Beatrice U. Mueller

Dominant-negative mutations of CEBPA , encoding CCAAT/enhancer binding protein-α (C/EBPα), in acute myeloid leukemia

The transcription factor C/EBPα (for CCAAT/enhancer binding protein-α; encoded by the gene CEBPA) is crucial for the differentiation of granulocytes. Conditional expression of C/EBPα triggers neutrophilic differentiation, and no mature granulocytes are observed in Cebpa-mutant mice. Here we identify heterozygous mutations in CEBPA in ten patients with acute myeloid leukemia (AML). We found that five mutations in the amino terminus truncate the full-length protein, but did not affect a 30-kD protein initiated further downstream. The mutant proteins block wild-type C/EBPα DNA binding and transactivation of granulocyte target genes in a dominant-negative manner, and fails to induce granulocytic differentiation. Ours is the first report of CEBPA mutations in human neoplasia, and such mutations are likely to induce the differentiation block found in AML.

AML1-ETO downregulates the granulocytic differentiation factor C/EBPalpha in t (8;21) myeloid leukemia

The transcription factor CCAAT/enhancer binding protein α, or C/EBPα, encoded by the CEBPA gene, is crucial for the differentiation of granulocytes. Conditional expression of C/EBPα triggers neutrophilic differentiation, and Cebpa knockout mice exhibit an early block in maturation. Dominant-negative mutations of CEBPA have been found in some patients with acute myeloid leukemia (AML), but not in AML with the t(8;21) translocation which gives rise to the fusion gene RUNX1–CBF2T1 (also known as AML1–ETO) encoding the AML1–ETO fusion protein. RUNX1–CBF2T1 positive-AML blasts had eight-fold lower CEBPA RNA levels and undetectable C/EBPα protein levels compared with other subgroups of AML patients. Conditional expression of RUNX1–CBF2T1 in U937 cells downregulated CEBPA mRNA, protein and DNA binding activity. AML1–ETO appears to suppress C/EBPα expression indirectly by inhibiting positive autoregulation of the CEBPA promoter. Conditional expression of C/EBPα in AML1–ETO-positive Kasumi-1 cells results in neutrophilic differentiation. We suggest that restoring C/EBPα expression will have therapeutic implications in RUNX1–CBF2T1-positive leukemias.

Risk Assessment in Patients with Acute Myeloid Leukemia and a Normal Karyotype

Purpose: The recognition of a number of leukemia-specific cytogenetic abnormalities and their role as independent prognostic factors have provided considerable insights into leukemia pathogenesis and have paved the way to adopt risk-adapted treatment. However, ∼50% of newly diagnosed acute myeloid leukemia (AML) have a normal karyotype. There has therefore been much interest in identifying molecular markers that could help to improve the prognostic stratification of patients with normal-karyotype AML. Experimental Design: Consecutive untreated AML patients (n = 67) from a single institution all with normal karyotype were analyzed for the presence of mutations in the myeloid transcription factor gene CEBPA (for CCAAT/enhancer binding protein-α), for internal tandem duplications (ITD) of the tyrosine kinase receptor gene FLT3 (for fms-like tyrosine kinase 3), and for expression of the BAALC gene (for brain and acute leukemia, cytoplasmic). Results: 17.9% of normal-karyotype AML had mutations in the CEBPA gene, and 28.4% had FLT3-ITD; 65.7% (44 of 67) had high BAALC expression and 34.3% (23 of 67) had low BAALC expression. Patients with CEBPA mutations had a very favorable course of their disease. Median disease-free survival (DFS) and overall survival (OS) were 33.5 and 45.5 months, respectively, compared with 10 (e.g., 12 months in patients without CEBPA mutations; P = 0.0017; P = 0.0007). AML patients with FLT3-ITD had significantly shorter median DFS (P = 0.0328) and OS (P = 0.0148) than patients without FLT3-ITD. High BAALC expression predicted for a shorter DFS (P = 0.0152) and OS (P = 0.0210) compared with AML with low BAALC expression; 53.7% of normal-karyotype AML had neither FLT3-ITD nor CEBPA mutations. We found that high BAALC expression in normal-karyotype AML with neither FLT3-ITD nor CEBPA mutations (18 of 67) indicates adverse prognosis for both DFS and OS (P = 0.0001; e.g., P = 0.0001) compared with the group with low BAALC expression and absent FLT3-ITD and CEBPA mutations (18 of 67). Thus, BAALC expression represents a novel prognostic marker particularly for normal-karyotype AML patients with neither FLT3-ITD nor CEBPA mutations. Conclusions: Assessment of CEBPA mutations, FLT3-ITD, and BAALC expression permits to split normal-karyotype AML into clinically distinct subgroups.

