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Query: UNIPROT:P06889 (Mol)
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Lack of expression of the fragile X mental retardation protein (FMRP) results in mental retardation and macroorchidism, seen as the major pathological symptoms in fragile X patients. FMRP is a cytoplasmic RNA-binding protein which cosediments with the 60S ribosomal subunit. Recently, two proteins homologous to FMRP were discovered: FXR1 and FXR2. These novel proteins interact with FMRP and with each other and they are also associated with the 60S ribosomal subunit. Here, we studied the expression pattern of the three proteins in brain and testis by immunohistochemistry. In adult brain, FMR1, FXR1 and FXR2 proteins are coexpressed in the cytoplasm of specific differentiated neurons only. However, we observed a different expression pattern in fetal brain as well as in adult and fetal testis, suggesting independent functions for the three proteins in those tissues during embryonic development and adult life.
Hum Mol Genet 1997 Aug
PMID:Differential expression of FMR1, FXR1 and FXR2 proteins in human brain and testis. 925 78

The fragile X syndrome results from a transcriptional silencing of the FMR1 gene and the absence of its encoded protein. FMRP is a cytoplasmic RNA-binding protein, whose specific cellular function is still unknown. We present evidence that virtually all detectable cytoplasmic FMRP in mouse NIH 3T3 and human HeLa cells is found strictly in association with mRNA in actively translating polyribosomes. Furthermore, FMRP released from polyribosomes is associated with ribonucleoprotein complexes with sedimentation coefficients of 60-70S and selection on oligo(dT)-cellulose reveals that this association is specific to poly(A)-containing mRNPs. This association with actively translating polyribosomes is not affected by alteration of translational processes induced by serum stimulation and starvation in NIH 3T3 cells, suggesting that FMR1 expression is not cell cycle regulated and that FMRP might have a house-keeping function. FXR2 protein, which is closely related to FMRP, is also detected associated with mRNPs in translating polyribosomes. The results strongly suggest that FMRP might be a mRNA chaperone interacting with mRNP complexes.
Hum Mol Genet 1997 Sep
PMID:The fragile X mental retardation protein is associated with poly(A)+ mRNA in actively translating polyribosomes. 928 83

Genomic methylation patterns of mammals can vary among individuals and are subject to dynamic changes during development. In order to gain a better understanding of this variation, we have analyzed patterns of cytosine methylation within a 200 bp region at the CpG island of the human FMR1 gene from leukocyte DNA. FMR1 is normally methylated during inactivation of the X chromosome in females and it is also methylated and inactivated upon expansion of CGG repeats in fragile-X syndrome. Patterns of methylation (epigenotypes) were determined by the sequencing of bisulfite-treated alleles from normal males and females and alleles from a family of five brothers who are methylation mosaics and are affected to various degrees by the fragile-X syndrome. Our data indicate that: (i) methylation of individual CpG cytosines is strikingly variable in hypermethylated epigenotypes obtained from a single individual, suggesting that maintenance of cytosine methylation is a dynamic process; (ii) methylation of non-CpG cytosines in the region studied may occur but is rare; (iii) mosaicism of methylation in the analyzed fragile-X males is remarkably similar to that found for the active X and inactive X alleles in normal females, suggesting that the methylation mosaicism of some fragile-X males reflects similar on and off states of FMR1 expression that exist in normal females; (iv) hypermethylation is slightly more pronounced on fragile-X alleles than on normal inactive X alleles of females; (v) the general dichotomy of hypo- and hypermethylated alleles persisted over the 5 year period that separated samplings of the fragile-X males; (vi) methylation variability was most pronounced at a consensus binding sequence for the alpha-PAL transcription factor, a sequence that may play a role in regulating expression of FMR1.
Hum Mol Genet 1997 Oct
PMID:Epigenetic variation illustrated by DNA methylation patterns of the fragile-X gene FMR1. 930 55

Fragile X mental retardation syndrome is associated with an expansion of a CGG repeat within the 5'UTR of the first exon of the FMR1 gene, abnormal methylation of the CpG island in the promoter region, and a transcriptional silencing of this gene. We studied transcriptional regulation of the FMR1 gene using protein footprint analysis of the active and inactive gene in vivo . We identified four footprints within the FMR1 promoter region which correspond to consensus binding sites of known transcription factors, alpha-PAL/NRF1, Sp1, H4TF1/Sp1-like and c-myc. These footprints were present in normal cells with a transcriptionally active FMR1 gene. The same footprints were present in different cell types: primary fibroblasts, lymphoblastoid cells and peripheral lymphocytes. However, for the 1.1 kb region analyzed, no footprints were detected in a variety of cell types derived from patients with fragile X syndrome which have a transcriptionally inactive FMR1 gene. A BLAST nucleotide search identified sequence similarities between the region of the FMR1 gene containing the footprints and an analogous region within the promoter region of the gene for the heterogeneous nuclear ribonucleoprotein (hnRNP) A2, a member of a family of ribonucleoproteins implicated in mRNA processing and nuclear-cytoplasm transport. The nucleotide sequences identified in the hnRNP-A2 promoter region correspond to the same consensus binding sites showing DNA-protein interactions in the FMR1 gene. Our previous functional studies and the studies of others demonstrate that FMR proteins, like hnRNP-A2, are also ribonucleoproteins which appear to be involved in mRNA transport. The results from our footprint studies suggest that the expression of the FMR1 gene is regulated by the binding of specific transcription factors to sequence elements in the 5' region of the gene and that this expression may be regulated by elements in common with the hnRNP-A2 gene. Common regulation of these two genes might play an important role in the cooperative processing and transport of mRNA from the nucleus to the translation machinery.
Hum Mol Genet 1997 Nov
PMID:Structural and functional characterization of the human FMR1 promoter reveals similarities with the hnRNP-A2 promoter region. 932 68

