Prenatal Fragile X testing

The most common inherited cause of mental retardation affecting
approximately 1 in 4000–6000 male and 1 in 8000–10,000 female.
The syndrome is associated with an abnormality of the
Fragile X Mental Retardation 1 (FMR1) gene located in a fragile site at Xq27.3. This alteration originates from an unstable expansion of CGG trinucleotide repeats within the 5′untranslated region of the FMR1 gene.
The normal Individuals have
approximately 5 to 40 CGG repeatsand individuals who are carriers for FXS have 55 to 200 CGG repeatswhich is classified
as premutation, it is associated with pathological conditions that premature ovarian failurein females and fragile X- associated tremor/ataxia syndrome (FXTAS) in males.
Also, the FXS is characterized by the CGG full mutation and
methylation of the cytosine bases, which leads to silencing of transcription and abnormal or absence of the encoded Fragile
X mental retardation protein (FMRP). FMRP is a major protein regulator of the translation of many mRNAs involved in
synaptic plasticity. Since the mutation is X-linked, males are more severely affected than females and males with FXS
predominantly exhibit mental retardation, autism, and behavioral alteration.FXS inheritance depends on the number of
CGG repeats within the promoter of the FMR1 gene and transition from the premutation to full mutationallele can occur
because of the expansion phenomenon during the transmission of the maternal X chromosome and rarely the paternal X
chromosome carrying a premutation allele to offspring (1-3).