Elevated FMR1-mRNA and lowered FMRP - A double-hit mechanism for psychiatric features in men with FMR1 premutations

Fragile X syndrome (FXS) is caused by a full mutation of theFMR1gene (>200 CGG repeats and subsequent methylation), such that there is little or noFMR1protein (FMRP) produced, leading to intellectual disability (ID). Individuals with the premutation allele (55-200 CGG repeats, generally unmethylated) have elevatedFMR1mRNA levels, a consequence of enhanced transcription, resulting in neuronal toxicity and a spectrum of premutation-associated disorders, including the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Here we described 14 patients who had both lowered FMRP and elevatedFMR1mRNA levels, representing dual mechanisms of clinical involvement, which may combine features of both FXS and FXTAS. In addition, the majority of these cases show psychiatric symptoms, including bipolar disorder, and/or psychotic features, which are rarely seen in those with just FXS.