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Fragile X-associated tremor and ataxia syndrome (FXTAS) is a late-onset, progressive neurodegenerative disorder characterized by tremors, ataxia and neuropsychological problems. This disease is quite common in the general population with approximately 20 million carriers worldwide. The risk of developing FXTAS increases dramatically with age, with about 45% of male carriers over the age of 50 being affected. FXTAS is caused by a CGG-repeat expansion (CGGexp) in the fragile X mental retardation 1 (FMR1) gene. CGGexp RNA is translated into the FMRpolyG protein by a mechanism called RAN translation. Although both gene and pathogenic trigger are known, no therapeutic interventions are available at this moment. Here, we present, for the first time, primary hippocampal neurons derived from the ubiquitous inducible mouse model which is used as a screening tool for targeted interventions. A promising candidate is the repeat binding, RAN translation blocking, small molecule 1a. Small molecule 1a shields the disease-causing CGGexp from being translated into the toxic FMRpolyG protein. Primary hippocampal neurons formed FMRpolyG-positive inclusions, and upon treatment with 1a, the numbers of FMRpolyG-positive inclusions are reduced. We also describe for the first time the formation of FMRpolyG-positive inclusions in the liver of this mouse model. Treatment with 1a reduced the insoluble FMRpolyG protein fraction in the liver but not the number of inclusions. Moreover, 1a treatment had a reducing effect on the number of Rad23b-positive inclusions and insoluble Rad23b protein levels. These data suggest that targeted small molecule therapy is effective in an FXTAS mouse model and has the potential to treat CGGexp-mediated diseases, including FXTAS.

Fragile X syndrome (FXS) is the most common monogenetic cause of intellectual disability and autism spectrum disorders. Mostly, FXS is caused by transcriptional silencing of the FMR1 gene due to a repeat expansion in the 5′ UTR, and consequently lack of the protein product FMRP. However, in rare cases FXS is caused by other types of variants in the FMR1 gene. We describe a missense variant in the FMR1 gene, identified through whole-exome sequencing, in a boy with intellectual disability and behavioral problems. The variant is located in the FMRP's nuclear export signal (NES). We performed expression and localization studies of the variant in hair roots and HEK293 cells. Our results show normal expression but significant retention of the FMRP in the cells’ nucleus. This finding suggests a possible FMRP reduction at its essential functional sites in the dendrites and the synaptic compartments and possible interference of other cellular processes in the nucleus. Together, this might lead to a FXS phenotype in the boy.

Human homologue of yeast UV excision repair protein Rad23b (HR23B) inclusions are found in a number of neurodegenerative diseases, including frontotemporal dementia (FTD), Huntington’s disease (HD), spinocerebellar ataxia type 3 and 7 (SCA3/7), fragile X associated tremor/ataxia syndrome (FXTAS) and Parkinson’s disease (PD). Here, we describe HR23B pathology in C9ORF72 linked FTD and amyotrophic lateral sclerosis (ALS) cases. HR23B presented in neuropils, intranuclear inclusions and cytoplasmic and perinuclear inclusions and was predominantly found in cortices (frontal, temporal and motor), spinal cord and hippocampal dentate gyrus. HR23B co-localized with poly-GA-, pTDP-43- and p62-positive inclusions in frontal cortex and in hippocampal dentate gyrus, the latter showing higher co-localization percentages. HR23B binding partners XPC, 20S and ataxin-3, which are involved in nucleotide excision repair (NER) and the ubiquitin-proteasome system (UPS), did not show an aberrant distribution. However, C9ORF72 fibroblasts were more sensitive for UV-C damage than healthy control fibroblasts, even though all factors involved in NER localized normally to DNA damage and the efficiency of DNA repair was not reduced. HR23Bs other binding partner NGly1/PNGase, involved in ER-associated degradation (ERAD) of misfolded proteins, was not expressed in the majority of neurons in C9FTD/ALS brain sections compared to non-demented controls. Our results suggest a difference in HR23B aggregation and co-localization pattern with DPRs, pTDP-43 and p62 between different brain areas from C9FTD/ALS cases. We hypothesize that HR23B may play a role in C9ORF72 pathogenesis, possibly by aberrant ERAD functioning. Electronic supplementary material The online version of this article (10.1186/s40478-019-0694-6) contains supplementary material, which is available to authorized users.

