Mouse models of the fragile X premutation and fragile X-associated tremor/ataxia syndrome

Abstract

Carriers of the fragile X premutation (FPM) have CGG trinucleotide repeat expansions of between 55 and 200 in the 5'-UTR of FMR1, compared to a CGG repeat length of between 5 and 54 for the general population. Carriers were once thought to be without symptoms, but it is now recognized that they can develop a variety of early neurological symptoms as well as being at risk for developing the late onset neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Several mouse models have contributed to our understanding of FPM and FXTAS, and findings from studies using these models are summarized here. This review also discusses how this information is improving our understanding of the molecular and cellular abnormalities that contribute to neurobehavioral features seen in some FPM carriers and in patients with FXTAS. Mouse models show much of the pathology seen in FPM carriers and in individuals with FXTAS, including the presence of elevated levels of Fmr1 mRNA, decreased levels of fragile X mental retardation protein, and ubiquitin-positive intranuclear inclusions. Abnormalities in dendritic spine morphology in several brain regions are associated with neurocognitive deficits in spatial and temporal memory processes, impaired motor performance, and altered anxiety. In vitro studies have identified altered dendritic and synaptic architecture associated with abnormal Ca(2+) dynamics and electrical network activity. FPM mice have been particularly useful in understanding the roles of Fmr1 mRNA, fragile X mental retardation protein, and translation of a potentially toxic polyglycine peptide in pathology. Finally, the potential for using these and emerging mouse models for preclinical development of therapies to improve neurological function in FXTAS is considered.

Keywords: CGG trinucleotide repeat; FMR1; FMRP; FXTAS; Fragile X premutation; Intranuclear inclusions; Mouse models; RNA toxicity.

Figure 1

Most individuals in the general population have between 5 and 54 CGG trinucleotide repeats in the 5′-UTR of FMR1. Repeat length in the fragile X premutation range is 55 to 200, resulting in an elevation in FMR1 mRNA levels, a moderate decrease in FMRP and an increased risk of developing FXTAS. Repeat size in the full mutation is >200; FMR1 transcription is silenced due to DNA hypermethylation, and the absence of FMRP results in fragile X syndrome. (Adapted from [9].) FMRP, fragile X mental retardation protein; FXTAS, fragile X-associated tremor/ataxia syndrome.

Figure 2

Potential mechanisms of CGG-repeat RNA toxicity in FMP carriers. (A) Protein sequestration model: RNA binding proteins are sequestered through their interactions with the expanded CGG-repeat RNA. These proteins can in turn recruit other proteins. The net result of sequestration of these proteins is that they are unavailable to carry out their normal functions and critical cellular processes are thereby altered or blocked. (? -Sequestration of SAM68 by CGG expanded repeats is indirect, presumably through protein-protein interactions). (B) Toxic polypeptide model: The 43S translation initiation complex stalls near the CGG repeat hairpin formed on the FMR1 RNA. This promotes the repeat-associated non-AUG translation of FMR1 mRNA using a near-AUG start site. This results in a frame shift and the production of polyglycine- and/or polyalanine-containing polypeptides that somehow interfere with normal cell function or may be directly toxic. FMRP, FMRP, fragile X mental retardation protein; ORF, open reading frame; polyA, polyalanine; polyG, polyglycine.

Figure 3

Ubiquitin-positive intranuclear inclusions in neurons and astrocytes of CGG_dut knock-in mice. White arrows point to red punctate intranuclear inclusions in pyramidal neurons in motor cortex (A), cortical astrocytes (B) and Bergmann glia in cerebellum (C). Intranuclear inclusions (red) were labeled by immunofluorescence for ubiquitin, neurons (green) for Kv2.1 potassium channels, and astrocytes and Bergmann glia (green) for GFAP. In (B), note an intranuclear inclusion in an adjacent neuron (arrowhead). Nuclei were stained with DAPI. (Adapted from [25]).

Figure 4

Ectopic expression of a CGG90 repeat results in Purkinje cell loss. (A) Cerebellum of control mouse without a CGG90 repeat (that is, L7Fmr1) showing normal distribution of Purkinje cells in the Purkinje cell layer. (B) Higher magnification of the Purkinje cell layer in control mouse. (C) Selective Purkinje cell loss in 32-week-old mouse expressing a CGG90 repeat under the L7 Purkinje cell-specific promoter (that is, L7CGG90Fmr1). (D) Purkinje cell loss in shown at higher magnification in L7CGG90Fmr1 mouse. gl, granule cell layer; ml, molecular layer; pcl, Purkinje cell layer. (Adapted from [34]).