Biometeorological consequences of environmental controls: overview.

We have studied the genetic alterations acquired during selection of a cloned human leukaemic cell line (CEM/VP-1) that is 15-fold more resistant to the anticancer topoisomerase Il-inhibitor etoposide than parental CCRF-CEM cells. CEM/VP-1 cells exhibit an 'atypical MDR' phenotype: cross resistance to other topo II inhibitors (but not Vinca alkaloids) and expression of a drug-resistant topo II activity. Cytogenetic and molecular studies revealed that the cell line carried multiple genetic changes affecting TOP2 genes encoding both topo IIa and P isoforms. to have been preferentially selected from a 1% diploid subpopulation present in the tetraploid parental cells. The same chromosomal abnormalities were present in resistant and sensitive cells except for an acquired 3p-change most likely deleting one TOP2P allele. PCR/DNA sequence analysis and allele-specific hybridisation showed that one of two TOP2a alleles expressed in CEM/VP-1 cells had acquired a Lys-797+Asn codon change. This mutation lies close to the catalytic Tyr-804 residue of the protein and may interfere with drug-induced trapping of the cleavable complex. Alternatively, it could exert a loss of function phenotype. CEM/VP-1 cells did not exhibit codon 449 or 486 TOP2a mutations in the ATP binding domain reported in two other resistant cell lines. Diploid selection and multiple changes observed in CEM/VP-1 cells appear to be consequences of the recessive phenotype of at-MDR. These results may be useful in approaching the mechanisms of clinical resistance. Resistance to multiple antitumour agents is a major problem in cancer chemotherapy. Some tumours are resistant to primary therapy, others become resistant during treatment. Progress in understanding the mechanisms of multidrug resistance (MDR) has come largely from studies of cultured cell lines and has identified at least two distinct phenotypes, so-called 'classical' and 'atypical' MDR (Moscow & Cowan, 1988). Cells exhibiting 'classical' MDR are resistant to a range of structurally unrelated lipophilic drugs including the Vinca alkaloids, anthracyclines, actinomycin D and colchicine (Endicott & Ling, 1989). Resistance arises from reduced in-tracellular drug levels due to expression of the transmem-brane P-glycoprotein efflux pump, the product of the MDR] gene. In contrast, 'atypical MDR' (at-MDR) involves cross resistance to drugs that inhibit the replicative enzyme, DNA topoisomerase II (Danks et al., 1987, 1988). This ATP-requiring dimeric protein is a structural component of metaphase chromosomes and is responsible for chromosome segregation via transient double strand DNA breaks (Liu, 1989). Topo II inhibiting drugs are thought to exert their cytotoxicity by trapping a 'cleavable complex' of the …