FRAXA coincides with a >200 CGG*CCG repeat tract in the 5' UTR of the FMR1 gene, and alleles prone to fragility are associated with Fragile X (FX) syndrome, one of the leading genetic causes of intellectual disability.
Multiplex XLMR pedigrees have been reported with only one mutated patient having autism and MR: different X-located MR genes have been shown to be involved (NLGN4, MECP2, OPHN1, ZNF674 and FRAXA) which does not suggest that they could be "autism genes".
Fragile X syndrome, the second most common genetic cause of mental retardation, is due to the expansion of a trinucleotide repeat (CGG)n within the first exon of the FMR-1 gene.
From the clinical point of view, the folate sensitive rare fragile site FRAXA is the most important fragile site as it is associated with the fragile X syndrome, the most common form of familial mental retardation, affecting about 1/4000 males and 1/6000 females.
In contrast, a relationship between the second main group of fragile sites characterized by repeat expansion, the rare fragile sites, and mental retardation has been proposed many years ago, but after the molecular cloning of FRAXA and FRAXE both unequivocally involved in mental retardation, no additional fragile sites linked with mental retardation have been cloned for over a decade.
In a second series, we screened 140 patients with MR and behaviour disturbance who did not fulfil the de Vries criteria for subtelomeric rearrangements and who had a normal karyotype and no detectable FRAXA mutation.
Determination of the CGG repeat number was achieved by polymerase chain reaction (PCR) on modified DNA from 129 unrelated Mexican mestizos (46 FRAXA-negative males with mental retardation and 83 healthy individuals).
Here we report molecular screening survey of 97 unrelated individuals diagnosed with non-specific mental retardation (MR), which produced positive test for FRAXA in two boys and none positive for the FRAXE mutation.
We have investigated a population consisted of 276 males with idiopathic mental retardation or learning disability and a control sample of 207 non-affected boys in order to determine if there was a possible phenotype consequence of the expanded unmethylated alleles for FRAXA/FRAXE loci.
Three individuals (2.6%) with the FRAXA form of the fragile X syndrome and one boy (0.9%) with FRAXE mental retardation were detected; a total of four newly diagnosed fragile X families were identified.
Fragile X syndrome linked to the FRAXA locus is the most common inherited genetic disease accounting for mental retardation and is usually caused by the expansion of an unstable CGG repeat in the first exon of the FMR1 gene on the X chromosome.
In view of these data, we screened MECP2 in a cohort of 185 patients found negative for the expansions across the FRAXA CGG repeat and reported the identification of mutations in four sporadic cases of MR. One of the mutations, A140V, which we found in two patients, has been described previously, whereas the two others, P399L and R453Q, are novel mutations.
The FRAXE fragile site, 600 kb distal to the more common FRAXA, has been reported to be expressed in subjects with mild non-syndromal mental retardation (MR).
The aim of this population screening study was to determine if Fra-X or FRAXE mutations are the cause of a number of cases of mental retardation in a sample of Mexican children with mental retardation of unknown cause (MRUC) and to stress the importance of performing molecular analysis of the FMR-1 gene in all patients with MRUC.
We have analysed the size of the non-expanded FRAXA CGG repeat in 385 male patients affected by mental retardation and in 182 unrelated normal chromosomes as control.
The FRAXE fragile site, 600 Kb distal to the more common FRAXA, has been reported to be expressed in subjects with mild nonsyndromal mental retardation.
The purpose of the study was to estimate the frequency of FRAXA mutation in individuals with nonspecific mental retardation without family history and phenotypic stigmata in the Hellenic population.
Expansion of a (CCG)n repeat in the FMR2 gene corresponds to the FRAXE fragile site which lies distal to FRAXA and is also associated with mental retardation, but it is less frequent and lacks a consistent phenotype.
To date, seven diseases have been identified as expanded repeat disorders: the fragile X syndrome of mental retardation both FRAXA and FRAXE loci), myotonic dystrophy, X-linked spinal and bulbar muscular atrophy, Huntington's disease, spinocerebellar ataxia type I, dentatorubral-pallidoluysian atrophy, and Machado-Joseph disease.
The FMR-1 gene for the human fragile-chi syndrome, a mental retardation disease inherited by non-Mendelian transmission, contains a genetically unstable CGG region in the 5' non-translated region.