Extrahelical (CAG)/(CTG) triplet repeat elements support proliferating cell nuclear antigen loading and MutLα...

Area of Science:

• Molecular Biology
• DNA Repair Mechanisms
• Genetics

Background:

• MutLα endonuclease activation is crucial for DNA mismatch repair.
• Triplet repeat expansions, like (CAG)n/(CTG)n, are linked to various genetic disorders.
• Previous studies suggested these expansions can occur independently of DNA replication.

Purpose of the Study:

• To investigate if extrahelical (CAG)n or (CTG)n elements can trigger MutLα activation.
• To explore the mechanism of MutLα activation on nonreplicating DNA containing triplet repeat extrusions.
• To understand the role of MutSβ, replication factor C, and proliferating cell nuclear antigen (PCNA) in this process.

Main Methods:

• Utilized relaxed closed circular DNA containing (CAG)n or (CTG)n extrusions.
• Assessed MutLα endonuclease activity in the presence of MutSβ, replication factor C, and PCNA.
• Examined mismatch repair in human cell nuclear extracts using heteroduplex DNA with triplet repeat extrusions.

Main Results:

• Extrahelical (CAG)n and (CTG)n elements of two to three repeat units efficiently activate MutLα endonuclease.
• These extrusions also support MutSβ-, replication factor C-, and PCNA-dependent MutLα activation.
• Mismatch repair is triggered by these extrusions in human cell extracts, albeit at a lower rate than with nicked DNA.

Conclusions:

• An extrahelical triplet repeat element is sufficient to initiate MutLα activation and mismatch repair on nonreplicating DNA.
• This provides a mechanism for triplet repeat processing in nonreplicating DNA, relevant to trinucleotide repeat expansion disorders.
• The findings may also have implications for understanding triplet repeat processing during DNA replication.