Carriers of FMR1 alleles with 55-200 repeats in the 5' UTR are at risk for Fragile X associated tremor and ataxia syndrome. The cause of the neuropathology is unknown but is thought to be RNAmediated. Maternally transmitted premutation alleles are also at risk of expansion of the repeat tract into the "full mutation" range (>200 repeats). The mechanism responsible for expansion is unknown. Full mutation alleles produce reduced amounts of the FMR1 gene product, FMRP, which leads to Fragile X mental retardation syndrome. We have developed a murine model for Fragile X premutation carriers that recapitulates key features seen in humans including a direct relationship between repeat number and Fmr1 mRNA levels, an inverse relationship with FMRP levels and Purkinje cell dropout that have not been seen in a previously described knock-in mouse model. In addition, these mice also show a differential deficit of FMRP in different parts of the brain that might account for symptoms of the full mutation that are seen in premutation carriers. As in humans, repeat instability is high with expansions predominating and, for the first time in a mouse model, large expansions into the full mutation range are seen that occur within a single generation. Thus, contrary to what was previously thought, mice may be good models not only for the symptoms seen in human carriers of FMR1 premutation alleles but also for understanding the mechanism responsible for repeat expansion, a phenomenon that is responsible for a number of neurological and neurodevelopmental disorders.

Expansion of a GAA⅐TTC repeat in the first intron of the frataxin (FXN) gene causes an mRNA deficit that results in Friedreich ataxia (FRDA). The region flanking the repeat on FRDA alleles is associated with more extensive DNA methylation than is seen on normal alleles and histone modifications typical of repressed genes. However, whether these changes are responsible for the mRNA deficit is controversial. Using chromatin immunoprecipitation and cell lines from affected and unaffected individuals, we show that certain marks of active chromatin are also reduced in the promoter region of the FXN gene in patient cells. Thus, the promoter chromatin may be less permissive for transcription initiation than it is on normal alleles. Furthermore, we show that the initiating form of RNA polymerase II and histone H3 trimethylated on lysine 4, a chromatin mark tightly linked to transcription initiation, are both present at lower levels on FRDA alleles. In addition, a mark of transcription elongation, trimethylated H3K36, shows a reduced rate of accumulation downstream of the repeat. Our data thus suggest that repeat expansion reduces both transcription initiation and elongation in FRDA cells. Our findings may have implications for understanding the mechanism responsible for FRDA as well as for therapeutic approaches to reverse the transcription deficit.Friedreich ataxia (FRDA), 3 an autosomal recessive disorder with early onset, is the most common inherited ataxia (1). In addition to ataxia, diabetes and hypertrophic cardiomyopathy are also commonly seen, with congestive heart failure being a frequent cause of early mortality. The disorder results from mutations in the frataxin (FXN) gene. Most affected individuals have large expansions of a GAA⅐TTC repeat in intron 1 of both FXN alleles (1). Expanded alleles produce lower than normal amounts of FXN mRNA, resulting in a deficiency of frataxin, a protein essential for mitochondrial iron metabolism (1). In general, the repeat number is directly related to the age of onset and the severity of symptoms (2).Brain, peripheral blood leukocytes, and lymphoblastoid cells from individuals with FRDA all show a similar FXN mRNA deficit, and the region flanking the repeat in these cells is aberrantly methylated and enriched for histone modifications characteristic of genes that are transcriptionally repressed (3-7). Because the repeat is relatively close to the promoter, the repeat-mediated chromatin changes could affect transcription initiation if they were to spread to the promoter. This could produce a promoter chromatin configuration that was less permissive for transcription initiation (8). Because intragenic DNA methylation can reduce transcription elongation (8), repeat expansion may also affect the ability of RNA polymerase II (RNAPII) to transcribe through the intron.An alternative explanation for the FXN mRNA deficit in FRDA has been suggested based on the observation that long GAA⅐TTC repeats block transcription elongation on unchromatinized templates in vitro or when propaga...

Expansion of the CGG•CCG-repeat tract in the 5′ UTR of the FMR1 gene to >200 repeats leads to heterochromatinization of the promoter and gene silencing. This results in Fragile X syndrome (FXS), the most common heritable form of mental retardation. The mechanism of gene silencing is unknown. We report here that a Class III histone deacetylase, SIRT1, plays an important role in this silencing process and show that the inhibition of this enzyme produces significant gene reactivation. This contrasts with the much smaller effect of inhibitors like trichostatin A (TSA) that inhibit Class I, II and IV histone deacetylases. Reactivation of silenced FMR1 alleles was accompanied by an increase in histone H3 lysine 9 acetylation as well as an increase in the amount of histone H4 that is acetylated at lysine 16 (H4K16) by the histone acetyltransferase, hMOF. DNA methylation, on the other hand, is unaffected. We also demonstrate that deacetylation of H4K16 is a key downstream consequence of DNA methylation. However, since DNA methylation inhibitors require DNA replication in order to be effective, SIRT1 inhibitors may be more useful for FMR1 gene reactivation in post-mitotic cells like neurons where the effect of the gene silencing is most obvious.

Expansion of a GAA‐repeat tract in the first intron of the frataxin (FXN) gene causes an mRNA deficit that results in Friedreich ataxia (FRDA). The region flanking the repeat on FRDA alleles is more extensively methylated than normal alleles and is enriched for repressive histone modifications. However, whether these epigenetic changes are responsible for the mRNA deficit is controversial. We show here that certain marks of active chromatin are also reduced in the promoter region of the FXN gene in patient cells. Thus, the promoter chromatin may be less permissive for transcription initiation than it is on normal alleles. Furthermore, we show that the initiating form of RNA polymerase II and histone H3 trimethylated on lysine 4, a chromatin mark tightly linked to transcription initiation, are both present at lower levels on FRDA alleles. Our data thus supports the idea that repeat‐induced chromatin modifications reduce transcription initiation in FRDA. Our findings may have implications both for understanding the mechanism responsible for FRDA as well as for therapeutic approaches to reverse the transcription deficit.