Fragile X syndrome-linked mutations impair FMRP accumulation in neuronal transport granules

RNA granules are biomolecular condensates that are reversibly formed through multivalent interactions among mRNAs and RNA-binding proteins (RBPs). RNA granules include stress granules and neuronal transport granules, which share common components and functions in translational regulation. Fragile X messenger ribonucleoprotein (FMRP) is an RBP encoded by FMR1, whose loss of function causes fragile X syndrome (FXS), a neurodevelopmental disorder. FMRP is a component of both stress granules and neuronal transport granules. Although several FXS-associated missense mutations in FMRP have been identified, it remains unclear whether these mutations affect the assembly of FMRP into RNA granules. Here, we found that the KH1 domain mutation G266E markedly reduced the accumulation of FMRP in stress granules in HEK293T cells, whereas the KH2 domain mutation I304N had little effect. In addition, both G266E and I304N reduced the accumulation of FMRP within neuronal transport granules in FMRP-deficient mouse cortical neurons. Because G266E and I304N have been reported to impair the RNA-binding and/or RNA-associated functions of FMRP, our findings suggest that the KH1 and KH2 domains are important for the assembly of FMRP into RNA granules. These findings further raise the possibility that impaired RNA granule assembly may contribute to the pathogenesis of FXS and other neurodevelopmental disorders.