1. ¹Institute of Medical Genetics, Catholic University, Rome, Italy
2. ²Institute of Hygiene, Catholic University, Rome, Italy
3. ³Department of Experimental Medicine and Biochemical Sciences, Faculty of Medicine, 'Tor Vergata' University, Rome, Italy
4. ⁴Developmental and Molecular Genetics Section, Faculty of Medicine, Catholic University, Leuven, Belgium

Correspondence: Dr G Neri, Institute of Medical Genetics, Catholic University, Largo F. Vito 1, Rome 00168, Italy. Tel: +39 06 3054 449; Fax: +39 06 3050 031; E-mail: gneri@rm.unicatt.it

Received 4 April 2008; Revised 17 June 2008; Accepted 19 June 2008; Published online 16 July 2008.

Abstract

Fragile X syndrome (FXS) is caused by the expansion of a CGG repeat in the 5'UTR of the FMR1 gene and the subsequent methylation of all CpG sites in the promoter region. We recently identified, in unrelated FXS families, two rare males with an unmethylated full mutation, that is, with an expanded CGG repeat (>200 triplets) lacking the typical CpG methylation in the FMR1 promoter. These individuals are not mentally retarded and do not appear to be mosaic for premutation or methylated full mutation alleles. We established lymphoblastoid and fibroblast cell lines that showed essentially normal levels of the FMR1-mRNA but reduced translational efficiency of the corresponding mRNA. Epigenetic analysis of the FMR1 gene demonstrated the lack of DNA methylation and a methylation pattern of lysines 4 and 27 on histone H3 similar to that of normal controls, in accordance with normal transcription levels and consistent with a euchromatic configuration. On the other hand, histone H3/H4 acetylation and lysine 9 methylation on histone H3 were similar to those of typical FXS cell lines, suggesting that these epigenetic changes are not sufficient for FMR1 gene inactivation. These findings demonstrate remarkable consistency and suggest a common genetic mechanism causing this rare FMR1 epigenotype. The discovery of such a mechanism may be important in view of therapeutic attempts to convert methylated into unmethylated full mutations, restoring the expression of the FMR1 gene.