DNA extrusion size determines pathway choice during CAG repeat expansion.

Area of Science:

• Molecular Biology
• Genetics
• DNA Repair Mechanisms

Background:

• DNA triplet repeat expansion is a key mutational mechanism underlying neurodegenerative disorders like Huntington's disease.
• The DNA mismatch repair protein MutSβ is thought to promote repeat expansion by recognizing hairpin structures.
• The FAN1 nuclease acts as a suppressor of repeat expansion by cleaving these structures.

Purpose of the Study:

• To investigate the role of DNA extrusion size in determining the pathway choice between FAN1 and MutSβ.
• To elucidate the molecular basis for the opposing roles of FAN1 and MutSβ in DNA triplet repeat stability.

Main Methods:

• Comparative analysis of FAN1 and MutSβ processing of DNA extrusions of varying sizes.
• In vitro assays to assess nuclease activity and protein-DNA interactions.
• Genetic studies to confirm the in vivo relevance of extrusion size-dependent pathway selection.

Main Results:

• Both FAN1 and MutSβ pathways process extrusions containing 2-3 triplet repeats.
• Single triplet extrusions are not cleaved by FAN1 and are exclusively processed by the MutSβ-dependent mismatch repair pathway.
• This size-dependent processing dictates whether DNA triplet repeats expand or remain stable.

Conclusions:

• The size of DNA extrusions is a critical determinant of DNA repair pathway choice.
• This size-based mechanism controls the balance between repeat expansion and stability, impacting disease pathogenesis.
• Findings offer new insights into DNA structural dynamics and pathway selection in DNA repair.