Disruption of the NHR4 domain structure in AML1-ETO abrogates SON binding and promotes

AML1-ETO is generated from t(8;21)(q22;q22), which is a commonform of chromosomal translocation associated with developmentof acute myeloid leukemia (AML). Although full-length AML1-ETOalone fails to promote leukemia because of its detrimental effectsoncellproliferation,analternativelysplicedisoform,AML1-ETO9a,without its C-terminal NHR3/NHR4 domains, strongly induces leu-kemia. However, full-length AML1-ETO is a major form of fusionproduct in many t(8;21) AML patients, suggesting additional mo-lecular mechanisms of t(8;21)-related leukemogenesis. Here, wereport that disruption of the zinc-chelating structure in the NHR4domain of AML1-ETO by replacing only one critical amino acidleads to rapid onset of leukemia, demonstrating that the NHR4domain with the intact structure generates inhibitory effects onleukemogenesis. Furthermore, we identified SON, a DNA/RNA-binding domain containing protein, as a novel NHR4-interactingprotein. Knock-down of SON by siRNA resulted in significantgrowth arrest, and disruption of the interaction between AML1-ETO and endogenous SON rescued cells from AML1-ETO-inducedgrowth arrest, suggesting that SON is an indispensable factor forcell growth, and AML1-ETO binding to SON may trigger signalsinhibiting leukemogenesis. In t(8;21) AML patient-derived primaryleukemic cells and cell lines, abnormal cytoplasmic localization ofSON was detected, which may keep cells proliferating in thepresence of full-length AML1-ETO. These results uncovered thecrucial role of the NHR4 domain in determination of cellular fateduring AML1-ETO-associated leukemogenesis.