Fusion between transcription factor CBF beta/PEBP2 beta and a myosin heavy chain in acute myeloid leukemia

Mutagenesis of the Runt domain defines two energetic hot spots for heterodimerization with the core binding factor beta subunit

Core-binding factors (CBFs) are a small family of heterodimeric transcription factors that play critical roles in several developmental pathways and in human disease. Mutations in CBF genes are found in leukemias, bone disorders, and gastric cancers. CBFs consist of a DNA-binding CBF alpha subunit (Runx1, Runx2, or Runx3) and a non-DNA-binding CBF beta subunit. CBF alpha binds DNA in a sequence-specific manner, whereas CBF beta enhances DNA binding by CBF alpha. Both DNA binding and heterodimerization with CBF beta are mediated by a single domain in the CBF alpha subunits known as the "Runt domain." We analyzed the energetic contribution of amino acids in the Runx1 Runt domain to heterodimerization with CBF beta. We identified two energetic "hot spots" that were also found in a similar analysis of CBF beta (Tang, Y.-Y., Shi, J., Zhang, L., Davis, A., Bravo, J., Warren, A. J., Speck, N. A., and Bushweller, J. H. (2000) J. Biol. Chem. 275, 39579-39588). The importance of the hot spot residues for Runx1 function was demonstrated in in vivo transient transfection assays. These data refine the structural analyses and further our understanding of the Runx1-CBF beta interface.

Dysplasia and high proliferation rate are common in acute myeloid leukemia with inv(16)(p13q22)

Acute myeloid leukemia (AML) with inv(16)(p13q22), also known as M4Eo, is a distinct type of AML with a favorable prognosis associated with abnormal bone marrow eosinophils. We reviewed the morphologic findings of archival bone marrow specimens with M4Eo, specifically assessing for dysplasia, and performed immunohistochemical studies to assess the growth fraction using the MIB-1 (Ki-67) antibody. We also assessed the apoptotic rate by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-nick end labeling. All assessable cases had more than 10% dysplastic forms in at least 1 lineage. Seventeen cases had 10% or more dysplastic forms, and 3 cases had more than 50% dysplastic forms in at least 2 lineages. Immunoreactivity for Ki-67 was higher in M4Eo than in other AML types (P = .000). The apoptotic rate in M4Eo was similar to other AML types (P = .724). Our data show that dysplasia is a prominent feature, but not a prognostic indicator, in M4Eo. M4Eo is associated with a significantly higher proliferation rate than other AML types.

Differences in heat sensitivity between normal and acute myeloid leukemic stem cells: feasibility of hyperthermic purging of leukemic cells from autologous stem cell grafts

OBJECTIVES

In autologous stem cell transplantation contamination of the graft with malignant cells is frequently noticed and necessitates the use of in vivo or in vitro purging modalities. The hematopoietic recovery after transplantation depends on the number of stem and progenitor cells in the transplant. Therefore, in the present study the effects of hyperthermic treatment on the human normal and acute myeloid leukemic (AML) stem cell compartment were investigated.

METHODS

Normal bone marrow and AML blasts were heat treated up to 120 minutes at 43 degrees C. The surviving fractions of the different stem cell subsets were determined using in vitro methylcellulose and cobblestone area-forming cell (CAFC) clonogenic assays, as well as the in vivo NOD/SCID repopulating assay. The leukemic nature of the colonies from AML cells was confirmed by RT-PCR analysis. In order to increase the therapeutic index of the hyperthermic purging modality, the heat treatment was preceded by a 3-hour incubation at 37 degrees C with the ether lipid ET-18-OCH(3) (25 microg/mL).

RESULTS

It could be demonstrated that normal progenitor cells are far more resistant to hyperthermia than leukemic progenitor cells (56%+/-7% vs 9.9%+/-2.6% survival after 60 minutes at 43 degrees C, respectively). Furthermore, normal hematopoietic stem cells appear to be extremely resistant to the heat treatment (94%+/-9% survival after 60 minutes at 43 degrees C). In contrast, in the leukemic stem cell compartment no significant differences in heat sensitivity between the stem cells and progenitor subsets could be observed (12.3%+/-2.9% vs 9.9%+/-2.6% survival after 60 minutes at 43 degrees C, respectively). The combined treatment resulted in a survival for normal progenitor and stem cells of 32%+/-6% and 85%+/-15% after 60 minutes at 43 degrees C, respectively. Under these conditions the number of leukemic stem cells was reduced to 1%+/-0.3%. After 120 minutes at 43 degrees C, no AML-colonies could be detected anymore.

CONCLUSIONS

Our data demonstrate that leukemic stem cells have an increased hyperthermic sensitivity compared to their normal counterparts and that this difference can be further increased in combination with ET-18-OCH(3). These striking differences in heat sensitivity warrant the use of hyperthermia as a clinically applicable purging modality in autologous stem cell transplantation.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

The t(8;21) fusion protein contacts co-repressors and histone deacetylases to repress the transcription of the p14ARF tumor suppressor

The t(8;21) is one of the most frequent chromosomal translocations associated with acute leukemia. The translocation fuses the DNA binding domain of AML1 to nearly all of the ETO co-repressor. ETO associates with the mSin3 and N-CoR co-repressors as well as histone deacetylases 1, 2, and 3. Although this is one of the most frequent chromosomal translocations in acute leukemia, accounting for 10-15% of the cases of acute myeloid leukemia (AML), the direct targets for transcriptional regulation that stimulate leukemogenesis are unknown. We found that AML1-ETO repressed the promoter of p14(ARF) tumor suppressor in transient transfection assays and reduced endogenous levels of p14(ARF) expression in multiple cell types. Chromatin immunoprecipitation assays demonstrated that AML1-ETO bound to the p14(ARF) promoter. In acute myeloid leukemia samples containing the t(8;21), levels of p14(ARF) mRNA were markedly lower when compared to other acute myeloid leukemias. Therefore, p14(ARF) is a direct transcriptional target of AML1-ETO.

Structural and functional characterization of Runx1, CBF beta, and CBF beta-SMMHC

Core binding factors (CBFs) are heterodimeric transcription factors consisting of a DNA-binding CBF alpha subunit and non-DNA-binding CBF beta subunit. DNA binding and heterodimerization is mediated by a single domain in the CBF alpha subunit called the Runt domain, while sequences flanking the Runt domain confer other biochemical activities such as transactivation. On the other hand, the heterodimerization domain in CBF beta is the only functional domain that has been identified in this subunit. The biophysical properties of the Runt domain and the CBF beta heterodimerization domain, and the structures of the isolated domains as well as of the Runt domain-DNA, Runt domain-CBF beta, and ternary Runt domain-CBF beta-DNA complexes, have been characterized over the past several years, and are summarized in this review.

