الفهرس 
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Abstract
The Fragile X Syndrome (FXS) is a genetic disease inherited through the X chromosome. It is the most common inherited cause of intellectual disability. It prevalence is now estimated to be 1 in 4000 males and 1 in 6000-8000 females and accounts for 10-12% of XLMR. FXS is caused by deficiency in the FMRP.
The most common mechanism ( 99% ) for this deficiency is an expansion of a CGG-rich triplet repeat sequence in 5’ UTR of the FMR1gene at Xq27.3 to full mutation (> 200 CGG repeats) which leads to its hypermethylation and silencing of gene transcription. Repeats ranging from 55-200 repeats are termed premutations. Carriers of premutation do not have FXS but affected by other medical problems as fragile X tremor/ataxia syndrome (FXTAS) especially in males and fragile X premature ovarian insufficiency (FXPOI) in females.
The prevalence of the premutation in the general population is 1:130-200 women and 1:250 to 450 men. Tremor/Ataxia Syndrome occurs in approximately 40% of men with the premutation and 16% of women, whereas FXPOI occurs in approximately 16 to 20% of women with the permutation
FXS is associated with a variety of neurological, cognitive and behavioral deficits, and less frequent dysmorphic features. Females with FXS are typically less affected than males because they have a second X chromosome with production of some FMRP. The phenotype of FXS is quite variable. The main and most consistent clinical feature is intellectual disability, whereas physical and behavioral features may vary with age and gender. Thus, the clinical diagnosis of FXS is difficult and requires a confirmatory molecular laboratory testing.
FMRP, a major negative translation regulator and widely expressed in various tissues, with the highest levels found in the brain and the testis. Although the exact role of FMRP is still largely unknown in other cell types, the evidence accumulated to date strongly suggests that one significant role of FMRP in neurons is to regulate synaptic plasticity via the transport and translation of specific mRNAs at dendritic spines. At the cellular level, FXS is associated with immature dendritic spine morphology.
The study was conducted on 56 cases with unexplained ID and with Hagerman 13-item checklist score more than 10, selected from 230 cases with ID after exclusion of other known causes of ID.
All patients were subjected to thorough history taking, detailed clinical examination, scoring with Hagerman 13-item checklist, IQ assessment, cytogenetic testing (both conventional and folate deficient) and testing by the betaine-based expanded long range PCR.
Cytogenetic results showed that;
All included patients showed normal showed normal karyotype and none had Fragile site on the X chromosome.