Relationship Between Neurologic Symptoms and Signs and FMR1 Genotype in Premutation Carriers

Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) is the most severe late-onset condition caused by a premutation in the FMR1 gene, characterized by expanded CGG triplet repeats of 55-200. Clinical presentations of FXTAS, including gait ataxia, kinetic tremor, cognitive decline, and rare Parkinsonism, are linked to white matter degeneration, predominantly in the middle cerebellar peduncles. Underlying pathophysiological mechanisms involve the sequestration of CGG-binding proteins due to elevated FMR1 mRNA and repeat-associated non-AUG (RAN)-initiated translation. Outside of the full presentation of FXTAS, some FMR1 premutation carriers exhibit only isolated clinical changes occurring in this syndrome. This study explored the relationship of molecular predictors of disease, either with these isolated features in patients who did not meet diagnostic criteria for FXTAS or with diagnosable FXTAS.

176 male (N = 111) and female (N = 65) premutation carriers were separated into three groups based on neurological/cognitive examination data: asymptomatic, non-syndromic/presenting isolated changes, and syndromic-FXTAS. These categories were then separately correlated with CGG repeat length and FMR1 mRNA expression levels.

Regression and distributions' analyses showed that the most consistent associations of both genomic markers were with neurological severity rankings, followed by a binary definition of FXTAS status. Among other minor presentations, Parkinsonism and cognitive impairment were significantly correlated with CGG size in male samples.

This data provides evidence for a linear relationship between FMR1 CGG size and mRNA levels, as well as both syndromic and non-syndromic forms of neurological manifestations, which represent aspects of the premutation-linked neurodegenerative process that persist with advancing age.