The Dimerization Domain and C-Terminal NCoR/SMRT Interacting Zinc-Finger Domain of t(8;21) Fusion Protein AML1-ETO Have Critical and Opposite Effects on Leukemogenesis.

Previous analyses demonstrated that full length AML1-ETO requires additional mutagenic events to promote leukemia. Here we describe a novel shorter form of the AML1-ETO fusion protein (AML1-ETOtr), which lacks C-terminal NHR3 and NHR4 [NCoR/SMRT interacting zinc-finger (ZnF)] domains. Unlike full length AML1-ETO, expression of AML1-ETOtr results in rapid development of acute myeloid leukemia (AML) in mice. Interestingly, naturally occurring alternatively spliced AML1-ETO variants exist in t(8;21) AML patient samples. AML1-ETOexon9a that includes one additional exon Exon 9a and AML1-ETOintron5 that uses a polyadenylation signal in Intron 5 are two such variants. AML1-ETOexon9a is structurally similar to AML1-ETOtr. AML1-ETOintron5 lacks the NHR2 heterodimerization domain as well as missing the NHR3 and NHR4 domains. To elucidate their putative role in leukemia development, we performed retroviral transduction and transplantation experiments with AML1-ETOexon9a, AML1-ETOintron5, and two zinc-finger domain (NHR4) mutants [AML1-ETOdZnF (deleted ZnF) and AML1-ETOmZnF (mutated ZnF)]. All of the mice transplanted with AML1-ETOexon9a (n = 8), AML1-ETOdZnF (n = 7), and AML1-ETOmZnF (n = 12) expressing cells rapidly developed AML, displaying a similar phenotype to AML1-ETOtr. None of AML1-ETOintron5 (n = 8) mice showed leukemia. These results suggest that the NHR2 domain of AML1-ETO is required whereas the NHR4 NCoR/SMRT interacting zinc finger domain is inhibitory for leukemia development. These observations suggest a novel mechanism of leukemogenesis for the AML1-ETO gene that can transcribe both tumor suppressive and/or oncogenic chimeric protein(s) by alternative RNA splicing.