HPRT(-) MUTATION SPECTRUM IN A CLOSELY-RELATED PAIR OF HUMAN BLADDER-TUMOR CELL-LINES AFTER GAMMA-IRRADIATION AT DIFFERENT DOSE-RATES

The spectrum of deletion sizes in mutants of two human bladder carcino ma cell lines has been examined. The cell lines were MGH-U1 and a radi ation-sensitive subline (U1-S40b) that has been developed in this labo ratory. Three groups, each of 20-30 mutants at the hprt locus were inv estigated: arising spontaneously, or induced after exposure to 10 Gy g amma-radiation either at high dose-rate (2 Gy/min) or low dose-rate (0 .01 Gy/min). Data on the mutation frequency of the two cell lines at l ow dose-rate were obtained to supplement previously published data at high dose-rate. The mutation frequency was lower in U1-S40b than in MG H-U1 both for high and low dose-rate irradiation. The presence of inta ct copies of each of the nine hprt exons was examined using multiplex PCR, supplemented by single-exon PCR. The incidence of small hprt muta tions (i.e. leading to no change in the size of the PCR products) was the same for spontaneous mutations in the two cell lines; for radiatio n-induced mutants it was higher in U1-S40b. The incidence of total del etions (i.e. no positive exon amplification) was lower in U1-S40b both for high and low dose-rate irradiation. The results are consistent wi th the hypothesis that large deletions tend to lead to the loss of adj acent essential genes and thereby to the death of potential mutants.