A knockin mouse model of spinocerebellar ataxia type 3 exhibits prominent aggregate pathology and aberrant splicing of the disease gene transcript

Abstract

Polyglutamine diseases, including spinocerebellar ataxia type 3 (SCA3), are caused by CAG repeat expansions that encode abnormally long glutamine repeats in the respective disease proteins. While the mechanisms underlying neurodegeneration remain uncertain, evidence supports a proteotoxic role for the mutant protein dictated in part by the specific genetic and protein context. To further define pathogenic mechanisms in SCA3, we generated a mouse model in which a CAG expansion of 82 repeats was inserted into the murine locus by homologous recombination. SCA3 knockin mice exhibit region-specific aggregate pathology marked by intranuclear accumulation of the mutant Atxn3 protein, abundant nuclear inclusions and, in select brain regions, extranuclear aggregates localized to neuritic processes. Knockin mice also display altered splicing of the disease gene, promoting expression of an alternative isoform in which the intron immediately downstream of the CAG repeat is retained. In an independent mouse model expressing the full human ATXN3 disease gene, expression of this alternatively spliced transcript is also enhanced. These results, together with recent findings in other polyglutamine diseases, suggest that CAG repeat expansions can promote aberrant splicing to produce potentially more aggregate-prone isoforms of the disease proteins. This report of a SCA3 knockin mouse expands the repertoire of existing models of SCA3, and underscores the potential contribution of alternative splicing to disease pathogenesis in SCA3 and other polyglutamine disorders.

The S/T-Rich Motif in the DNAJB6 Chaperone Delays Polyglutamine Aggregation and the Onset of Disease in a Mouse Model

“…Thus, polyQ expansion does not render the protein a target for accelerated degradation, as for misfolded, disease-associated proteins such as SOD1 in ALS (Crippa et al, 2010) or CFTR in cystic fibrosis (Villella et al, 2013). For the polyQ expansion to become exposed and initiate aggregation, the fulllength proteins have to be shortened, either by proteases (Ellerby et al, 1999;Graham et al, 2006;Haacke et al, 2006;Koch et al, 2011;Raspe et al, 2009;Venkatraman et al, 2004) or via alternative splicing (Ramani et al, 2015;Sathasivam et al, 2013), or need to undergo conformational changes such that flanking sequences open up or align the polyQ stretch for b-hairpin-mediated nucleation (Hoop et al, 2014;Kar et al, 2013).…”

Antisense oligonucleotide therapy rescues aggresome formation in a novel Spinocerebellar Ataxia type 3 human embryonic stem cell line

“…Misfolding of expanded polyQ proteins and subsequent accumulation as high molecular weight oligomers and aggregates is a key pathological feature of polyQ diseases 4,27,28 . SCA3 post-mortem patient brains and transgenic mouse models of SCA3 exhibit widespread neuronal intranuclear and cytosolic inclusions of ATXN3 in brain regions affected in disease [28][29][30][31][32] .…”

Physiological and pathophysiological characteristics of ataxin-3 isoforms

“…Ataxin-3c seems to be the predominant protein isoform in human and murine brain tissue, but all isoforms were reported to be expressed (19). Interestingly, the expanded CAG repeat in ATXN3 seems to be associated with an increased generation of the ataxin-3a transcript (20).…”