Differential expression of topoisomerase I and RAD52 protein in yeast reveals new facets of the mechanism of action of bisdioxopiperazine compounds

A screening procedure which permits identification of compounds based on th eir activities against specific biological targets directly in a living org anism, Saccharomyces cerevisiae, has been established as part of our new dr ug discovery programme. Use of this assay has provided the first direct evi dence that TOPI and RAD52 proteins are involved in the mode of action of bi sdioxopiperazine ICRF compounds, which thus express a made of action quite distinctive from the other known TOP2 inhibitors evaluated. The functional assay is based on a comparison of pairs of yeast differing in their phenoty pes by specific traits: the expression or lack of expression of ectopic hum an DNA topoisomerase I, with or without that of the RAD52 gene. Amongst a s eries of anticancer agents, inhibitors bf topoisomerase I (camptothecin) we re identified as such in yeast expressing human topoisomerase I, whilst the presence or absence of RAD52 protein permitted the discrimination of compo unds generating double-stranded DNA breaks, either directly (bleomycin) or involving DNA adduct formation (cisplatin), or indirectly with DNA damage m ediated via inhibition of the topoisomerase II enzyme (etoposide). Notably, however, both the RAD52 protein and the lack of TOP1 enzyme appeared impli cated in the cytotoxic activities of the series of bisdioxopiperazine ICRF compounds tested. This functional assay in a living organism therefore appe ars to provide a valuable tool for probing distinctive and specific models) of action of diverse anticancer agents. (C) 1999 Cancer Research Campaign.