Cbf beta is involved in maturation of all lineages of hematopoietic cells during embryogenesis except erythroid

The transcription factor Cbf beta forms a heterodimeric complex with members of the Runx family of proteins. Together, Cbf beta and Runx1 play a critical role in the establishment of definitive hematopoiesis in mouse embryos. Previously, we used a Cbfb-GFP "knock-in" mouse model to demonstrate that Cbf beta is expressed in hematopoietic stem cells of the mouse fetal liver and aorta-gonad-mesonephros (Blood 100 (2002), 2449). We also examined the expression pattern of Cbf beta in different lineages of adult hematopoietic cells and found that it is expressed uniformly in all lineages except B lymphocytes and erythroid cells. Cbf beta expression decreases during maturation of B cells in the adult bone marrow, and is not expressed in nucleated erythroid precursors. Here, we examine the expression of Cbf beta in various hematopoietic lineages, including myeloid, lymphoid, and erythroid during late stages of embryonic development, and compare it to the pattern observed in adults. We find that there are subtle differences in expression of Cbf beta-GFP in embryonic hematopoietic cells compared to their adult counterparts, but that the overall pattern is the same. Our data complement recently published data on hematopoetic defects observed in transgenic Cbfb-null mouse embryos partially rescued by ectopic expression of Cbfb (Nature Genet. 32 (2002), 633; Nature Genet. 32 (2002), 645). and supports the emerging view that Cbf beta and Runx proteins are required for normal maturation of hematopoietic cells as well as establishment of definitive hematopoiesis.

The role of a Runt domain transcription factor AML1/RUNX1 in leukemogenesis and its clinical implications

A Runt domain transcription factor AML1/RUNX1 is essential for generation and differentiation of definitive hematopoietic stem cells. AML1 is the most frequent target of chromosomal translocations in acute leukemias. Several chimeric proteins such as AML1-MTG8 and TEL-AML1 have transdominant properties for wild-type AML1 and acts as transcriptional repressors. The transcriptional repression in AML1 fusion proteins is mediated by recruitment of nuclear corepressor complex that maintains local histone deacetylation. Inhibition of the expression of AML1-responsive genes leads to a block in hematopoietic cell differentiation and consequent leukemic transformation. On the other hand, mutations in the Runt domain of the AML1 are identified in both sporadic acute myeloblastic leukemia (AML) without AML1 translocation and familial platelet disorder with predisposition to AML. These observations indicate that a decrease in AML1 dosage resulting from chromosomal translocations or mutations contributes to leukemogenesis. Furthermore, dysregulated chromatin remodeling and transcriptional control appears to be a common pathway in AML1-associated leukemias that could be an important target for the development of new therapeutic agents.

The core-binding factor leukemias: lessons learned from murine models

The recent development of murine models of core-binding factor leukemias has provided important insights into the underlying molecular pathology of this common subtype of acute myeloid leukemia. Evidence from these models supports the idea that acute myeloid leukemia 1/core-binding factor beta-subunit (AML1/CBFbeta) has a critical role in the control of the self-renewal capacity of hematopoietic stem cells and their progeny. Moreover, the accumulated data demonstrate that the expression of translocation-encoded AML1 or CBFbeta fusion proteins are insufficient by themselves to induce a full leukemic phenotype. The models that have been developed should prove to be of value for defining the range of mutations that can cooperate with AML1/CBFbeta fusion proteins, and for assessing novel therapies targeted toward the pathways that are altered by the expression of these fusion proteins.

The inv(16) fusion protein associates with corepressors via a smooth muscle myosin heavy-chain domain

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 beta to the coiled-coil region of a smooth muscle myosin heavy chain (CBFbeta/SMMHC). CBFbeta interacts with AML1 to form a heterodimer that binds DNA; this interaction increases the affinity of AML1 for DNA. The CBFbeta/SMMHC fusion protein cooperates with AML1 to repress the transcription of AML1-regulated genes. We show that CBFbeta/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 CBFbeta/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.

Genetics of myeloid leukemias

Human leukemias are typified by acquired recurring chromosomal translocations. Cloning of these translocation breakpoints has provided important insights into pathogenesis of disease as well as novel therapeutic approaches. Chronic myelogenous leukemias (CML) are caused by constitutively activated tyrosine kinases, such as BCR/ABL, that confer a proliferative and survival advantage to hematopoietic progenitors but do not affect differentiation. These activated kinases are validated targets for therapy with selective tyrosine kinase inhibitors, a paradigm that may have broad applications in treatment of hematologic malignancies as well as solid tumors. Chromosomal translocations in acute myeloid leukemias (AML) most often result in loss-of-function mutations in transcription factors that are required for normal hematopoietic development. These latter mutations, however, are not sufficient to cause AML. The available evidence indicates that activating mutations in the hematopoietic tyrosine kinases FLT3 and c-KIT, and in N-RAS and K-RAS, confer proliferative advantage to hematopoietic progenitors and cooperate with loss-of-function mutations in hematopoietic transcription factors to cause an acute leukemia phenotype characterized by proliferation and impaired differentiation. The data supporting this hypothesis and the clinical and therapeutic implications of these observations are reviewed.

Core-binding factor beta interacts with Runx2 and is required for skeletal development

Core-binding factor beta (CBFbeta, also called polyomavirus enhancer binding protein 2beta (PEBP2B)) is associated with an inversion of chromosome 16 and is associated with acute myeloid leukemia in humans. CBFbeta forms a heterodimer with RUNX1 (runt-related transcription factor 1), which has a DNA binding domain homologous to the pair-rule protein runt in Drosophila melanogaster. Both RUNX1 and CBFbeta are essential for hematopoiesis. Haploinsufficiency of another runt-related protein, RUNX2 (also called CBFA1), causes cleidocranial dysplasia in humans and is essential in skeletal development by regulating osteoblast differentiation and chondrocyte maturation. Mice deficient in Cbfb (Cbfb(-/-)) die at midgestation, so the function of Cbfbeta in skeletal development has yet to be ascertained. To investigate this issue, we rescued hematopoiesis of Cbfb(-/-) mice by introducing Cbfb using the Gata1 promoter. The rescued Cbfb(-/-) mice recapitulated fetal liver hematopoiesis in erythroid and megakaryocytic lineages and survived until birth, but showed severely delayed bone formation. Although mesenchymal cells differentiated into immature osteoblasts, intramembranous bones were poorly formed. The maturation of chondrocytes into hypertrophic cells was markedly delayed, and no endochondral bones were formed. Electrophoretic mobility shift assays and reporter assays showed that Cbfbeta was necessary for the efficient DNA binding of Runx2 and for Runx2-dependent transcriptional activation. These findings indicate that Cbfbeta is required for the function of Runx2 in skeletal development.

