REACTIVATION OF THE FMR1 GENE OF THE FRAGILE X SYNDROME: ROLE OF RNA INTERFERENCE AND IDENTIFICATION OF PATHOGENIC PATHWAYS BY GENE EXPRESSION PROFILING

The fragile X syndrome (FXS) is the most common form of inherited mental retardation, due to inactivation of the FMR1 gene, located on the long arm of the X chromosome. As a consequence, this disorder is transmitted as an X-linked trait by carrier mothers to their affected sons. Females can also be affected, but not as severely as males. The mutation that typically causes FXS consists of the expansion of a repetitive sequence of CGGs (Cytosine-Guanine-Guanine) in the promoter region of the FMR1 gene. This structural change triggers a further modification of the DNA (methylation) and of histones (methylation and deacetylation), the proteins on which the DNA is wound. This so-called epigenetic modification is the actual cause of the inactivation of FMR1 , in spite of the fact that its coding sequence is intact. One could say that the gene, like a lamp, is simply switched off, not broken. In a series of previous experiments we were able to switch the gene back on in an in vitro system, using drugs that remove DNA methylation (5-azadeoxycytidine; valproic acid) and restore histone acetylation (sodium butyrate). Now we plan to elucidate other mechanisms that regulate the activity of FMR1 , in the first place a phenomenon called RNA interference. It was recently shown by other authors that small fragments of RNA (a molecule that is copied from DNA and is very similar to it) can regulate gene activity. We intend to study RNA interference not only as a regulatory mechanism of FMR1 , but also of other genes, called methyltransferases, that cause DNA methylation and therefore contribute to the inactivation of FMR1 . At the same time, with the help of powerful new technologies that measure the activity of several thousands of genes simultaneously, we shall analyse all those genes whose expression is different in FXS cells, compared to normal cells. Elucidation of this intricate network of interactions promises to open new avenues towards the cure of FXS.