Background: The Repeat Expansion Diseases are caused by the intergenerational expansion of a specific tandem repeat. GAA/TTC-repeat expansion in the first intron of the frataxin gene results in a transcription defect that leads to a deficit of frataxin, a protein involved in mitochondrial iron homeostasis. This results in Friedreich ataxia (FRDA), a relentlessly progressive neurodegenerative disease. In addition to the consequences of cerebellar degeneration, FRDA symptoms include diabetes and a frequently fatal hypertrophic cardiomyopathy. Thus the frataxin deficit has consequences outside of the CNS as well. How an intronic repeat affects gene expression is unknown. Previous work from our group and others showed that during transcription in vitro the repeat was able to form a triple-stranded DNA structure (triplex) that trapped the RNA polymerase on the template and reduced the overall yield of transcript (Grabczyk and Usdin, 2000 a,b). However, whether such a triplex forms in the endogenous frataxin locus in vivo is unknown.[unreadable] [unreadable] [unreadable] Progress report: We have shown that the GAA/TTC-repeat expansion responsible for FRDA causes the locus to become heterochromatinized (Greene et. al., 2007). We have identified both an unusual pattern of aberrant DNA methylation that is superimposed on the basal methylation found in this region even in unaffected individuals. We have also identified a number of key histone modifications that characterize silenced alleles. We are in the process of identifying compounds that are able to successfully reverse the key histone modifications responsible for silencing.[unreadable] [unreadable] [unreadable] The epigenetic changes on the FRDA alleles show similarities to those seen on the heterochromatinized genes responsible for another Repeat Expansion Disease, Fragile X Syndrome (FXS). FXS results from expansion of a different repeat that is also normally transcribed but not translated. Our recent data supports and extends our original hypothesis that the underlying mechanism for repeat mediated gene silencing in these 2 disorders may be more similar than originally anticipated. It also raises the possibility that similar approaches may be useful in ameliorating disease symptoms in both disorders. Furthermore, our data suggest that DNA methylation is secondary to other epigenetic changes in the FRDA locus and, since the FRDA repeat contains no CpG residues, the repeat cannot be the nidus for DNA methylation as originally suggested for the FXS repeats. Thus it may be that the trigger for DNA methylation in both FRDA and FXS lies elsewhere and that a different common mechanism may be responsible for not only these 2 diseases but other Repeat Expansion Diseases where the repeat is transcribed.
NIH ExPorter
NIH ExPorter