Dose- and time-response relationships for lethal mutations and chromosomalinstability induced by ionizing radiation in an immortalized human keratinocyte cell line

Purpose: To investigate the relationship between two well-established delay ed effects of ionizing radiation, experiments were conducted to determine t he induction and expression of lethal mutations (delayed reproductive death ) and chromosomal instability with respect to dose and time in a human immo rtalized keratinocyte cell line. Methods: HPV-G cells were gamma- or alpha-irradiated and maintained in cult ure for up to 72 population doublings. At intervals, measurements were made of cloning efficiency and the cells examined for apoptosis and cytogenetic aberrations. Results: The descendants of cells surviving 1 or 3 Gy gamma-irradiation, bu t not 0.5 Gy gamma-irradiation, exhibited a reduced colony-forming efficien cy. The reduction persisted at a constant rate of 15-20% clonogenic cell lo ss per population doubling for up to 72 population doublings. Apoptosis was demonstrated in all colonies in the 1 and 3 Gy groups at 30 and 72 populat ion doublings post-irradiation but not in the 0.5 Gy group. A significant p ersistent reduction in colony-forming ability (similar to 80%) was demonstr ated in the progeny of cells irradiated with 0.5 Gy alpha-particles. After 30 population doublings, the proportion of chromosomally aberrant cells was significantly greater than control values for all doses of both high- and low-LET radiations. The major cytogenetic aberrations (chromatid breaks, ch romosome fragments and minutes) were consistent with the transmission of ch romosomal instability. The expression of instability declined between 30 an d 72 population doublings in the 0.5 Gy and 3 Gy gamma-irradiation groups, but persisted up to 72 population doublings in the 1 Gy group. The expressi on of chromosomal instability was greater in the descendants of alpha-irrad iated cells and showed little evidence of reduction with time. Conclusions: Unstable aberrations characteristic of radiation-induced chrom osomal instability may commonly result in apoptosis and account for a compo nent of the delayed reproductive death/lethal mutation phenotype in HPV-G c ells. However, the absence of lethal mutations in the descendants of 0.5 Gy gamma-irradiated cells indicates a low-LET threshold effect for this parti cular endpoint. Overall, and particularly at low doses, there is no direct correlation between the two endpoints, indicating the absence of a simple r elationship between these manifestations of radiation-induced genomic insta bility.