In the heterothallic ascomycete Podospora anserina, the mating-type locus is occupied by two mutually exclusive sequences termed mat+ and mat-. The mat+ sequence contains only one gene, FPR1, while the mat- sequence contains three genes: FMR1, SMR1 and SMR2. Previous studies have demonstrated that FPR1 and FMR1 are required for fertilization. Further analyses have led to the hypothesis that mat+ and mat- genes establish a mat+ and mat- nuclear identity, allowing recognition between nuclei of opposite mating type within the syncytial cells formed after fertilization. This hypothesis was based on the phenotypes of strains bearing mutations in ectopic mat genes. Here we present an analysis of mutations in resident mat- genes which suggests that, unlike FMR1 and SMR2, SMR1 is not involved in establishing nuclear identity. In fact, mutations in these two genes impair nuclear recognition, leading to uniparental progeny, while mutations in SMR1 block the sexual process, probably at a step after nuclear recognition. The nuclear identity hypothesis has also been tested through internuclear complementation tests. In these experiments, the mat- mutants were crossed with a mat+ strain carrying the wild-type mat- genes. Our rationale was that internuclear complementation should not be possible for nuclear identity genes: the relevant genes should show nucleus-restricted expression, and diffusion of their products to other nuclei should not occur. This test confirmed that SMR1 is not a bona fide mat gene since it can fulfill its function whatever its location, in either a mat- or a mat+ nucleus, and even when present in both nuclei. SMR2, but not FMR1, behaves like a nuclear identity gene with respect to internuclear complementation tests. A model is proposed that tentatively explains the ambiguous behaviour of the FMR1 gene and clarifies the respective functions of the three mat- proteins.
Mol Gen Genet 1997 Sep
PMID:What is a bona fide mating-type gene? Internuclear complementation of mat mutants in Podospora anserina. 934 8

Fragile X syndrome is the most frequent cause of heritable mental retardation. Most patients have a mutation in the 5' untranslated region of the FMR1 gene, consisting of the amplification of a polymorphic (CGG)nrepeat sequence, and cytogenetically express the folate-sensitive fragile site FRAXA in Xq27.3. Fragile X patients harbour an expanded sequence with >200 CGG repeats (full mutation), accompanied by methylation of most cytosines of the sequence itself and of the upstream CpG island. This abnormal hypermethylation of the promoter suppresses gene transcription, resulting in the absence of the FMR1 protein. Rare individuals of normal intelligence were shown to carry a completely or partially unmethylated full mutation and to express the FMR1 protein. Given this observation and knowing that the open reading frame of the mutated FMR1 gene is intact, we decided to investigate whether its activity could be restored in vitro by inducing DNA demethylation with 5-azadeoxycytidine (5-azadC) in fragile X patients' lymphoblastoid cells. We report that treatment with 5-azadC causes reactivation of fully mutated FMR1 genes with 300-800 repeats, as shown by the restoration of specific mRNA and protein production. This effect correlates with the extent of promoter demethylation, determined by restriction analysis with methylation-sensitive enzymes. These results confirm the critical role of FMR1 promoter hypermethylation in the pathogenesis of the fragile X syndrome, provide an additional explanation for the normal IQ of the rare males with unmethylated full mutations and pave the way to future attempts at pharmacologically restoring mutant FMR1 gene activity in vivo.
Hum Mol Genet 1998 Jan
PMID:In vitro reactivation of the FMR1 gene involved in fragile X syndrome. 938 10