Figure S6 Validation of HR23B pathology by a second independent antibody. HR23B staining using Abcam antibody in C9ORF72 FTD cases (n = 5) and non-demented control (n = 3) post-mortem brain sections. Staining pattern is consistent with HR23B GeneTex antibody (see Figs. 1 and 2). All scale bars are 20 μm (PDF 1543 kb)

Figure S4 Pur-alpha staining in C9FTD cases and non-demented controls. Pur-alpha staining reveals abundant stress granules in both C9ORF72 FTD cases (n = 5) and non-demented controls (n = 3) post-mortem brain sections. All scale bars are 20 μm. (PDF 1826 kb)

Table S1. Neuropathological scores of C9ORF72 FTD patients. Neuronal loss score was based on hematoxylin and eosin (HE) staining and pathological report and scored as absent (0), mild (1), moderate (2) or severe (3). Pathological scores were based on the degree of pathology as absent (0), rare (1), occasional (2), moderate (3), or numerous (4). Brain areas: F = frontal cortex, T = temporal cortex, M = motor cortex, H = hippocampus dentate gyrus, C = cerebellum. NCI = neuronal cytoplasmic inclusion, NII = neuronal intranuclear inclusion, DNs = dystrophic neurites. (DOCX 23 kb)

Figure S3. FMRP staining in C9FTD cases and non-demented controls. FMRP staining does not reveal any differences between C9ORF72 FTD cases (n = 5) and non-demented control (n = 3) post-mortem brain sections. Occasional inclusions are found in the hippocampus dentate gyrus in both C9FTD cases and controls. All scale bars are 20 μm. (PDF 1883 kb)

Figure S2. ADARB2 staining in C9FTD cases and non-demented controls. A) Staining of ADARB2 in C9ORF72 FTD cases (n = 5) and non-demented control (n = 3) post-mortem brain sections shows some intranuclear inclusions in hippocampus CA and DG. All scale bars are 20 μm B) Immunofluorescence staining of ADARB2 (red) and p62 (green) in hippocampal dentate gyrus reveals ADARB2 punctuated staining and some intranuclear inclusions in both C9ORF72 FTD cases (n = 5) and non-demented controls (n = 3). Scale bars in fluorescent pictures are 10 μm. (PDF 2721 kb)

Figure S5. HR23B pathology in different brain areas of C9FTD cases. Staining of HR23B in several brain areas of C9FTD cases and non-demented controls. Pathology burden was highest in cortices (frontal, temporal and motor) and was mostly cytoplasmic (inclusions and neuropils) and intranuclear (cateye). Hippocampus dentate gyrus (DG) harbors perinuclear inclusions, and hippocampus cornu ammonis (CA) had some cells with strong nuclear staining. Pathology was low in cerebellum granular layer with only some nuclear and perinuclear inclusions and almost absent in cerebellum molecular layer. Our C9FTD cases did not show HR23B pathology in spinal cord neurons. All scale bars are 20â Îźm (PDF 2260 kb)

Figure S1. Ran-GAP staining in C9FTD cases and non-demented controls. Ran-GAP is predominantly localized to the nucleus and nuclear membrane. Unevenly shaped nuclear membranes occur in both C9ORF72 FTD cases (n = 5) and non-demented controls (n = 3). All scale bars are 20 μm. (PDF 2202 kb)

Figure S7. Nucleotide excision repair is not affected in C9ORF72 human fibroblasts. A) Dose-response curve for 4 healthy control human fibroblast lines (81E253, 86E1375, 06E0717 and 99E0774) and 4 C9ORF72 human fibroblast lines (13E634, 13E659, 17E0225, 17E0278) and the NER deficient XP25RO human fibroblast line treated with increasing dose of UV-C light (0–12 J/m2). B) Immunofluorescence staining showing the recruitment of NER factors XPC, XPB, XPA, XPF and XPG to local DNA damage (visualized by CPD antibody), induced by 60 J/m2 UV-C irradiation through a microporous filter. Fibroblast lines used for pictures: 13E634, 13E659 and 81E253 C) Human fibroblasts lines were treated with 16 J/m2 UV-C light and incubated with EdU for 1 h to measure unscheduled DNA synthesis (UDS) as measure of DNA repair. The NER-deficient XPC25RO cell line is shown as negative control. Ctrl 1–4 are lines 81E253, 86E1375, 06E0717 and 99E0774 in this order. C9 1–4 are lines 13E634, 13E659, 17E0225, 17E0278 in this order. (PDF 934 kb)

