Elucidation of the role of 53BP1 in DNA double strand break (DSB) repair and tumor suppression

The protein p53 binding protein one (53BP1) was discovered in a yeast two-hybrid screen that used the DNA binding domain of p53 as bait. Cloning of full-length 53BP1 showed that this protein contains several protein domains which help make up the protein, which include two tandem BRCT domains and a amino-terminal serine/glutamine cluster domain (SCD). These are two protein domains are often seen in factors that are involved in the cellular response to DNA damage and control of cell cycle checkpoints and we hypothesize that 53BP1 is involved in the cellular response to DNA damage. In support of this hypothesis we observe that 53BP1 is phosphorylated and undergoes a dramatic nuclear re-localization in response to DNA damaging agents. 53BP1 also interacts with several factors that are important in the cellular response to DNA damage, such as the BRCA1 tumor suppressor, ATM and Rad3 related (ATR), and the phosphorylated version of the histone variant H2AX. Mice deficient in 53BP1 display increased sensitivity ionizing radiation (IR), a DNA damaging agent that introduces DNA double strand breaks (DSBs). In addition, 53BP1-deficient mice do not properly undergo the process of class switch recombination (CSR). We also observe that when a defect in 53BP1 is combined with a defect in p53; the resulting mice have an increased rate of formation of spontaneous tumors, notably the formation of B and T lineage lymphomas. The T lineage tumors arise by two distinct mechanisms: one driven by defects in cell cycle regulation and a second driven by defects in the ability to repair DNA DSBs. The B lineage tumors arise by the inability to repair DNA damage and over-expression of the oncogene c-myc. With these observations, we conclude that not only does 53BP1 function in the cellular response to DNA damage, but it also works in concert with p53 to suppress tumor formation