MutLγ promotes repeat expansion in a Fragile X mouse model while EXO1 is protective

Area of Science:

• Genetics
• Molecular Biology
• Genomic Instability

Background:

• Fragile X-related disorders (FXDs) are repeat expansion diseases caused by CGG-repeat expansion in the FMR1 gene.
• The precise molecular mechanisms driving these expansions remain incompletely understood.
• Previous research implicated mismatch repair (MMR) complexes (MutSβ, MutSα) and Polβ in expansion processes.

Purpose of the Study:

• To elucidate the roles of specific MMR pathway components, MLH1/MLH3 (MutLγ) and EXO1, in FMR1 CGG-repeat expansions.
• To investigate the mechanism of repeat expansion and the protective role of EXO1.

Main Methods:

• Utilized a mouse model for Fragile X-related disorders.
• Generated and analyzed mice with targeted mutations in MLH1/MLH3 and EXO1 (including a nuclease-dead mutant).
• Assessed germ line and somatic repeat expansions in these genetically modified mouse models.

Main Results:

• MLH1/MLH3 (MutLγ) is essential for both germ line and somatic CGG-repeat expansions.
• EXO1 is not required for expansion; rather, its absence leads to more extensive expansions.
• A nuclease-inactive EXO1 mutant partially mitigated the expansion increase seen in Exo1 null mice, suggesting both nuclease-dependent and -independent protective roles for EXO1.

Conclusions:

• FMR1 CGG-repeat expansion in this model proceeds through a pathway dependent on MutLγ but independent of EXO1's requirement for expansion.
• EXO1 acts as a suppressor of repeat expansion through both its nuclease activity and other functions.
• These findings enhance understanding of repeat expansion mechanisms and potential genetic modifiers of FXD risk in humans.