Induced expression of expanded CGG RNA causes mitochondrial dysfunctionin vivo

Abstract

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder affecting carriers of premutation forms of the FMR1 gene, resulting in a progressive development of tremor, ataxia and neuropsychological problems. The disease is caused by an expanded CGG repeat in the FMR1 gene, leading to an RNA gain-of-function toxicity mechanism. In order to study the pathogenesis of FXTAS, new inducible transgenic mouse models have been developed that expresses either 11CGGs or 90CGGs at the RNA level under control of a Tet-On promoter. When bred to an hnRNP-rtTA driver line, doxycycline (dox) induced expression of the transgene could be found in almost all tissues. Dox exposure resulted in loss of weight and death within 5 d for the 90CGG RNA expressing mice. Immunohistochemical examination of tissues of these mice revealed steatosis and apoptosis in the liver. Decreased expression of GPX1 and increased expression of cytochrome C is found. These effects were not seen in mice expressing a normal sized 11CGG repeat. In conclusion, we were able to show in vivo that expression of an expanded CGG-repeat rather than overexpression of a normal CGG-repeat causes pathology. In addition, we have shown that expanded CGG RNA expression can cause mitochondrial dysfunction by regulating expression levels of several markers. Although FTXAS patients do not display liver abnormalities, our findings contribute to understanding of the molecular mechanisms underlying toxicity of CGG repeat RNA expression in an animal model. In addition, the dox inducible mouse lines offer new opportunities to study therapeutic interventions for FXTAS.

Reversibility of neuropathology and motor deficits in an inducible mouse model for FXTAS

“…This model recapitulates many of the features seen in human FXTAS: not only increased expression of Fmr1 mRNA, decreased levels of FMRP and intranuclear aggregates in neurons and astrocytes, but also poor motor function, impaired memory and progressive spatial processing deficits (13). Recently, we reported initial findings in our new Tet-On doxycycline-inducible mouse model for FXTAS (14). In this model, expression of either a control size CGG repeat (11CGGs) or a pathogenic expanded repeat (90CGGs) can be activated using the Tet-On system.…”

“…In order to address such questions, we describe the successful development and initial characterization of an inducible mouse model for the fragile X-premutation and FXTAS. In this model, an expanded CGG RNA is expressed under control of a Tet-On promoter activated by the addition of dox to drinking water (14). This inducible model shows no evidence of expression in the absence of dox (i.e.…”

“…However, we recently reported that dox-induced expression of expanded CGG RNA can cause mitochondrial damage in the liver, followed by early death of mice within 1 week of exposure. These experiments were carried out in bigenic mice (TRE-90CGG-eGFP/hnRNP-rtTA), in which expression of the expanded CGG RNA utilized an hnRNP promoter to drive rtTA expression (14). Importantly, these bigenic mice express elevated levels of mutant-expanded CGG RNA in the liver, but show a total absence of ubiquitin-positive inclusions.…”

“…We opted for the prion protein or PrP promoter, as it drives expression confined to the central nervous system (both neurons and glia cells). We expected that the use of this driver line would avert the liver failure we observed when using the more ubiquitous driver line hnRNP-rtTA (14). Indeed, the double transgenic TRE-nCGG-eGFP/PrP-rtTA mice could be treated with dox in their drinking water for as long as 28 weeks without signs of overt toxicity.…”

“…Recently, we have shown that induced expression of an expanded 90CGG RNA produces mitochondrial stress and apoptosis in the liver in a Tet-On mouse model for FXTAS (14). Here, we used the same strategy to create bigenic transgenic mice in which exposure to dox induces expression of either a control size 11CGG or an expanded 90CGG repeat RNA fused to an enhanced green fluorescent protein (eGFP) reporter in the brain.…”