MNDA BINDS NPM B23 AND THE NPM-MLF1 CHIMERA GENERATED BY THE T(3-5) ASSOCIATED WITH MYELODYSPLASTIC SYNDROME AND ACUTE MYELOID-LEUKEMIA/

The myeloid cell nuclear differentiation antigen (MNDA) is a nuclear p rotein expressed specifically in developing cells of the human myelomo nocytic lineage, including the end-stage monocytes/macrophages and gra nulocytes. Nuclear localization, lineage-and stage-specific expression , association with chromatin, and regulation by interferon a indicate that this protein is involved in regulating gene expression uniquely a ssociated with the differentiation process and/or function of the mono cyte/macrophage. MNDA does not bind specific DNA sequences, but rather a set of nuclear proteins that includes nucleolin (C23). Both in vitr o binding assays and co-immunoprecipitation were used to demonstrate t hat MNDA also binds protein B23 (nucleophosmin/NPM). Three reciprocal chromosome translocations found in certain cases of leukemia/lymphoma involve fusions with the NPM/B23 gene, t(5;17) NPM-RAR alpha, t(2;5) N PM-ALK, and the t(3;5) NPM-MLF1. In the current study, MNDA was not ab le to bind the NPM-ALK chimera originating from the t(2;5) and contain ing residues 1-117 of NPM. However, MNDA did bind the NPM-MLF1 product of the t(3;5) that contains the N-terminal 175 residues of NPM. The a dditional 58 amino acids (amino acids 117-175) of the NPM sequence tha t are contained in the product of the NPM-MLF1 fusion gene relative to the product of the NPM-ALK fusion appear responsible for MNDA binding . This additional NPM sequence contains a nuclear localization signal and clusters of acidic residues believed to bind nuclear localization signals of other proteins. Whereas NPM and nucleolin are primarily loc alized within the nucleolus, MNDA is distributed throughout the nucleu s including the nucleolus, suggesting that additional interactions def ine overall MNDA localization.