Human Molecular Genetics, Vol 7, 109-113, Copyright © 1998 by Oxford University Press
P Chiurazzi, MG Pomponi, R Willemsen, BA Oostra and G Neri
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)nrepeat 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.
ARTICLES
In vitro reactivation of the FMR1 gene involved in fragile X syndrome
Istituto di Genetica Medica, Universita Cattolica, and Centro Ricerche per la Disabilita Mentale e Motoria, Associazione Anni Verdi, Largo F. Vito 1, 00168 Rome, Italy.
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