Abstract
53BP1 accumulates at sites of DNA double strand breaks (DSBs), and plays a key role in repair of DSBs. We found that (1) 53BP1 plays a role in a novel pathway distinct from the Ku-dependent and Artemis-dependent non-homologous end-joining (NHEJ) pathways (Genes Cells, 11: 935, 2006), and that (2) RAD18 interacts with 53BP1 and is recruited at DSB sites in a 53BP1-dependent manner specifically during the G1-phase of the cell cycle. RAD18 monoubiquitinates 53BP1and enhances retention of 53BP1 in chromatin (Nucleic Acids Res, 37: 2176, 2009). Depletion of TRF2 (one of shelterin components) arises dysfunctional and unprotected telomeres. The unprotected telomeres, like DSBs, activate the ATM kinase, and are ligated by the NHEJ pathway to generate fused chromosomes (Nature Cell Biol., 7: 712, 2005). To ask whether 53BP1 and Rad18 are required for telomere-telomere fusions between unprotected telomeres, we removed TRF2 from telomeres with shTRF2 in wild type mouse embryo fibroblasts (MEFs), 53BP1-/-MEFs (a gift from Dr. Junjie Chen) and Rad18-/-MEFs. We assessed the frequencies of telomere-telomere fusions by FISH analyses with the telomere probe. We confirmed that the frequency of telomere-telomere fusions is reduced in 53BP1-/-MEFs. We will show roles of 53BP1, Rad18 and the interaction of these proteins in fusions of unprotected telomeres.
