Transcription, translation and fragile X syndrome
Introduction
The FMR1 (FRAGILE X MENTAL RETARDATION 1) gene is directly associated with three distinct diseases: fragile X syndrome, fragile X-associated tremor/ataxia syndrome, and premature ovarian failure [1, 2] (see also review by D Toniolo [3], this issue). All are caused by FMR1 alleles with expanded CGG trinucleotide repeats in the 5′ untranslated region. Although normal alleles contain, on average, 30 repeats, fragile X syndrome is caused by a massive expansion beyond 200 repeats (i.e. the full mutation), and both fragile X-associated tremor/ataxia syndrome and premature ovarian failure are associated with premutation alleles (i.e. 55–200 repeats). Both abnormal alleles result in transcriptional dysregulation of the FMR1 gene. Whereas fragile X syndrome is almost exclusively caused by a complete transcriptional shutdown of the gene, the premutation-associated diseases are caused by excess transcript levels leading to — at least for fragile X-associated tremor/ataxia syndrome — toxic effects of repeat-containing mRNA [1, 4, 5]. Given that mutations in FMR1 manifest two vastly different transcriptional defects, much work has gone into understanding the cis sequences and trans-acting proteins that normally influence this gene in addition to its chromatin structure.
Fragile X syndrome occurs in approximately 1 in 4000 males and in 1 in 8,000 females and presents as developmental delay around 36 months of age. Speech delay is frequent, along with behavior problems such as over-activity and anxiety. Many parallel phenotypes with autism are seen, such as gaze avoidance, stereotyped repetitive behavior, resistance to change in routines or environment, and preservation. Premutation males, and to a lesser degree females, can have fragile X-associated tremor ataxia syndrome with cerebellar tremor/ataxia, cognitive decline and generalized brain atrophy presenting beyond the fifth decade of life. Approximately 24% of premutation females also experience premature ovarian failure (i.e. cessation of menses at <40 years).
FMRP, the protein encoded by FMR1, is an RNA binding protein involved in the control of local protein synthesis. FMRP shuttles from the nucleus to the cytoplasm, where it associates with polyribosomes through large mRNP particles [6] and suppresses translation of a selective group of mRNAs to which it binds [7, 8]. In vivo, the lack of Fmrp in mice is associated with elevations in the rates of protein synthesis in certain regions of the brain [9]. Various approaches have been taken to identify the mRNAs associated with FMRP and its homologs, and these mRNAs include, among others, MAP1B (MICROTUBULE-ASSOCIATED PROTEIN 1B), the FMR1 message itself, and others involved in neuronal development and plasticity [10, 11, 12]. FMRP recognizes two three-dimensional structures in the RNAs: an intramolecular G-quartet and an intricate tertiary structure termed an FMRP-kissing complex [11, 13]. Interactions have also been discovered between FMRP and components of the microRNA pathway in addition to the microRNAs themselves, suggesting a mechanistic link in the regulation of protein synthesis [14, 15••]. Current theories suggest that FMRP is involved in the control of local translation within dendrites in response to synaptic activity, and that loss of FMRP results in defects in protein synthesis-dependent plasticity [16].
Two key aspects of FMR1 biology are in need of further mechanistic insight. One involves the FMR1 promoter and the process of transcriptional silencing in the full mutation and transcriptional enhancement in the premutation. The second is the precise function of FMRP in the neuron and the neuronal consequence of its loss in fragile X syndrome.
In this review, we focus on recent advances in our understanding of the transcription of the FMR1 gene and the influence of FMRP on translation.
Section snippets
FMR1 promoter function and chromatin structure
Loss of transcription of FMR1 in fragile X syndrome is the best understood of the FMR1-related disease processes. Repeat expansion results in cytosine methylation of the repeats in addition to the CpG island in the promoter. It appears that the full mutation-bearing FMR1 is recognized as repeated DNA and subjected to ‘heterochromatinization’, much as is transposon or centromeric DNA. Interestingly, it has been shown that long CGG-repeat tracts, as RNA, are substrates for the binding of the
FMRP and translational control
Synaptic transmission occurs at the dendritic spines, and in individuals with fragile X syndrome these spines are abnormally long and appear to be immature [42, 43]. This has led to the notion that FMRP is involved in synaptic maturation and spine-pruning. In the dendritic spines, long-term potentiation (LTP) and long-term depression (LTD) — two forms of synaptic plasticity — are triggered by synaptic activity through processes that require local protein synthesis [44]. The messages that are
FMRP and miRNAs work together to inhibit translation
MicroRNAs (miRNAs) are a class of non-coding RNAs that control translation of their target mRNAs by base-pairing with partially complementary transcript sequences [59]. miRNAs use the RNA-induced silencing complex (RISC) to effect their function. FMRP interacts with the Argonaute proteins AGO1 and AGO2, which are components of RISC, and with miRNAs [14, 15••, 60], so it has been proposed that the translational suppression associated with FMRP occurs through miRNAs. In support of this idea, AGO1
Current model of FMRP function
Synthesizing the current data on translational suppression by FMRP, we propose a model in which FMRP is transported into the nucleus, where it associates with specific RNA transcripts and forms messenger ribonucleoprotein complexes. These complexes are transported out of the nucleus, enabling them to interact with components of the RISC complex, thereby inhibiting the translation of the messages therein. Using kinesin as its motor, these translationally silent complexes can be transported to
Conclusions
We now understand in detail the consequence of repeat-mediated transcriptional shut-off of FMR1, and this knowledge, especially in comparison with transcriptional upregulation of premutation-sized repeats at this locus, will add to our basic understanding of gene regulation. Because this transcriptional shut-off causes fragile X syndrome through the loss of a single protein, it places FMRP in a central role for learning and memory. Synaptic plasticity requires tightly controlled and highly
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
Acknowledgements
This research is supported by National Institutes of Health grants HD35576, HD20521 and HD24064. We apologize for work that could not be cited owing to space limitations.
