Evaluation of R6/2 HD transgenic mice for therapeutic studies in Huntington's disease: behavioral testing and impact of diabetes mellitus.

R6/2 transgenic mice express exon 1 of the human Huntington's disease (HD) gene with an increased CAG repeat length. They develop a progressive neurological phenotype, die within 12-14 weeks of age and were also found to develop diabetes mellitus. Since R6/2 mice are broadly used to screen for potential therapies in HD, the aim of this study was (a) to search for behavioral tests that are best applicable to monitor the behavioral abnormalities in therapy studies and (b) to investigate the extent to which diabetes influences the disease phenotype. We found that the rotarod test for motor coordination and the open field test for spontaneous explorative behavior were useful to monitor the progressive behavioral deterioration of R6/2 mice. An accelerating rotarod paradigm was superior over testing with a rotarod at various fixed speeds since it leads to similar results with less repetitive daily trials so that exhaustion cannot contribute substantially to their decline in performance. With the Morris water maze, however, it was only possible to monitor cognitive decline in visuo-spatial learning in the first weeks of disease since, at later stages, mice were not able to learn the task adequately. A latent diabetes mellitus was found in all transgenic mice demonstrated by a pathological glucose tolerance. Only 26% of the mice, however, were found to develop a manifest diabetes with increasing blood glucose levels on normal diet over the disease period. R6/2 mice with manifest and latent diabetes showed no significant differences in survival, weight loss, motor coordination, or spontaneous explorative behavior. These results suggest that diabetes mellitus is not a major contributing factor to the disease phenotype.