Molecular genetic diagnosis of pediatric cancer: current and emerging methods

A wide array of diagnostic tests are available to evaluate molecular abnormalities in pediatric cancer. Classic cytogenetics, FISH, flow cytometry, PCR, and Southern blot analysis are in widespread use throughout pediatric hospitals. Examples of the application of these methods in pediatric cancer diagnosis are reviewed. Newer methods such as CGH, SKY, gene expression microarrays and proteomic methods are under active investigation andwill almost certainly lead to significant advances in our ability to diagnose and treat pediatric cancer.

Multimerization via its myosin domain facilitates nuclear localization and inhibition of core binding factor (CBF) activities by the CBFbeta-smooth muscle myosin heavy chain myeloid leukemia oncoprotein

In CBFbeta-SMMHC, core binding factor beta (CBFbeta) is fused to the alpha-helical rod domain of smooth muscle myosin heavy chain (SMMHC). We generated Ba/F3 hematopoietic cells expressing a CBFbeta-SMMHC variant lacking 28 amino acids homologous to the assembly competence domain (ACD) required for multimerization of skeletal muscle myosin. CBFbeta-SMMHC(DeltaACD) multimerized less effectively than either wild-type protein or a variant lacking a different 28-residue segment. In contrast to the control proteins, the DeltaACD mutant did not inhibit CBF DNA binding, AML1-mediated reporter activation, or G(1) to S cell cycle progression, the last being dependent upon activation of CBF-regulated genes. We also linked the CBFbeta domain to 149 or 83 C-terminal CBFbeta-SMMHC residues, retaining 86 or 20 amino acids N-terminal to the ACD. CBFbeta-SMMHC(149C) multimerized and slowed Ba/F3 proliferation, whereas CBFbeta-SMMHC(83C) did not. The majority of CBFbeta-SMMHC and CBFbeta-SMMHC(149C) was detected in the nucleus, whereas the DeltaACD and 83C variants were predominantly cytoplasmic, indicating that multimerization facilitates nuclear retention of CBFbeta-SMMHC. When linked to the simian virus 40 nuclear localization signal (NLS), a significant fraction of CBFbeta-SMMHC(DeltaACD) entered the nucleus but only mildly inhibited CBF activities. As NLS-CBFbeta-SMMHC(83C) remained cytoplasmic, we directed the ACD to CBF target genes by linking it to the AML1 DNA binding domain or to full-length AML1. These AML1-ACD fusion proteins did not affect Ba/F3 proliferation, in contrast to AML1-ETO, which markedly slowed G(1) to S progression dependent upon the integrity of its DNA-binding domain. Thus, the ACD facilitates inhibition of CBF by mediating multimerization of CBFbeta-SMMHC in the nucleus. Therapeutics targeting the ACD may be effective in acute myeloid leukemia cases associated with CBFbeta-SMMHC expression.

Molecular analysis of a new variant of the CBF beta-MYH11 gene fusion

The inv(16)(p13q22) is observed in 16% of patients with acute myelogenous leukemia (AML). It is classically found in the AML M4Eo subtype, which has distinctive morphological abnormalities in the bone marrow including myelomonocytic differentiation and an increase in atypical bone marrow eosinophils. A gene fusion involving CBFbeta and MYH11 is invariably created by the inv(16)(p13q22) and is thought to be a necessary genetic lesion in this form of leukemia. The most common fusion point occurs at CBFbeta nucleotide (nt) 495 and MYH11 nt 1921; however, several rare variants have been described. We report a patient with AML M4Eo whose leukemic cells contained two distinct CBFbeta-MYH11 transcripts, one rare and the other previously undescribed. Both gene fusion products were cloned and sequenced and the breakpoints were identified. These were at CBFbeta nt 495 and MYH11 nt 994 and CBFbeta nt486 and MYH11 nt 1591. The CBFbeta(495)/MYH11(994) fusion is seen in 5-7% of AML M4Eo, while the CBFbeta(486)/MYH11(1591) fusion is novel. We postulate that these two fusions arose from a single rearranged chromosome 16 by way of alternative splicing. These fusions were associated with a good prognosis in this patient. Molecular diagnostic facilities should be aware of the existence of the CBFbeta(486)/MYH11(1591) variant and its potential association with the previously described type E fusion.

Role of Cbfb in hematopoiesis and perturbations resulting from expression of the leukemogenic fusion gene Cbfb-MYH11

Core-binding factor beta (CBFbeta) and CBFalpha2 form a heterodimeric transcription factor that plays an important role in hematopoiesis. The genes encoding either CBFbeta or CBFalpha2 are involved in chromosomal rearrangements in more than 30% of cases of acute myeloid leukemia (AML), suggesting that CBFbeta and CBFalpha2 play important roles in leukemogenesis. Inv(16)(p13;q22) is found in almost all cases of AML M4Eo and results in the fusion of CBFB with MYH11, the gene encoding smooth muscle myosin heavy chain. Mouse embryos heterozygous for a Cbfb-MYH11 knock-in gene lack definitive hematopoiesis, a phenotype shared by Cbfb(-/-) embryos. In this study we generated a Cbfb-GFP knock-in mouse model to characterize the normal expression pattern of Cbfbeta in hematopoietic cells. In midgestation embryos, Cbfbeta was expressed in populations enriched for hematopoietic stem cells and progenitors. This population of stem cells and progenitors was not present in mouse embryos heterozygous for the Cbfb-MYH11 knock-in gene. Together, these data suggest that Cbfb-MYH11 blocks embryonic hematopoiesis at the stem-progenitor cell level and that Cbfb is essential for the generation of hematopoietic stem and progenitor cells. In adult mice, Cbfbeta was expressed in stem and progenitor cells, as well as mature myeloid and lymphoid cells. Although it was expressed in erythroid progenitors, Cbfbeta was not expressed during the terminal stages of erythropoiesis. Our data indicate that Cbfb is required for myeloid and lymphoid differentiation; but does not play a critical role in erythroid differentiation.

