DEFECTIVE G(2) CHECKPOINT FUNCTION IN CELLS FROM INDIVIDUALS WITH FAMILIAL CANCER SYNDROMES

The early events in the G(2) checkpoint response to ionizing radiation (IR) were analyzed in diploid normal human fibroblasts (NHFs) and fib roblasts from patients with two heritable cancer syndromes. Exposure t o gamma-radiation of asynchronously growing NHFs resulted in a rapid r eduction in the number of cells in mitosis (G(2) delay) and was accomp anied by a quantitatively similar reduction in the p34(CDC2)/cyclin B in vitro histone H1 kinase activity as compared with sham-treated cont rols. This G(2) delay was strong by 1 h following exposure to IR, maxi mal by 2 h, and was accompanied by an accumulation of tyrosine-phospho rylated p34(CDC2) molecules. In contrast, fibroblasts from individuals with ataxia telangiectasia displayed significantly less reduction of the mitotic index or histone H1 kinase activity after IR. Low passage fibroblasts from individuals with Li-Fraumeni syndrome having one wild -type and one mutated p53 allele were similar to NHFs in their immedia te G(2) checkpoint response to IR, as were NHFs expressing the human p apilloma virus type 16 E6 gene product (functionally inactivating p53) and low passage cells from p53-deficient mouse embryos. However, the p53-deficient fibroblasts were genomically unstable and became defecti ve in their early G(2) checkpoint response to IR. Furthermore, immorta l Li-Fraumeni syndrome fibroblasts lacking wild-type p53 displayed an attenuated G(2) checkpoint response. These results link the early even ts in G(2) checkpoint response to IR in NHFs with a rapid inhibition o f p34(CDC2)/cyclin B protein kinase activity and demonstrate that whil e not required for this immediate G(2) delay, lack of p53 can lead to subsequent genetic alterations that result in defective G(2) checkpoin t function.