Processive methylation of hemimethylated CpG sites by mouse Dnmt1 DNA methyltransferase

Area of Science:

• Epigenetics
• Molecular Biology
• Genomics

Background:

• DNA methyltransferase 1 (Dnmt1) is crucial for maintaining DNA methylation patterns during mammalian genome replication.
• Dnmt1 localizes to replication foci and interacts with PCNA, preferring hemimethylated CpG sites.

Purpose of the Study:

• To elucidate the mechanism underlying Dnmt1's DNA methylation maintenance function.
• To investigate the role of Dnmt1's N-terminal region and its processivity on different DNA substrates.

Main Methods:

• Purification of recombinant full-length and truncated Dnmt1 (Dnmt1-(291-1620)).
• Assessing enzyme processivity on unmethylated and hemimethylated DNA substrates.
• Analyzing methylation patterns using bisulfite sequencing and hairpin-PCR.

Main Results:

• Full-length Dnmt1 exhibits high processivity and >95% fidelity on hemimethylated DNA, with lower processivity on unmethylated DNA.
• A truncated Dnmt1 lacking PCNA/DNA binding sites showed similar methylation properties, indicating the N-terminal region is dispensable for these functions.
• Dnmt1 methylates hemimethylated CpG sites on one strand within a single processive run.

Conclusions:

• Dnmt1's inherent enzymatic properties, particularly its processivity and preference for hemimethylated DNA, are key to its maintenance function.
• The N-terminal region of Dnmt1 is not essential for its preferential activity or processivity.
• These findings highlight the molecular basis for faithful epigenetic inheritance in mammals.