Human cytogenetics: 46 chromosomes, 46 years and counting

Human cytogenetics was born in 1956 with the fundamental, but empowering, discovery that normal human cells contain 46 chromosomes. Since then, this field and our understanding of the link between chromosomal defects and disease have grown in spurts that have been fuelled by advances in cytogenetic technology. As a mature enterprise, cytogenetics now informs human genomics, disease and cancer genetics, chromosome evolution and the relationship of nuclear structure to function.

Minimal residual disease quantification in patients with acute myeloid leukaemia and inv(16)/CBFB-MYH11 gene fusion

We have designed a real-time CBFB-MYH11 reverse transcription polymerase chain reaction (RT-PCR) assay to quantify minimal residual disease (MRD) in patients with inv(16)-positive acute myeloid leukaemia (AML). Six patients were followed for a median of 17.5 months after diagnosis during which 120 evaluable samples were analysed. The CBFB-MYH11 expression at diagnosis varied only fourfold between the six patients and was virtually identical to that observed in the CBFB-MYH11-positive cell line ME-1. For two cases, a patient-specific real-time PCR for CBFB-MYH11 quantification at genomic DNA level was designed. Similar disease levels were found at the RNA and genomic DNA level during and after treatment, indicating that CBFB-MYH11 gene expression was unaltered during treatment and that the percentage of malignant cells can be accurately quantified at the RNA level. Following successive courses of chemotherapy, the reduction of malignant cells was found to be significantly more pronounced (80-250-fold greater) in peripheral blood compared with bone marrow in five out of six cases tested. Treatment with gemtuzumab ozogamicin as sole agent at relapse did not result in a selective decrease of tumour cells in three cases analysed. We conclude that real-time PCR is a powerful method of monitoring MRD levels and quantifying the antileukaemic effect of separate (experimental) courses of chemotherapy.

Core-binding factor alpha 1 (Cbfa1) induces osteoblastic differentiation of C2C12 cells without interactions with Smad1 and Smad5

Core-binding factor alpha(1) (Cbfa1) is an essential transcription factor for osteoblastic differentiation and osteogenesis. Bone morphogenetic protein (BMP) is also a powerful inducer of differentiation of pluripotent mesenchymal cells to osteoblast lineage and bone formation. Recent studies suggest that Cbfa1 plays a critical role during BMP-induced osteoblastic differentiation through association with cytoplasmic BMP signaling molecules, Smads. However, other studies have suggested that Cbfa1 may exhibit its osteogenic function without interaction with Smads. Therefore, it remains unclear whether association with Smad is essential for Cbfa1 function. In this study we examine the effects of Cbfa1 on osteoblastic differentiation in the presence or absence of interactions with Smad1 or Smad5 using C2C12 undifferentiated mesenchymal cells. Cbfa1 expression was induced upon stimulation with BMP-2 in C2C12 cells. Introduction of Cbfa1 into C2C12 cells induced osteoblastic differentiation and promoted transactivation of osteocalcin gene promoter without forming the complex with Smad1 or Smad5. Furthermore, in C2C12 cells in which the association of Cbfa1 with Smad1/Smad5 was prevented by the overexpression of the natural antagonist, Smad6, Cbfa1 still induced osteoblastic differentiation and transactivated osteocalcin gene promoter, regardless of BMP-2 stimulation. These results suggest that the interactions with Smad1 or Smad5 are not essential for Cbfa1 to demonstrate its osteogenic actions. However, interactions with Smad1/Smad5 enhance these osteogenic actions of Cbfa1. Of note, BMP-2-induced or Smad-induced osteoblastic differentiation was inhibited by dominant-negative Cbfa1, suggesting that the function of Cbfa1 is critical for BMP-2-induced osteoblastic differentiation. Our results suggest that Cbfa1 is essential and also sufficient to induce osteoblastic differentiation in undifferentiated mesenchymal cells, and establishment of an association with Smad1/Smad5 enhances the osteogenic actions of Cbfa1. On the other hand, Cbfa1 expression requires the activation of Smad1/Smad5 by BMP-2.

Hematopoietic stem cell expansion and distinct myeloid developmental abnormalities in a murine model of the AML1-ETO translocation

The t(8;21)(q22;q22) translocation, which fuses the ETO gene on human chromosome 8 with the AML1 gene on chromosome 21 (AML1-ETO), is one of the most frequent cytogenetic abnormalities associated with acute myelogenous leukemia (AML). It is seen in approximately 12 to 15% of AML cases and is present in about 40% of AML cases with a French-American-British classified M2 phenotype. We have generated a murine model of the t(8;21) translocation by retroviral expression of AML1-ETO in purified hematopoietic stem cells (HSC). Animals reconstituted with AML1-ETO-expressing cells recapitulate the hematopoietic developmental abnormalities seen in the bone marrow of human patients with the t(8;21) translocation. Primitive myeloblasts were increased to approximately 10% of bone marrow by 10 months posttransplant. Consistent with this observation was a 50-fold increase in myeloid colony-forming cells in vitro. Accumulation of late-stage metamyelocytes was also observed in bone marrow along with an increase in immature eosinophilic myelocytes that showed abnormal basophilic granulation. HSC numbers in the bone marrow of 10-month-posttransplant animals were 29-fold greater than in transplant-matched control mice, suggesting that AML1-ETO expression overrides the normal genetic control of HSC pool size. In summary, AMLI-ETO-expressing animals recapitulate many (and perhaps all) of the developmental abnormalities seen in human patients with the t(8;21) translocation, although the animals do not develop leukemia or disseminated disease in peripheral tissues like the liver or spleen. This suggests that the principal contribution of AML1-ETO to acute myeloid leukemia is the inhibition of multiple developmental pathways.

