Tetrahelical forms of the fragile X syndrome expanded sequence d(CGG)(n) are destabilized by two heterogeneous nuclear ribonucleoprotein-related telomeric DNA-binding proteins.
(25/907)
Formations of hairpin and tetrahelical structures by the trinucleotide repeat sequence d(CGG)(n) might contribute to its expansion in fragile X syndrome. Here we show that tetraplex structures of d(CGG)(n) are destabilized by two mammalian heterogeneous nuclear ribonucleoprotein-related tetraplex telomeric DNA-binding and -stabilizing proteins, quadruplex telomeric DNA-binding protein 42 (qTBP42) (Sarig, G., Weisman-Shomer, P., Erlitzki, R., and Fry, M. (1997) J. Biol. Chem. 272, 4474-4482) and unimolecular quadruplex telomeric DNA-binding protein 25 (uqTBP25) (Erlitzki, R., and Fry, M. (1997) J. Biol. Chem. 272, 15881-15890). Blunt-ended and 3'-tailed or 3'- and 5'-tailed bimolecular tetraplex structures of d(CGG)(n) and guanine-sparse 20-/46-mer partial DNA duplex were progressively destabilized by increasing amounts of qTBP42 or uqTBP25 in time-dependent and ATP- or Mg(2+)-independent reactions. By contrast, tetraplex structures of telomeric and IgG sequences or guanine-rich double-stranded DNA resisted destabilization by qTBP42 or uqTBP25. Increased stability of tetraplex d(CGG)(n) in the presence of K(+) or Na(+) ions or at lowered reaction temperature diminished the destabilizing activity of uqTBP25. The contrasting stabilization of tetraplex telomeric DNA and destabilization of tetraplex d(CGG)(n) by qTBP42 and uqTBP25 suggested that sequence or structural differences between these tetraplexes might serve as cues for the differential stabilizing/destabilizing activities. (+info)
Sequence-specific RNA binding by a Nova KH domain: implications for paraneoplastic disease and the fragile X syndrome.
(26/907)
The structure of a Nova protein K homology (KH) domain recognizing single-stranded RNA has been determined at 2.4 A resolution. Mammalian Nova antigens (1 and 2) constitute an important family of regulators of RNA metabolism in neurons, first identified using sera from cancer patients with the autoimmune disorder paraneoplastic opsoclonus-myoclonus ataxia (POMA). The structure of the third KH domain (KH3) of Nova-2 bound to a stem loop RNA resembles a molecular vise, with 5'-Ura-Cyt-Ade-Cyt-3' pinioned between an invariant Gly-X-X-Gly motif and the variable loop. Tetranucleotide recognition is supported by an aliphatic alpha helix/beta sheet RNA-binding platform, which mimics 5'-Ura-Gua-3' by making Watson-Crick-like hydrogen bonds with 5'-Cyt-Ade-3'. Sequence conservation suggests that fragile X mental retardation results from perturbation of RNA binding by the FMR1 protein. (+info)
Imprinting effect in premature ovarian failure confined to paternally inherited fragile X premutations.
(27/907)
Fragile X premutations are considered to be a risk factor for premature ovarian failure (POF), which is usually defined as menopause at age <40 years. Since premutations may be inherited from either the mother or the father, we evaluated the influence of the inheritance pattern on the duration of reproductive life in female carriers. The occurrence of POF and age at menopause in women with a paternally inherited fragile X premutation (PIP) were compared to those in women with a maternally inherited fragile X premutation (MIP). We identified 148 women in whom the parental origin of the premutation could be determined. In 109 of these women we were able to establish whether POF had occurred: 82 women had a PIP, and 27 had a MIP. Twenty-three of the women (28%) with a PIP had POF, versus only 1 (3.7%) with a MIP (two -tailed Fisher's exact test; P=. 007). Kaplan-Meier analysis of all 148 premutations showed that the age at menopause was significantly lower in the women with a PIP than in the woman with a MIP (Breslow test in Kaplan-Meier analysis; P=.003). Our data strongly suggest that, when POF occurs in fragile X premutation carriers, a considerable proportion of the premutations are inherited paternally (parent-of-origin effect). We hypothesize that this may be owing to a paternal genomic imprinting effect. (+info)
A unique form of mental retardation with a distinctive phenotype maps to Xq26-q27.
(28/907)
We report a novel X-linked mental retardation (XLMR) syndrome, with characteristic facial dysmorphic features, segregating in a large North Carolina family. Only males are affected, over four generations. Clinical findings in the seven living affected males include a moderate degree of mental retardation (MR), coarse facies, puffy eyelids, narrow palpebral fissures, prominent supraorbital ridges, a bulbous nose, a prominent lower lip, large ears, obesity, and large testicles. Cephalometric measurements suggest that the affected males have a distinctive craniofacial skeletal structure, when compared with normative measures. Obligate-carrier females are unaffected with MR, but the results of cephalometric skeletal analysis suggest craniofacial dysmorphisms intermediate between affected males and normative control individuals. Unaffected male relatives show no clinical or cephalometric resemblance to affected males. The blood-lymphocyte karyotype and the results of DNA analysis for fragile-X syndrome and of other routine investigations are normal. Linkage analysis for polymorphic DNA markers spanning the X chromosome established linkage to Xq26-q27. Maximum LOD scores were obtained at marker DXS1047 (maximum LOD score = 3.1 at recombination fraction 0). By use of haplotype analysis, we have localized the gene for this condition to an 18-cM genetic interval flanked by ATA59C05 and GATA31E08. On the basis of both the clinical phenotype and the mapping data, we were able to exclude other reported XLMR conditions. Therefore, we believe that a unique recessive XLMR syndrome with a distinctive and recognizable phenotype is represented in this family. (+info)
Survey of the fragile X syndrome CGG repeat and the short-tandem-repeat and single-nucleotide-polymorphism haplotypes in an African American population.
