RUNX family members are DNA-binding transcription factors that regulate the expression of genes involved in cellular differentiation and cell cycle progression. The RUNX family includes three mammalian RUNX proteins (RUNX1, -2, -3) and two homologues in Drosophila. Experiments in Drosophila and mouse indicate that the RUNX proteins are required for gene silencing of engrailed and CD4, respectively. RUNX-mediated repression involves recruitment of corepressors such as mSin3A and Groucho as well as histone deacetylases. Furthermore, RUNX1 and RUNX3 associate with SUV39H1, a histone methyltransferase involved in gene silencing. RUNX1 is frequently targeted in human leukemia by chromosomal translocations that fuse the DNA-binding domain of RUNX1 to other transcription factors and corepressor molecules. The resulting leukemogenic fusion proteins are transcriptional repressors that form stable complexes with corepressors, histone deacetylases and histone methyltransferases. Thus, transcriptional repression and gene silencing through RUNX1 contribute to the mechanisms of leukemogenesis of the fusion proteins. Therapies directed at the associated cofactors may be beneficial for treatment of these leukemias.

Inversion (16) is one of the most frequent chromosomal translocations found in acute myeloid leukemia (AML), occurring in over 8% of AML cases. This translocation results in a protein product that fuses the first 165 amino acids of core binding factor ␤ to the coiled-coil region of a smooth muscle myosin heavy chain (CBF␤/SMMHC). CBF␤ interacts with AML1 to form a heterodimer that binds DNA; this interaction increases the affinity of AML1 for DNA. The CBF␤/SMMHC fusion protein cooperates with AML1 to repress the transcription of AML1-regulated genes. We show that CBF␤/SMMHC contains a repression domain in the C-terminal 163 amino acids of the SMMHC region that is required for inv(16)-mediated transcriptional repression. This minimal repression domain is sufficient for the association of CBF␤/SMMHC with the mSin3A corepressor. In addition, the inv(16) fusion protein specifically associates with histone deacetylase 8 (HDAC8). inv(16)-mediated repression is sensitive to HDAC inhibitors. We propose a model whereby the inv(16) fusion protein associates with AML1 to convert AML1 into a constitutive transcriptional repressor.Human acute leukemias often arise from chromosomal translocations targeting regulatory genes that affect cell growth, differentiation, and apoptosis (30). These translocations often fuse transcription factors to other proteins resulting in oncogenic chimeric proteins. inv (16) is found in approximately 8% of acute myeloid leukemia (AML) cases (35), making it one of the most frequent translocations in AML. inv(16) fuses the first 165 amino acids (aa) of core binding factor ␤ (CBF␤) to the C-terminal coiled-coil region of a smooth muscle myosin heavy chain (SMMHC [the gene name is MYH11]) (28). CBF␤ interacts with the AML1 (also known as RUNX1) transcription factor to increase the affinity of AML1 for DNA (15,40,47) and to stimulate the ability of AML1 to either activate or repress transcription (22,32). Both components of the CBF␤-AML1 transcription factor complex are disrupted by chromosomal translocations. The translocations that target AML1 include t(8;21) and t(12;21) that result in the leukemogenic fusion proteins AML1/ETO and TEL/AML1 (10, 16).The CBF␤-AML1 complex regulates genes encoding cytokines and their receptors, genes involved in differentiation such as T-cell receptors, neutrophil peptide 3, and myeloperoxidase as well as the p14 ARF tumor suppressor (4,12,14,27,36,39,43,46,52). Furthermore, CBF␤/SMMHC slows the cell cycle transition from G 1 to S in hematopoietic cells; thus, cell cycle regulatory genes such as cdk4 may also be targeted by this complex (5).Animal studies indicate that CBF␤ and AML1 are required for hematopoiesis. CBF␤ and AML-1 knockout mice share a phenotype; these mice lack fetal liver hematopoiesis and are embryonic lethal at embryonic day 12.5 (e12.5) to e13.5 (41,49). A similar phenotype is observed in mice expressing CBF␤/ SMMHC from a "knocked-in" CBFB-MYH11 gene (7), suggesting that inv(16) creates a dominant repressor of AML1-and CBF␤-regulated genes. Chimer...

