Cytogenetic abnormality in acute myeloid leukemia: translocation(8;21) in AML-M2
Cytogenetic abnormality in acute myeloid leukemia: translocation(8;21) in AML-M2
Acute myeloid leukemia (AML) is a heterogenous disease, with individual cases showing variability in clinical presentation. This heterogeneity extends to the molecular genetic lesions underlying the pathogenesis of AML. Although nonrandom clonal chromosomal aberrations are present in the majority of cases, each abnormality affects only a limited subset of cases. One of the most frequent cytogenetic abnormalities in AML is t(8;21)(q22;q22) that found in approximately 10±15% of AML cases. Patients with this subtype of AML typically present with FAB AML-M2 morphology, in which the leukemic blasts have prominent Auer rods, strong myeloperoxidase positivity, homogenous salmon coloured granules, cytoplasmic vacuolization, and prominent bone marrow eosinophilia.
The 8;21 translocation was shown to rearrange the AML1 gene (also referred to as CBFA2 or PEBP2aB) on chromosome 21q22 and the ETO gene (also referred to as MTG8) on chromosome 8q22 (Fig 1A). The AML1 gene encodes the DNA-binding subunit of the AML1/CBFb core binding factor transcription complex, whereas ETO encodes the mammalian homologue of the Drosophila protein Nervy. Although t(8;21) which encodes an AML1-ETO fusion protein is the critical genetic lesion. The chromosomal breakpoints resulting from the t(8;21) cluster within a single intron of both AML1 and ETO and generate similar AML1-ETO chimeric genes in every case. The encoded fusion protein consists of the N-terminal 177 amino acids of AML1 fused in frame to almost the complete ETO protein (Fig 1B).
Fig 1. British Journal of Haematology, 1999, 106, 296-308
The AML1/CBFβ transcription factor complex: normal function
AML1 is the DNA-binding subunit of the core-binding transcription factor (CBF) and binds to the enhancer core sequence TGTGGT, and its affinity for DNA binding is increased through heterodimerization through the RHD with a second non-DNA-binding subunit CBFβ (Fig 2)
(Fig 2) Haematologica 1997; 82:364-370
AML1/CBFβ has been shown to function as a transcriptional activator that is critical for the tissue-specific expression of a number of haemopoietic specific genes, including those for myeloperoxidase(MPO), subunits of the T-cell antigen receptor (TCR), the cytokines IL-3 , GM-CSF and others. Although binding of AML1/CBFβ to the core enhancer sequence is important for the haemopoietic specific expression of these genes, their expression is also dependent on the presence of adjacent binding sites for lineage-restricted transcription factors.
The ETO: a putative transcriptional regulator
ETO is the mammalian homologue of the Drosophila gene nervy, and subsequent work has identified two other mammalian members of this family, MTGR1 and MTG16. The ETO is expressed as a nuclear phosphoprotein in brain and CD34 + haemopoietic progenitors, whereas the related family member MTGR1 is ubiquitously expressed. Although ETO is a nuclear zinc-finger-containing protein, there is no experimental evidence to suggest that it can bind directly to DNA. Nevertheless, the structure of ETO suggests that it is likely to function as a regulator of transcription. The ETO can directly interact with the nuclear co-repressors N-CoR and Sin3A, and through these interactions can recruit an active histone deacetylase (HDAC) (Fig 3)
Fig. 3 British Journal of Haematology, 1999, 106, 296-308
The AML1-ETO chimaeric protein: mechanisms of cell transformation
The AML1-ETO fusion protein retains many of the important functional domains of both AML1 and ETO, including the RHD of AML1, and the ETO sequences that mediate homo- and heterodimerization with ETO/MTG family members and interaction with nuclear co-repressors. The AML1-ETO continues to bind the core enhancer DNA sequence and to heterodimerize with CBFβ (Fig 4). Similarly, like the wild-type AML1 protein, AML1-ETO is a nuclear phosphoprotein that regulates the nuclear accumulation of CBFβ; however, several of its critical functional properties differ from those of wild-type AML1. First, the transcriptional activation domains of AML1 are deleted and replaced by ETO sequences known to interact with nuclear co-repressors, suggests that the chimaeric protein should function not as a transcriptional activator, but instead as a transcriptional repressor.
Second, AML1-ETO binds CBFβ more avidly than AML1, and therefore accumulates CBFb more efficiently in the nucleus than the wild-type protein. Finally, AML1-ETO associates only weakly with the nuclear matrix and directly inhibits wild-type AML1 from binding to this site, thereby inhibiting its ability to stimulate DNA replication.
The ability of AML1-ETO to repress transcription is dependent on both the RHD of AML1 and the HHR and zinc fingers of ETO. Transcriptional repression requires direct DNA binding by the AML1-ETO chimaeric protein through the core enhancer sequence and appears to be mediated through the recruitment of thenuclear co-repressor complex by ETO (Fig 4).
(Fig 4) British Journal of Haematology, 1999, 106, 296±308
Mutation within the Zn-fingers of ETO that eliminate N-CoR binding abrogate the ability of the chimaeric protein to repress AML1-mediated transcription. Repression is an active process with the AML1-ETO/N-CoR/Sin3A/HDAC complex leading to the deacetylation of histones and the alteration of the chromatin structure of AML1 responsive genes such that transcription is inhibited. In addition, the recruitment of these nuclear co-repressors by AML1-ETO may also induce transcriptional repression through a HDAC independent mechanism.
AML-ETO induces haemopoietic cell transformation by (1) actively repressing AML1-mediated transcriptional activation thereby blocking the normal activity of AML1; (2) repressing transcription by other AML1 family members; (3) repressing the activity of transcription factors other than AML1, such as C/EBP-α ; (4) interfering with the normal functions of ETO and other ETO/MTG family members expressed in haemopoietic cells; and (5) aberrantly activating the transcription of AML1-regulated and novel AML1-ETO-specific target genes. A critical step in understanding the mechanism of AML1-ETO-mediated transformation will be to identify the genes whose transcription is altered by expression of this chimaeric oncoprotein. In addition, AML1/CBFβ may either directly or indirectly regulate transcription of the p53 tumour suppressor. Inhibition of p53 expression by the AML1-ETO fusion protein would be predicted to contribute directly to cell transformation. Interestingly, p53 inactivating mutations are exceeding rare in cases of AML with alterations in the CBF complex. Thus,inhibition of p53 induction by the CBF fusion proteins may serve as an alternative mechanism of p53 inactivation.
Reference
1. Francesco Lo Coco, Simona Pisegna, Daniela Diverio, The AML1 gene : A transcription factor involved in the pathogenesis of myeloid and lymphoid leukemias, Haematologica 1997; 82:364-370
2. The AML1-ETO chimaeric transcription factor in Acute Myeloid Leukemia:Biology and clinical significance,British Journal of Haematology, 1999, 106, 296-308
