Curvilinear Association Between Language Disfluency and

Area of Science:

• Neurogenetics
• Human Genetics
• Linguistics

Background:

• Research on the Fragile X Mental Retardation 1 (FMR1) gene has historically concentrated on CGG expansions causing premutation (55-200 repeats) and fragile X syndrome (>200 repeats).
• Emerging evidence indicates that CGG repeat lengths within the typical 'normal' range may also influence phenotypes.
• The relationship between CGG repeat length and language production, particularly across the full allele spectrum, remains underexplored.

Purpose of the Study:

• To investigate the association between language production ability and FMR1 CGG repeat length across normal, intermediate, and premutation ranges.
• To examine language disfluencies as an index of language production skills.
• To test the Inhibition Deficit Hypothesis by assessing verbal inhibition skills as a correlate of language disfluency.

Main Methods:

• A cohort of 61 adult women (mothers of children with developmental disorders) with varying FMR1 CGG repeat lengths (normal, intermediate, premutation) was recruited.
• Language samples were analyzed for disfluency frequency, and verbal inhibition was assessed using the Hayling Sentence Completion Test.
• FMR1 CGG repeat size was determined from blood samples, and statistical models analyzed the relationship between repeat length and disfluency, controlling for covariates.

Main Results:

• A significant curvilinear association was found between FMR1 CGG repeat length and language disfluency.
• Higher rates of disfluency were observed in individuals with low-normal (around <25 repeats) and mid-premutation (around 90-110 repeats) alleles.
• Language disfluency was not associated with deficits in verbal inhibition, challenging the Inhibition Deficit Hypothesis and suggesting a primary language-based deficit.

Conclusions:

• FMR1 CGG repeat length influences language production ability across a spectrum of allele sizes, including those considered normal.
• The findings suggest that CGG repeat variations in FMR1 can lead to observable differences in speech fluency.
• Elevated language disfluency in FMR1-associated conditions may stem from primary language deficits rather than solely inhibitory control issues.