Long-term Effects of Wildtype Huntingtin Lowering in the Primate Corticostriatal Tract and Thalamus

The purpose of this project is to fill gaps in our knowledge about the effects of wildtype huntingtin lowering in the primate brain over an extended period of time, especially in regions of the brain that are of particular relevance to Huntington’s disease (HD) and to the functions of wildtype huntingtin. Evidence from mouse models of HD indicate that lowering huntingtin specifically in the neurons of the cortex that are part of the corticostriatal system may be necessary for optimal therapeutic efficacy. Although prior research has established that lowering wildtype huntingtin by 45% in the striatum of the primate brain is well-tolerated for six months, this and other research has not lowered huntingtin in layer V neurons of the primate cortex whose axons comprise the corticostriatal tract. Wildtype huntingtin is involved in the rate of production and transport from cortical neurons to the striatum of the brain-derived neurotrophic factor (BDNF) on which striatal neurons depend. It is not known whether reduction of huntingtin in the cortex and corticostriatal tract of the primate brain will have adverse effects on the survival and functioning of striatal neurons. AIM 1 of this study is to determine the long-term tolerability of wildtype huntingtin lowering in the corticostriatal tract, quantitatively measuring the effects of this lowering on striatal functioning (glutamate release, dopamine turnover) and levels of BDNF. As research tools, the project uses an shRNA already known to lower huntingtin in the primate and a serotype of AAV already known to retrogradely transduce cortical neurons from a striatal point of infusion. AIM 2 of this study is to determine if reduction of wildtype huntingtin in the primate thalamus, whether occurring incidentally from striatal infusion of vectors (as in AIM 1) or directly by vector infusion targeting the thalamus, will result in an increased thalamic calcium accumulation. Pathologic calcification of the thalamus arises in mice nine months after partial, conditional knockout of wildtype huntingtin, regardless of the age of the mouse when the knockout occurs. In the planned study, eight adult rhesus monkeys per experimental group (four of each sex) will receive intracranial injections of AAV6 encoding either a huntingtin-lowering shRNA or a control shRNA, into either the striatum or thalamus. Monkeys will be assessed by periodic behavioral and neurological exams. The glutamatergic functioning of their striatum or thalamus will be assessed in vivo nine months after AAV administration. Post-mortem analysis will include assessment of dopamine turnover and BDNF levels in striatal tissue and calcium in the thalamus. The data obtained from the nonhuman primate brain by this project will provide information that cannot be clearly obtained from human clinical trials of huntingtin-lowering treatments of HD, even through post-mortem analyses of the diseased brain. This information will be pertinent to managing and interpreting the safety and efficacy of any current or future therapies for HD that use non-allele-specific means to lower huntingtin by whatever agent or route of delivery.