Rearrangement of the AML1/CBFA2 gene in myeloid leukemia with the 3;21 translocation: expression of co-existing multiple chimeric genes with similar functions as transcriptional repressors, but with opposite tumorigenic properties

Repression of RUNX1 activity by EVI1: a new role of EVI1 in leukemogenesis

Recurring chromosomal translocations observed in human leukemia often result in the expression of fusion proteins that are DNA-binding transcription factors. These altered proteins acquire new dimerization properties that result in the assembly of inappropriate multimeric transcription complexes that deregulate hematopoietic programs and induce leukemogenesis. Recently, we reported that the fusion protein AML1/MDS1/EVI1 (AME), a product of a t(3;21)(q26;q22) associated with chronic myelogenous leukemia and acute myelogenous leukemia, displays a complex pattern of self-interaction. Here, we show that the 8th zinc finger motif of MDS1/EVI1 is an oligomerization domain involved not only in interaction of AME with itself but also in interactions with the parental proteins, RUNX1 and MDS1/EVI1, from which AME is generated. Because the 8th zinc finger motif is also present in the oncoprotein EVI1, we have evaluated the effects of the interaction between RUNX1 and EVI1 in vitro and in vivo. We found that in vitro, this interaction alters the ability of RUNX1 to bind to DNA and to regulate a reporter gene, whereas in vivo, the expression of the isolated 8th zinc finger motif of EVI1 is sufficient to block the granulocyte colony-stimulating factor-induced differentiation of 32Dcl3 cells, leading to cell death. As EVI1 is not detected in normal bone marrow cells, these data suggest that its inappropriate expression could contribute to hematopoietic transformation in part by a new mechanism that involves EVI1 association with key hematopoietic regulators, leading to their functional impairment.

t(3;21)(q26;q22) in myeloid leukemia: an aggressive syndrome of blast transformation associated with hydroxyurea or antimetabolite therapy

BACKGROUND

The t(3;21)(q26;q22) translocation is associated with myeloid leukemias and results in a chimeric oncoprotein containing AML1/RUNX1 variably fused to EAP, MDS1, and/or EVI1.

METHODS

The current study describes what to the authors' knowledge is the first large case series reported to date of 26 t(3;21)(q26;q22)-associated leukemias, in which 24 cases arose after chemotherapy. Conventional G-band karyotyping and flow cytometry immunophenotyping were performed. Reverse transcriptase-polymerase chain reaction (RT-PCR) was performed to detect fusion transcripts between AML1 and EAP, MDS1, or EVI1, followed by DNA sequencing.

RESULTS

In all 16 patients with chronic myeloproliferative disorders, including 14 with chronic myelogenous leukemia (CML), the occurrence of t(3;21) heralded myeloid blast transformation. Fifteen (93%) patients had been previously treated with hydroxyurea. Eight patients with chronic myeloproliferative disorders (CMPD) were found to have t(3;21) with t(9;22) as the sole cytogenetic abnormality; in 5 other patients this was accompanied by trisomy 8. Among 10 cases of t(3;21)-associated acute myeloid leukemia, 8 were secondary tumors after chemotherapy for other neoplasms that had been treated with regimens including fludarabine and 5-fluorouracil in 3 patients each and etoposide in 2 patients. The immunophenotype of the blasts in all 22 tested cases was similar, with uniform expression of myeloid markers and CD34 and variable expression of CD7 and CD9, but minimal morphological myeloid maturation. Dysplastic micromegakaryocytes and bone marrow fibrosis were observed predominantly in CMPD cases. RT-PCR followed by DNA sequencing showed that the AML1-/MDS1-/EVI1 (AME) fusion transcript was detected in all 5 cases assessed. Among the patients with CMPD, 8 died of disease (at a median of 6.5 mos) and 5 achieved disease remission with bone marrow transplantation. Among patients with acute myeloid leukemia/myelodysplastic syndrome, 7 died of disease (at a median of 2 mos) and 2 had persistent leukemia with short follow-up.

CONCLUSIONS

Activation of AME through t(3;21) defines a highly aggressive, therapy-related leukemic blast syndrome. Prior treatment with hydroxyurea or other antimetabolites is implicated as a contributory cause.