Heterogeneity within AML with CEBPA mutations; only CEBPA double mutations, but not single CEBPA mutations are associated with favourable prognosis

CCAAT/enhancer binding protein alpha (CEBPA) mutations in AML are associated with favourable prognosis and are divided into N- and C-terminal mutations. The majority of AML patients have both types of mutations. We assessed the prognostic significance of single (n=7) and double (n=12) CEBPA mutations among 224 AML patients. Double CEBPA mutations conferred a decisively favourable overall (P=0.006) and disease-free survival (P=0.013). However, clinical outcome of patients with single CEBPA mutations was not different from CEBPA wild-type patients. In a multivariable analysis, only double – but not single – CEBPA mutations were identified as independent prognostic factors. These findings indicate heterogeneity within AML patients with CEBPA mutations.

Somatic CEBPA Mutations Are a Frequent Second Event in Families With Germline CEBPA Mutations and Familial Acute Myeloid Leukemia

PURPOSE The transcription factor CCAAT/enhancer binding protein-alpha (CEBPA) is crucial for normal myeloid differentiation. Mutations in the CEBPA gene are found in subsets of patients with acute myeloid leukemia (AML). Recently, three families were reported in whom several family members had germline CEBPA mutations and subsequently developed AML. Whereas familial AML is considered a rare event, the frequency of CEBPA germline mutations in AML is not known. PATIENTS AND METHODS In this study, we screened 187 consecutive AML patients for CEBPA mutations at diagnosis. We detected 18 patients (9.6%) with CEBPA mutations. We then analyzed remission samples and constitutive DNA from these patients. RESULTS We found that two (11.1%) of 18 AML patients with CEBPA mutations carried a germline N-terminal frameshift CEBPA mutation. Interestingly, additional members in the families of both of these patients have been affected by AML, and the germline CEBPA mutations were also observed in these patients. Additional somatic mutations in AML patients with germline CEBPA mutations in the two families comprised in-frame C-terminal CEBPA mutations in two patients, two nonsilent CEBPA point mutations in one patient, and monosomy 7 in one patient. CONCLUSION This study shows, for the first time to our knowledge, that germline CEBPA mutations are frequently observed among AML patients with CEBPA mutations. Including the families with germline CEBPA mutations reported previously, additional somatic CEBPA mutations represent a frequent second event in AML with germline CEBPA mutations. Our data strongly indicate that germline CEBPA mutations predispose to AML and that additional somatic CEBPA mutations contribute to the development of the disease.

Transcriptional dysregulation during myeloid transformation in AML

The current paradigm on leukemogenesis indicates that leukemias are propagated by leukemic stem cells. The genomic events and pathways involved in the transformation of hematopoietic precursors into leukemic stem cells are increasingly understood. This concept is based on genomic mutations or functional dysregulation of transcription factors in malignant cells of patients with acute myeloid leukemia (AML). Loss of the CCAAT/enhancer binding protein-α (CEBPA) function in myeloid cells in vitro and in vivo leads to a differentiation block, similar to that observed in blasts from AML patients. CEBPA alterations in specific subgroups of AML comprise genomic mutations leading to dominant-negative mutant proteins, transcriptional suppression by leukemic fusion proteins, translational inhibition by activated RNA-binding proteins, and functional inhibition by phosphorylation or increased proteasomal-dependent degradation. The PU.1 gene can be mutated or its expression or function can be blocked by leukemogenic fusion proteins in AML. Point mutations in the RUNX1/AML1 gene are also observed in specific subtypes of AML, in addition to RUNX1 being the most frequent target for chromosomal translocation in AML. These data are persuasive evidence that impaired function of particular transcription factors contributes directly to the development of human AML, and restoring their function represents a promising target for novel therapeutic strategies in AML.

