R-loops: targets for nuclease cleavage and repeat instability.

Area of Science:

• Genetics
• Molecular Biology
• Genomics

Background:

• R-loops are stable RNA:DNA hybrids formed during transcription.
• Expandable repeat sequences, implicated in neurodegenerative diseases, can form stable R-loops.
• R-loops are increasingly recognized as a source of genome instability.

Purpose of the Study:

• To investigate the mechanisms by which R-loops contribute to DNA breaks and repeat instability at CAG/CTG repeat tracts.
• To identify factors responsible for R-loop-dependent DNA breaks.
• To elucidate the link between R-loops, DNA breaks, and repeat instability in the context of neurodegenerative diseases.

Main Methods:

• Analysis of R-loop formation at expanded CAG/CTG repeat tracts.
• Identification of factors causing R-loop-dependent DNA breaks, including cytosine deamination and MutLγ (Mlh1-Mlh3) endonuclease.
• Investigation of the role of base excision repair in R-loop-induced repeat instability.

Main Results:

• R-loops at expanded CAG/CTG repeats cause DNA breaks and repeat instability.
• Cytosine deamination and the MutLγ endonuclease were identified as two novel mechanisms for R-loop-induced DNA breaks.
• Base excision repair following R-loop-dependent nicking leads to repeat instability.

Conclusions:

• R-loops contribute to genome instability through DNA breaks and repeat instability at G-rich repeat sequences.
• These findings reveal new mechanisms linking RNA:DNA hybrids to DNA damage and repeat expansion diseases.
• Understanding R-loop-mediated fragility is crucial for developing therapeutic strategies for repeat expansion disorders.