MOZ and MORF Lysine Acetyltransferases
Domain Organizations of MOZ and MORF
MOZ and MORF are large proteins (∼250 kDa) with multiple functional domains (Fig. 1). The MYST domain contains a C2HC nucleosome-binding domain and the histone acetyltransferase (HAT) domain. MOZ and MORF are members of the MYST family, which has five MYST members in Drosophila or humans (Fig. 1). The MYST domain is the only structural feature common to all members of this family. MOZ and MORF only show sequence similarity to the very N-terminal region and MYST domain of Enok (Enoki mushroom), the most related Drosophila protein (Scott et al. 2001). Different from Drosophila, zebra fish possesses orthologs of MOZ and MORF. Thus, different from TIP60, MOF, and HBO1, both MOZ and MORF are vertebrate specific.
Other protein domains of MOZ and MORF include an NEMM (N-terminal part of Enok, MOZ, or MORF) domain, tandem PHD (plant homeodomain linked) zinc fingers, a long acidic stretch, and a SM (serine/methionine rich) region. The C-terminal part of the NEMM domain shows some sequence similarity to histones H1 and H5. The H15-like domain was shown to promote nuclear targeting (Kitabayashi et al. 2001a). The tandem PHD fingers are similar to those of Requiem and homologs (Borrow et al. 1996; Nabirochkina et al. 2002), which may recognize methyl-lysine-containing motifs (Kouzarides 2007). Indeed, MOZ can bind directly to the trimethyl K4 of histone H3 (Paggetti et al. 2010). Both the N terminus of MOZ and MORF possess a strong transcriptional repression domain (Champagne et al. 1999). The SM domain constitutes an activation domain. The SM domain of MOZ is weaker than that of MORF, raising the interesting possibility that insertion of a PQ-stretch in the SM domain of MOZ reduces its activation potential (Champagne et al. 2001). The possession of multiple functional domains by MOZ and MORF suggests that they may regulate transcription in both acetylation-dependent and acetylation-independent manners.
MOZ and MORF Acetyltransferase Complexes
MOZ and MORF may function in the cell through their presence in the ING5 complex as the catalytic subunits (Doyon et al. 2006). ING5 is the fifth member of the novel ING (inhibitor of growth) tumor suppressor family (Soliman and Riabowol 2007). The other subunits of the MOZ/MORF complexes are EAF6 (homolog of yeast Esa1-associated factor 6) and BRPF1 (bromodomain-PHD finger protein 1) (Doyon et al. 2006). Different from MOZ and MORF alone, ING5-MOZ/MORF complexes acetylate only histone H3 at lysine 14 (Doyon et al. 2006). BRPF1 binds to the MYST domains of MOZ and MORF, stimulates acetyltransferase and coactivator activities, and bridges interaction with ING5 and EAF6 (Ullah et al. 2008). Coexpression of MOZ, BRPF1, and ING5 has synergistic effect on the Runx2 promoter. MOZ/MORF-ING5 complexes are important for DNA replication (Doyon et al. 2006).
MOZ and MORF Act as Transcriptional Coactivators of Transcription Factors
Both MOZ and MORF are widely expressed in various mouse and human tissues (Borrow et al. 1996; Champagne et al. 1999; Katsumoto et al. 2006; Thomas et al. 2006), so similar to GCN5/PCAF and p300/CBP, they may function as coactivators for many transcription factors. Many evidences have demonstrated that MOZ and MORF act as coactivators for different transcription factors such as Runxs, Hox, ETV6, PU.1, and P53; all of them play pivotal roles in different development and cellular processes.
MOZ and MORF can bind to Runx2 (runt-related transcription factor 2) through the SM domain and potentiate Runx2-dependent transcriptional activation (Pelletier et al. 2002). Runx2 plays an important role in T-cell lymphomagenesis (Pelletier et al. 2002) and is essential in controlling osteoblast differentiation and bone formation (Ducy et al. 2000; Wheeler et al. 2000). The SM domain of MORF can also bind to Runx1 (Pelletier et al. 2002). Runx1-interacting domains in MOZ include the SM-rich region and the basic domain between 312 and 664 (Kitabayashi et al. 2001a). Runx1 is essential for generation of hematopoietic stem cells (Okuda et al. 1996; Wang et al. 1996) and is important for differentiation of megakaryocytes and lymphocytes (Ichikawa et al. 2004; Growney et al. 2005).
