What is the primary disorder characterized by GAA trinucleotide repeat expansion?

The primary disorder associated with GAA trinucleotide repeat expansion is Friedrich ataxia. This genetic condition is caused by an expanded GAA repeat within the FXN gene on chromosome 9, which leads to a deficiency of the frataxin protein. Frataxin is crucial for mitochondrial function and iron regulation; its deficit results in neurodegenerative symptoms, particularly progressive ataxia, loss of deep tendon reflexes, and scoliosis, starting in childhood or early adulthood.

In contrast, Huntington's disease is associated with CAG repeat expansions, Fragile X syndrome is connected with CGG repeat expansions, and myotonic dystrophy involves an expansion of CTG repeats. Each of these disorders has distinct genetic mechanisms and clinical presentations, which helps to clarify why Friedrich ataxia is the correct choice for the disorder characterized by GAA repeat expansions.