Lymphoid blastic crisis of chronic myelogenous leukaemia with inv(16)(p13;q22)

We report a case of chronic myelogeneous leukaemia (CML) in B-lineage lymphoid blastic crisis (BC) having chromosome abnormality, inv(16)(p13;q22) in addition to Philadelphia chromosome, in 20/20 marrow metaphase. Inv(16)(p13;q22) was not observed in cells of chronic phase or accelerate phase. Abnormalities of chromosome 16, including inv(16)(p13;q22), del(16)(q22) and t(16;16)(p13;q22), have been reported mostly in acute myelomonocytic leukaemia (AML), (FAB M4-Eo), and some in CML-BC and myelodysplastic syndrome (MDS) cases. Most of the cases showed increase of myelomonocytic components and abnormal eosinophils with dysplastic granules in the bone marrow (BM). However, our case was diagnosed as lymphoid BC without increase of myelomonocytic components, although some abnormal eosinophilia was seen. To date, lymphoid BC of CML having inv(16)(p13;q22) abnormality has not been reported. The case presented here could be a clue to understand the pathophysiology of inv(16)(p13;q22) leukaemia.

A novel t(16;20)(q22;p13) in polycythemia vera

We report a patient with polycythemia vera whose bone marrow cells carried a novel t(16;20)(q22;p13) as detected by karyotype analysis using G- and Q-banding techniques. The reciprocal translocation was confirmed by fluorescence in situ hybridization (FISH) using DNA libraries of chromosomes 16 and 20. To our knowledge, t(16;20)(q22;p13) has not been reported previously. The core binding factor beta (CBFbeta) gene located on 16q22 is known to be frequently involved in acute myelocytic leukemia. On the other hand, the 20p13 locus contains a gene encoding protein kinase CK2alpha, which is closely related to cell proliferation and cell cycle regulation. The t(16;20)(q22;p13) may be one of the cytogenetic aberrations in myeloproliferative disorders, and therefore, our observation warrants further studies on a possible involvement of the genes resulting from this translocation.

Transcription factor fusions in acute leukemia: variations on a theme

The leukemia-associated fusion proteins share several structural or functional similarities, suggesting that they may impart a leukemic phenotype through common modes of transcriptional dysregulation. The fusion proteins generated by these translocations usually contain a DNA-binding domain, domains responsible for homo- or hetero-dimerization, and domains that interact with proteins involved in chromatin remodeling (e.g., co-repressor molecules or co-activator molecules). It is these shared features that constitute the 'variations on the theme' that underling the aberrant growth and differentiation that is the hallmark of acute leukemia cells.

AML1 stimulates G1 to S progression via its transactivation domain

Inhibition of AML1-mediated transactivation potently slows G1 to S cell cycle progression. In Ba/F3 cells, activation of exogenous AML1 (RUNX1)-ER with 4-hydroxytamoxifen prevents inhibition of G1 progression mediated by CBFbeta-SMMHC, a CBF oncoprotein. We expressed three AML1-ER variants with CBFbeta-SMMHC in Ba/F3 cells. In these lines, CBFbeta-SMMHC expression is regulated by the zinc-responsive metallothionein promoter. Deletion of 72 AML1 C-terminal residues, which includes a transrepression domain, did not alter the activity of AML1-ER, whereas further deletion of 98 residues, removing the most potent AML1 transactivation domain (TAD), prevented rescue of cell cycle inhibition. Notably, the two variants which did not stimulate G1 exacerbated CBFbeta-SMMHC-mediated cell cycle arrest, suggesting that they dominantly inhibit AML1 activities. In addition, the two variants which stimulated G1 also induced apoptosis in 5-15% of the cells, an effect consistent with excessive G1 stimulation. These observations indicate that AML1 activates transcription of one or more genes critical for the G1 to S transition via its C-terminal transactivation domain. Inactivation of AML in acute leukemia is expected to slow proliferation unless additional genetic alterations co-exist which accelerate G1.

The role of drug efflux pumps in acute myeloid leukemia

A major problem in the treatment of patients with acute myeloid leukemia (AML) is the occurrence of resistance to structurally and functionally unrelated chemotherapeutic agents, called multidrug resistance (MDR). One of the known MDR mechanisms is the overexpression of adenosine triphosphate (ATP)-dependent efflux pumps. Permeability-glycoprotein (P-gp), the best characterized of the human drug efflux pumps, has been shown to be associated with poor treatment outcome in AML patients. Besides P-gp, in addition the multidrug resistance protein 1 (MRP1) appeared to contribute to the observed resistance in AML. Alternative transporter proteins, such as the MRP1 homologues MRP2, MRP3, MRP5 and MRP6, and the breast cancer resistance protein (BCRP), have been shown to be expressed at variable levels in AML patient cells. The latter proteins have been described to confer resistance to chemotherapeutic agents, such as daunorubicin, mitoxantrone, etoposide and 6-mercaptopurine, which are generally used in the treatment of AML patients; however, theyhave not yet proven to play a role in drug resistance in AML. The present review gives an overview of the current knowledge concerning these drug transporters, with a focus on the role of the transporter proteins in AML.

Genetic profile of acute myeloid leukemia

Understanding genomic events and the cascade of their effects in cell function is crucial for identifying distinct subsets of acute myeloid leukemia and developing new therapeutic strategies. Conventional cytogenetics, fluorescence in situ hybridization investigations and molecular studies have provided much information over the past few years. This review will focus on major genomic mechanisms in acute myeloid luekemia and on the genes implicated in the pathogenesis of specific subtypes.

Mutational analyses of the AML1 gene in patients with myelodysplastic syndrome

The AML1 gene is the most frequent target of translocations associated with human leukemias. We recently found somatic point mutations of the AML1 gene, V105ter and R139G, in two cases of myelodysplastic syndrome (MDS). Both mutations are present in the region encoding the Runt domain of AML1, and cause loss of the DNA-binding ability of the resultant products. Of these mutants, V105ter has also lost the ability to heterodimerize with PEBP2beta/CBFbeta. On the other hand, the R139G mutant acts as a dominant negative inhibitor through competing with wild-type AML1 for interaction with PEBP2beta/CBFbeta. In this review, we summarize mutational changes of the AML1 gene in hematological malignancies, especially in MDS and discuss the mechanism whereby the mutant acts as a dominant negative inhibitor of wild-type AML1.