The fragile X premutation is a CGG trinucleotide repeat expansion between 55 and 200 repeats in the 5′-untranslated region of the fragile X mental retardation 1 (FMR1) gene. Human carriers of the premutation allele are at risk of developing the late-onset neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). Characteristic neuropathology associated with FXTAS includes intranuclear inclusions in neurons and astroglia. Previous studies recapitulated these histopathological features in neurons in a knock-in mouse model, but without significant astroglial pathology. To determine the role of astroglia in FXTAS, we generated a transgenic mouse line (Gfa2-CGG99-eGFP) that selectively expresses a 99-CGG repeat expansion linked to an enhanced green fluorescent protein (eGFP) reporter in astroglia throughout the brain, including cerebellar Bergmann glia. Behaviorally these mice displayed impaired motor performance on the ladder-rung test, but paradoxically better performance on the rotarod. Immunocytochemical analysis revealed that CGG99-eGFP co-localized with GFAP and S-100ß, but not with NeuN, Iba1, or MBP, indicating that CGG99-eGFP expression is specific to astroglia. Ubiquitin-positive intranuclear inclusions were found in eGFP-expressing glia throughout the brain. In addition, intracytoplasmic ubiquitin-positive inclusions were found outside the nucleus in distal astrocyte processes. Intriguingly, intranuclear inclusions, in the absence of eGFP mRNA and eGFP fluorescence, were present in neurons of the hypothalamus and neocortex. Furthermore, intranuclear inclusions in both neurons and astrocytes displayed immunofluorescent labeling for the polyglycine peptide FMRpolyG, implicating FMRpolyG in the pathology found in Gfa2-CGG99 mice. Considered together, these results show that Gfa2-CGG99 expression in mice is sufficient to induce key features of FXTAS pathology, including formation of intranuclear inclusions, translation of FMRpolyG, and deficits in motor function. Electronic supplementary material The online version of this article (10.1186/s40478-019-0677-7) contains supplementary material, which is available to authorized users.

Table S2. Co-localization of HR23B with DPRs/pTDP-43/p62 differs between brain areas. The percentage is the amount of HR23B inclusions also positive for other pathological hallmarks (not the other way around). For example 11/153 for poly-GA in C9FTD patient 1 means that out of 153 HR23B inclusions, 11 were also positive forpoly- GA, which is 7%. F = frontal cortex. H = hippocampus dentate gyrus. *1 = All co-localizations are fibrils of poly-GP and HR23B in frontal cortex. Perinuclear inclusions of poly-GP did not stain positive for HR23B. *2 = In total 2 poly-PA inclusions have been found in frontal cortex of 5 C9FTD patients, too less to quantify. We therefore state N/A = non-applicable in this table. *3 = Non-demented cases had some p62 and some HR23B inclusions per person per section, which sometimes overlapped. No pTDP-43 inclusions were found so no co-localization of pTDP-43 with HR23B in non-demented cases. (DOCX 19 kb)

