Maternal FMR1 premutation allele expansion and contraction in fraternal twins.

Fragile X syndrome results from an expansion of the CGG trinucleotide repeat in the 5' untranslated region of the Fragile X Mental Retardation 1 (FMR1) gene. Expansion of a maternal premutation allele is the mechanism by which a full mutation allele arises; contraction of a maternal premutation allele is rare. Here we report on both an expansion and contraction of a maternal FMR1 premutation allele in fraternal twins. The propositus was the product of a 29-week gestation twin pregnancy and was referred for FMR1 testing due to developmental delay. A FMR1 full mutation with complete methylation was observed on Southern blot analysis. Evaluation of the maternal FMR1 gene by PCR revealed a normal and premutation allele with CGG repeat numbers of 30 and 93, respectively. Subsequent FMR1 testing on the twin sister of the propositus detected CGG repeat numbers of 30 and 54. The FMR1 CGG repeat number of the reproductive partner was 30. The FMR1 CGG repeat 30 allele in the twin sister was determined to be of paternal origin and the FMR1 allele with a CGG repeat number of 54 was of maternal origin. This observation is particularly interesting not only because of the concomitant donation of a FMR1 expanded and contracted premutation allele in a twin pregnancy but also because of the significant degree of contraction (39 repeats) of the maternal premutation allele.