Msh2-Msh3 interferes with Okazaki fragment processing to promote trinucleotide repeat expansions

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Trinucleotide repeat (TNR) expansions cause over 40 neurodegenerative and neuromuscular diseases.
• The precise molecular mechanisms driving TNR expansions remain unclear, despite evidence implicating DNA replication and repair errors.

Purpose of the Study:

• To investigate the role of the mismatch repair complex Msh2-Msh3 in promoting TNR expansions.
• To elucidate the molecular mechanisms underlying TNR expansions during DNA replication.

Main Methods:

• In vivo studies in Saccharomyces cerevisiae (yeast) to assess Msh2-Msh3's role in CTG and CAG repeat expansions.
• Biochemical assays to examine the interaction between Msh2-Msh3 and key DNA replication/processing proteins (Rad27, Cdc9) with TNR sequences.

Main Results:

• Msh2-Msh3 was demonstrated to promote both CTG and CAG repeat expansions in vivo.
• Biochemical evidence showed Msh2-Msh3 directly interferes with Okazaki fragment processing by flap endonuclease1 (Rad27) and DNA ligase I (Cdc9) in the presence of TNR sequences.
• This interference leads to small, incremental expansion events.

Conclusions:

• This study provides the first mechanistic evidence linking replication and repair proteins to TNR sequence expansion.
• The findings highlight the direct role of Msh2-Msh3 in promoting TNR expansions through interference with lagging-strand DNA replication processes.