Large-Brained Animal Models of Huntington's Disease: Sheep.

The limitations of using small-brained rodents to model diseases that affect large-brain humans are becoming increasingly obvious as novel therapies emerge. Huntington's disease (HD) is one such disease. In recent years, the desirability of a large-brained, long-lived animal model of HD for preclinical testing has changed into a necessity. Treatment involving gene therapy in particular presents delivery challenges that are currently unsolved. Models using long-lived, large-brained animals would be useful, not only for refining methods of delivery (particularly for gene and other therapies that do not involve small molecules) but also for measuring long-term "off-target" effects, and assessing the efficacy of therapies. With their large brains and convoluted cortices, sheep are emerging as feasible experimental subjects that can be used to bridge the gap between rodents and humans in preclinical drug development. Sheep are readily available, economical to use, and easy to care for in naturalistic settings. With brains of a similar size to a large rhesus macaque, they have much to offer. The only thing that was missing until recently was the means of testing their neurological function and behavior using approaches and methods that are relevant to HD. In this chapter, I will outline the present and future possibilities of using sheep and testing as large animal models of HD.