American Board of Psychiatry and Neurology (ABPN) Practice Exam

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What type of genetic mutation is associated with fragile X syndrome?

  1. Translocation

  2. Duplication

  3. Microdeletion

  4. Trinucleotide repeat expansion

The correct answer is: Trinucleotide repeat expansion

Fragile X syndrome is specifically associated with a trinucleotide repeat expansion mutation. In this condition, there is an expansion of the CGG repeat within the FMR1 gene located on the X chromosome. In individuals who are affected by fragile X syndrome, this repeat can exceed 200 copies, leading to methylation of the gene and subsequent silencing, which prevents the production of the FMRP protein essential for normal brain development and functioning. This type of mutation is distinct in that it involves an increase in a specific trinucleotide sequence, which can further lead to genetic instability as it is passed through generations. This results in varying degrees of expression of the disorder, with females often showing a wide range of cognitive abilities, while males typically exhibit more pronounced intellectual disabilities. Understanding this mechanism is crucial, as it differentiates fragile X syndrome from other genetic disorders that may arise from different types of genetic mutations, such as translocations, duplications, or microdeletions. Each of these other mutation types has its own unique set of implications and pathophysiological mechanisms, which are not characteristic of fragile X syndrome.