Deletion of all CGG repeats plus flanking sequences in FMR1 does not abolish gene expression

The fragile X syndrome is due to the new class of dynamic mutations. It is associated with an expansion of a trinucleotide repeat (CGG) in exon 1 of the fragile X mental retardation gene 1 gene (FMR1). Here we present a fragile X family with an unique female patient who was rendered hemizygous for the FRAXA locus due to a large deletion of one X chromosome. In addition, the other X had a microdeletion in FMR1. PCR and sequence analysis revealed that the microdeletion included all CGG repeats plus 97 bp of flanking sequences, leaving transcription start site and translation start site intact. Despite this total lack of CGG repeats in the FMR1 gene, Western blot analysis showed expression of FMRP, and the patient's phenotype was essentially normal. X-inactivation studies of the androgen-receptor (AR) locus and haplotype determination of microsatellite markers gave evidence that the deletion probably originated from regression of a fully mutated FMR1 gene. Although the minimal number of CGG repeats hitherto reported in FRAXA is six, and at least four other genes associated with CGG repeats are known, suggesting an as yet unknown function of these repeats, our study clearly demonstrates that the absence of CGG repeats does not abolish expression of the FMR1 gene in lymphoblastoid cells.