Disease-Associated Mutation A554V Disrupts Normal Autoinhibition of DNMT1.

DNA methyltransferase 1 (DNMT1) is the enzyme primarily responsible for propagation of the methylation pattern in cells. Mutations in DNMT1 have been linked to the development of adult-onset neurodegenerative disorders; these disease-associated mutations occur in the regulatory replication foci-targeting sequence (RFTS) domain of the protein. The RFTS domain is an endogenous inhibitor of DNMT1 activity that binds to the active site and prevents DNA binding. Here, we examine the impact of the disease-associated mutation A554V on normal RFTS-mediated inhibition of DNMT1. Wild-type and mutant proteins were expressed and purified to homogeneity for biochemical characterization. The mutation increased DNA binding affinity ~8-fold. In addition, the mutant enzyme exhibited increased DNA methylation activity. Circular dichroism (CD) spectroscopy revealed that the mutation does not significantly impact the secondary structure or relative thermal stability of the isolated RFTS domain. However, the mutation resulted in changes in the CD spectrum in the context of the larger protein; a decrease in relative thermal stability was also observed. Collectively, this evidence suggests that A554V disrupts normal RFTS-mediated autoinhibition of DNMT1, resulting in a hyperactive mutant enzyme. While the disease-associated mutation does not significantly impact the isolated RFTS domain, the mutation results in a weakening of the interdomain stabilizing interactions generating a more open, active conformation of DNMT1. Hyperactive mutant DNMT1 could be responsible for the increased DNA methylation observed in affected individuals.