Fragile X gene instability: anchoring AGGs and linked microsatellites

Area of Science:

• Genetics
• Molecular Biology
• Human Genetics

Background:

• The FMR1 gene's CGG repeat region is crucial for X chromosome stability.
• Interspersed AGG sequences may prevent replication errors (slippage) in the CGG repeats.
• Fragile X syndrome is associated with expansions in the FMR1 CGG repeat region.

Purpose of the Study:

• To develop and apply a method for detecting AGG positions within the FMR1 gene's CGG repeat region.
• To investigate the correlation between AGG positions, CGG repeat numbers, and fragile X premutations.
• To explore the evolutionary history of CGG repeat instability in primates.

Main Methods:

• Partial MnlI restriction analysis was employed to map AGG positions.
• X chromosomes from 187 males (controls and fragile X premutations) were analyzed.
• Microsatellite allele lengths and heterozygosity were assessed in control individuals.

Main Results:

• AGG positions were highly polymorphic in controls (91% heterozygosity), with most having two AGGs.
• Fragile X premutation samples showed a significant reduction in AGGs (63% had none, 37% had one).
• Instability of CGG repeats was found to be polar, occurring primarily at the 3' end.

Conclusions:

• The number and position of AGG sequences significantly influence FMR1 CGG repeat stability.
• Reduced AGG sites are strongly associated with fragile X premutations, indicating a role in expansion.
• Polar instability within the FMR1 gene is an evolutionarily conserved phenomenon dating back to primate ancestors.