Telomeres of human chromosomes contain a G-rich 3′-overhang that adopts an intramolecular G-quadruplex structure in vitro which blocks the catalytic reaction of telomerase. Agents that stabilize G-quadruplexes have the potential to interfere with telomere replication by blocking the elongation step catalyzed by telomerase and can therefore act as antitumor agents. We have identified by Fluorescence Resonance Energy Transfer a new series of quinoline-based G-quadruplex ligands that also exhibit potent and specific anti-telomerase activity with IC ₅₀ in the nanomolar concentration range. Long term treatment of tumor cells at subapoptotic dosage induces a delayed growth arrest that depends on the initial telomere length. This growth arrest is associated with telomere erosion and the appearance of the senescent cell phenotype (large size and expression of β-galactosidase activity). Our data show that a G-quadruplex interacting agent is able to impair telomerase function in a tumor cell thus providing a basis for the development of new anticancer agents.

The extremities of eukaryotic chromosomes are called telomeres. They have a structure unlike the bulk of the chromosome, which allows the cell DNA repair machinery to distinguish them from 'broken' DNA ends. But these specialised structures present a problem when it comes to replicating the DNA. Indeed, telomeric DNA progressively erodes with each round of cell division in cells that do not express telomerase, a specialised reverse transcriptase necessary to fully duplicate the telomeric DNA. Telomerase is expressed in tumour cells but not in most somatic cells and thus telomeres and telomerase may be proposed as attractive targets for the discovery of new anticancer agents.

Bloom's syndrome is a rare genetic disorder associated with loss of genomic integrity and a large increase in the incidence of many types of cancer at an early age. The Bloom's syndrome gene product, BLM, belongs to the RecQ family of DNA helicases, which also includes the human Werner's and Rothmund-Thomson syndrome gene products and the Sgs1 protein of Saccharomyces cerevisiae. This family shows strong evolutionary conservation of protein structure and function. Previous studies have shown that Sgs1p interacts both physically and genetically with topoisomerase III. Here, we have investigated whether this interaction has been conserved in human cells. We show that BLM and hTOPO IIIα, one of two human topoisomerase III homologues, co-localize in the nucleus of human cells and can be co-immunoprecipitated from human cell extracts. Moreover, the purified BLM and hTOPO IIIα proteins are able to bind specifically to each other in vitro, indicating that the interaction is direct. We have mapped two independent domains on BLM that are important for mediating the interaction with hTOPO IIIα. Furthermore, through characterizing a genetic interaction betweenBLM and TOP3 in S. cerevisiae, we have identified a functional role for the hTOPO IIIα interaction domains in BLM.