Comprehensive behavioral and molecular characterization of a new knock-in mouse model of Huntington's disease: zQ175

Abstract

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor, cognitive and psychiatric manifestations. Since the mutation responsible for the disease was identified as an unstable expansion of CAG repeats in the gene encoding the huntingtin protein in 1993, numerous mouse models of HD have been generated to study disease pathogenesis and evaluate potential therapeutic approaches. Of these, knock-in models best mimic the human condition from a genetic perspective since they express the mutation in the appropriate genetic and protein context. Behaviorally, however, while some abnormal phenotypes have been detected in knock-in mouse models, a model with an earlier and more robust phenotype than the existing models is required. We describe here for the first time a new mouse line, the zQ175 knock-in mouse, derived from a spontaneous expansion of the CAG copy number in our CAG 140 knock-in colony [1]. Given the inverse relationship typically observed between age of HD onset and length of CAG repeat, since this new mouse line carries a significantly higher CAG repeat length it was expected to be more significantly impaired than the parent line. Using a battery of behavioral tests we evaluated both heterozygous and homozygous zQ175 mice. Homozygous mice showed motor and grip strength abnormalities with an early onset (8 and 4 weeks of age, respectively), which were followed by deficits in rotarod and climbing activity at 30 weeks of age and by cognitive deficits at around 1 year of age. Of particular interest for translational work, we also found clear behavioral deficits in heterozygous mice from around 4.5 months of age, especially in the dark phase of the diurnal cycle. Decreased body weight was observed in both heterozygotes and homozygotes, along with significantly reduced survival in the homozygotes. In addition, we detected an early and significant decrease of striatal gene markers from 12 weeks of age. These data suggest that the zQ175 knock-in line could be a suitable model for the evaluation of therapeutic approaches and early events in the pathogenesis of HD.

Figure 1. Body weight (mean ± SEM) of wild type, heterozygous and homozygous mice as a function of age for female (A) and male (B) mice.

Figure 2. Kaplan-Meier survival curve in WT vs. homozygous mice as a function of genotype and age.

Figure 3. Total distance covered in the Open Field per 5 minute bin (mean ± SEM) of wild type, heterozygous and homozygous mice as a function of age and test time during the dark phase of the diurnal cycle.

Figure 4. Rearing frequency in the Open Field (mean ± SEM) of wild type, heterozygous and homozygous mice as a function of age during the dark phase of the diurnal cycle.

Figure 5. Latency to fall from the rotarod (mean ± SEM) of wild type, heterozygous and homozygous mice as a function of age during the dark phase of the diurnal cycle.

Figure 6. Grip strength (mean ± SEM) of wild type, heterozygous and homozygous mice as a function of age during the light phase of the diurnal cycle.

Figure 7. Proportion of mice climbing in the rearing climbing assay in the wild type, heterozygous and homozygous group during the dark phase of the diurnal cycle.

Figure 8. Percent correct choices for WT, heterozygous and homozygous mice per test day during acquisition of the procedural swim tank at 58 weeks of age.

Figure 9. Overall visit frequency during PhenoCube testing at 16 weeks, broken down into 1 hour bins (A) and summarized across the two complete light/dark periods from lights-on on day 2 (B).

Figure 10. Overall visit frequency during PhenoCube testing at 37 weeks, broken down into 1 hour bins (A) and summarized across the two complete light/dark periods from lights-on on day 2 (B).

Figure 11. The relative striatal mRNA expression level of wildtype (WT), heterozyous (HET) and homozygous (HOMO) zQ175 mice at 12, 18 and 41 weeks of age, analyzed by qPCR.

Relative mRNA levels are normalized to age matched and gender matched wild type controls. For normalization, the geometric mean of UBbc, Eif4a2 and ATP5B was used. Gender separation was performed with the 12 and 41 week groups. *, p<0.05; **, p<0.01; ***, p<0.001. The number on each bar graph represents “n” for each group.