THE EFFECT OF DIFFERENT CHEMOTHERAPEUTIC-AGENTS ON THE ENRICHMENT OF DNA MISMATCH REPAIR-DEFICIENT TUMOR-CELLS

Loss of DNA mismatch repair is a common finding in hereditary non-poly posis colon cancer as well as in many types of sporadic human tumours. We compared the effect of loss of DNA mismatch repair on drug sensiti vity as measured by a clonogenic assay with its effect on the ability of the same drug to enrich for mismatch repair-deficient cells in a pr oliferating tumour cell population. Mixed populations containing 50% D NA mismatch repair-deficient cells constitutively expressing green flu orescent protein and 50% mismatch repair-proficient cells were exposed to different chemotherapeutic agents. 6-Thioguanine, to which DNA mis match repair-deficient cells are known to be resistant, was included a s a control. The results in the cytotoxicity assays and in the enrichm ent experiments were concordant. Treatment with either carboplatin, ci splatin, doxorubicin, etoposide or 6-thioguanine resulted in enrichmen t for mismatch repair-deficient cells, and clonogenic assays demonstra ted resistance to these agents, which varied from 1.3-to 4.8-fold. Tre atment with melphalan, paclitaxel, perfosfamide or tamoxifen failed to enrich for mismatch repair-deficient cells, and no change in sensitiv ity to these agents was detected in the clonogenic assays. These resul ts identify the topoisomerase II inhibitors etoposide and doxorubicin as additional agents for which loss of DNA mismatch repair causes drug resistance. The concordance of the results from the two assay systems validates the enrichment assay as a rapid and reliable method for scr eening for the effect of loss of DNA mismatch repair on sensitivity to additional drugs.