Fragile X Syndrome

Fragile X Syndrome (FXS) is a hereditary neurodevelopmental disorder linked to the X chromosome and the most common inherited cause of intellectual disability. It is the second most frequent cause of intellectual disability overall, following Down Syndrome. The genetic etiology involves an increase in the number of CGG trinucleotide repeats in the Fragile X Mental Retardation 1 (FMR1) gene located at the q27.3 region of the X chromosome. When these repeats exceed 200, the FMR1 gene becomes silenced through methylation, leading to suppression of FMRP (Fragile X Mental Retardation Protein) production. Due to the critical role of FMRP in synaptic plasticity and neuronal development, its absence or deficiency results in the characteristic neurological and behavioral symptoms of the syndrome. The phenotypic presentation of the syndrome can vary across generations and by sex.

The prevalence of Fragile X Syndrome is approximately 1 in 3600 in males and ranges from 1 in 4000 to 1 in 6000 in females. Premutation carrier frequency is reported at 1 in 130–200 in females and 1 in 250–450 in males. Although prevalence varies by geographic region, screening studies in Europe, America, and Asia have found FXS rates among individuals with intellectual disability ranging from 0.9% to 6%.

The FMR1 gene is located at the Xq27.3 locus and contains CGG trinucleotide repeat sequences ranging from 6 to 44 in normal individuals. Repeats numbering between 55 and 200 are classified as a premutation, while those exceeding 200 are defined as a full mutation. Expansion of CGG repeats leads to transcriptional silencing of the gene, mediated by epigenetic mechanisms such as DNA methylation and histone modification.

FMRP is an RNA-binding protein that regulates the translation of target mRNAs in neuronal cells, thereby controlling synaptic protein synthesis. It plays a vital role in brain development and synaptic plasticity. Deficiency of FMRP results in excessive protein synthesis at synaptic connections, leading to neurodevelopmental abnormalities.

In males with a full mutation, typical phenotypic features include a long and narrow face, large ears, protruding jaw, macroorchidism, joint hypermobility, flat feet, and mitral valve prolapse. Neurological manifestations include intellectual disability, speech delay, autism spectrum disorder (25%), epilepsy (10–20%), and attention deficit hyperactivity disorder. In the neonatal period, hypotonia and motor delays are notable.

Due to Lyonization (X-chromosome inactivation), symptoms in females carrying a full mutation are generally milder. Approximately 50% of females show no obvious phenotypic features; in the remainder, psychiatric symptoms such as learning difficulties, attention deficits, and social anxiety may be observed. The prevalence of epilepsy is approximately 5%.

Premutation carriers are typically asymptomatic. However, certain age-related syndromes may develop in these individuals:

In female premutation carriers (15–27%), menstruation may cease before age 40. A definitive correlation between CGG repeat number and risk has not been established.

In premutation carriers, particularly males, neurodegenerative features such as intention tremor, cerebellar ataxia, gait disturbances, and dementia may emerge around age 60. The risk of FXTAS in male premutation carriers is approximately 40%.

FXS should be suspected in individuals presenting with intellectual disability, autism spectrum disorder, and macroorchidism. In males, characteristic facial features (long face, large ears) and enlarged testes are notable. Family history and phenotypic findings serve as important diagnostic clues.

GTG banding and fragile site visualization are classical techniques. However, molecular tests offer greater sensitivity for diagnosis and carrier detection.

FXS must be distinguished from syndromes with phenotypic similarities. It may be confused with Sotos Syndrome (tall stature, acromegalic facies), Prader-Willi Syndrome (obesity, hypogonadism), and Klinefelter Syndrome (XXY karyotype). Clinical history, physical examination, and genetic testing aid in differential diagnosis.

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