The pathogenesis of acute myeloid leukemias (AMLs) is linked to oncogenic fusion proteins, generated as a consequence of primary chromosome translocations or inversions (1
). Many different types of translocations have been described in AMLs, the most frequent being the t(8;21), t(15;17), inv(16
), and t(9;11), which, taken together with their variants, account for approximately 40% of AML cases (2
Despite genetic heterogeneity, there is increasing evidence for common molecular and biological mechanisms in AMLs. In particular, one of the components of each fusion protein is invariably a transcription factor, frequently involved in the regulation of differentiation (3
). As a consequence, AML-associated fusion proteins function as aberrant transcriptional regulators with the potential to interfere with the processes of myeloid differentiation. Indeed, ectopic expression of different fusion proteins into hemopoietic precursors induces a state of partial refractoriness to terminal differentiation and increases cell survival (4
). It has been suggested, therefore, that AML-associated fusion proteins contribute to the leukemic phenotype by inducing a differentiation block: a biological activity consistent with the main phenotypic trait of AMLs (i.e., the accumulation of hemopoietic precursors blocked at particular stages of myeloid development).
Analysis of mice transgenic for various fusion proteins revealed, however, a more complex scenario. Transgenic mice develop leukemias after long latency, suggesting that while AML-associated fusion proteins induce a preleukemic state, other genetic events are necessary for progression to a frank leukemia (7
). It is unclear whether fusion proteins also induce a mutator phenotype, thereby favoring accumulation of further genetic alterations. In the preleukemic state, the myeloid compartment of the transgenic animals appears morphologically normal, and the only detectable abnormalities are an increase in the self-renewal capacity of hemopoietic progenitors and minor alterations of differentiation markers (8
). This suggests that the effect of AML-associated fusion proteins on the differentiation program cannot be explained solely by their ability to block differentiation. Thus, further investigations are needed to characterize the biological contribution of AML-associated fusion proteins to the leukemogenic process in vivo.
A powerful approach to identify novel biological activities of AML fusion proteins is the characterization of their transcriptional targets through microarray analyses of patient samples. Analysis of blast samples with t(8;21), t(15;17), and inv(16
) has revealed a unique correlation between AML-specific cytogenetic aberrations and gene expression profiles (11
). Genes that are coregulated by different AML fusion proteins would not be identified by these studies, however, which are based on cluster analysis of different AML samples. To investigate common pathogenetic mechanisms in AMLs, we expressed different fusion proteins (AML1/ETO, which represents a fusion of the Acute Myeloid Leukemia 1 and the Eight-Twenty One gene products and is generated by the 8;21 translocation; PML/RAR, a fusion of the Promyelocytic Leukemia and the Retinoic Acid Receptor α [RARα] gene products, generated by the 15;17 translocation; and PLZF/RAR, a fusion of the Promyelocytic Leukemia Zinc Finger and the RARα gene products, generated by the 11;17 translocation) in the same genetic background and measured global gene expression using oligonucleotide chips. These experiments showed that a relevant number of regulated genes are targets of more than one AML fusion protein. Functional clustering revealed that AML fusion proteins influence the activity of genes involved in the control of diverse functions, including differentiation, cell survival, DNA repair, signal transduction, and several metabolic pathways. We further demonstrated that expression of AML fusion proteins activates Notch signaling through overexpression of the Jagged1 ligand, a function known to favor expansion of the hemopoietic stem cell (HSC) compartment. We also show that AML fusion protein expression can inhibit DNA repair function, suggesting that an increase in the rate of secondary mutations is a direct consequence of fusion protein expression.