(29/907)
Previous studies have shown that specific short-tandem-repeat (STR) and single-nucleotide-polymorphism (SNP)-based haplotypes within and among unaffected and fragile X white populations are found to be associated with specific CGG-repeat patterns. It has been hypothesized that these associations result from different mutational mechanisms, possibly influenced by the CGG structure and/or cis-acting factors. Alternatively, haplotype associations may result from the long mutational history of increasing instability. To understand the basis of the mutational process, we examined the CGG-repeat size, three flanking STR markers (DXS548-FRAXAC1-FRAXAC2), and one SNP (ATL1) spanning 150 kb around the CGG repeat in unaffected (n=637) and fragile X (n=63) African American populations and compared them with unaffected (n=721) and fragile X (n=102) white populations. Several important differences were found between the two ethnic groups. First, in contrast to that seen in the white population, no associations were observed among the African American intermediate or "predisposed" alleles (41-60 repeats). Second, two previously undescribed haplotypes accounted for the majority of the African American fragile X population. Third, a putative "protective" haplotype was not found among African Americans, whereas it was found among whites. Fourth, in contrast to that seen in whites, the SNP ATL1 was in linkage equilibrium among African Americans, and it did not add new information to the STR haplotypes. These data indicate that the STR- and SNP-based haplotype associations identified in whites probably reflect the mutational history of the expansion, rather than a mutational mechanism or pathway. (+info)
The human 20-kDa 5'-(CGG)(n)-3'-binding protein is targeted to the nucleus and affects the activity of the FMR1 promoter.
(30/907)
Previous reports have described the human DNA CGG repeat-binding protein (CGGBP1 or p20), which binds specifically to nonmethylated, but not to methylated, 5'-(CGG)(n)-3' repeats in the promoter of the fragile X mental retardation 1 (FMR1) gene. The results of transfection experiments into human HeLa cells using a p20-green fluorescent protein fusion construct indicate that the p20 protein is targeted to the nucleus. By deletion analyses, a nuclear localization signal has been found between amino acids 80 and 84. Deletions between amino acids 69 and 71 and between 95 and 167 interfere with 5'-(CGG)(n)-3' binding. The results of electrophoretic mobility shift assays using DNA with 5'-(CGG)(n)-3' repeats of different lengths render it likely that oligomers of the p20 protein bind to the repeat. In cotransfection experiments, the activity of the FMR1 promoter is reduced by the presence of p20. Upon transfection of the p20 cDNA construct into HeLa cells, transcription of the endogenous FMR1 gene is decreased. The green fluorescent protein-p20 fusion protein associates preferentially with the telomeres of the short arms of human chromosomes 13, 14, 15, 21, and 22. Their telomeres carry the genes for the 28 S rRNA, which contain 5'-(CGG)(n)-3' repeats. The translated region of the p20 gene from three healthy, five fragile X syndrome, and five premutation-carrying individuals has been sequenced, but mutations have not been detected. (+info)
Identification and evaluation of mental retardation.
(31/907)
Mental retardation in young children is often missed by clinicians. The condition is present in 2 to 3 percent of the population, either as an isolated finding or as part of a syndrome or broader disorder. Causes of mental retardation are numerous and include genetic and environmental factors. In at least 30 to 50 percent of cases, physicians are unable to determine etiology despite thorough evaluation. Diagnosis is highly dependent on a comprehensive personal and family medical history, a complete physical examination and a careful developmental assessment of the child. These will guide appropriate evaluations and referrals to provide genetic counseling, resources for the family and early intervention programs for the child. The family physician is encouraged to continue regular follow-up visits with the child to facilitate a smooth transition to adolescence and young adulthood. (+info)
Interruption of the fragile X syndrome expanded sequence d(CGG)(n) by interspersed d(AGG) trinucleotides diminishes the formation and stability of d(CGG)(n) tetrahelical structures.
(32/907)
Fragile X syndrome is caused by expansion of a d(CGG) trinucleotide repeat sequence in the 5' untranslated region of the first exon of the FMR1 gene. Repeat expansion is thought to be instigated by formation of d(CGG)(n)secondary structures. Stable FMR1 d(CGG)(n)runs in normal individuals consist of 6-52 d(CGG) repeats that are interrupted every 9-11 triplets by a single d(AGG) trinucleotide. By contrast, individuals having fragile X syndrome premutation or full mutation present >54-200 or >200-2000 monotonous d(CGG) repeats, respectively. Here we show that the presence of interspersed d(AGG) triplets diminished in vitro formation of bimolecular tetrahelical structures of d(CGG)(18)oligomers. Tetraplex structures formed by d(CGG)(n)oligomers containing d(AGG) interspersions had lower thermal stability. In addition, tetraplex structures of d(CGG)(18)oligomers interspersed by d(AGG) triplets were unwound by human Werner syndrome DNA helicase at rates and to an extent that exceeded the unwinding of tetraplex form consisting of monotonous d(CGG)(18). Diminished formation and stability of tetraplex structures of d(AGG)-containing FMR1 d(CGG)(2-50)tracts might restrict their expansion in normal individuals. (+info)