Loss of normal G(1) checkpoint control is an early step in carcinogenesis,independent p53 status

Recent studies have described a diminished radiation-induced G(1) arrest in some wild-type (wt) p53 human tumor cell lines compared to normal human fi broblasts, However, the significance of this finding was unclear, particula rly because tumor cell lines may have accumulated additional genetic change s after long periods in culture. Because malignant transformation of indivi dual cells is thought to be an early step in carcinogenesis, we have used a model system of normal and transformed mouse fibroblast 10T1/2 cell clones to examine whether loss of G(1) checkpoint control may be an early event i n tumor development and to study the relationships between G(1) arrest, rad iosensitivity, and genetic alterations. Twelve transformed clones were esta blished from type III foci induced by irradiation of normal 10T1/2 cells an d were compared with six clones derived from wt 10T1/2 cells. Three of the transformed clones expressed mutant p53; two of these had the same point mu tation at codon 132 (exon 5), and one had a point mutation at codon 135. Th e remaining transformed and normal clones had wt p53 status. The radiosensi tivity of transformed clones, as measured by a clonogenic assay, was simila r to that of normal clones; the three clones with mutant p53 did not differ from the others. There was no relationship between G(1) arrest and radiose nsitivity. Normal 10T1/2 cell clones showed a transient G(1) arrest lasting similar to 9 h after 6 Gy of irradiation. This G(1) arrest was either abse nt or markedly reduced in all of the transformed clones, regardless of p53 status. These results suggest that diminished G(1) checkpoint control is an early event in the process of carcinogenesis that is associated with the m alignant transformation of individual cells and is independent of p53 statu s.