Regulatory interaction between NBS1 and DNMT1 responding to DNA damage.

NBS1 is the causative gene product of Nijmegen breakage syndrome (NBS), a recessive genetic disorder resulting in chromosomal instability and immunodeficiency. We isolated DNMT1 cDNA by two-hybrid screening by using NBS1 as bait to study its function in DNA replication and damage checkpoint. DNMT1 encodes DNA methyltransferase 1, which maintains the genomic methylation pattern and also regulates the checkpoint pathway via interactions with various factors, such as CHK1, p53, Rb and ATM. The interaction between NBS1 and DNMT1 was observed under conditions of hydroxyl urea treatment, resulting in replication stall and mitomycin C treatment resulting in DNA damage. Additionally, we mapped their binding regions to the N-terminus of NBS1 (including the forkhead-associated domain) and amino acids 1401-1503 in the target recognition domain in the C-terminus of DNMT1. Under DNA replication stall conditions, DNMT1 was recruited to the survivin promoter by p53, and it repressed survivin expression via hetrochromatin formation; this regulation was dependent on the NBS1 genotype. These results suggest that DNMT1 function in the regulatory response is controlled by NBS1.