et al., 2006; Jude et al., 2007; McMahon et al., 2007; Perez-Campo et al., 2009).
In conclusion, this study identifies an intimate association between two different histone-modifying enzymes. The interaction between MOZ and MLL facilitates the epigenetic modification and may lead to subsequent chromatin remodeling. It is tempting to hypothesize that these epigenetic modifications may act in order to dictate the myeloid commitment of human cord blood CD34+ cells through HOX gene regulation. MOZ and MLL may belong to some common protein complexes. In a leukemic context, owing to the fact that the other partner of fusion proteins involving MOZ or MLL are HATs that interact with many transcription factors, it is tempting to suggest that steric obstructions could occur, modifying the interactions between MOZ, MLL and WDR5.
Materials and methods
The mouse HoxA7 and the human MIP-1a luciferase reporter genes were provided by Robert Slany (University of Erlangen-Nürnberg, Erlangen, Germany) and Paul Shore (University of Manchester, Manchester, UK), respectively. The HA-MOZ and the Myc-MOZ were provided by Issai Kitabayashi (NCCRI, Tokyo, Japan) and Edward Chan (Indiana University Cancer Center, Indianapolis, IN, USA), respectively, whereas FLAG-MOZ and FLAG-MOZ mutants were previously described (Ullah et al., 2008). The HA-MLL expression vector was a gift from Michael Cleary (Stanford University School of Medicine, Stanford, CA, USA). Antibodies and peptides are described in Supplementary Methods section.
The human U-937 and K562 lines were maintained in 10% fetal calf serum (FCS) in RPMI 1640 Glutamax (Bio Whittaker, Santa Monica, CA, USA) with 1% penicillin, streptomycin and amphotericin B. The human HEK293T cells were maintained in 10% FCS (Bio Whittaker) in Dulbecco's modified Eagle's medium supplemented with 4.5 g l−1 glucose (Bio Whittaker).
Human cord blood CD34+ cells
CD34+ cells were prepared from human umbilical cord blood obtained ethically and with informed consent of donors with no history of hematological disorders, and supplied by the Etablissement Français du Sang (Bourgogne Franche-Comté, France). Culture of CD34+ cells is described in Supplementary Methods section.
Myeloid colony-forming assays
For myeloid culture, a total of 1 × 103 CD34+ cells or CD34+ siRNA-treated cells (48 h after transfection) were added to methyl-cellulose supplemented with other compounds (Supplementary Methods).
Reporter gene assays
Transfection of HEK293T cells was performed using the jetPEI (Polyplus transfection, Illkirch, France), according to the manufacturer's instructions. Transient transfection of U-937 cells was carried out using the SuperFect transfection reagent (Qiagen, Hilden, Germany) according to the manufacturer's instructions. After 48 h, luciferase activity was analyzed in cell lysates using the Luciferase Reporter and β-galactosidase Assays systems (Promega, Madison, WI, USA). Other details are described in Supplementary Methods section.
Adherent HEK293T and cytospun K562 or CD34+ cells were fixed in 4% paraformaldehyde, permeabilized in 0.1% Triton X-100 and saturated in 1 × phosphate-buffered saline, and in 4% bovine serum albumin. Then, immunofluorescence assays were performed as described in Supplementary Methods.
SDS–polyacrylamide gel electrophoresis, immunoprecipitation and western blotting
Immunoprecipitation, SDS–polyacrylamide gel electrophoresis and western blotting were carried out as described in Supplementary Methods.
Briefly, 2 × 106 of HEK293T or CD34+ cells were fixed with 1% formaldehyde to crosslink DNA with proteins, then lysed and sonicated. ChIP procedure was carried out with modifications according to the manufacturer's instructions (Upstate Biotechnology, Temecula, CA, USA) and as described in Supplementary Methods.
For siRNA-mediated gene knockdown, 10 × 106 CD34+ cells were transfected by nucleoporation according to the manufacturer's protocol (Amaxa, Köln, Germany) using siRNAs described in Supplementary Methods.
Real-time reverse transcriptase–PCR
Real-time reverse transcriptase–PCR is described in Supplementary Methods section.
Histone tail peptide-binding assays
HEK293T were transfected with the MOZ expression plasmid (Supplementary Methods). Cell extracts were prepared as indicated for immunoprecipitation assays. For direct in vitro histone tail peptide pull-down assays, in vitro translation of Myc-MOZ was carried out in rabbit reticulocyte lysates (T7 Quick coupled transcription/translation system, Promega, Madison, WI, USA). Histone tail peptide-binding assays were carried out as described in Supplementary Methods.
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We gratefully acknowledge Amandine Bataille, Amandine Chlemaire and Franck Ménétrier for immunocytofluorescence assays; André Bouchot for epifluorescence microscopy; and Christine Arnould for confocal analysis (SERCOBIO, Université de Bourgogne), and the Etablissement Français du Sang (EFS) of Bourgogne Franche-Comté, which kindly supplied the cord blood buffy coats. We also thank Mustapha Oulad-Abdelghani for producing the anti-MOZ antibody. We thank Robert Slany, Issai Kitabayashi, Paul Shore, Michael Cleary and Edward Chan for providing plasmids. We appreciate the fine work performed by Magali Belt in correction of English text. This work was supported by funds from the Fondation de France (Leukemia Committee to LD), the Ligue contre le Cancer (Côte d’Or committee to LD), the Ligue contre le Cancer (Rhône committee to LD), the Conseil Régional de Bourgogne (FABER to LD), the Faculty of Medicine (to J-NB) in Dijon, the National Institute of Cancer (to ES), the Ligue Nationale contre le Cancer (Label to ES), the Agence Nationale de la Recherche of France (to ES and LD) and the Canadian Cancer Society (to XJY). JP was supported by fellowships from the Ministère de l’Enseignement Supérieur et de la Recherche of France and the Association pour la Recherche sur le Cancer (ARC). AL was supported by fellowships from the Inserm and the Région Bourgogne. RA and AJ were supported by fellowships from the Ligue contre le Cancer, Saône-et-Loire committee and national committee, respectively. BL was supported by fellowships from the Ministère de l’Enseignement Supérieur et de la Recherche of France.
The authors declare no conflict of interest.
Supplementary Information accompanies the paper on the Oncogene website
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Paggetti, J., Largeot, A., Aucagne, R. et al. Crosstalk between leukemia-associated proteins MOZ and MLL regulates HOX gene expression in human cord blood CD34+ cells. Oncogene 29, 5019–5031 (2010). https://doi.org/10.1038/onc.2010.254
- HOX genes
- human CD34+ cells
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