Multipoint interphase FISH analysis of chromosome 3 abnormalities in 28 childhood AML patients

We detected non-random 3p losses and 3q gains on well-determined regions in both murine and human tumors using a microcell hybrid-based model system called 'elimination test'. We suggest that these are general malignancy-associated aberrations not necessarily linked to a particular tissue of origin. To examine chromosome 3 abnormalities, in 28 childhood acute myeloid leukemia bone marrow samples, we performed interphase multipoint-fluorescence in situ hybridization using 84 chromosome 3-specific probes and detected clonal chromosome 3 aberrations in nine cases, which is of a higher frequency than the previously reported one. In 3/28 children, a chromosome 3 abnormality was detected which was not visible using conventional cytogenetic analysis. We did not detect any 3p deletion. Increased copy number of 3q was found in four cases with trisomy of whole chromosome 3 and one case with 3q tetrasomy (isodisomy). We identified rare structural rearrangements in childhood acute myeloblastic leukemia, involving 3q21 and 3q26 loci around RPN1 and MDS1/EVI1 respectively. The poor outcome in pediatric patients with 3q rearrangements appears to be quite uniform.

The Distal Zinc Finger Domain of AML1/MDS1/EVI1 Is an Oligomerization Domain Involved in Induction of Hematopoietic Differentiation Defects in Primary Cells In vitro

AML1/MDS1/EVI1 (AME) is a chimeric transcription factor produced by the (3;21)(q26;q22) translocation. This chromosomal translocation is associated with de novo and therapy-related acute myeloid leukemia and with the blast crisis of chronic myelogenous leukemia. AME is obtained by in-frame fusion of the AML1 and MDS1/EVI1 (ME) genes. The mechanisms by which AME induces a neoplastic transformation in bone marrow cells are unknown. AME interacts with the corepressors CtBP and HDAC1, and it was shown that AME is a repressor in contrast to the parent transcription factors AML1 and ME, which are transcription activators. Studies with murine bone marrow progenitors indicated that the introduction of a point mutation that destroys the CtBP-binding consensus impairs but does not abolish the disruption of cell differentiation and replication associated with AME expression, suggesting that additional events are required. Several chimeric proteins, such as AML1/ETO, BCR/ABL, and PML/RARa, are characterized by the presence of a self-interaction domain critical for transformation. We report that AME is also able to oligomerize and displays a complex pattern of self-interaction that involves at least three oligomerization regions, one of which is the distal zinc finger domain. Although the deletion of this short domain does not preclude the self-interaction of AME, it significantly reduces the differentiation defects caused in vitro by AME in primary murine bone marrow progenitors. The addition of a point mutation that inhibits CtBP binding completely abrogates the effects of AME on differentiation, suggesting that AME induces hematopoietic differentiation defects through at least two separate but cooperating pathways.

The leukemia-associated transcription repressor AML1/MDS1/EVI1 requires CtBP to induce abnormal growth and differentiation of murine hematopoietic cells

The leukemia-associated fusion gene AML1/MDS1/EVI1 (AME) encodes a chimeric transcription factor that results from the (3;21)(q26;q22) translocation. This translocation is observed in patients with therapy-related myelodysplastic syndrome (MDS), with chronic myelogenous leukemia during the blast crisis (CML-BC), and with de novo or therapy-related acute myeloid leukemia (AML). AME is obtained by in-frame fusion of the AML1 and MDS1/EVI1 genes. We have previously shown that AME is a transcriptional repressor that induces leukemia in mice. In order to elucidate the role of AME in leukemic transformation, we investigated the interaction of AME with the transcription co-regulator CtBP1 and with members of the histone deacetylase (HDAC) family. In this report, we show that AME physically interacts in vivo with CtBP1 and HDAC1 and that these co-repressors require distinct regions of AME for interaction. By using reporter gene assays, we demonstrate that AME represses gene transcription by CtBP1-dependent and CtBP1-independent mechanisms. Finally, we show that the interaction between AME and CtBP1 is biologically important and is necessary for growth upregulation and abnormal differentiation of the murine hematopoietic precursor cell line 32Dc13 and of murine bone marrow progenitors.

Mutations of the AML1 gene in acute myeloid leukemia of FAB types M0 and M7

The AML1 gene encodes a transcription factor that, together with its heterodimeric partner CBFB, regulates a number of target genes that are essential for normal hemopoiesis. In acute myeloid leukemia (AML), AML1 is disrupted not only by chromosomal translocations but also by mutations in the runt domain, which binds both DNA and CBFB. Acquired mutations have been described predominantly in the AML FAB type M0. To date, most patients appear to have biallelic disease, suggesting a complete lack of normal AML1 function. Inherited loss of function mutations thought to lead to haploinsufficiency also have been described in patients who have a familial disorder with predisposition to AML (FPD/AML), indicating the role of AML1 in megakaryopoiesis. Using single-strand conformation polymorphism analysis, we studied the AML1 runt domain in 41 patients with M0 AML and identified potentially pathologic mutations in five (12%). Biallelic disease could be confirmed in only one patient, using loss of heterozygosity studies. At least three of the mutations would lead to truncated proteins similar to those reported in FPD/AML, suggesting that haploinsufficiency plays a role in the pathogenesis of this minimally differentiated type of leukemia. The incidence of acquired mutations in AML patients with acute megakaryoblastic leukemia (FAB type M7) was the same as that reported in other non-M0 patients, with only one mutation detected in 20 (5%) patients studied.

