Mad2 Be Aneuploid with E1A

Abstract

Advanced cancers typically contain cells with abnormal chromosome numbers. However, the mechanisms that underlie aneuploidy--and whether it drives malignancy or is a by-product of mutations that lead to malignancy--are unclear. Although various lines of evidence suggest that aneuploidy may result from defects in the mitotic spindle checkpoint, human cancers are rarely associated with loss-of-function mutations in spindle checkpoint genes. Hernando et al. noted that the viral oncoprotein E1A [which prevents the product of the retinoblastoma (Rb) gene from regulating E2F transcription factors and thereby stimulates cell cycle progression and tumorigenesis] increased expression of MAD2 , a spindle checkpoint protein, in microarray analyses. MAD2 mRNA and protein expression were enhanced in mouse embryo fibroblasts (MEFs) infected with retroviruses that express E1A , and Mad2 protein was increased in MEFs lacking Rb and in human IMR90 fibroblasts transduced with E1A or E2F . Further, chromatin precipitation analysis indicated that E2F-1 bound to the MAD2 promoter in IMR90 cells that express E1A . In normal IMR90 cells, Mad2 expression was regulated during the cell cycle, whereas in IMR90 cells expressing E1A or in which Rb expression was suppressed through RNA interference, and in tumor lines with Rb defects, Mad2 was overexpressed throughout the cell cycle. IMR90 cells transduced with E1A or MAD2 , or in which Rb was suppressed, showed slowed mitoses and abnormal chromosome segregation and accumulated aneuploid cells. Partial suppression of Mad2 in E1A - or E2F -transduced cells reduced the number of aneuploid cells. Thus, defects in the Rb pathway leading to aberrant Mad2 expression appear to constitute one route to aneuploidy. E. Hernando, Z. Nahlé, G. Juan, E. Diaz-Rodriguez, M. Alaminos, M. Hemann, L. Michel, V. Mittal, W. Gerald, R. Benezra, S. W. Lowe, C. Cordon-Cardo, Rb inactivation promotes genomic instability by uncoupling cell cycle progression from mitotic control. Nature 430 , 797-802 (2004). [Online Journal]