The inv(16) is one of the most frequent chromosomal translocations associated with acute myeloid leukemia (AML). The inv(16) fusion protein acts by dominantly interfering with AML-1͞core binding factor ␤-dependent transcriptional regulation. Here we demonstrate that the inv(16) fusion protein cooperates with AML-1B to repress transcription. This cooperativity requires the ability of the translocation fusion protein to bind to AML-1B. Mutational analysis and cell fractionation experiments indicated that the inv(16) fusion protein acts in the nucleus and that repression occurs when the complex is bound to DNA. We also found that the inv(16) fusion protein binds to AML-1B when it is associated with the mSin3A corepressor. An AML-1B mutant that fails to bind mSin3A was impaired in cooperative repression, suggesting that the inv(16) fusion protein acts through mSin3 and possibly other corepressors. Finally, we demonstrate that the C-terminal portion of the inv(16) fusion protein contains a repression domain, suggesting a molecular mechanism for AML-1-mediated repression.T he inv (16) is one of the most frequent translocations in acute myeloid leukemia (AML) (1). It fuses most of core binding factor ␤ (CBF␤͞CBFB͞PEBP2B) to the C terminus of a smooth muscle myosin heavy chain (SMMHC), MYH11 (2). CBF␤ is a transcription factor that does not bind DNA directly but interacts with the AML-1 DNA-binding transcription factor to increase its ability to bind DNA and regulate transcription (3, 4). AML-1 is one of the most frequently mutated genes in human leukemia (5). It is disrupted by the t(8;21), t(3;21), and t(16;21) in AML and by the t(12;21) in childhood B cell acute lymphocytic leukemia (ALL). By disrupting CBF␤, the inv(16) also disrupts AML-1 functions (6). Together, these chromosomal translocations account for nearly one-quarter of all AML cases and one-fifth of all childhood B cell ALL-containing discernible chromosomal abnormalities (7,8).The largest form of AML1, termed AML-1B(9), activates transcription of numerous tissue-specific genes, including genes encoding cytokines and cytokine receptors, and differentiationspecific genes such as T cell receptors, neutrophil peptide-3, and myeloperoxidase (10). CBF␤ cooperates with AML-1 to activate transcription (11). Targeted deletion of the AML1 (Cbfa2͞ Pebp2aB2) or CBF␤ genes led to identical phenotypes: embryonic lethality at day 12.5-13.5 post coitus with a complete lack of fetal liver hematopoiesis (12-15). Thus, CBF␤ is required for AML-1 function.The targeting of AML-1 by multiple chromosomal translocations in acute leukemia suggests a convergent mechanism of leukemogenesis. The t(8;21) fusion protein creates a transcriptional repressor protein by fusing the AML-1 DNA-binding domain to ETO (MTG8), a corepressor that recruits histone deacetylases to inhibit transcription (5). Likewise, the t(16;21) fuses the AML-1 DNA-binding domain to an ETO family member (16). The t(3;21) fuses the AML-1 DNA-binding domain to the Evi I transcriptional repressor (17). Alternatively...

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AML-1 is one of the most frequently translocated genes in human leukemia. AML-1 binds DNA and activates or represses transcription, while the chromosomal translocation fusion proteins in acute myeloid leukemia subvert these functions. The t(8;21) is the second most frequent translocation in acute myeloid leukemia and creates a fusion between the DNA binding domain of AML-1 and the ETO (also known as MTG8) corepressor. The t(12;21) is found in up to 25% of pediatric B cell acute lymphoblastic leukemias and fuses the ETS family transcription factor TEL to the amino terminus of AML-1. In addition, the inv(16), the most frequent translocation in acute myeloid leukemia, fuses the AML-1 cofactor CBFbeta to the smooth muscle myosin heavy chain MYH11. Both the t(8;21) and t(12;21) create transcriptional repressors that impair AML-1 target gene expression. We demonstrated that the fusion proteins encoded by these translocations contact the nuclear hormone corepressors (N-CoR/SMRT), mSin3A, and histone deacetylases. We have also found that both TEL and AML-1 interact with mSin3A. TEL also binds N-CoR and histone deacetylase-3, indicating that wild-type TEL is a transcriptional repressor. The t(12;21) fuses the mSin3A interaction domain of TEL to AML-1 to transform AML-1 from a regulated to an unregulated transcriptional repressor. The recognition that AML-1 interacts with mSin3A to repress transcription suggested that the inv(16) fusion protein might also repress the transcription of AML-1-target genes. In fact, the inv(16) encodes a protein that cooperates with AML-1 to repress transcription. The inv(16) fusion protein was found in a ternary complex with AML-1 and mSin3A, suggesting that the inv(16) also acts by recruiting transcriptional corepressors and histone deacetylases.