Novel point mutation within intron 10 of FMR-1 gene causing fragile X syndrome

The majority of cases involving fragile X syndrome are due to expansion of a (CGG)n trinucleotide repeat at the 5' untranslated region of the FMR-1 gene. Deletion and intragenic loss of function mutations of the FMR-1 gene also have been reported. Here, we report a C to T point mutation at the 14th nucleotide in intron 10 of the FMR-1 gene in three unrelated fragile X patients. However, the (CGG)n repeat of FMR-1 in those patients does not expand. To determine the effect of this mutation on the patients' FMR-1 transcripts, total RNA from peripheral blood cells was reverse transcribed and amplified by polymerase chain reaction (RT-PCR). Direct and subcloned sequencing of the RT-PCR products revealed that the transcripts from the allele with C to T mutation skip exon 10 entirely, resulting in a joining of exons 9 and 11. Deletion of exon 10 results in frame-shift and premature termination of translation, which removes the highly conserved region that encoding the KH2 and RGG box domains of FMRP. Interestingly, a male of the three patients has another G to A substitution in exon 15. However, the intron 10 mutation is sufficient for development of fragile X syndrome.