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Genome-wide alteration of 5-hydroxymethylcytosine in a mouse model of fragile X-associated tremor/ataxia syndrome

ABSTRACT: Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder in which patients carry premutation alleles of 55-200 CGG repeats on the FMR1 gene. To date, whether alterations in epigenetic regulation modulate FXTAS has gone unexplored. 5-hydroxymethylcytosine (5hmC) converted from 5-methylcytosine (5mC) by the ten-eleven translocation (TET) family of proteins, has been found recently to play key roles in neuronal functions. Here we undertook genome-wide profiling of cerebellar 5hmC in a FXTAS mouse model (rCGG mice) and found that rCGG mice at 16 weeks showed overall reduced 5hmC levels genome-wide compared to age-matched wild-type littermates. However, we also observed gain-of-5hmC regions in repetitive elements, as well as cerebellum-specific enhancers, but not general enhancers. Genomic annotation and motif prediction of wild-type- and rCGG-specific differential 5-hydroxymethylated regions (DhMRs) revealed their high correlation with genes and transcription factors that are important in neuronal developmental and functional pathways. DhMR-associated genes partially overlapped with genes that were differentially associated with ribosomes in CGG mice identified by bacTRAP ribosomal profiling. Taken together, our data strongly indicate a functional role for 5hmC-mediated epigenetic modulation in the etiology of FXTAS, possibly through the regulation of transcription.

ORGANISM(S): Mus musculus

PROVIDER: GSE49463 | GEO | 2013/10/17

SECONDARY ACCESSION(S): PRJNA213971

REPOSITORIES: GEO

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Project description:The molecular mechanism(s) leading to Purkinje neuron loss in the neurodegenerative disorder Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS) are limited by the complex morphology of this cell type. Purkinje neurons are notoriously difficult to isolate and maintain in culture presenting considerable difficultly to identify molecular changes in response to riboCGG repeat-containing mRNA that induces neurotoxicity in FXTAS. Several studies have uncovered a number of RNA binding proteins involved in translation that aberrantly interact with the toxic RNA; however, whether these interactions alter the translational profile of cells has not been investigated. Here we employ bacTRAP translational profiling to demonstrate that Purkinje neurons ectopically expressing 90 CGG repeats exhibit a dramatic change in their translational profile even prior to the onset of riboCGG-induced phenotypes. This approach identified nearly 500 transcripts that are differentially associated with ribosomes in r(CGG)90-expressing mice. Functional annotation cluster analysis revealed broad ontologies enriched in the r(CGG)90 list, including RNA binding and response to stress. Intriguingly, a transcript for the Tardbp gene, implicated in a number of other neurodegenerative disorders, exhibits altered association with ribosomes in the presence of r(CGG)90 repeats. We therefore tested and showed that reduced association of Tardbp mRNA with the ribosomes results in a loss of TDP-43 protein expression in r(CGG)90–expressing Purkinje neurons. Furthermore, we showed that TDP-43 could modulate the rCGG repeat-mediated toxicity in a Drosophila model that we developed previously. These findings together suggest translational dysregulation may be an underlying mechanism of riboCGG-induced neurotoxicity and provide insight into the pathogenicity of FXTASBAC-trap studies of Purkinje cels in normal and mutant mice Looking for cell type specific gene expression differences caused by mutation in FMR1 using BAC-Trap RNA capture assay. A cutoff value of 4.5 for mean probe_set mean across all 18 hybridizations was used to determine a cutoff for expression in Purkinje cells. This 16040 corresponded to a mean value of at least 3.4323 in the 4week time point.

Project description:Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder associated with a premutation repeat expansion (55-200 CGG repeats) in the 5 prime noncoding region of the FMR1 gene. Solitary intranuclear inclusions within FXTAS neurons and astrocytes constitute a hallmark of the disorder, yet our understanding of how and why these bodies form is limited. Here, we have discovered that FXTAS inclusions emit a distinct autofluorescence spectrum, which forms the basis of a novel, unbiased method for isolating FXTAS inclusions, by preparative fluorescence-activated cell sorting (FACS). Using a combination of autofluorescence-based FACS and liquid chromatography/tandem mass spectrometry (LC-MS/MS) based proteomics, we have identified more than two hundred proteins that are enriched within the inclusions relative to FXTAS whole nuclei. Whereas no single protein species dominates inclusion composition, highly enriched levels of conjugated small ubiquitin-related modifier 2 (SUMO 2) protein and p62/sequestosome-1 (p62/SQSTM1) protein were found within the inclusions. Many additional proteins involved with RNA binding, protein turnover, and DNA damage repair were enriched within inclusions relative to total nuclear protein. The current analysis has also allowed the first direct detection, through peptide sequencing, of endogenous FMRpolyG peptide, the product of repeat-associated non-ATG (RAN) translation of the FMR1 mRNA. However, this peptide was found only at extremely low levels, and not within whole FXTAS nuclear preparations, raising the question as to whether endogenous RAN products exist at quantities sufficient to contribute to FXTAS pathogenesis. The abundance of the inclusion-associated ubiquitin- and SUMO-based modifiers supports a model for inclusion formation as the result of increased protein loads and elevated oxidative stress leading to mal-adaptive autophagy. These results highlight the need to further investigate FXTAS pathogenesis in the context of endogenous systems.

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Genome-wide alteration of 5-hydroxymethylcytosine in a mouse model of fragile X-associated tremor/ataxia syndrome

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