A transgenic mouse model of spinocerebellar ataxia type 3 resembling late disease onset and gender-specific instability of CAG repeats
Introduction
Spinocerebellar ataxia type 3 (SCA3), or Machado–Joseph disease (MJD), is an autosomal-dominantly inherited neurodegenerative disorder caused by the expansion of a CAG repeat in the MJD1 gene (Kawaguchi et al., 1994). While, in unaffected individuals, the number of CAG repeats is usually less than 45 (Padiath et al., 2005), it increases up to 86 repeats in SCA3 patients (Riess et al., 2008). The expanded CAG repeat results in an expanded polyglutamine stretch in the encoded ataxin-3 protein. SCA3 therefore belongs to the group of polyglutamine diseases, which includes other types of spinocerebellar ataxias as well as SBMA, DRPLA, and Huntington's disease (reviewed in Gatchel and Zoghbi, 2005). The protein harboring the expanded polyglutamine tract has a strong tendency for aggregation (Scherzinger et al., 1999), which usually manifests in the nucleus of neuronal cells in SCA3. Interestingly, while ataxin-3 is ubiquitously expressed throughout the whole brain, the formation of intranuclear inclusion bodies appears in specific brain regions (Paulson, 1997b, Schmidt, 1998). There is an ongoing discussion whether these intranuclear inclusions are toxic or perhaps even protective for neuronal cells (Michalik and Van Broeckhoven, 2003, Sisodia, 1998).
Clinically, SCA3 presents with a highly heterogenous phenotype, leading to differentiation into clinical subtypes (reviewed in Riess et al., 2008). In addition, the disease is characterized by a late onset, usually appearing in the late third decade of life (Dürr, 1996, Schöls, 1997), and by slow progression. In contrast to the disease course in human patients, most of the previously generated mouse models of SCA3 are characterized by an early onset and rapid progression (Bichelmeier, 2007, Cemal, 2002, Chou, 2008, Goti, 2004, Ikeda, 1996) with rather distinct regional manifestation. Thus, not all the pathogenic aspects of the disease in humans are reproduced by these models.
Here, we present a mouse model for SCA3 with late onset of symptoms that resembles major genetic and pathogenetic characteristics in humans and demonstrates that motor symptoms precede the occurrence of intranuclear inclusion bodies. For this new transgenic mouse model, we utilized the well-characterized rat huntingtin promoter (Holzmann, 1998, Holzmann, 2001) to express a full-length ataxin-3 construct containing 148 CAG repeats ubiquitously throughout the brain.
In transgenic mice, the instability of the CAG repeat expansion was obvious, and clearly correlated with the sex of the transmitting parent. We additionally observed a biphasic course of the disease and detected hyperactivity in HDPromMJD148 mice long before the onset of motor deficits, but the formation of intranuclear inclusion bodies was no prerequisite for the onset of symptoms.
The novel mouse model of SCA3 presented in this study is a valuable addition to previous published mouse models, since its slowly progressing phenotype allows us to study aspects of the disease that cannot be analyzed in other models of this disease.
Section snippets
Transgenic mice
To generate a novel transgenic mouse model for SCA3, a 764 bp XbaI restriction fragment of the rat huntingtin promoter corresponding to nucleotide positions − 777 to − 14 was cloned to the 5′ end of the full-length ataxin-3 cDNA (isoform ataxin-3c) (Goto, 1997, Schmitt, 1997) in the pBluescript SK vector. The transcriptional activity of this promoter fragment has been demonstrated before (Holzmann et al., 1998). The SV40 early mRNA polyadenylation signal was amplified from the vector pEGFP-C
The huntingtin promoter led to transgene expression throughout the whole brain
To control the expression of a full-length construct of ataxin-3 containing 148 CAG repeats, a 764-bp fragment of the well-characterized rat huntingtin promoter was used (Fig. 1A). This promoter fragment contains numerous conserved putative transcription factor binding sites, which have led to robust expression in a variety of cell lines derived from different rodents (Holzmann et al., 1998). A comparable fragment of this promoter has been successfully used before to generate a transgenic rat
Discussion
Spinocerebellar ataxia type 3 is a neurodegenerative disease with late onset, slow progression and a heterogeneous clinical phenotype (Riess et al., 2008). As previously generated mouse models of SCA3 do not reproduce all aspects of the disease in humans (Bichelmeier, 2007, Cemal, 2002, Chou, 2008, Goti, 2004, Ikeda, 1996) we generated a novel mouse model for SCA3 to reflect more closely the late manifesting and slowly progressing phenotype. To control the expression of a full-length ataxin-3
Acknowledgments
The technical help of Gabi Frommer-Kästle in electron microscopy is greatly appreciated. This study was supported by the German Research Foundation (DFG) to OR, and by a grant from the European Union (6th Framework Programme, EUROSCA).
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These authors contributed equally to this project.
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Present address: Prince of Wales Medical Research Institute, Randwick, Australia.