Fig. 1. Motor dysfunction, cognitive deficits and striatal neuropathology in YAC128 mice are modulated by strain background. (A) Rotarod performance at 12 months of age was worse in YAC128 mice on all three background strains compared to WT mice. The magnitude of the rotarod deficit was greatest in FVB/N mice and approximately equal in 129 and C57BL/6 mice. (B) Open field activity during a 30 min open field trial at 12 months of age was decreased in YAC128 mice on the FVB/N and C57BL/6 strains. The magnitude of this deficit was greater in FVB/N mice than in C57BL/6 mice. (C) Motor learning is impaired in YAC128 mice at 12 months of age on the FVB/N background. FVB-YAC128 mice show a complete inability to learn the rotarod task while 129-YAC128 mice perform worse than WT mice during testing and training. (D) The body weight of YAC128 mice is significantly increased compared to WT mice only on the FVB/N strain at 12 months of age. (E) Striatal volume loss in YAC128 mice is penetrant on all three strains at 12 months of age. The greatest degree of volume loss occurs in FVB/N followed by 129 mice and then C57BL/6 mice. (F) Striatal neuronal loss is detected in both FVB-YAC128 mice and 129-YAC128 mice compared to WT mice but is not observed in the C57BL/6 strain background. Overall, motor dysfunction, hypoactivity, motor learning deficits, increased body weight, striatal volume loss and striatal neuronal loss are all modulated by strain. Error bars indicate standard error of the mean. *p < 0.05, ***p < 0.001.

Fig. 2. Huntingtin expression is not influenced by genetic background. (A) An examination of wild-type and mutant htt expression between YAC128 mice on the FVB/N, 129 and C57BL/6 genetic background reveals no difference in the level of wild-type or mutant protein expression. Loading was controlled for by blotting for GAPDH. (B) Western blotting of whole brain lysates with 1C2 antibody demonstrates that YAC128 mice on all three strain backgrounds contain the putatively toxic 586 amino acid caspase 6 cleavage fragment of mutant htt. As a positive control, FVB-YAC128 brain lysate was cut ex vivo with caspase 6 (YAC128 + C6). As expected, 1C2 does not detect the caspase 6 fragment in a WT mouse. Â-tubulin is included as a loading control.

Fig. 3. The nuclear localization of mutant huntingtin is modulated by strain. Sections from 1 month, 2 month and 4 month old YAC128 mice on FVB/N, 129 and C57BL/6 strain backgrounds were stained with EM48 antibody to compare the nuclear localization of mutant htt. At 1 month of age, faint staining is observed in the striatum of FVB-YAC128 mice but not in 129-YAC128 mice or B6-YAC128 mice (see triangles). At 2 months of age, mutant htt is detected in the nucleus in both FVB-YAC128 mice and 129-YAC128 mice. Mutant htt immunostaining in the nucleus is not detected in B6-YAC128 mice until 4 months of age. At this age, EM48 staining intensity is greater in FVB-YAC128 and 129-YAC128 mice than in B6-YAC128 mice. These observations are confirmed by quantification of nuclear huntingtin staining intensity. N = 2 mice per strain per time point. Error bars indicate standard error of the mean.

Fig. 4. Characterization of YAC128 mice on the 129 strain background as a possible model for therapeutic trials. (A) 129-YAC128 mice exhibit a motor learning deficit on the rotarod at 2 months of age since 129-YAC128 mice take longer to reach the same level of performance as 129-WT mice. (B) Longitudinal assessment of body weight in 129-YAC128 mice reveals no difference from 129-WT mice between 2 and 12 months of age. (C) Open field activity during a 5 min open field trial is significantly increased in 129-YAC128 mice at 2 and 4 months compared to 129-WT mice. However, 129-YAC128 mice do not exhibit hypoactivity or hyperactivity at later ages. (D) YAC128 mice on the 129 strain show a deficit in motor function on the rotarod beginning at 4 months of age and continuing to 12 months. (E) At 12 months, the brain and the testis in 129-YAC128 mice show significant atrophy compared to 129-WT mice while the other organs tested showed no significant change in weight. (F) Also at 12 months of age, 129-YAC128 mice show striatal neuronal volume loss (volume), striatal neuronal loss (counts) and striatal neuronal atrophy (cross-section) compared to 129-WT mice. Overall, YAC128 mice on the 129 strain reproduce almost all of the HD-like symptoms observed on the FVB/N strain background except hypoactivity and weight gain. N = 23 WT, 28 YAC128. Error bars indicate standard error of the mean. *p < 0.05, **p < 0.01, ***p < 0.001.

Progression of HD-like phenotypes in YAC128 mice on the C57BL/6 strain background from 12 to 18 months of age

For comparison, data from 12 month old YAC128 mice on the FVB/N strain are included. Percentages indicate the difference between YAC128 and WT mice for a given group with significance indicated below.

Summary of potential phenotypes for quantitative trait loci (QTL) mapping: ideal candidates are present on one strain (susceptible) and absent in another (resistant)

Longitudinal comparison of YAC128 mice on FVB/N and 129 strain backgrounds

Columns indicate comparison to strain matched WT mice.