Minimal residual disease in acute myeloid leukaemia

Relapse remains the main cause of treatment failure in acute myeloid leukaemia (AML). Studies to date suggest that monitoring of minimal residual disease (MRD) in AML is useful in identifying patients at high risk of relapse from those in durable remission. This chapter describes the methodological advances in the detection of MRD and, in particular, focuses on the development of highly sensitive RT-PCR techniques, including real-time, for quantifying MRD. Preliminary results on the clinical utility of MRD monitoring in AML with t(8;21) and inv(16) are promising and provide the basis for further evaluation by quantitative real-time analysis in prospective clinical trials. For AML without a specific fusion transcript, the WT1 gene is an alternative molecular target. The clinical value of quantitative MRD monitoring in AML, however, will need to be confirmed in future studies.

Expression of a conditional AML1-ETO oncogene bypasses embryonic lethality and establishes a murine model of human t(8;21) acute myeloid leukemia

The AML1/CBFbeta transcription factor complex, a frequent target of chromosomal translocations in leukemia, is essential for the generation of definitive hematopoietic stem cells. Paradoxically, expression of the acute myeloid leukemia-associated AML1-ETO fusion protein in mice results not in leukemia, but in embryonic lethality due to an absence of normal hematopoiesis. To bypass the embryonic lethality, we generated a mouse strain with a conditional AML1-ETO knockin allele that contains a loxP bracketed transcriptional stop cassette 5' to the AML1-ETO fusion site. Activation of this allele in vivo by Cre-mediated recombination resulted in an enhanced replating efficiency of myeloid progenitors, but it did not block their differentiation, nor was it sufficient to induce leukemia. However, induction of cooperating mutations resulted in the development of an acute myeloid disease that mimicked many of the features of human AML1-ETO-expressing leukemia.

Real-time quantitation of minimal residual disease in inv(16)-positive acute myeloid leukemia may indicate risk for clinical relapse and may identify patients in a curable state

The inv(16) cytogenetic subtype of acute myeloid leukemia (AML) has a relatively good prognosis. Many patients achieve complete remission (CR). The prognostic uncertainty of negative qualitative reverse transcription-polymerase chain reaction (RT-PCR) assays suggests the need to identify prognostically significant critical thresholds by real-time RT-PCR. A reliable and sensitive (10(-5)) real-time RT-PCR assay was set up for the evaluation of relevant CBFbeta-MYH11/ABL transcript ratios and was applied to the 21 patients with inv(16) AML routinely referred for cytogenetic and molecular monitoring in Seràgnoli Institute (Bologna, Italy) since 1990. Among the 18 patients who underwent ablative chemotherapy, all achieved CR with a 3-year disease-free survival probability of 63% (95% CI, 40%-87%) and no recorded events after 26 months. Five patients had relapses; 2 died of disease and 3 entered second CR. Analysis of the 125 bone marrow (or peripheral blood) samples studied by real-time RT-PCR showed that transcript ratios of samples taken during CR at any time before a relapse were always greater than 0.12%, whereas those of samples taken during first or second CR from patients who did not subsequently have relapses were always less than 0.25%. This suggests that transcript ratios greater than 0.25% may correspond to high risk for relapse, whereas ratios below 0.12% might indicate the patient is in a curable state. If confirmed, such thresholds could open the way to a new phase in post-CR therapeutic decision making for patients with inv(16) AML.

Detection of minimal residual disease

A high percentage of patients with leukemia, lymphoma, and solid tumors achieve a complete clinical remission after initial treatment, but the majority of these patients will finally relapse from residual tumor cells detectable in clinical remission only by the most sensitive methods. The in vitro amplification of tumor-specific DNA or RNA sequences by polymerase chain reaction (PCR) allows identification of a few neoplastic cells in 10(4) to 10(6) normal cells. Depending on the underlying malignant disease and therapeutic treatment, the presence of residual tumor cells in an individual patient may herald relapse, but a long-term stable situation or slowly vanishing tumor cells are also possible. Molecular monitoring of residual leukemia and lymphoma cells by quantitative PCR techniques has provided important information about the effectiveness of treatment and the risk of recurrent disease as shown by minimal residual disease (MRD) analysis in patients with various malignant diseases. Such diseases include childhood acute lymphoblastic leukemia, after induction therapy; acute promyelocytic leukemia, during and after chemotherapy; and chronic myelogenous leukemia, during treatment with alpha-interferon and after allogeneic bone marrow transplantation. Evaluation of the predictive value of the detection of MRD has to take into account its evolution and course, the pathogenesis, biology, and natural course of the underlying malignant disease, the molecular genetic lesion, and finally, the type of treatment. Quantification of minimal residual cells by the recently developed real-time quantitative PCR technique will surely have a major impact on our therapeutic strategies for patients with leukemia, lymphomas, and solid tumors. Based on quantitative PCR data, the terms molecular remission and molecular relapse have to be exactly defined and validated in prospective clinical trials to assess the biological and clinical significance of MRD in various types of malignancies.

The role of EBERs in oncogenesis

Epstein-Barr virus (EBV)-encoded small non-polyadenylated RNAs (EBERs) are the most abundant viral transcripts in latently EBV-infected cells. However, until recently, their roles in viral infection were totally unknown. It now appears that EBERs play a key role in maintaining the malignant phenotypes of Burkitt's lymphoma (BL) cells. The EBERs confer clonability in soft agarose, tumourigenicity in mice, and resistance to apoptosis against various stimuli in BL. Furthermore, EBERs induce transcription of interleukin-10, which acts as an autocrine growth factor of BL. These studies open the way toward the new concept that RNA molecules can act in oncogenesis.

Chromosome translocations: dangerous liaisons revisited

Although it has been clear for more than a century that the chromosomes in human tumour cells are often wildly abnormal, there has been controversy as to whether these changes are primary events or are merely secondary epiphenomena that reflect the genomic instability of these cells. The prevailing view for most of this period was that chromosome changes were secondary events. What happened to change this view?

