Association between FMR1 mRNA levels and clinical phenotypes in Fragile X syndrome

Fragile X syndrome (FXS) is caused by a CGG repeat expansion in the FMR1 gene on chromosome X that leads to hypermethylation and reduces or completely silences the expression of FMR1 mRNA and its protein product, Fragile X Messenger Ribonucleoprotein (FMRP). Since the clinical manifestations of FXS can be variable, there is interest in identifying molecular markers that reflect phenotypic severity. The extent of CGG repeat expansion and resulting hypermethylation varies substantially between patients, suggesting that FMR1 gene silencing differs between individuals with FXS and could contribute to the phenotypic variability. Previous studies have identified FMR1 mRNA as a highly reproducible, sensitive and quantitative measure of gene activity. This study builds on these previous studies and assesses the relationship between FMR1 mRNA and clinical outcomes in FXS.

We recruited 300 study participants, including males and females across the following diagnostic groups: FXS, fragile X premutation carriers (PMCs), typically developing controls (TDC), and (non-FXS) developmental disability controls (DD). Peripheral blood samples were used for quantifying both FMR1 mRNA and FMRP (from a Luminex-based assay). IQ and adaptive behavior were evaluated with the Stanford-Binet 5 and Vineland-3 tests, respectively. Analyses examined group- and sex-level associations between FMR1 mRNA and FMRP, IQ, adaptive behavior, and age across diagnostic groups.

FMR1 mRNA was substantially reduced in participants with FXS, particularly males, consistent with expected gene silencing. Within the FXS group, higher FMR1 mRNA levels were associated with higher IQ and better adaptive behavior. These associations were not observed in the TDC or DD comparison groups, suggesting that the relationship is specific to FXS pathology rather than developmental impairment more broadly. FMR1 mRNA also correlated positively with FMRP in FXS, supporting its validity as a molecular marker. Age-related increases in FMR1 mRNA were also observed in females with FXS.

Our findings that FMR1 mRNA correlates with both FMRP levels and clinical outcomes in participants with FXS support its utility as a clinically meaningful molecular marker of phenotypic severity. As an accessible surrogate of disease severity, FMR1 mRNA may help improve prognostic assessment and support more individualized treatment planning in FXS.