References (65)
- et al.
The fragile-X premutation: a maturing perspective
Am J Hum Genet
(2004) - et al.
RNA-mediated neurodegeneration caused by the fragile X premutation rCGG repeats in Drosophila
Neuron
(2003) - et al.
FMRP associates with polyribosomes as an mRNP, and the I304N mutation of severe fragile X syndrome abolishes this association
Mol Cell
(1997) - et al.
Microarray identification of FMRP-associated brain mRNAs and altered mRNA translational profiles in fragile X syndrome
Cell
(2001) - et al.
Fragile X mental retardation protein targets G quartet mRNAs important for neuronal function
Cell
(2001) - et al.
RNA cargoes associating with FMRP reveal deficits in cellular functioning in Fmr1 null mice
Neuron
(2003) - et al.
The fragile X syndrome repeats form RNA hairpins that do not activate the interferon-inducible protein kinase, PKR, but are cut by dicer
Nucleic Acids Res
(2003) - et al.
Synergistic effect of histone hyperacetylation and DNA demethylation in the reactivation of the FMR1 gene
Hum Mol Genet
(1999) - et al.
Sp1 and AP2 transcription factors are required for the human fragile mental retardation promoter activity in SK-N-SH neuronal cells
Neurosci Lett
(1999) - et al.
FMR1 enhancer is regulated by cAMP through a cAMP-responsive element
DNA Cell Biol
(1997)
Occupancy and synergistic activation of the FMR1 promoter by Nrf-1 and Sp1 in vivo
Hum Mol Genet
Is nuclear respiratory factor 2 a master transcriptional coordinator for all ten nuclear-encoded cytochrome c oxidase subunits in neurons?
Gene
Differential translation and fragile X syndrome
Genes Brain Behav
Brahma links the SWI/SNF chromatin-remodeling complex with MeCP2-dependent transcriptional silencing
Nat Genet
Group I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to cyclic AMP response element binding protein (CREB) through a common Ca2+ - and protein kinase C-dependent pathway
J Neurochem
Fragile X mental retardation protein is necessary for neurotransmitter-activated protein translation at synapses
Proc Natl Acad Sci USA
Fragile X syndrome: (what's) lost in translation?
Proc Natl Acad Sci USA
Biochemical evidence for the association of fragile X mental retardation protein with brain polyribosomal ribonucleoparticles
Proc Natl Acad Sci USA
Synaptic protein synthesis associated with memory is regulated by the RISC pathway in Drosophila
Cell
A brain-specific microRNA regulates dendritic spine development
Nature
The fragile X mental retardation protein and group I metabotropic glutamate receptors regulate levels of mRNA granules in brain
Proc Natl Acad Sci USA
Localization of FMRP-associated mRNA granules and requirement of microtubules for activity-dependent trafficking in hippocampal neurons
Genes Brain Behav
Fragile X syndrome
Am J Med Genet C Semin Med Genet
The FMR1 CGG repeat mouse displays ubiquitin-positive intranuclear neuronal inclusions; implications for the cerebellar tremor/ataxia syndrome
Hum Mol Genet
Evidence that fragile X mental retardation protein is a negative regulator of translation
Hum Mol Genet
The fragile X mental retardation protein inhibits translation via interacting with mRNA
Nucleic Acids Res
Postadolescent changes in regional cerebral protein synthesis: an in vivo study in the FMR1 null mouse
J Neurosci
Kissing complex RNAs mediate interaction between the fragile-X mental retardation protein KH2 domain and brain polyribosomes
Genes Dev
Fragile X-related protein and VIG associate with the RNA interference machinery
Genes Dev
Biochemical and genetic interaction between the fragile X mental retardation protein and the microRNA pathway
Nat Neurosci
The mGluR theory of fragile X mental retardation
Trends Neurosci
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