Involvement of 53BP1, a p53 Binding Protein, in Chk2 Phosphorylation of p53 and DNA Damage Cell Cycle Checkpoints

摘要

53BPI, a p53 binding protein, is involved in DNA damage response. It is phosphorylated in response to DNA damage and rapidly relocalize to sites of damage, forming nuclear foci that colocalize with those formed by phosphorylated histone H2AX (-H2AX), Mre11-Rad50-Nbs1 complex, MDC1, Brca1 and other DNA damage signaling proteins. Our studies aimed to determine the role of 53BP1 in DNA damage response and tumor suppression. We studied the function of 53BP1 in mammalian cells by knocking down expression of 53BP1 using small interfering RNA (siRNA) against 53BP1. We also generated 53BP1-defective (53BP1tr/tr) mouse model with expression of a defective C-terminal truncation of the m53BP1 protein. We have shown that 53BP1 is a key transducer in the DNA damage response signaling. Inhibition of 53BP1 by siRNA in human cancer cell lines resulted in defective S-phase and G2/M checkpoints in response to ionizing irradiation (IR). 53BP1 interacts with tumor suppressors p53, Chk2 and Brca1, and is involved in regulation of these proteins in response to IR. 53BP1tr/tr mice were growth retarded and IR-sensitive. Mouse embryonic fibroblast (MEF) cells generated from 53BP1tr/t mice were hypersensitive to IR, and exhibited higher level of chromosomal abnormalities when treated with genotoxic stress, indicating a role of 53BP1 in maintaining genomic stability. Thus 53BP1 is likely to play an important role in maintenance of genomic stability and cancer prevention. These studies allowed us gain further insights into the DNA damage response pathway and the tumor suppression pathway in mammalian cells.