Mosaicism in a fragile X male including a de novo deletion in the FMR1 gene

In most cases the fragile X syndrome is caused by an amplification of the C GG trinucleotide repeat in the 5' untranslated region of the FMR1 gene, in combination with the hypermethylation of the proximal CpG island. Recently, also a few cases with deletions or a mosaic of a deletion and a full mutat ion in the FMR1 gene, leading to the same phenotype, have been described. H ere we report the molecular analysis of a patient with typical fragile X ph enotype and mosaicism of the FMR1 genomic region consisting of a premutatio n, a full mutation of the CGG repeats, and a 215 bp deletion, diagnosed by Southern blot hybridisation and polymerase chain reaction (PCR), Sequence a nalysis of the deletion demonstrated that the 5' breakpoint of the deletion is located within a putative hotspot region 75-53 bp proximal to the CGG r epeat. (C) 1999 Wiley-Liss, Inc.