Abasic sites induce triplet-repeat expansion during DNA replication in vitro

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Triplet-repeat expansion (TRE) is implicated in neurological and neuromuscular diseases like Fragile X syndrome.
• DNA slippage during replication is a known cause of TRE, but its origins are unclear.
• Abasic sites are common DNA lesions that can stall replication forks.

Purpose of the Study:

• To investigate the role of abasic sites in inducing triplet-repeat expansion (TRE) during DNA replication.
• To determine how the location and sequence context of abasic sites affect TRE.
• To elucidate the mechanism by which DNA damage contributes to genetic instability.

Main Methods:

• In vitro DNA replication assays using Escherichia coli Klenow polymerase I.
• Synthesis of template DNA containing an abasic site analog within a triplet-repeat tract.
• Sequencing of expanded DNA tracts to confirm repeat additions.

Main Results:

• A single abasic site analog in the template strand induced significant TRE.
• TRE induction decreased as the abasic site moved towards the middle of the repeat tract.
• TRE was sequence-dependent, with AAT/ATT repeats showing higher expansion rates than CAG/CTG repeats.
• Expansions required primer-template complementarity and resulted in multiple repeat additions.

Conclusions:

• Abasic sites are potent inducers of TRE during DNA replication.
• The position and sequence context of DNA damage influence TRE frequency.
• DNA damage is a significant factor in the generation of TRE in vivo, potentially contributing to disease pathogenesis.