Studies in mouse and zebra fish indicate that MOZ plays a key role in controlling HOX (homeobox) gene expression (Miller et al. 2004; Camos et al. 2006; Crump et al. 2006; Katsumoto et al. 2006; Laue et al. 2008; Voss et al. 2009), so unknown DNA-binding transcription factor may recruit MOZ to achieve this goal. In human cord blood CD34+ cells, MOZ interact with MLL (mixed lineage leukemia) that encodes a histone methyltransferase. Both are recruited to multiple HOX promoters and synergistically stimulate HOX genes expression (Paggetti et al. 2010). HOX genes encode transcription factors that are involved in embryogenesis and morphogenesis (McGinnis and Krumlauf 1992) and exert crucial functions in normal hematopoiesis (van Oostveen et al. 1999).
The ets-family transcription factor ETV6 (TEL) is essential for the establishment of adult hematopoiesis in the bone marrow (Wang et al. 1998) and is involved in the balanced chromosomal translocations in various hematological malignancies and in some soft tissue tumors (Sato et al. 1997; Rowley 1999). MOZ and MORF are able to interact with portions of ETV6 through the MYST domains (Putnik et al. 2007).
MOZ also can physically and functionally interact with PU.1, a transcription factor that is essential for maintenance of hematopoietic stem cells and development of myeloid and lymphoid lineages (Scott et al. 1994; McKercher et al. 1996; Kim et al. 2004; Iwasaki et al. 2005). In addition, MOZ forms a complex with p53 to induce p21 expression and cell cycle arrest (Rokudai et al. 2009). MOZ can also function as a coactivator of the Nrf2-MafK and NF-κB (Chan et al. 2007; Ohta et al. 2007). MORF is present in the transcriptional coactivator complex associated with the nuclear receptor peroxisome proliferator-activated receptor-α (PPARα) (Surapureddi et al. 2002).
Moz in Development
Consistent with the regulatory role of MOZ/MORF on the activities of transcription factors that are essential in different processes, studies on mutant models indicate that MOZ and MORF play key roles in hematopoiesis, skeletogenesis, neurogenesis, and other developmental processes (Thomas et al. 2000; Miller et al. 2004; Crump et al. 2006; Katsumoto et al. 2006; Merson et al. 2006; Thomas et al. 2006).
Homozygous Moz mutant mice lacking the Moz protein expression die at birth. Fetal liver hematopoietic cells from the Moz mutant fail to contribute to the hematopoietic system of recipients after transplantation and display defects in the stem cell compartment (Thomas et al. 2006). Moz-/- mice without the expression of Moz die around embryonic day 15 (E15). In Moz-/- E14.5 embryos, HSCs (hematopoietic stem cells) and progenitors are decreased, and maturation of erythroid cells is inhibited in Moz-deficient fetal liver that is due to the synergistic effects of decreased expression of c-Mpl, HoxA9, and c-Kit (Katsumoto et al. 2006). Therefore, Moz is essential for the maintenance of hematopoietic stem cells.
A mouse strain carrying a point mutation that inactivate the HAT activity of the Moz protein remain alive during all the gestation period, but about 40% of the homozygote mice die within the first 6 month. These mice exhibit significant defects in the number of HSCs and committed precursors and B-cell development defect. The reduced number of HSCs is caused by the failure of HAT-/- cells to expand. These results indicate that the HAT activity of Moz play a critical role in the proliferation and maintenance of hematopoietic precursors (Perez-Campo et al. 2009).
Studies on Moz-/- mutant mice (Thomas et al. 2006) also show that Moz is required for normal levels of H3K9 acetylation and gene expression at a large number of Hox loci and for correct body segment identity specification of 19 body segments. In addition, Moz is required for Ing5 and H3 K4 methyltransferase Mll1 recruitment to the Hox loci (Voss et al. 2009). In zebra fish, it was found that Brpf1 and Moz tightly cooperate to promote histone acetylation and maintenance of anterior Hox gene expression and determine pharyngeal segmental identities (Laue et al. 2008).
