Fragile X-associated tremor/ataxia syndrome (FXTAS) is an incurable neurodegenerative disorder caused by expansion of CGG repeats in the FMR1 5’UTR. The RNA containing expanded CGG repeats (rCGGexp) causes cell damage by interaction with complementary DNA, forming R-loop structures, sequestration of nuclear proteins involved in RNA metabolism and initiation of translation of polyglycine-containing protein (FMRpolyG), which forms nuclear insoluble inclusions. Here we show the therapeutic potential of short antisense oligonucleotide steric blockers (ASOs) targeting directly the rCGGexp. In nuclei of FXTAS cells ASOs affect R-loop formation and correct miRNA biogenesis and alternative splicing, indicating that nuclear proteins are released from toxic sequestration. In cytoplasm, ASOs significantly decrease the biosynthesis and accumulation of FMRpolyG. Delivery of ASO into a brain of FXTAS mouse model reduces formation of inclusions, improves motor behavior and corrects gene expression profile with marginal signs of toxicity after a few weeks from a treatment.

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.

The protein doublecortin is mainly expressed in migrating neuroblasts and immature neurons. The Xlinked gene MECP2, associated to several neurodevelopmental disorders such as Rett Syndrome, encodes the protein methyl-CpG-binding protein 2 (MeCP2), a regulatory protein that has been implicated in neuronal maturation and refinement of olfactory circuits. Here we explored doublecortin immunoreactivity in the brain of young-adult female Mecp2-heterozygous and male Mecp2-null mice and their wild-type littermates. The distribution of doublecortin-immunorective somata in neurogenic brain regions was consistent with previous reports in rodents, and no qualitative differences were found between genotypes or sexes. Quantitatively, we found a significant increase in doublecortin cell density in in the piriform cortex of Mecp2-null males as compared to WT littermates. A similar increase was seen in a newly-identified population of doublecortin cells in the olfactory tubercle. In these olfactory structures, however, the percentage of doublecortin immature neurons that also expressed NeuN was not different between genotypes. By contrast, we found no significant differences between genotypes in doublecortinimmunorectivity in the olfactory bulbs. Nonetheless, in the periglomerular layer of Mecp2-null males we observed a specific decrease of immature neurons co-expressing doublecortin and NeuN. Overall, no differences were evident between Mecp2-heterozygous and WT females. Also, no differences could be detected between genotypes in the density of doublecortin-immunoreactive cells in the hippocampus or striatum of either males or females. Our results suggest that MeCP2 is involved in neuronal maturation in a region-dependent manner.

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Deficits in arginine-vasopressin (AVP) and oxytocin (OT), two neuropeptides closely implicated in the modulation of social behaviours, have been reported in some early developmental disorders and autism spectrum disorders. Mutations in the X-linked methyl CpG binding protein 2 (MECP2) gene are associated to Rett syndrome and other neuropsychiatric conditions. Thus, we first analysed AVP and OT expression in the brain of Mecp2-mutant mice by immunohistochemistry. Our results reveal no significant differences in these systems in young adult Mecp2-heterozygous females, as compared to WT littermates. By contrast, we found a significant reduction in the sexually dimorphic, testosterone-dependent, vasopressinergic innervation in several nuclei of the social brain network and of oxytocinergic innervation in the lateral habenula of Mecp2-null males, as compared to WT littermates. Analysis of urinary production of pheromones show that Mecp2-null males lack the testosterone-dependent pheromone darcin, strongly suggesting low levels of androgens in these males. In addition, resident-intruder tests revealed lack of aggressive behaviour in Mecp2-null males and decreased chemoinvestigation of the intruder. By contrast, Mecp2-null males exhibited enhanced social approach, as compared to WT animals, in a 3-chamber social interaction test. In summary, Mecp2-null males, which display internal testicles, display a significant reduction of some male-specific features such as vasopressinergic innervation within the social brain network, male pheromone production and aggressive behaviour. Thus, atypical social behaviours in Mecp2null males may be caused, at least in part, by the effect of lack of MeCP2 over sexual differentiation.