Bypass of senescence after disruption of p21CIP1/WAF1 gene in normal diploid human fibroblasts.

Most somatic cells die after a finite number of cell divisions, a phenomenon described as senescence. The p21(CIP1/WAF1) gene encodes an inhibitor of cyclin-dependent kinases. Inactivation of p21 by two sequential rounds of targeted homologous recombination was sufficient to bypass senescence in normal diploid human fibroblasts. At the checkpoint between the prereplicative phase of growth and the phase of chromosome replication, cells lacking p21 failed to arrest the cell cycle in response to DNA damage, but their apoptotic response and genomic stability were unaltered. These results establish the feasibility of using gene targeting for genetic studies of normal human cells.