Ribonucleoprotein (RNP) granules are dense aggregations composed of RNA-binding proteins (RBPs) and a part of the transcriptome that is still uncharacterized. We performed a large-scale study of interaction networks and discovered that RBPs phase-separating in RNP granules share specific RNA partners with high structural content, long untranslated regions (UTRs) and nucleotide repeat expansions. Our analysis suggests that RNAs can promote formation of RNP granules by acting as scaffolds for protein assembly. To experimentally validate our findings, we investigated the scaffolding ability of CGG repeats contained in the 5’ UTR of FMR1 transcript implicated in Fragile X-associated Tremor / Ataxia Syndrome (FXTAS). Using a novel high-throughput approach we identified RBPs recruited by FMR1 5’ UTRs of different lengths. We employed human primary tissues as well as primate and mouse models of FXTAS to characterize protein sequestration in FMR1 aggregates and focused on a previously unreported partner, TRA2A, which induces impairment of splicing in genes linked to mental retardation and intellectual disability. SIGNIFICANCE Proteins and RNAs phase-separate into granules under physiological (ribosome assembly and shuttling through neurons) and pathological (Amyotrophic Lateral Sclerosis and Fragile X Tremor/Ataxia Syndrome) conditions. We report here that granule-forming proteins share more RNA targets than other proteins and form a dense network of interactions. RNAs interacting with granule-forming proteins have longer UTRs, are more structured and contain nucleotide repeats. Our study suggests that specific RNAs can act as the ‘glue’ of ribonucleoprotein assemblies. To experimentally validate the results of our analysis, we investigated how nucleotide repeats contained in the 5’ UTR of FMR1 are able to sequester proteins in intranuclear inclusions. We predicted protein interactions in silico, confirmed them in vitro and further characterized the ribonucleoprotein aggregates in vivo.

Intellectual disability (ID) comprises a large group of heterogeneous disorders, often without a known molecular cause. X-linked ID accounts for 5–10% of male ID cases. We investigated a large, three-generation family with mild ID and behavior problems in five males and one female, with a segregation suggestive for X-linked inheritance. Linkage analysis mapped a disease locus to a 7.6 Mb candidate region on the X-chromosome (LOD score 3.3). Whole-genome sequencing identified a 2 bp insertion in exon 2 of the chromosome X open reading frame 56 gene (CXorf56), resulting in a premature stop codon. This insertion was present in all intellectually impaired individuals and carrier females. Additionally, X-inactivation status showed skewed methylation patterns favoring the inactivation of the mutated allele in the unaffected carrier females. We demonstrate that the insertion leads to nonsense-mediated decay and that CXorf56 mRNA expression is reduced in the impaired males and female. In murine brain slices and primary hippocampal neuronal cultures, CXorf56 protein was present and localized in the nucleus, cell soma, dendrites, and dendritic spines. Although no other families have been identified with pathogenic variants in CXorf56, these results suggest that CXorf56 is the causative gene in this family, and thus a novel candidate gene for X-linked ID with behavior problems.

A CGG-repeat expansion in the premutation range in the Fragile X mental retardation 1 gene (FMR1) has been identified as the genetic cause of Fragile X-associated tremor/ataxia syndrome (FXTAS), a late-onset neurodegenerative disorder that manifests with action tremor, gait ataxia and cognitive impairments. In this study, we used a bigenic mouse model, in which expression of a 90CGG premutation tract is activated in neural cells upon doxycycline administration-P90CGG mouse model. We, here, demonstrate the behavioural manifestation of clinically relevant features of FXTAS patients and premutation carrier individuals in this inducible mouse model. P90CGG mice display heightened anxiety, deficits in motor coordination and impaired gait and represent the first FXTAS model that exhibits an ataxia phenotype as observed in patients. The behavioural phenotype is accompanied by the formation of ubiquitin/FMRpolyglycine-positive intranuclear inclusions, as another hallmark of FXTAS, in the cerebellum, hippocampus and amygdala. Strikingly, upon cessation of transgene induction the anxiety phenotype of mice recovers along with a reduction of intranuclear inclusions in dentate gyrus and amygdala. In contrast, motor function deteriorates further and no reduction in intranuclear inclusions can be observed in the cerebellum. Our data thus demonstrate that expression of a 90CGG premutation expansion outside of the FMR1 context is sufficient to evoke an FXTAS-like behavioural phenotype. Brain region-specific neuropathology and (partial) behavioural reversibility make the inducible P90CGG a valuable mouse model for testing pathogenic mechanisms and therapeutic intervention methods.

