Clinical Information

Fragile X syndrome is an X-linked disorder with variable expression in males and females. It is caused by an expansion of the CGG trinucleotide repeat in the FMR1 gene, located on the X chromosome. This trinucleotide repeat is polymorphic in the general population, with the number of repeats ranging from 5 to 44. These normal alleles are passed from generation to generation with the number of repeats remaining constant. Small expansions, called premutations, most often range from 59 to 200 CGG repeats. Premutation carriers do not exhibit features of fragile X syndrome, but are at risk for other FMR1-related disorders such as fragile X tremor/ataxia syndrome (FXTAS) and premature ovarian failure (POF). Transmission of a premutation by a male to his daughter usually results in little or no change in the CGG repeat number. Transmission of a premutation by a female to her son or daughter usually results in further expansion, either to a larger premutation or a full mutation. The risk for a female premutation carrier to have a child affected with fragile X syndrome by expansion to a full mutation increases with the number of CGG repeats in the premutation. Full mutations can be 200 to thousands of repeats long, and are associated with abnormal methylation of a region adjacent to the FMR1 gene. This is thought to interfere with normal FMR1 gene expression, resulting in fragile X syndrome. There are multiple clinical phenotypes associated with expansion (premutations and full mutations) in the FMR1 gene.

Fragile X Syndrome:

Approximately 1/4000 individuals (male and female) are affected with fragile X syndrome. Most affected males exhibit moderate mental retardation, with affected females having milder (if any) cognitive deficiency. Neuropsychiatric diagnoses such as autism spectrum and anxiety disorders are common. Characteristic physical features include a long face with prominent jaw, protruding ears, connective tissue abnormalities, and large testicles in postpubertal males.

Fragile X Tremor/Ataxia Syndrome (FXTAS):

FXTAS is a neurodegenerative disorder that is clinically distinct from fragile X syndrome. Both male and female premutation carriers are at risk for FXTAS. However, the disorder is much less common, milder in presentation, and shows a later age of onset in females. Clinical hallmarks of the disorder include intention tremor, gait ataxia, dementia, and neuropsychiatric symptoms. The risk for FXTAS increases as the number of CGG repeats increases, and the majority of individuals with FXTAS have CGG repeat expansions of 70 or more. Penetrance of clinical symptoms is associated with increasing age, with the majority of affected males showing symptoms between age 70 and 90.

Premature Ovarian Failure (POF):

Female premutation carriers are at risk for increased follicular stimulating hormone (FSH) levels, early menopause, and POF. Penetrance and early onset of female reproductive symptoms correlates with increasing size of the CGG repeat, and reaches its highest penetrance at approximately 80 to 90 repeats. Of note, penetrance actually remains stable or may even decrease at approximately 100 repeats. There is no risk for increased penetrance of the POF phenotype due to maternal or paternal inheritance of the expanded CGG repeat.