Replicating Huntington's disease phenotype in experimental animals

Huntington's disease (HD) is an inherited, autosomal dominant, neurodegenerative disorder characterized by involuntary choreiform movements, cognitive decline and a progressive neuronal degeneration primarily affecting the striatum. There is at present no effective therapy against this disorder. The gene responsible for the disease (IT15) has been cloned and the molecular defect identified as an expanded polyglutamine tract in the N-terminal region of a protein of unknown function, named huntingtin (The Huntington's Disease Collaborative Research Group, 1993. Cell 72, 971-983). An intense, search for the cell pathology attached to this molecular defect is currently under way [see Sharp and Ross (1996, Neurobiol. Dis. 3, 3-15) for review]. Huntingtin interacts with a number of proteins, some of which have well identified functions, and it has thus been suggested that alterations in glycolysis, vesicle trafficking or apoptosis play a role in the physiopathology of HD. On the other hand data derived from positron emission tomography (PET), magnetic resonance spectroscopy and post-mortem biochemical evidence for a defect in succinate oxidation have suggested the implication of a primary impairment of mitochondrial energy metabolism. All these hypotheses are not necessarily to be opposed and recent findings indicate that the HD mutation could possibly directly alter mitochondrial functions which would in turn activate apoptotic pathways. To test this mitochondrial hypothesis, we studied the effects in rodents and non-human primates of a chronic blockade of succinate oxidation by systemic administration of the mitochondrial toxin 3-nitropropionic acid (3NP). Extensive behavioural and neuropathological evaluations showed that a partial but prolonged energy impairment induced by 3NP is sufficient to replicate most of the clinical and pathophysiological hallmarks of HD, including spontaneous choreiform and dystonic movements, frontal-type cognitive deficits, and progressive heterogeneous striatal degeneration at least partially by apoptosis. 3NP produces the preferential degeneration of the medium-sized spiny GABAergic neurons with a relative sparing of interneurons and afferents, as was observed in HD striatum. The present manuscript reviews the different aspects of this neurotoxic treatment in rodents and non-human primates, and its interest as a phenotypic model of HD to understand the degenerative process of HD and test new therapeutic strategies.