Neuronal intranuclear inclusion disease (NIID) is a rare heterogeneous progressive neurodegenerative disease characterized by the presence of eosinophilic hyaline intranuclear inclusions in neuronal and glial cells of the CNS, peripheral cells of the autonomic nervous system, visceral organs and skin. The clinical presentation is broadly heterogeneous and includes limb weakness, dementia, seizures, ataxia, and parkinsonism. High-intensity signal in the corticomedullary junction on brain MRI is a characteristic finding in NIID. We describe a 65-year-old patient presenting with mild cognitive impairment, evolving in dementia with behavioral disturbances and parkinsonism. Brain MRI showed mild global cortical atrophy, more pronounced in the cingulate and temporal cortex and mild leukoaraiosis, but no high-intensity signal in corticomedullary junction on diffusion weighted imaging. Neuropathological examination showed p62- and optineurin-positive neuronal intranuclear inclusions in the hippocampus and in some subcortical structures. Glial cells did not present any intranuclear inclusions, and no spongiotic changes proximal to the U-fibers or diffuse myelin pallor were disclosed in the white matter. We report on a case with pathological features of NIID showing different neuroimaging and pathological findings. We noted an absence of typical MRI abnormalities, lack of intranuclear inclusions in glial cells, and prominent involvement of hippocampal neurons, refining the clinico-pathological spectrum of the disease.

Summary Recent evidence indicates that specific RNAs promote the formation of ribonucleoprotein condensates by acting as scaffolds for RNA-binding proteins (RBPs). We systematically investigated RNA-RBP interaction networks to understand ribonucleoprotein assembly. We found that highly contacted RNAs are structured, have long UTRs, and contain nucleotide repeat expansions. Among the RNAs with such properties, we identified the FMR1 3′ UTR that harbors CGG expansions implicated in fragile X-associated tremor/ataxia syndrome (FXTAS). We studied FMR1 binding partners in silico and in vitro and prioritized the splicing regulator TRA2A for further characterization. In a FXTAS cellular model, we validated the TRA2A-FMR1 interaction and investigated implications of its sequestration at both transcriptomic and post-transcriptomic levels. We found that TRA2A co-aggregates with FMR1 in a FXTAS mouse model and in post-mortem human samples. Our integrative study identifies key components of ribonucleoprotein aggregates, providing links to neurodegenerative disease and allowing the discovery of therapeutic targets., Graphical Abstract, Highlights • Highly contacted granule RNAs are structured and contain long, repetitive UTRs • Mutations related with FXTAS increase FMR1 scaffolding propensity • TRA2A co-aggregates with FMR1 in FXTAS mouse model and in post-mortem human samples • TRA2 sequestration has both transcriptomic and post-transcriptomic implications, Cid-Samper et al. analyze protein-RNA networks and identify properties of RNA scaffolds within biological condensates. They find that CGG repeats in the 3′ UTR of FMR1 attract several proteins, including the splicing factor TRA2A that co-aggregates in fragile X-associated tremor/ataxia syndrome (FXTAS).

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder affecting carriers of the fragile X-premutation, who have an expanded CGG repeat in the 5'-UTR of the FMR1 gene. FXTAS is characterized by progressive development of intention tremor, ataxia, parkinsonism and neuropsychological problems. The disease is thought to be caused by a toxic RNA gain-of-function mechanism, and the major hallmark of the disease is ubiquitin-positive intranuclear inclusions in neurons and astrocytes. We have developed a new transgenic mouse model in which we can induce expression of an expanded repeat in the brain upon doxycycline (dox) exposure (i.e. Tet-On mice). This Tet-On model makes use of the PrP-rtTA driver and allows us to study disease progression and possibilities of reversibility. In these mice, 8 weeks of dox exposure was sufficient to induce the formation of ubiquitin-positive intranuclear inclusions, which also stain positive for the RAN translation product FMRpolyG. Formation of these inclusions is reversible after stopping expression of the expanded CGG RNA at an early developmental stage. Furthermore, we observed a deficit in the compensatory eye movements of mice with inclusions, a functional phenotype that could be reduced by stopping expression of the expanded CGG RNA early in the disease development. Taken together, this study shows, for the first time, the potential of disease reversibility and suggests that early intervention might be beneficial for FXTAS patients.