Background: The Repeat Expansion Diseases are caused by intergenerational expansions of a specific tandem repeat. More than 20 such diseases that belong to this group have been identified thus far. The Fragile X-related disorders (FXDs) arise from expansion of a CGG.CCG-repeat in the 5' UTR of the X-linked FMR1 gene. Carriers of alleles with 55-200 repeats, so-called premutation (PM) alleles, are at risk for a neurodegenerative disorder, Fragile X-associated tremor/ataxia syndrome (FXTAS) and a form of ovarian dysfunction known as FX-associated primary ovarian insufficiency (FXPOI). Furthermore, in females, the premutation allele can undergo expansion on intergenerational transfer that can result in their children having alleles with >200 repeats. This expanded allele is known as a full mutation (FM) and individuals who inherit such alleles almost always have Fragile X syndrome (FXS), which is the leading heritable cause of intellectual disability. FM alleles become silenced. This results in a deficiency of the protein product of this gene, FMRP, which is involved in the regulation of translation of a subset of mRNAs. The FMRP deficiency in brain results in aberrant dendritic spine morphology and a defective response to synaptic activation. The mechanism of gene silencing is unknown, but may show parallels to Friedreich ataxia, a related disorder that also shows repeat-mediated gene silencing. Progress report: Some of our past work focused on the identification of steps in the Fragile X gene silencing process including some that precede DNA methylation and some that occur very late in the silencing process (Biacsi, Kumari and Usdin, 2008; Kumari and Usdin, 2014). Some of our recent work has involved using patient-derived somatic cells to identify biomarkers that may be useful for monitoring therapeutic efficacy of compounds with the potential to ameliorate the symptoms of FXS (Kumari et. al., 2014). We have also generated induced pluripotent stem cell (iPSCs) lines from individuals with FXS to derive disease-relevant cell types for use in understanding disease pathology and screening for small molecules able to ameliorate the disease phenotype. In a proof of principle, we developed an HTRF assay for FMRP suitable for use in a high throughput screen and used it and various patient-derived cell lines to screen various small molecule libraries for compounds capable of reactivation of the FMR1 gene silenced in FXS. This screen identified a number of compounds that had a modest effect on reversing gene silencing (Kumari et. al., 2014). These data suggest that screen of a larger library may yield useful compounds.
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