Changes in genomic stability induced by inactivation of different components of the p53 pathway

To assess the role of different p53-regulated signaling pathways in the con trol of genomic integrity, we studied the frequency of various chromosome a bnormalities in sublines of mouse primary embryonic fibroblasts with the kn ock-outs genes of the p53, p21(WAF) pRb, and p19(ARF) genes. Protein p21(WA F) is transactivated by p53 and is responsible for the cell arrest in the G (1) phase of the damaged cells; protein pRb is a target for p21(WAF) which controls the G(1)S-phase transition; and p19(ARF) protein is responsible fo r p53 activation in cells with certain anomalies. Inactivation of either of the genes studied proved to increase significantly the frequency of change s in the karyotype. However, the resultant chromosome instability differed: the frequency of the chromosome breaks, both spontaneous and induced with ethylmethane sulfonate (EMS), was highest in p53-deficient cells and lowest in Rb-deficient cells. These distinctions were not caused by differences i n the cell-cycle checkpoint control: in all sublines, the cell arrest in G( 1) was abolished and the checkpoint function in G(2) remained normal. Howev er, the induction of apoptosis in EMS-treated cells differed in the studied sublines. The lowest numbers of apoptotic nuclei were determined in p53-/- cultures, whereas the highest were in the Rb-/- cultures. It is apparent t hat the degree of genetic instability is determined by a combined effect of apoptosis and inactivation of the cell-cycle checkpoints.