Fragile X syndrome, the most frequent form of inherited mental retardation, is due to the absence of Fragile X Mental Retardation Protein (FMRP), an RNA-binding protein involved in several steps of RNA metabolism. To date, two RNA motifs have been found to mediate FMRP/RNA interaction, the G-quartet and the “kissing complex,” which both induce translational repression in the presence of FMRP. We show here a new role for FMRP as a positive modulator of translation. FMRP specifically binds Superoxide Dismutase 1 (Sod1) mRNA with high affinity through a novel RNA motif, SoSLIP (Sod1 mRNA Stem Loops Interacting with FMRP), which is folded as three independent stem-loop structures. FMRP induces a structural modification of the SoSLIP motif upon its interaction with it. SoSLIP also behaves as a translational activator whose action is potentiated by the interaction with FMRP. The absence of FMRP results in decreased expression of Sod1. Because it has been observed that brain metabolism of FMR1 null mice is more sensitive to oxidative stress, we propose that the deregulation of Sod1 expression may be at the basis of several traits of the physiopathology of the Fragile X syndrome, such as anxiety, sleep troubles, and autism. The most common form of inherited mental retardation, Fragile X syndrome, is caused by the absence of the Fragile X Mental Retardation Protein (FMRP), which can bind mRNA. In mice devoid of FMRP, several hundreds of mRNAs with altered expression and localization have been reported. The impact of these abnormalities is particularly important in the brain, where the absence of FMRP affects the structure and function of connections between neurons, resulting in reduced cognitive abilities. In recent years, scientists studying Fragile X syndrome have focused on identifying the RNAs that are specifically bound by FMRP. We and others have identified two specific RNA structures bound by FMRP/RNA. Importantly, when FMRP binds these structures, the translation of the RNA into its protein product is inhibited. In this new study we have identified a third FMRP-binding RNA structure that is found in the mRNA that encodes superoxide dismutase, an oxidative-stress-mitigating enzyme. Most significantly, FMRP enhances translation of the superoxide dismutase mRNA when it interacts with the structure. These findings suggest that absence of FMRP might result in reduced levels of superoxide dismutase, which in turn leads to increased oxidative stress in the brain. Interestingly, oxidative stress in the brain has already been linked to anxiety, sleeping difficulties, and autism, all of which typically affect individuals with Fragile X syndrome.

Background: Although much progress has been made recently in characterising the proteins involved in duodenal iron trafficking, regulation of intestinal iron transport remains poorly understood. It is not known whether the level of mRNA expression of these recently described molecules is genetically regulated. This is of particular interest however as genetic factors are likely to determine differences in iron status among mouse strains and probably also contribute to the phenotypic variability seen with disruption of the haemochromatosis gene. Aims: To investigate this issue, we examined concomitant variations in duodenal cytochrome b (Dcytb), divalent metal transporter 1 (DMT1), ferroportin 1 (FPN1), hephaestin, stimulator of Fe transport (SFT), HFE, and transferrin receptor 1 (TfR1) transcripts in response to different dietary iron contents in the four mouse strains C57BL/6, DBA/2, CBA, and 129/Sv. Subjects: Six mice of each strain were fed normal levels of dietary iron, six were subjected to the same diet supplemented with 2% carbonyl iron, and six were fed an iron deficient diet. Methods: Quantification of mRNAs isolated from the duodenum was performed using real time reverse transcription-polymerase chain reaction. Results: There was a significant increase in mRNA expression of Dcytb, DMT1, FPN1, and TfR1 when mice were fed an iron deficient diet, and a significant decrease in mRNA expression of these molecules when mice were fed an iron supplemented diet. Strain to strain differences were observed not only in serum transferrin saturations, with C57BL/6 mice having the lowest values, but also in hepatic iron stores and in duodenal mRNA expression of Dcytb, DMT1, FPN1, hephaestin, HFE, and TfR1. Conclusions: The results favour some degree of genetic control of mRNA levels of these molecules.

Fragile X syndrome (FXS) – the leading cause of inherited mental retardation – is an X-linked disease caused by loss of expression of the FMR1 (fragile X mental retardation 1) gene. In addition to impairment of higher-cognitive functions, FXS patients show a variety of physical and other mental abnormalities. FMRP, the protein encoded by the FMR1 gene, is thought to play a key role in translation, trafficking and targeting of mRNA in neurons. To better understand FMRP's functions, the protein partners and mRNA targets that interact with FMRP have been sought. These and functional studies have revealed links with processes such as cytoskeleton remodelling via the RhoGTPase pathway and mRNA processing via the RNA interference pathway. In this review, we focus on recent insights into the function of FMRP and speculate on how the absence of FMRP might cause the clinical phenotypes seen in FXS patients. Finally, we explore potential therapies for FXS.

The AFF (AF4/FMR2) family of genes includes four members: AFF1/AF4, AFF2/FMR2, AFF3/LAF4 and AFF4/AF5q31. AFF2/FMR2 is silenced in FRAXE intellectual disability, while the other three members have been reported to form fusion genes as a consequence of chromosome translocations with the myeloid/lymphoid or mixed lineage leukemia (MLL) gene in acute lymphoblastic leukemias (ALLs). All AFF proteins are localized in the nucleus and their role as transcriptional activators with a positive action on RNA elongation was primarily studied. We have recently shown that AFF2/FMR2 localizes to nuclear speckles, subnuclear structures considered as storage/modification sites of pre-mRNA splicing factors, and modulates alternative splicing via the interaction with the G-quadruplex RNA-forming structure. We show here that similarly to AFF2/FMR2, AFF3/LAF4 and AFF4/AF5q31 localize to nuclear speckles and are able to bind RNA, having a high apparent affinity for the G-quadruplex structure. Interestingly, AFF3/LAF4 and AFF4/AF5q31, like AFF2/FMR2, modulate, in vivo, the splicing efficiency of a mini-gene containing a G-quadruplex structure in one alternatively spliced exon. Furthermore, we observed that the overexpression of AFF2/3/4 interferes with the organization and/or biogenesis of nuclear speckles. These findings fit well with our observation that enlarged nuclear speckles are present in FRAXE fibroblasts. Furthermore, our findings suggest functional redundancy among the AFF family members in the regulation of splicing and transcription. It is possible that other members of the AFF family compensate for the loss of AFF2/FMR2 activity and as such explain the relatively mild to borderline phenotype observed in FRAXE patients.

FRAXE is a form of mild to moderate mental retardation due to the silencing of the FMR2 gene. The cellular function of FMR2 protein is presently unknown. By analogy with its homologue AF4, FMR2 was supposed to have a role in transcriptional regulation, but robust evidences supporting this hypothesis are lacking. We observed that FMR2 co-localizes with the splicing factor SC35 in nuclear speckles, the nuclear regions where splicing factors are concentrated, assembled and modified. Similarly to what was reported for splicing factors, blocking splicing or transcription leads to the accumulation of FMR2 in enlarged, rounded speckles. FMR2 is also localized in the nucleolus when splicing is blocked. We show here that FMR2 is able to specifically bind the G-quartet-forming RNA structure with high affinity. Remarkably, in vivo , in the presence of FMR2, the ESE action of the G-quartet situated in mRNA of an alternatively spliced exon of a minigene or of the putative target FMR1 appears reduced. Interestingly, FMR1 is silenced in the fragile X syndrome, another form of mental retardation. All together, our findings strongly suggest that FMR2 is an RNA-binding protein, which might be involved in alternative splicing regulation through an interaction with G-quartet RNA structure.