Fragile X "gray zone" alleles: AGG patterns, expansion risks, and associated haplotypes

Area of Science:

• Genetics
• Molecular Biology
• Human Genetics

Background:

• Fragile X syndrome is associated with CGG repeat expansions in the FMR1 gene.
• Gray-zone alleles (35-59 repeats) present an intermediate risk for expansion.
• The presence and number of AGG interruptions within the CGG repeat region are thought to stabilize these alleles.

Purpose of the Study:

• To investigate the AGG interspersion patterns in fragile X gray-zone alleles.
• To determine the relationship between CGG repeat length, AGG interruptions, and pure CGG (PCGG) stretch length.
• To explore associations between PCGG length and flanking microsatellites.

Main Methods:

• Analysis of 101 chromosomes with CGG repeat lengths from 35 to 59.
• Characterization of AGG interspersion patterns within the CGG repeat region.
• Determination of pure CGG (PCGG) repeat length.
• Genotyping of flanking microsatellites DXS548 and FRAXAC1.

Main Results:

• A significant inverse relationship was observed between FMR1 repeat length and the number of AGG interruptions.
• Alleles with 45-59 repeats were more likely to have 0 or 1 AGG interruption and a longer PCGG stretch (>33 repeats).
• Specific microsatellite haplotypes (20-19 and 25-21) were strongly associated with PCGG lengths <=20 and >20, respectively.

Conclusions:

• Gray-zone alleles with 45-59 repeats and fewer AGG interruptions possess longer pure CGG stretches, increasing their risk of unstable inheritance.
• The observed associations with microsatellite haplotypes may indicate a founder effect or general instability of CGG and microsatellite repeats.