C/EBPalpha and the pathophysiology of acute myeloid leukemia

Purpose of reviewThe transcription factor C/EBPα controls differentiation and proliferation in normal granulopoiesis in a stage-specific manner. Loss of C/EBPα function in myeloid cells in vitro and in vivo leads to a block to myeloid differentiation similar to that which is observed in malignant cells from patients with acute myeloid leukemia. The finding of C/EBPα alterations in subgroups of acute myeloid leukemia patients suggests a direct link between critically decreased C/EBPα function and the development of the disorder. Recent findingsConditional mouse models provide direct evidence that loss of C/EBPα function leads to the accumulation of myeloid blasts in the bone marrow. Targeted disruption of the wild type C/EBPα protein, while conserving the dominant-negative 30 kDa isoform of C/EBPα, induces an AML-like disease in mice. In hematopoietic stem cells C/EBPα serves to limit cell self-renewal. Finally, C/EBPα function is disrupted at different levels in specific subgroups of acute myeloid leukemia patients. SummaryThere is evidence that impaired C/EBPα function contributes directly to the development of acute myeloid leukemia. Normal myeloid development and acute myeloid leukemia are now thought to reflect opposite sides of the same hematopoietic coin. Restoring C/EBPα function represents a promising target for novel therapeutic strategies in acute myeloid leukemia.

Complexity of CEBPA Dysregulation in Human Acute Myeloid Leukemia

The transcription factor CCAAT enhancer binding protein alpha (CEBPA) is crucial for normal development of granulocytes. Various mechanisms have been identified how CEBPA function is dysregulated in patients with acute myeloid leukemia (AML). In particular, dominant-negative mutations located either at the N- or the C terminus of the CEBPA gene are observed in roughly 10% of AML patients, either in the combination on separate alleles or as sole mutation. Clinically significant complexity exists among AML with CEBPA mutations, and patients with double CEBPA mutations seem to have a more favorable course of the disease than patients with a single mutation. In addition, myeloid precursor cells of healthy carriers with a single germ-line CEBPA mutation evolve to overt AML by acquiring a second sporadic CEBPA mutation. This review summarizes recent reports on dysregulation of CEBPA function at various levels in human AML and therapeutic concepts targeting correction of CEBPA activity. The currently available data are persuasive evidence that impaired CEBPA function contributes directly to the development of AML, whereas restoring CEBPA function represents a promising target for novel therapeutic strategies in AML. (Clin Cancer Res 2009;15(17):5303–7)

The tumour-suppressive miR-29a/b1 cluster is regulated by CEBPA and blocked in human AML.

Background:CCAAT/enhancer-binding protein-α (CEBPA) is crucial for normal granulopoiesis and is frequently disrupted in acute myeloid leukaemia (AML). Increasing evidence suggests that CEBPA exerts its effects, in parts, by regulating specific microRNAs (miRNAs), as previously shown for miR-223. The aim of this study was to investigate the genome-wide pattern of miRNAs regulated by CEBPA in myeloid cells.Methods:In Kasumi-1 cells, conditionally expressing CEBPA, we assessed the expression of 470 human miRNAs by microarray analysis. We further investigated the microarray results by qRT-PCR, luciferase reporter assays, and chromatin immunoprecipitation assays.Results:In all, 18 miRNAs were more than two-fold suppressed or induced after CEBPA restoration. Among these 18 miRNAs, we focused on CEBPA-mediated regulation of the tumour-suppressive miR-29b. We observed that miR-29b is suppressed in AML patients with impaired CEBPA function or loss of chromosome 7q. We found that CEBPA selectively regulates miR-29b expression on its miR-29a/b1 locus on chromosome 7q32.3, whereas miR-29b2/c on chromosome 1q32.2 is not affected.Conclusion:This study reports the activation of the tumour-suppressive miR-29b by the haematopoietic key transcription factor CEBPA. Our data provide a rationale for miR-29b suppression in AML patients with loss of chromosome 7q or CEBPA deficiency.

Complexity of miR-223 regulation by CEBPA in human AML.

microRNA-223 (miR-223) can trigger normal granulopoiesis. miR-223 expression is regulated by two distinct CEBPA (CCAAT/enhancer binding protein-alpha) sites. Here, we report that miR-223 is largely suppressed in cells from acute myeloid leukemia (AML) patients. By sequencing, we found that miR-223 suppression in AML is not caused by DNA sequence alterations, nor is it mediated by promoter hypermethylation. The analysis of the individual contribution of both CEBPA sites to miR-223 regulation identified the site upstream of the miR-223 primary transcript as the predominant regulatory element. Our results suggest that miR-223 suppression in AML is caused by impaired miR-223 upstream factors.