Table 4.

Important molecular mechanisms involved in AML1/ETO-induced leukemic transformation.

Molecule	Function	Effect of AML1/ETO on the molecule	Final effect on t(8;21) cells
Altered gene transcription
PU.1 [82]	Transcription factor	Decreased expression by AML1/ETO	Inhibition of differentiation
C/EBPα [83, 84]	Transcription factor	Downregulation of C/EBPα; the normal function of this factor is regulation of differentiation and inhibition of proliferation	Increased proliferation and inhibition of differentiation
C/EBP β [85]	Transcription factor	The normal function is transcriptional upregulation of C/EBPα	Altered regulation of differentiation and proliferation through reduced expression of C/EBPα
POU4F1 [86]	Transcription factor	The POU4F1 levels are significantly correlated with the fusion protein levels. One study described differential regulation of 140 genes by this factor, and half of them are also AML1/ETO targets	POU4F1 probably contribute to the gene expression signature associated with t(8;21) AML
PAX5 [87, 88]	Transcription factor	Increased expression at the mRNA and protein level	Aberrant expression of B lymphocyte markers, including CD19, CD79a
RNA-rependent mechanisms and ribosomal functions
μRNA [62, 89]	Regulatory RNA molecules	The fusion protein selects a set of μRNAs (miR); it occupies the miR-24-23-27 locus and upregulates their expression	Modulation of proliferation and differentiation through the effects on miR24
		miR24 downregulates the mitogen-activated protein kinase phosphatase 7 and enhances the downstream signaling through phosphorylation of c-jun and p38 kinases
		Silencing of miR223 through epigenetic mechanisms	Altered regulation of myelopoiesis through effects on mir223
RNA polymerase I [74]	Transcriptional regulators	The fusion protein seems to localize to the nucleolar organizing regions during mitosis, associates with metaphase chromosomes and occupies ribosomal DNA repeats during interphase together with UBF1 (see Table 3)	In contrast to AML1 the fusion protein seems to be a positive regulator of rDNA transcription. Transcription regulated by RNA polymerase 1 seems to increase the proliferation of transformed cells (discussed in [74])
DNA damage and repair
OGG1[20]	DNA repair	OGG1 is an important part of the DNA base excision repair pathway, its expression is downregulated by the fusion protein	High OGG1 levels are associated with an adverse prognosis; the downregulation may increase chemosensitivity
DNA damage [90]	Carcinogen-DNA adducts	Increased formation of aromatic hydrocarbon-DNA adducts and upregulation of the cytochrome P450 1A1 enzyme that metabolizes polycyclic aromatic hydrocarbons	This effect may contribute to an increased susceptibility to additional genetic damage
Increased intracellular ROS [105]	Altered signalling. DNA damage?	AML1-ETO causes increased intracellular levels of reactive oxygen species (ROS) in Drosophila	ROS are important for induction of the AML1-ETO associated phenotype and may also increase the risk of additional genetic abnormalities
Increased mutation frequency [91]		Predisposition to leukemia progression	Predisposed for additional genetic effects that are required for leukemogenesis.
Cytokine-mediated growth regulation
IL3 [76]	Hematopoietic growth factor	Decreased gene expression	Decreased growth factor-dependent proliferation
M-CSF receptor [92]	Growth factor	M-CSF is a growth-enhancing hematopoietic growth factor	Increased cytokine-dependent AML cell proliferation
G-CSF receptor [93]	Growth factor	G-CSF is a growth-enhancing hematopoietic growth factor	Increased cytokine-dependent AML cell proliferation
BCL2 [94]	Antiapoptotic signaling	Upregulation by the AML1-ETO fusion protein	Increased antiapoptotic signaling
C/EBPε [92, 93]	Transcriptional regulator	Induction of G-CSF receptor expression; upregulation of the myeloid-specific promoter for the M-CSF receptor	Increased growth factor receptor expression and thereby increased cytokine-dependent proliferation by t(8;21) cells
NF1 [95]	Tumor suppressor	Decreased expression of the Neurofibromatosis1 (NF1) tumor suppressor	Decreased protein levels are associated with increased response of primary AML cells to GM-CSF
Tyrosine receptor kinase A [96]	A part of the nerve growth factor receptor (NGF)	Upregulation of this growth factor both at the mRNA and protein level	NGF is released by bone marrow stromal cells; in addition, AML1-ETO expressing cells show increased proliferation in response to growth factors
Cell-cycle regulation
p21 [97]	Negative cell-cycle regulator	Increased mRNA and protein levels of p21	p21 is a cell-cycle inhibitor, this effect may contribute to the absence of leukemogenesis in the presence of t(8;21) alone
p27kip [98]	Negative cell-cycle regulator	Increased expression caused by either a direct effect of the fusion protein or by Cx43	Cell-cycle inhibition
SSX21P [99]	Cell-cycle regulation?	Low expression of this molecule is associated with low expression of CDC20; possibly causing attenuation of the spindle checkpoint	Altered cell-cycle regulation, increased risk of aneuploidy?
Disrupted spindle checkpoint [100]	Aneuploidy	Disruption of the spindle checkpoint during cell-cycle progression	Increased risk of aneuploidy.
Regulation of apoptosis and stress responses
Annexin A1 [101]	Proapoptotic, antiproliferative	Downregulated at the gene expression level by the fusion protein	The molecule has proapoptotic and antiproliferative effects; these functions are thus inhibited
Connexin 43 (Cx43) [98, 102]	Gap junction component	Increased expression of Connexin 43 in cells with t(8;21), possibly both a direct and an indirect effect mediated via c-Jun	Cx43 often inhibits cell proliferation both through gap junction dependent and independent mechanisms; this effect may contribute to the lack of leukemogenesis by the full-length fusion protein
p53 [91]	Tumor suppressor	Activation of the p53 pathway	Possibly increased chemosensitivity and thereby contribution to the good prognosis of these patients
TXNIP [103]	Part of stress responses	Involved in reactive oxygen stress responses, AML cells with t(8;21) have increased protein levels of this molecule. The mechanism is not known	High levels inhibit the proliferation of myeloid progenitor cells; this may contribute to the good prognosis of these patients