MORF in Development
Mice homozygous mutants for Querkopf fail to thrive in the postnatal period, display craniofacial abnormalities due to defects in the calvarial bones, and have defects in cerebral cortex development (Thomas et al. 2000). Homozygous mice show smaller cerebral cortex, lack of large pyramidal cells in layer V of the cortex, and have reduction in the number of GAD67-positive interneurons throughout the cortex, suggesting that Querkopf is an essential in regulating cell differentiation in the cortex (Thomas et al. 2000). Querkopf-deficient mice also display defects in adult neurogenesis in vivo. Isolated neural stem/progenitor cells exhibit decreased self-renewal capacity and reduced ability to produce differentiated neurons. Thus, Querkopf is also essential for adult neurogenesis.
Moz and Morf in Oncogenesis
MOZ fusion proteins caused by chromosome rearrangement enable the transformation of non-self-renewing myeloid progenitors into leukemia stem cells (Huntly et al. 2004). Fusion partners of MOZ include CBP (Borrow et al. 1996), CBP paralog p300 (Chaffanet et al. 2000; Kitabayashi et al. 2001b), the p300/CBP-interacting nuclear receptor coactivators TIF2 (transcription intermediary factor 2) (Carapeti et al. 1998; Liang et al. 1998), and nuclear receptor coactivators 2 (NCOA3) (Esteyries et al. 2008). MORF translocation is also associated with childhood AML or therapeutic myelodysplastic syndromes, in which the MORF gene is fused to that of CBP (Panagopoulos et al. 2001; Kojima et al. 2003). MORF gene is also disrupted in multiple cases of uterine leiomyomata. GCN5 is the fusion partner candidate (Moore et al. 2004). Deregulation of gene expression resulting from these translocations may be involved in the tumor pathogenesis. MOZ is involved in regulating cell cycle arrest in the G1 phase (Rokudai et al. 2009). DNA damage increases the level of p53-MOZ complex. In MOZ-/- mouse fibroblasts, DNA damage fails to induce the expression of p21 and G1 cell cycle arrest. MOZ-CBP inhibits p53-mediated transcription. Thus, the inhibition of p53/MOZ-mediated transcription contributes to the leukemogenesis and tumor pathogenesis (Rokudai et al. 2009).
MOZ/MORF has been shown to be coactivators of Runx genes. Runx2 functions as a novel oncogenic effector for T-cell lymphoma (Vaillant et al. 1999; Blyth et al. 2001). Runx1 is an important regulator of fetal liver hematopoiesis, and its gene is frequently rearranged in leukemia patients (Speck et al. 1999; Westendorf and Hiebert 1999). The expression levels of MORF/MOZ may affect roles of Runx proteins in the development of T-cell lymphoma and leukemogenesis. In addition, MOZ-TIF2 and MOZ-CBP stimulate the expression of macrophage colony-stimulating factor receptor (CSF1R) through the interaction with PU.1, and thus induce AML (Aikawa et al. 2010).
MOZ and MORF belong to the MYST family of histone acetyltransferase. The multifunctional domains also confer the function of transcriptional co-regulators. Thus, MOZ and MORF can regulate gene expression by acetylation-dependent and acetylation-independent manners. MOZ and MORF acetylate histone H3 at lysine 23. MOZ and MORF act as coactivator for different transcription factors such as Runxs, Hox, ETV6, PU.1, and P53; all of them play pivotal roles in different development and cellular processes. Accordingly, studies on mutant models indicate that MOZ and MORF play key roles in hematopoiesis, skeletogenesis, neurogenesis, body segment identity specification, and cell cycle control. MOZ and MORF are targets of chromosome rearrangement that are associated with leukemia and other types of tumor. Deregulation of gene expression resulting from these translocations may be involved in the tumor pathogenesis.
This research was supported by operating grants from Canadian Institutes of Health Research (CIHR) and Canadian Cancer Society (to X. J. Y.).
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