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Expression of Mitotic Spindle Checkpoint Protein hsMAD1 Correlates with Cellular Proliferation and Is Activated by a Gain-of-Function p53 Mutant

Molecular Virology Section, Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, NIH, Bethesda, Maryland 20892-0460

Human mitotic arrest deficiency protein 1, hsMAD1, is a component of the mitotic spindle assembly checkpoint (MSC) that monitors fidelity of chromosomal segregation and guards against emergence of cellular aneuploidy. Because aneuploidy is a pervasive characteristic of human cancers, understanding how MSC genes are regulated is important. Here, we have analyzed human genomic sequences upstream of the 5' most hsMAD1 coding exon and have identified a 1.5-kb fragment with promoter activity. The hsMad1 promoter, consistent with characteristics of housekeeping genes, is highly GC rich and is devoid of a TATA-box. Mutational analyses revealed a core region spanning -73 to -31 as being essential for hsMad1 transcription. Surprisingly, although MSC function, prototypically induced by microtubule inhibitors, is active selectively during mitosis, we found the hsMad1 promoter to be expressed predominantly in G₁ and to respond not to microtubule inhibitor but to mitogenic stimulus. In primary, as well as transformed cells, intracellular levels of hsMAD1 correlated with the proliferative status of cells. The hsMad1 promoter was also activated preferentially by a gain-of-function p53 mutant. Taken together, our results suggest that hsMAD1 might link p53 function to the generation of cellular aneuuploidy and that heightened activation of hsMad1 by gain-of-function p53 mutants could contribute to the worse prognosis of certain cancers.

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