FMR1: a gene with three faces

Abstract

The FMR1 gene is involved in three different syndromes, the fragile X syndrome (FXS), premature ovarian insufficiency (POI) and the fragile X-associated tremor/ataxia syndrome (FXTAS) at older age. Fragile X syndrome is caused by an expansion of a CGG repeat above 200 units in the FMR1 gene resulting in the absence of the FMR1 mRNA and protein. The FMR1 protein is proposed to act as a regulator of mRNA transport and of translation of target mRNAs at the synapse. FXS is seen as a loss of function disorder. POI and FXTAS are found in individuals with an expanded repeat between 50 and 200 CGGs and are associated with increased FMR1 mRNA levels. The presence of elevated FMR1 mRNA in FXTAS suggests that FXTAS may represent a toxic RNA gain-of-function effect. The molecular basis of POI is yet unknown. The role of the FMR1 gene in these disorders is discussed.

Figure 1. The CGG repeat in the FMR1 gene

Schematic representation of normal, PM (premutation) and FM (full mutation) alleles of the FMR1 gene and the effect of the expansion on transcription and translation. Methylation due to extensive elongation of the CGG repeat in the 5′-ÚTR of the FMR1 gene is depicted as a lock.

Figure 2. Schematic representation of the chromatin structure of the FMR1 gene

In the normal situation the active gene has an open chromatin structure. When the CGG repeat (red line) is expanded, deacetylation and methylation of the promoter and CGG region takes place leading to a packaged and less accessible chromatin structure causing inactivation of the FMR1 gene. Treatment with 5-azadC results in demethylation and acetylation leading to an open chromatin structure and transcription will be (partly) restored.

Fig. 3. Localization of EGFP-FMRP in dendritic spines

Primary hippocampal mouse neurons of E18 Fmr1 KO mice were co-transfected with β-actin-EGFP-Fmr1 (A) and β-actin-mCherry (B). The dendrite, including many spines, of one neuron is depicted. Note the presence of EGFP-FMRP in a spinehead in the overlay (arrow in C). Images were acquired using a Zeiss LSM510 confocal microscope (scalebar= 5 μm). Courtesy by Femke de Vrij.

Figure 4. The mGluR theory

Hypothetical model for the action of FMRP at the synapse, adapted from reference [76]. Treatment with MPEP, an mGluR5 antagonist, results in the rescue of some phenotypic features because mGluR5 stimulation is reduced and subsequently local translation at the synapse is no longer exaggerated. Ultimately, the number of internalized AMPA receptors is reduced and restored to normal levels. A. Stimulation of mGluR5, a metabotropic glutamate receptor, induces local mRNA translation. This results in novel protein synthesis that on its turn stimulates the internalization of the ionotropic AMPA receptor, essential for in long-term plasticity. FMRP acts as a negative regulator of transcription [51, 76], reducing the internalization of the ionotropic glutamate receptor. B. In neurons from fragile X patients the absence of FMRP leads to an increase internalization of the ionotropic glutamate receptors which results in enhanced LTD. C. Rescue of normal translation due to the mGluR antagonist MPEP, slowing down the internalization of the ionotropic glutamate receptors.

Figure 5. Simplified model of the translation control pathways at the dendritic spines

Stimulation of mGluR leads to active PP2A (<1min) which dephosphorylates FMRP, and this results in rapid translation of FMRP-associated mRNAs. Within 5 min, mTOR is activated, inhibiting PP2A and activating S6K1, leading to FMRP phosphorylation and translational inhibition of FMRP target messages. Based on [66, 67].

Figure 6. A schematic representation of the RNA gain of function mechanism proposed for the pathogenesis of FXTAS

The FMR1 gene is transcribed in the nucleus and transported to the ribosomes. The expanded CGG repeat present in the 5′ UTR of the FMR1 mRNA hampers translation, leading to lower levels of FMRP. The presence of the expanded CGG repeat results in enhanced transcription via a thusfar unknown mechanism and leads to elevated FMR1 mRNA levels. In an attempt to get rid of the excess of FMR1 mRNAs, the cell might attract chaperones or elements of the ubiquitin/proteasome system. Also CGG-binding proteins might be recruited. These processes could lead to the formation of intranuclear inclusions. Sequestration of proteins into the inclusion might prevent them from exerting their function, thereby disturbing normal cellular function, which in the end might cause neurodegeneration. However, it cannot be excluded that the formation of inclusions has a neuroprotective effect, such that neurons that are capable of capturing the toxic transcripts in the inclusions are the cells that survive.