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 first genetic event may destabilize cells and thereby facilitate the induction of subsequent mutations within the same cell. It has recently been shown that exposure to ionizing radiation can in itself induce a persistent, heritable genetic instability in cells. To further investigate this phenomenon, we utilized a mutationally unstable population derived from a single Chinese hamster ovary (CHO) cell that survived X irradiation. We exposed these cells to a second dose of radiation, selected hypoxanthine phosphoribosyl transferase (HPRT) mutant subclones, and identified the type of mutations involved. We found complete deletions, continuous tract partial deletions, single-exon deletions, discontinuous-exon deletions ("skip mutations"), and point mutations (changes of less than 100 bp) among the isolated HPRT mutants. We hypothesized that the skip mutation clones might be more likely to demonstrate genomic instability. To test this hypothesis, mutant subclones were screened for three markers of genetic instability: alteration of minisatellite sequences, change in telomere length, and induction of chromosomal aberrations. Clones with skip mutations and single-exon deletions possessed elevated frequencies of minisatellite alterations and chromosomal aberrations, particularly rings and dicentrics. All mutant clones showed longer telomere terminal restriction fragment lengths than did wild-type cells. These results are consistent with the hypothesis that irradiation may induce a global instability phenotype, since the multiple alterations observed are mechanistically distinct, heritable cellular modifications that arose in the clonogenic progeny of the irradiated cells. Skip mutations may be one manifestation of this instability, but their presence was not specifically associated with the other genetic alterations.