Multiple manifestations of X-ray-induced genomic instability in Chinese hamster ovary (CHO) cells

Carcinogenesis is postulated to follow a multistep cascade in which the fir st genetic event may destabilize cells and thereby facilitate the induction of subsequent mutations within the same cell. It has recently been shown t hat exposure to ionizing radiation can in itself induce a persistent, herit able genetic instability in cells. To further investigate this phenomenon, we utilized a mutationally unstable population derived from a single Chines e hamster ovary (CHO) cell that survived X irradiation. We exposed these ce lls to a second dose of radiation, selected hypoxanthine phosphoribosyl tra nsferase (HPRT) mutant subclones, and identified the type of mutations invo lved. We found complete deletions, continuous tract partial deletions, sing le-exon deletions, discontinuous-exon deletions ("skip mutations"), and poi nt mutations (changes of less than 100 bp) among the isolated HPRT mutants. We hypothesized that the skip mutation clones might be more likely to demo nstrate genomic instability. To test this hypothesis, mutant subclones were screened for three markers of genetic instability: alteration of minisatel lite sequences, change in telomere length, and induction of chromosomal abe rrations, Clones with skip mutations and single-exon deletions possessed el evated frequencies of minisatellite alterations and chromosomal aberrations , particularly rings and dicentrics. All mutant clones showed longer telome re terminal restriction fragment lengths than did wild-type cells. These re sults are consistent with the hypothesis that irradiation may induce a glob al instability phenotype, since the multiple alterations observed are mecha nistically distinct, heritable cellular modifications that arose in the clo nogenic progeny of the irradiated cells. Skip mutations may be one manifest ation of this instability, but their presence was not specifically associat ed with the other genetic alterations. (C) 2001 Wiley-Liss, Inc.