ReviewAdvances in understanding of fragile X pathogenesis and FMRP function, and in identification of X linked mental retardation genes
Introduction
Mutations in X-linked genes account, at least in part, for the excess of males in schools and institutions for mentally handicapped (reviewed in [1]). In recent years, many genes have been identified that are indeed implicated in syndromic forms of X-linked mental retardation (XLMR; forms characterized by specific clinical symptoms) or in ‘non specific’ (or non syndromic) XLMR. The latter is characterized by extensive non-allelic genetic heterogeneity, as 12 genes have been already found mutated in families with non specific XLMR and linkage analysis suggests that the total number of such genes may reach 30–50 [2]. Several of the identified genes code for proteins that function in the Rho/Rac GTPase signaling pathway that is implicated in neuronal maturation [2].
The most frequent form of XLMR is the fragile X mental retardation syndrome, caused by an unstable expansion of the CGG repeat within the FMR1 gene, which leads to transcriptional shut off and loss of expression of the FMRP, an RNA-binding protein. Since 1991, extensive work has been devoted to analysis of the mutation mechanisms, and to an understanding of FMRP function (see 3., 4., 5.). In this review, we focus on FMRP function, with an emphasis on the recent significant advances addressing the following questions: does FMRP bind to specific sets of mRNAs and what is the effect of this binding on metabolism and translation of such target mRNAs; what are the protein partners of FMRP that may modulate its functions; what are the mechanisms underlying the effect of FMRP deficiency on cognitive functions; what are the functional differences between FMRP and its close homologs FXR1P and FXR2P? We also discuss recent observations that premutation, the intermediate form of the fragile X CGG expansion, is associated in some carriers with phenotypes—premature ovarian failure in females, late-onset movement disorder in males—that are unlikely to result from FMRP deficiency, and suggest an effect of the presence of an abnormal FMR1 mRNA. We briefly update the status of identified XLMR genes since publication of a recent review [2], and notably the discovery of a homeobox gene, the Aristaless homolog ARX, as mutated in many families with syndromic or non-syndromic forms of XLMR.
Section snippets
Key facts on fragile X syndrome and FMRP
The fragile X mental retardation syndrome is the most common monogenic form of mental retardation, affecting ∼1 in 4000 males, and on average less severely, 1 in 7000 females (see [6] for a review). In addition to the cognitive deficits, affected boys often manifest a hyperactive and/or autistic-like behavior. Physical signs in males frequently include evocative facial features, post pubertal macroorchidism, and minor signs of connective tissue dysplasia [7]. The disease is caused by an
Unexpected phenotypes in premutation carriers
The premutation was initially thought to have either little or no effect on FMR1 expression, and thus to have no associated phenotypic manifestations. It had been reported, however, that FMR1 mRNA transcribed from a premutated gene, and that carries the moderate CGG expansion, is less efficiently translated [25]. This appeared somewhat in contradiction with the observation that rare males with unmethylated large expansions have a normal intelligence, indicative of the expression of functional
Mutation mechanisms
In the years following the discovery of the CGG repeat mutation, many papers analyzed the phenomenology of the various states of the expansion, its multistep evolution in human populations, and the stabilizing role of AGG interruptions in the CGG repeat (see [3] for review). More mechanistic analyses of triplet repeat instability have been performed in simpler bacterial or yeast systems (more often on CTG/CAG repeats than on CGG repeats). Analysis in mammalian systems has been impaired by the
mRNAs interacting with FMRP and translational regulation
Several laboratories have recently focused their attention on the identification of mRNAs that can specifically bind to FMRP. An early study had reported that FMRP binds 4% of brain mRNAs [46], but the identity of these mRNAs had remained elusive. The fact that FMRP manifests non-specific RNA binding, as shown by its ability to bind to polyU and polyG, certainly hindered identification of specific mRNA targets. The mRNA of FMR1 itself had, however, been found to bind to FMRP, and to be present
FMRP-interacting proteins
Another approach towards an understanding of FMRP function has been the search for proteins that interact directly with FMRP or that are components of the FMRP-containing mRNP complex (48., 57.). Several proteins have been characterized to date, as follows.
First, FXR1P and FXR2P are close homologs of FMRP and are able to both homo- and heterodimerize with it [58]. The two proteins contain the same functional motifs as FMRP and are supposed to have similar functions. Although the three proteins
Physiological function of FMRP in the brain
The Fmr1 knockout mouse has been used intensively to study FMRP function in brain. This model presents rather subtle differences in behavior and spatial learning compared to wild-type mice that are subject to strain background effects 24., 69., 70., 71.. Using various experimental paradigms, the Fmr1 knockout mouse has been reported to show increased sensitivity to audiogenic epileptic seizures 72., 73., and greater response to low-intensity auditory stimuli 73., 74., whereas abnormal responses
Newly identified XLMR genes
The field of X-linked mental retardation has been thoroughly described in a book [1], and advances in the identification and functional characterization of XLMR genes have been reviewed recently [2]. We here briefly update on novel findings in this fast-moving field.
The most significant result is certainly the discovery of a homeobox gene, the Aristaless homolog ARX, as mutated in 20 families with either syndromic forms of XLMR: with infantile spasms, called ISSX (MIM 308350) or with dystonic
Conclusions
Important progress has been made in the past year, notably concerning the specificity of interaction of FMRP with some mRNAs. Lists of potential mRNA targets are now available, but they must be validated using other functional approaches—as was done in a single case in Drosophila, to reveal how FMRP interaction with specific mRNAs affects their expression. Several lines of evidence suggest that FMRP may modulate synaptogenesis through an action on cytoskeletal proteins, by interaction with
Acknowledgements
We wish to thank our colleagues A Schenck, H Moine, C Schaeffer, A Giangrande, for fruitful collaboration and discussion and S Metz for help. Research on FMRP in the authors laboratory is supported by the National Institutes of Health, HFSP (Human Frontiers Science Program, Université Louis Pasteur, and by general grants from INSERM, CNRS, Institut Universitaire de France and CHRU (Centre Hospitalier Régional Universitaire). B Bardoni was supported by FRAXA foundation.
References and recommended reading
Papers of particular interest, published within the annual period of review,have been highlighted as:
• of special interest
•• of outstanding interest
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