Mariano A R, Colombo E, Luzi L, Martinelli P, Volorio S, Bernard L, Meani N, Bergomas R, Alcalay M, Pelicci P G
Department of Experimental Oncology, European Institute of Oncology, Milan, Italy.
Oncogene. 2006 Jul 20;25(31):4376-80. doi: 10.1038/sj.onc.1209453. Epub 2006 Feb 27.
Nucleophosmin (NPM) is a nucleus-cytoplasmic shuttling protein that is implicated in centrosome duplication, cell cycle progression and stress response. At the steady state, NPM localizes mainly in the nucleolus, where it forms a complex with different cellular proteins. One-third of acute myeloid leukemias (AML) are characterized by aberrant cytoplasmic localization of NPM, due to mutations within its last coding exon (exon 12) that cause a frameshift and the formation of novel C-termini. We report here our investigations on the molecular basis for the aberrant localization of mutated NPM. Alignment of the C-terminus of the various NPM mutants revealed the obligatory presence of four amino-acid residues that match a CRM1-dependent nuclear export signal (NES). Single alanine-substitutions at these sites provoked nuclear re-localization, while fusion of the mutated C-terminus to a heterologous nuclear protein induced CRM1-dependent cytoplasmic localization. Molecular characterization of one exceptional AML carrying cytoplasmic NPM and germ line exon 12 revealed a somatic mutation in the splicing donor site of exon 9 that caused the formation of a functional NES. It appears, therefore, that AMLs are frequently characterized by gain-of-function mutations of NPM that create functional NES, suggesting that alterations of nuclear export might represent a general mechanism of leukemogenesis and a novel target for therapeutic intervention.
核磷蛋白（NPM）是一种穿梭于细胞核与细胞质之间的蛋白质，参与中心体复制、细胞周期进程及应激反应。在稳态时，NPM主要定位于核仁，在那里它与不同的细胞蛋白形成复合物。三分之一的急性髓系白血病（AML）的特征是NPM在细胞质中异常定位，这是由于其最后一个编码外显子（外显子12）内的突变导致移码并形成新的C末端。我们在此报告对突变型NPM异常定位的分子基础的研究。对各种NPM突变体的C末端进行比对，发现必须存在四个与CRM1依赖性核输出信号（NES）匹配的氨基酸残基。在这些位点进行单个丙氨酸取代会引发核重新定位，而将突变的C末端与异源核蛋白融合会诱导CRM1依赖性细胞质定位。对一例携带细胞质NPM和生殖系外显子12的特殊AML进行分子特征分析，发现外显子9的剪接供体位点存在体细胞突变，导致形成功能性NES。因此，似乎AML的特征通常是NPM的功能获得性突变，从而产生功能性NES，这表明核输出的改变可能是白血病发生的一般机制和治疗干预的新靶点。