In at least 98% of fragile X syndrome cases, the disease results from expansion of the CGG repeat in the 5' end of FMR1. The use of microsatellite markers in the FMR1 region has revealed a disparity of risk between haplotypes for CGG repeat expansion. Although instability appears to depend on both the haplotype and the AGG interspersion pattern of the repeat, these factors alone do not completely describe the molecular basis for the linkage disequilibrium between normal and fragile X chromosomes, in part due to instability of the marker loci themselves. In an effort to better understand the mechanism of dynamic mutagenesis, we have searched for and discovered a single nucleotide polymorphism in intron 1 of FMR1 and characterized this marker, called ATL1, in 564 normal and 152 fragile X chromosomes. The G allele of this marker is found in 40% of normal chromosomes, in contrast to 83% of fragile X chromosomes. Not only is the G allele exclusively linked to haplotypes over-represented in fragile X syndrome, but G allele chromosomes also appear to transition to instability at a higher rate on haplotypes negatively associated with risk of expansion. The two alleles of ATL1 also reveal a highly significant linkage disequilibrium between unstable chromosomes and the 5' end of the CGG repeat itself, specifically the position of the first AGG interruption. The data expand the number of haplotypes associated with FMR1 and specifically allow discrimination, by ATL1 alleles, of single haplotypes with differing predispositions to expansion. Such haplotypes should prove useful in further defining the mechanism of dynamic mutagenesis.
Hum Mol Genet 1998 Nov
PMID:Re-examination of factors associated with expansion of CGG repeats using a single nucleotide polymorphism in FMR1. 981 38

The fragile X syndrome results from transcriptional silencing of the FMR1 gene and the absence of its encoded FMRP protein. Two autosomal homologues of the FMR1 gene, FXR1 and FXR2, have been identified and the overall structures of the corresponding proteins are very similar to that of FMRP. Using antibodies raised against FXR1P, we observed that two major protein isoforms of relative MW of 78 and 70 kDa are expressed in different mammalian cell lines and in the majority of mouse tissues. In mammalian cells grown in culture as well as in brain extracts, both P78and P70isoforms are associated with mRNPs within translating polyribosomes, similarly to their closely related FMRP homologues. In muscle tissues as well as in murine myoblastic cell lines induced to differentiate into myotubes, FXR1P78and P70isoforms are replaced by novel unpredicted isoforms of 81-84 kDa and a novel FXR1 exon splice variant was detected in muscle RNA. While P81-84isoforms expressed after fusion into myotubes in murine myoblast cell lines grown in culture are associated with polyribosomes, this is not the case when isolated from muscle tissues since they sediment with lower S values. Immunohistochemical studies showed coexpression of FMRP and FXR1P70and P78in the cytoplasm of brain neurons, while in muscle no FMRP was detected and FXR1P81-84were mainly localized to structures within the muscle contractile bands. The complex expression pattern of FXR1P suggests tissue-specific expression for the various isoforms of FXR1 and the differential expression of FMRP and FXR1Ps suggests that in certain types of cells and tissues, complementary functions may be fulfilled by the various FMRP family members.
Hum Mol Genet 1998 Dec
PMID:Novel isoforms of the fragile X related protein FXR1P are expressed during myogenesis. 981 30

Fragile X syndrome is the most common form of inherited mental retardation currently known, associated with a wide range of developmental disabilities in both males and females, caused by a large expansion of a (CGG)n repeat in the first exon of the FMR1 gene. Fragile X syndrome occurs in all racial and ethnic groups, and it is a condition of major epidemiological importance among mentally handicapped males. Therefore, this disease must be considered in the differential diagnosis of any child with developmental delay, mental retardation or learning disability. The fragile X syndrome is due to the shutdown of the FMR1 gene transcription, and the pathogenesis of this syndrome is a consequence of absence of the protein product of the FMR1 gene (FMRP). Since the great majority of fragile X patients have the same type of mutation in a specific location of the gene, molecular analysis is extremely accurate for diagnosis of the disease, and important for genetic counseling of family members. Others genetic disorders are also caused by expanded trinucleotide repeats.
Int J Mol Med 1999 Jun
PMID:Fragile X syndrome (review). 1034 Dec 96

Expanded trinucleotide repeats underlie a growing number of human diseases. The human FMR1 (CGG)(n) array can exhibit genetic instability characterized by progressive expansion over several generations leading to gene silencing and the development of the fragile X syndrome. While expansion is dependent upon the length of uninterrupted (CGG)(n), instability occurs in a limited germ line and early developmental window, suggesting that lineage-specific expression of other factors determines the cellular environment permissive for expansion. To identify these factors, we have established normal- and premutation-length human FMR1 (CGG)(n) arrays in the yeast Saccharomyces cerevisiae and assessed the frequency of length changes greater than 5 triplets in cells deficient in various DNA repair and replication functions. In contrast to previous studies with Escherichia coli, we observed a low frequency of orientation-dependent large expansions in arrays carrying long uninterrupted (CGG)(n) arrays in a wild-type background. This frequency was unaffected by deletion of several DNA mismatch repair genes or deletion of the EXO1 and DIN7 genes and was not enhanced through meiosis in a wild-type background. Array contraction occurred in an orientation-dependent manner in most mutant backgrounds, but loss of the Sgs1p resulted in a generalized increase in array stability in both orientations. In contrast, FMR1 arrays had a 10-fold-elevated frequency of expansion in a rad27 background, providing evidence for a role in lagging-strand Okazaki fragment processing in (CGG)(n) triplet repeat expansion.
Mol Cell Biol 1999 Aug
PMID:Stability of the human fragile X (CGG)(n) triplet repeat array in Saccharomyces cerevisiae deficient in aspects of DNA metabolism. 1040 56


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