Fragile X mental retardation 1 (FMR1) premutations: instability and associated phenotypes

Fragile X syndrome (FXS) is the most common hereditary form of intellectual disability with an estimated frequency of 1/4000 males and 1/8000 females. FXS is caused by a (CGG)n expansion of over 200 repeats, in the 5’UTR of the FMR1 gene, which as a result is usually methylated and the gene silenced. Based on CGG repeat length, four classes of alleles can be distinguished: normal (5-44), intermediate (45-54), premutation (55-200; PM) and full mutation (>200; PM) alleles. Premutations expand to full mutation alleles only via maternal transmission and larger premutations have an increased risk of expansion to full mutation. Paternal premutations and full mutations are inherited in the premutation range. The aim of this study is to gain insights into instability of FMR1 CGG repeat alleles and associated phenotypes in 128 Portuguese FXS families.