The t(3;21) fusion product, AML1/Evi-1 blocks AML1-induced transactivation by recruiting CtBP

AML1/Evi-1 is a chimeric protein that is derived from t(3;21), found in blastic transformation of chronic myelogenous leukemia. It is composed of the N-terminal AML1 portion with the DNA-binding Runt domain and the C-terminal Evi-1 portion. It has been shown to dominantly repress AML1-induced transactivation. The mechanism for it has been mainly attributed to competition with AML1 for the DNA-binding and for the interaction with PEBP2beta (CBFbeta), a partner protein which heterodimerizes with AML1. It was recently found that Evi-1 interacts with C-terminal binding protein (CtBP) to repress TGFbeta-induced transactivation. Here, we demonstrate that AML1/Evi-1 interacts with CtBP in SKH1 cells, a leukemic cell line which endogenously overexpresses AML1/Evi-1 and that AML1/Evi-1 requires the interaction with CtBP to repress AML1-induced transactivation. The association with CtBP is also required when AML1/Evi-1 blocks myeloid differentiation of 32Dcl3 cells induced by granulocyte colony-stimulating factor. Taken together, it is suggested that one of the mechanisms for AML1/Evi-1-associated leukemogenesis should be an aberrant recruitment of a corepressor complex by the chimeric protein.

Proviral insertions induce the expression of bone-specific isoforms of PEBP2alphaA (CBFA1): evidence for a new myc collaborating oncogene

The til-1 locus was identified as a common retroviral integration site in virus-accelerated lymphomas of CD2-myc transgenic mice. We now show that viral insertions at til-1 lead to transcriptional activation of PEBP2alphaA (CBFA1), a transcription factor related to the Drosophila segmentation gene product, Runt. Insertions are upstream and in the opposite orientation to the gene and appear to activate a variant promoter that is normally silent in T cells. Activity of this promoter was detected in rodent osteogenic sarcoma cells and primary osteoblasts, implicating bone as the normal site of promoter activity. The isoforms encoded by the activated gene all encompass the conserved runt DNA-binding domain and share a novel N terminus different from the previously reported PEBP2alphaA products. Minor products include isoforms with internal deletions due to exon skipping and a novel C-terminal domain unrelated to known runt domain factors. The major isoform expressed from the activated til-1 locus (G1) was found to account for virtually all of the core binding factor activity in nuclear extracts from its corresponding lymphoma cell line. Another member of this gene family, AML1(CBFA2), is well known for its involvement in human hemopoietic tumors. These results provide evidence of a direct oncogenic role for PEBP2alphaA and indicate that the Myc and Runt family genes can cooperate in oncogenesis.

Functional characterization of ETV6 and ETV6/CBFA2 in the regulation of the MCSFR proximal promoter

The ETV6/CBFA2 (TEL/AML1) fusion gene occurs as a result of the chromosome translocation t(12;21)(p13;q22) in up to 30% of children diagnosed with B cell precursor (cd10+, cd19+) acute lymphoblastic leukemia. Leukemic cells that have acquired the t(12;21) usually demonstrate loss of the remaining normal ETV6 (TEL) allele. Using reporter gene assays we have functionally characterized both the normal ETV6 and ETV6/CBFA2 fusion proteins in the regulation of the MCSFR proximal promoter. Neither ETV6 or ETV6/CBFA2 has any significant, detectable effect on the promoter by itself. However, both ETV6 and ETV6/CBFA2 inhibit the activation of the promoter by CBFA2B(AML1B) and C/EBPa. We have shown that a 29-bp region of the MCSFR promoter containing the binding sites for CBFA2B and C/EBPa is sufficient for the inhibition by ETV6 and ETV6/CBFA2. Mutational analysis of the MCSFR promoter revealed that binding of both CBFA2B and C/EBPa to their respective sites is necessary for the inhibition by ETV6 and ETV6/CBFA2. Deletion of the helix-loop-helix (HLH) region from the cDNAs of ETV6 and ETV6/CBFA2 decreased but did not completely abrogate the ability of either construct to inhibit promoter activation. We also found that the ETS DNA binding region of ETV6 is necessary for inhibition of the promoter. Addition of ETS1 and FLI1, two ETS family members that have homology in the 5' HLH region, but not Spi1, an ETS family member without the 5' HLH region, also inhibited reporter gene expression. Our data show that the inhibition mediated by ETV6 and ETV6/CBFA2, in the context of the MCSFR promoter, depend on interactions with other proteins, not just CBFA2B. Our results also indicate that the transactivation characteristics of ETV6/CBFA2 are a combination of positive and negative regulatory properties.