Which of the following is not true for Fragile X syndrome?
**Core Concept**
Fragile X syndrome is a genetic disorder caused by an expansion of the CGG repeat in the FMR1 gene on the X chromosome. This expansion leads to methylation and silencing of the gene, resulting in the absence of the fragile X mental retardation protein (FMRP). FMRP plays a crucial role in regulating protein synthesis and synaptic plasticity in the brain.
**Why the Correct Answer is Right**
Fragile X syndrome is characterized by a range of physical and developmental symptoms, including intellectual disability, autism, and distinctive physical features. The expansion of the CGG repeat is a key factor in the pathogenesis of the disorder, leading to the silencing of the FMR1 gene and the absence of FMRP. This absence disrupts normal protein synthesis and synaptic function in the brain, contributing to the cognitive and behavioral symptoms of the disorder.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Fragile X syndrome is indeed caused by an expansion of the CGG repeat in the FMR1 gene. The expansion leads to methylation and silencing of the gene, resulting in the absence of FMRP.
**Option B:** This option is incorrect because individuals with Fragile X syndrome often exhibit intellectual disability and cognitive impairment. The absence of FMRP disrupts normal protein synthesis and synaptic function in the brain, contributing to these symptoms.
**Option C:** This option is incorrect because Fragile X syndrome is a genetic disorder caused by an expansion of the CGG repeat in the FMR1 gene. This expansion is a key factor in the pathogenesis of the disorder.
**Option D:** This option is incorrect because Fragile X syndrome is indeed associated with autism and autism spectrum disorders. The absence of FMRP disrupts normal synaptic function and plasticity in the brain, contributing to these symptoms.
**Clinical Pearl / High-Yield Fact**
The CGG repeat expansion in the FMR1 gene is a key diagnostic feature of Fragile X syndrome. A repeat expansion of 55 or more CGG repeats is considered a full mutation, while a repeat expansion of 45-54 CGG repeats is considered a premutation. The premutation can lead to fragile X-associated tremor/ataxia syndrome (FXTAS) in older individuals.
**Correct Answer: D. Fragile X syndrome is not typically associated with seizures as a primary symptom.**