Non-invasive detection of neurochemical changes prior to overt pathology in a mouse model of spinocerebellar ataxia type 1

Spinocerebellar ataxias (SCAs) are a clinically and genetically heterogeneous group of autosomal dominantly inherited neurodegenerative diseases characterized by loss of cerebellar Purkinje cells (PCs) (Schols et al. 2004). Six SCA subtypes, including SCA1, are caused by CAG trinucleotide repeat expansions in the respective genes, resulting in polyglutamine expansions in the expressed proteins (Zoghbi and Orr 2000). Cerebellar and, in many cases, brainstem degeneration in SCAs result in progressive loss of motor coordination and affect gaze, speech, gait and balance. As with all neurodegenerative diseases, there is a great need for biomarkers to monitor disease onset and progression directly in the brain. In particular, outcome measures that are complementary to clinical scales are needed (Klockgether 2011) to evaluate the effects of potential therapies in the brain (Klockgether and Paulson 2011). Successful implementation of such interventions will be facilitated by early detection of cerebellar abnormalities, potentially preceding clinical abnormalities. To this end, we have previously demonstrated the sensitivity of 1H magnetic resonance spectroscopy (MRS) to progressive neurodegeneration in a transgenic mouse model of SCA1 (Ӧz et al. 2010b). That model, designated as the SCA1[82Q] line, over-expresses the mutant human ataxin-1 protein with an 82 glutamine stretch under the control of a PC specific promoter (Burright et al. 1995) and displays neuronal dysfunction apparent as dendritic atrophy starting at 6 weeks. Neurodegeneration in the SCA1[82Q] line progresses to severe cerebellar pathology by 1 year. A subset of the neurochemicals detected by 1H MRS (N-acetylaspartate, myo-inositol and glutamate) showed progressive alterations relative to control mice and significantly correlated with pathology scores in our study using the SCA1[82Q] mice (Ӧz et al. 2010b). Remarkably, the same neurochemicals also correlated with the ataxia score in patients (Ӧz et al. 2010a), indicating these metabolites as biomarkers of disease progression and substantiating an ability to translate the mouse findings to patients. In the SCA1[82Q] model, we identified another set of neurochemicals (total creatine, glutamine, and taurine) that marked the earliest biochemical changes. Namely, these metabolites were altered in the SCA1[82Q] mice at 6 weeks, but converged with the levels of control groups as the animals aged. In a separate study, we utilized conditional expression of the transgene in SCA1[82Q] mice to establish the sensitivity of MRS biomarkers to disease reversal (Ӧz et al. 2011). While the SCA1[82Q] line was highly valuable to demonstrate the sensitivity of 1H MRS measures to progressive neurodegeneration in SCA1, this line expresses the mutant ataxin-1 mRNA at around 50–100 times endogenous levels in PCs, therefore the disease is restricted to PCs and a cerebellar phenotype, and the mice live a normal life span. On the other hand, patients with SCA1 also develop non-cerebellar, e.g. cognitive, bulbar motor and even extrapyramidal features as the disease progresses, and die prematurely. To overcome the limitations of the transgenic SCA1 model and to guarantee accurate temporal and spatial expression patterns at endogenous levels, a knock-in mouse model of SCA1 was developed whereby the human mutation was introduced into the corresponding mouse gene (Watase et al. 2002): The Sca1154Q/2Q line has a 154 polyglutamine repeat in the endogenous ataxin-1 protein (the mouse Sca1 gene contains two CAGs at this site while the rest of the gene is highly homologous to the human SCA1 gene). These mice have learning and memory deficits and suffer muscle wasting and premature death similar to human patients. In addition, PC loss occurs only at the end stage of disease and the cerebellar pathology is milder than the transgenic SCA1[82Q] mice. Therefore, we expanded our correlative MRS and histology work into the knock-in line to take advantage of these features of the Sca1154Q/2Q mice, in particular the milder cerebellar pathology. We hypothesized that metabolite levels measured by ultra-high field MRS would be sensitive to disease even prior to the development of clear pathological changes. To test this hypothesis, we compared the cerebellar neurochemical profiles of Sca1154Q/2Q mice longitudinally to those of wild-type (WT) littermates and assessed the cerebella of the mice at the same time points by histology.