Most fragile X syndrome patients have expansion of a (CGG)_n sequence with >200 repeats (full mutation) in the FMR1 gene responsible for this condition. Hypermethylation of the expanded repeat and of the FMR1 promoter is almost always present and apparently suppresses transcription, resulting in absence of the FMR1 protein. We recently showed that transcriptional reactivation of FMR1 full mutations can be achieved by inducing DNA demethylation with 5-azadeoxycytidine (5-azadC). The level of histone acetylation is another important factor in regulating gene expression; therefore, we treated lymphoblastoid cell lines of non-mosaic full mutation patients with three drugs capable of inducing histone hyperacetylation. We observed a consistent, although modest, reactivation of the FMR1 gene with 4-phenylbutyrate, sodium butyrate and trichostatin A, as shown by RT-PCR. However, we report that combining these drugs with 5-azadC results in a 2- to 5- fold increase in FMR1 mRNA levels obtained with 5-azadC alone, thus showing a marked synergistic effect of histone hyperacetylation and DNA demethylation in the reactivation of FMR1 full mutations.

Fragile X syndrome is the most frequent cause of heritable mental retardation. Most patients have a mutation in the 5′ untranslated region of the FMR1 gene, consisting of the amplification of a polymorphic (CGG)_n repeat sequence, and cytogenetically express the folate-sensitive fragile site FRAXA in Xq27.3. Fragile X patients harbour an expanded sequence with >200 CGG repeats (full mutation), accompanied by methylation of most cytosines of the sequence itself and of the upstream CpG island. This abnormal hypermethylation of the promoter suppresses gene transcription, resulting in the absence of the FMR1 protein. Rare individuals of normal intelligence were shown to carry a completely or partially unmethylated full mutation and to express the FMR1 protein. Given this observation and knowing that the open reading frame of the mutated FMR1 gene is intact, we decided to investigate whether its activity could be restored in vitro by inducing DNA demethylation with 5-azadeoxycytidine (5-azadC) in fragile X patients' lymphoblastoid cells. We report that treatment with 5-azadC causes reactivation of fully mutated FMR1 genes with 300–800 repeats, as shown by the restoration of specific mRNA and protein production. This effect correlates with the extent of promoter demethylation, determined by restriction analysis with methylation-sensitive enzymes. These results confirm the critical role of FMR1 promoter hypermethylation in the pathogenesis of the fragile X syndrome, provide an additional explanation for the normal IQ of the rare males with unmethylated full mutations and pave the way to future attempts at pharmacologically restoring mutant FMR1 gene activity in vivo.

Research from JAMA Neurology — Characterization of the Pattern of Cognitive Impairment in Myotonic Dystrophy Type 1

The most common of the heterogeneous group of the extra structurally abnormal chromosomes (ESACs) is the inv dup(15), whose presence results in tetrasomy 15p and partial tetrasomy 15q. Inv dup(15), containing the Prader-Willi/Angelman syndrome (PWSAS) region, are constantly associated with phenotypic abnormalities and mental retardation. We report on four additional patients with inv dup(15), whose behavioral pattern, and neurologic and physical findings further delineate the phenotype of this neurogenetic syndrome. We also provide FISH analyses on chromosomes of the observed ESACs and discuss the role of a number of genes located within the tetrasomic region.