Analysis of X-ray-induced HPRT mutations in CHO cells: insertion and deletions.

Molecular alterations were examined in the hypoxanthine guanine phosphoribosyltransferase (hprt) gene of 41 independent X-ray-induced thioguanine-resistant (TGR) Chinese hamster ovary (CHO) cell clones. Rapid screening of the clones by multiplex polymerase chain reaction (PCR) for the presence or absence of exons revealed that the causes of the mutant phenotype were total gene deletion (26/41), partial gene deletion (4/41), and an insertion (1/41). No alterations of exon number or sizes were apparent in 10 of the mutants. Southern blot analysis confirmed the deletion data and revealed an additional class of mutants that had a gene disruption but retained all hprt exons (2/41). Therefore, at least 80% of the ionizing radiation-induced mutations were due to mechanisms involving DNA breakage and rejoining. The distribution of deletion sizes suggests that the two DNA breaks required for a deletion are not independent events. A possible mechanism is presented. In addition, the DNA sequence of the insertion mutation was determined. The insertion (229 bp) is coupled with a deletion (31 bp). An imperfect inverted repeat with flanking hprt DNA was identified and may be involved in the insertion event.