Molecular cloning and characterization of the human topoisomerase II alphaand II beta genes: evidence for isoform evolution through gene duplication

Human DNA topoisomerase II is essential for chromosome segregation and is t he target for several clinically important anticancer agents. It is express ed as genetically distinct alpha and beta isoforms encoded by the TOP2 alph a and TOP beta 2 genes that map to chromosomes 17q21-22 and 3p24, respectiv ely. The genes display different patterns of cell cycle- and tissue-specifi c expression, with the a isoform markedly upregulated in proliferating cell s. In addition to the fundamental role of TOP2 alpha and TOP2 beta genes in cell growth and development, altered expression and rearrangement of both genes are implicated in anticancer drug resistance. Here, we report the com plete structure of the human topoisomerase II alpha gene, which consists of 35 exons spanning 27.5 kb. Sequence data for the exon-intron boundaries we re determined and examined in the context of topoisomerase II alpha protein structure comprising three functional domains associated with energy trans duction, DNA breakage-reunion activity and nuclear localization. The organi zation of the 3' half of human TOP2 beta, including sequence specifying the C-terminal nuclear localization domain, was also elucidated. Of the 15 int rons identified in this 20 kb region of TOP2 beta, the first nine and the l ast intron align in identical positions and display the same phases as intr ons in TOP2 alpha. Though their extreme 3' ends differ, the striking conser vation suggests the two genes diverged recently in evolutionary terms consi stent with a gene duplication event. Access to TOP2 alpha and TOP2 beta gen e structures should aid studies of mutations and gene rearrangements associ ated with anticancer drug resistance. (C) 1999 Elsevier Science B.V. All ri ghts reserved.