The Future of Fragile X Syndrome: CDC Stakeholder Meeting Summary
Fragile X syndrome (FXS) is the most common known inherited cause of intellectual disability (ID). Males and females with FXS exhibit a wide range of intellectual ability and may experience various degrees of emotional, behavioral, sensory, learning, and social difficulties. In 1991, the gene responsible for FXS was identified on the X chromosome at q27.3 and named fragile x mental retardation 1 ( FMR1 ) gene.1 FXS and fragile X–associated disorders (FXD) are caused by a trinucleotide repeat (CGG) expansion mutation in the promoter region (exon 1) of FMR1 . Affected individuals with the full FXS mutation have >200 repeats. When the full mutation is present, FMR1 methylation occurs during gestation, which causes silencing of gene transcription.2 This in turn leads to a reduction or absence of fragile X mental retardation protein (FMRP), which is needed for brain development and function. Most males with FXS have ID. A small number of males have less impaired function due to methylation patterns or mosaicism. In females, FMRP levels depend on the X activation ratio, or the percent of cells expressing the normal allele on the active X chromosome,3 resulting in a range of normal intellectual ability to moderate ID. Over the past 2 decades, scientists have made significant advancements in identifying and describing genetic, molecular, and cellular underpinnings of FXS, allowing for a more precise diagnosis of this condition.
Riley C, Mailick M, Berry-Kravis E, Bolen J. The Future of Fragile X Syndrome: CDC Stakeholder Meeting Summary. Pediatrics. 2017;139 (Suppl 3):S147‐S152. doi:10.1542/peds.2016-1159B