ETO, a target of t(8;21) in acute leukemia, makes distinct contacts with multiple histone deacetylases and binds mSin3A through its oligomerization domain

t(8;21) and t(16;21) create two fusion proteins, AML-1-ETO and AML-1-MTG16, respectively, which fuse the AML-1 DNA binding domain to putative transcriptional corepressors, ETO and MTG16. Here, we show that distinct domains of ETO contact the mSin3A and N-CoR corepressors and define two binding sites within ETO for each of these corepressors. In addition, of eight histone deacetylases (HDACs) tested, only the class I HDACs HDAC-1, HDAC-2, and HDAC-3 bind ETO. However, these HDACs bind ETO through different domains. We also show that the murine homologue of MTG16, ETO-2, is also a transcriptional corepressor that works through a similar but distinct mechanism. Like ETO, ETO-2 interacts with N-CoR, but ETO-2 fails to bind mSin3A. Furthermore, ETO-2 binds HDAC-1, HDAC-2, and HDAC-3 but also interacts with HDAC-6 and HDAC-8. In addition, we show that expression of AML-1-ETO causes disruption of the cell cycle in the G(1) phase. Disruption of the cell cycle required the ability of AML-1-ETO to repress transcription because a mutant of AML-1-ETO, Delta469, which removes the majority of the corepressor binding sites, had no phenotype. Moreover, treatment of AML-1-ETO-expressing cells with trichostatin A, an HDAC inhibitor, restored cell cycle control. Thus, AML-1-ETO makes distinct contacts with multiple HDACs and an HDAC inhibitor biologically inactivates this fusion protein.

RUNX1/AML1: a central player in hematopoiesis

It has been well established that a number of transcription factors play critical roles in regulating the fate of hematopoietic stem cell populations. One of them is the leukemia-associated transcription factor acute myeloid leukemia 1 (AML1; also known as runt-related transcription factor 1, or RUNX1). This gene was originally cloned from the breakpoint of the t(8;21) reciprocal chromosome translocation and was later recognized as one of the most frequent targets of leukemia-associated gene aberrations. Gene-targeting experiments revealed that transcriptionally active AML1 is essential for the establishment of definitive hematopoiesis. More specifically, this gene functions in the emergence of the hematopoietic progenitor cells from the hemogenic endothelium by budding in the aorta-gonad-mesonephros region, and its expression points to the sites with strong potential for the emergence of hematopoietic stem cells. This review discusses aspects of the biologic properties of AML1 in early hematopoietic development.

MTG8 proto-oncoprotein interacts with the regulatory subunit of type II cyclic AMP-dependent protein kinase in lymphocytes

AML1-MTG8 chimeric oncogene is generated in acute myelogenous leukemia with t(8;21), and seems to be responsible for the pathogenesis of the disease. However, the role of MTG8 is ambiguous. Here we found that MTG8 interacted with the regulatory subunit of type II cyclic AMP-dependent protein kinase (PKA RIIalpha). The binding site of MTG8 was NHR3 domain, and that of RIIalpha was the N-terminus for interacting with PKA anchoring proteins (AKAPs). NHR3 contains a putative alpha-amphipathic helix which is characteristic in binding of AKAPs with RII. Indirect immunofluorescence microscopy showed that MTG8 and RIIalpha were overlapped at the centrosome-Golgi area in lymphocytes. These findings suggest that MTG8 may function as an AKAP at the centrosome-Golgi area in lymphocytes.

Common themes in the pathogenesis of acute myeloid leukemia

The pathogenesis of acute myeloid leukemia is associated with the appearance of oncogenic fusion proteins generated as a consequence of specific chromosome translocations. Of the two components of each fusion protein, one is generally a transcription factor, whereas the other partner is more variable in function, but often involved in the control of cell survival and apoptosis. As a consequence, AML-associated fusion proteins function as aberrant transcriptional regulators that interfere with the process of myeloid differentiation, determine a stage-specific arrest of maturation and enhance cell survival in a cell-type specific manner. The abnormal regulation of transcriptional networks occurs through common mechanisms that include recruitment of aberrant co-repressor complexes, alterations in chromatin remodeling, and disruption of specific subnuclear compartments. The identification and analysis of common and specific target genes regulated by AML fusion proteins will be of fundamental importance for the full understanding of acute myeloid leukemogenesis and for the implementation of disease-specific drug design.

Aneurysmal bone cyst with chromosomal changes involving 7q and 16p

An aneurysmal bone cyst was submitted to cytogenetic analysis. The modal chromosome number was 46. The composite karyotype was 40 approximately 48,XY,-Y[4],-6[3],del(7)(q32)[3],-9[3],+12[2],+13[2], inv(16)(p13.1q24)[4],-17[3],-19[4],-20[3][cp13]. The clonal structural changes detected were del(7)(q32) and inv(16)(p13.1q24). The breakpoints involved affected areas to which important genes for cell cycle regulation have been mapped. There is only one report in the literature of three aneurysmal bone cysts presenting clonal karyotypic alterations. The cytogenetic study of the aneurysmal bone cyst reported here showed different results when compared to those previously described in the literature.

Prospective monitoring of minimal residual disease in acute myeloid leukemia with inversion(16) by CBFbeta/MYH11 RT-PCR: implications for a monitoring schedule and for treatment decisions

Minimal residual disease in patients with acute myeloid leukemia (AML) with inversion(16) can be monitored by CBFbeta/MYH11 RT-PCR. While the association between molecular remission (MR) in bone marrow (BM) and peripheral blood (PB) and long-term clinical remission (CR) seems to be established, there are insufficient data on the kinetics of CBFbeta/MYH11. We have performed a prospective study in order to generate a reasonable and sufficient schedule for PCR-monitoring. 11 patients with AML and inversion (16) in complete hematological remission have been prospectively monitored by CBFbeta/MYH11 RT-PCR in their BM and PB during an observation period of 7 to 67 months (median 32 months). Patients were followed during consolidation chemotherapy with repetitive cycles of high-dose Ara-C and after autologous or allogeneic stem cell transplantation in 2nd CR or refractory AML. MR never coincided with achievement of CR but occurred between 2 and 8 months after hematological remission. All patients in continuous CR were PCR-negative after 1-8 (median 4) months. Two patients relapsed despite MR for 10 to 15 months. Molecular relapse preceded hematological relapse by 3 to 5 months. Three out of four patients who were not in MR after 8 months relapsed. Allogeneic stem cell transplantation was able to eradicate minimal residual disease in 4/4 patients. In 2 patients a temporary reconversion to PCR-positivity was reversed by reduction of immunosuppression. 1 patient did not become PCR-negative until compete withdrawal of immunosuppression. We suggest that BM and PB should be examined after the last consolidation treatment. In case of MR, PB should be examined every 1 to 2 months and BM examination should be done only in case of PCR-positivity in PB in order to confirm the molecular relapse and to identify an impending cytogenetic and/or hematological relapse. CBFbeta/MYH11 RT-PCR monitoring is able to predict relapse 3 to 5 months prior to overt hematological relapse, offers a window of opportunity for preemptive therapy of molecular relapse and confers implications for immunotherapy in the setting of allografting.

