The risk of fragile X premutation expansion is lower in carriers detected by general prenatal screening than in carriers from known fragile X families.

The Fragile X syndrome is the most common cause of inherited mental retardation. For a female premutation carrier, the risk of having a child with a full mutation is positively correlated with the size of the premutation. The current study was performed to evaluate the risk of premutation expansion in the offspring of average-risk carriers detected by general prenatal screening. Over a 4-year period, 9,660 women underwent DNA screening for FMR1 mutation/premutation at the Tel Aviv Sourasky Medical Center. A premutation was defined as a CGG repeat number >50 in the 5' untranslated region (UTR) of exon 1 in the FMR1 gene. The study included only individuals with no family history of X-linked mental retardation or known FMR1 mutations. A premutation was found in 85 women (1 in 114), 68 of whom consented to have prenatal diagnoses in 74 pregnancies. The abnormal allele was transmitted to the offspring in 44 pregnancies. Of these, no change in allele size was noted in 35 pregnancies (79.6%), and expansion within premutation range was evident in 4 pregnancies (9%). In 5 pregnancies (11.4%), expansion to the full mutation was noted. This occurred only in carriers having more than 90 repeats. We conclude that the likelihood of Fragile X premutation expansion to full mutation is significantly lower in individuals ascertained by general prenatal carrier testing than in those from known Fragile X families.