Fragile X premutation and associated health conditions: A review.

Fragile X syndrome (FXS) is the most common single gene disorder, which causes autism and intellectual disability. The fragile X mental retardation 1 (FMR1) gene is silenced when cytosine-guanine-guanine (CGG) triplet repeats exceed 200, which is the full mutation that causes FXS. Carriers of FXS have a CGG repeat between 55 and 200, which is defined as a premutation and transcription of the gene is overactive with high levels of the FMR1 mRNA. Most carriers of the premutation have normal levels of fragile X mental retardation protein (FMRP) and a normal intelligence, but in the upper range of the premutation (120-200) the FMRP level may be lower than normal. The clinical problems associated with the premutation are caused by the RNA toxicity associated with increased FMR1 mRNA levels, although for some mildly lowered FMRP can cause problems associated with FXS. The RNA toxicity causes various health problems in the carriers including but not limited to fragile X-associated tremor/ataxia syndrome, fragile X-associated primary ovarian insufficiency, and fragile X-associated neuropsychiatric disorders. Since some individuals with neuropsychiatric problems do not meet the severity for a diagnosis of a "disorder" then the condition can be labeled as fragile X premutation associated condition (FXPAC). Physicians must be able to recognize these health problems in the carriers and provide appropriate management.