AML1-ETO expression is directly involved in the development of acute myeloid leukemia in the presence of additional mutations

The t(8;21) is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). The translocation, which involves the AML1 gene on chromosome 21 and the ETO gene on chromosome 8, generates an AML1-ETO fusion transcription factor. To examine the effect of the AML1-ETO fusion protein on leukemogenesis, we made transgenic mice in which expression of AML1-ETO is under the control of the human MRP8 promoter (hMRP8-AML1-ETO). AML1-ETO is specifically expressed in myeloid cells, including common myeloid progenitors of hMRP8-AML1-ETO transgenic mice. The transgenic mice were healthy during their life spans, suggesting that AML1-ETO alone is not sufficient for leukemogenesis. However, after treatment of newborn hMRP8-AML1-ETO transgenic mice and their wild-type littermates with a strong DNA-alkylating mutagen, N-ethyl-N-nitrosourea, 55% of transgenic mice developed AML and the other 45% of transgenic mice and all of the wild-type littermates developed acute T lymphoblastic leukemia. Our results provide direct evidence that AML1-ETO is critical for causing myeloid leukemia, but one or more additional mutations are required for leukemogenesis. The hMRP8-AML1-ETO-transgenic mice provide an excellent model that can be used to isolate additional genetic events and to further understand the molecular pathogenesis of AML1-ETO-related leukemia.

Redundant function of Runt Domain binding partners, Big brother and Brother, during Drosophila development

The Core Binding Factor is a heterodimeric transcription factor complex in vertebrates that is composed of a DNA binding α-subunit and a non-DNA binding β-subunit. The α-subunit is encoded by members of the Runt Domain family of proteins and the β-subunit is encoded by the CBFβ gene. In Drosophila, two genes encoding α-subunits, runt and lozenge, and two genes encoding β-subunits, Big brother and Brother, have been previously identified. Here, a sensitized genetic screen was used to isolate mutant alleles of the Big brother gene. Expression studies show that Big brother is a nuclear protein that co-localizes with both Lozenge and Runt in the eye imaginal disc. The nuclear localization and stability of Big brother protein is mediated through the formation of heterodimeric complexes between Big brother and either Lozenge or Runt. Big brother functions with Lozenge during cell fate specification in the eye, and is also required for the development of the embryonic PNS. ds-RNA-mediated genetic interference experiments show that Brother and Big brother are redundant and function together with Runt during segmentation of the embryo. These studies highlight a mechanism for transcriptional control by a Runt Domain protein and a redundant pair of partners in the specification of cell fate during development.

Quantification of CBFbeta/MYH11 fusion transcript by real time RT-PCR in patients with INV(16) acute myeloid leukemia

Amplification of the CBFbeta/MYH11 fusion transcript by a qualitative reverse transcription-polymerase chain reaction (RT-PCR) has been used to detect minimal residual disease (MRD) and assess the risk for disease relapse in inv(16)(p13q22) acute myeloid leukemia (AML). This strategy has, however, produced conflicting results and because of an uncertain predictive value, its use in the clinical setting cannot be recommended. The objective of the current study was to evaluate if quantification by Real Time RT-PCR could be useful to determine levels of CBFbeta/MYH11 fusion transcripts predictive of clinical outcome in inv(16)(p13q22) AML at diagnosis or during remission. Bone marrow (BM) samples from 16 patients with inv(16) AML enrolled on a German multicenter trial (AML HD93) were analyzed for levels of CBFbeta/MYH11 fusion transcripts by Real Time RT-PCR at diagnosis (n= 14), during remission (n= 10) and at relapse (n=6). The CBFbeta/MYH11 transcript copy number in each sample was normalized to copies of an internal control housekeeping transcript (ie 18S). The copy number measured at diagnosis or relapse were 3 to 4 log higher that those measured during remission, following completion of induction treatment. A high CBFbeta/MYH11 transcript copy number at diagnosis had a significant correlation with a high percentage of BM blasts (Spearman's coefficient = -0.66; P= 0.03), and a borderline correlation with a short complete remission (CR) duration (Spearman's coefficient = -0.51; P= 0.07). No difference in levels of CBFbeta/MYH11 fusion transcripts measured during intensification therapy was found between patients destined to relapse and those who continued in CCR (P= 0.75). Following completion of the entire chemotherapy program, patients that during CR showed a CBFbeta/MYH11 fusion transcript copy number >10 had a significantly shorter CR duration (P= 0.002) and higher risk for disease relapse (P= 0.05) than patients with a CBFbeta/MYH11 fusion transcript copy number <10. The results of the current study, therefore, suggest that it is possible to determine in remission samples a threshold of CBFbeta/MYH11 transcript copy number above which relapse occurs and below which continuous CR is likely.

Function of the inv(16) fusion gene CBFB-MYH11

Inv(16)(p13q22) is associated with acute myeloid leukemia subtype M4Eo, which is characterized by the presence of myelomonocytic blasts and atypical eosinophils. This chromosomal rearrangement results in the fusion of CBFB and MYH11 genes. Mouse models indicate that the fusion gene, Cbfb-MYH11, inhibits differentiation of hematopoietic cells. Although expression of Cbfb-MYH11 is not sufficient for leukemogenesis, a combination of Cbfb-MYH11 and additional mutations can lead specifically to the development of myeloid leukemia. Normally, CBFbeta interacts with CBFalpha to form a transcriptionally active nuclear complex. In vitro studies indicate that expression of CBFB-MYH11 leads to sequestration of CBFalpha2 in the cytoplasm. It also has been shown to inhibit CBF-mediated transactivation, slow cell cycle progression, delay the apoptotic response to DNA damaging agents, and protect CBFalpha2 from degradation. The importance of these functions in vivo remains to be determined.

Core binding factor and its role in normal hematopoietic development

The core binding factors are a small family of transcription factors comprising a DNA binding CBFalpha subunit and a non-DNA binding CBFbeta subunit. One gene encoding a CBFalpha subunit, RUNX1 (also known as AML1, CBFA2, and PEBPA2A), and the gene encoding CBFbeta (CBFB) are essential for hematopoiesis and are frequently mutated in human leukemias. Both genes are required for the generation of hematopoietic stem cells (HSCs) during embryonic development. Expression studies in fish and frogs and functional analyses in flies indicate that a role for these genes in hematopoiesis is evolutionarily conserved.