Huntington disease arises from a combinatory toxicity of polyglutamine and copper binding.

Huntington disease (HD) is a progressive neurodegenerative disorder caused by dominant polyglutamine (polyQ) expansion within the N terminus of huntingtin (Htt) protein. Abnormal metal accumulation in the striatum of HD patients has been reported for many years, but a causative relationship has not yet been established. Furthermore, if metal is indeed involved in HD, the underlying mechanism needs to be explored. Here using a Drosophila model of HD, wherein Htt exon1 with expanded polyQ (Htt exon1-polyQ) is introduced, we show that altered expression of genes involved in copper metabolism significantly modulates the HD progression. Intervention of dietary copper levels also modifies HD phenotypes in the fly. Copper reduction to a large extent decreases the level of oligomerized and aggregated Htt. Strikingly, substitution of two potential copper-binding residues of Htt, Met8 and His82, completely dissociates the copper-intensifying toxicity of Htt exon1-polyQ. Our results therefore indicate HD entails two levels of toxicity: the copper-facilitated protein aggregation as conferred by a direct copper binding in the exon1 and the copper-independent polyQ toxicity. The existence of these two parallel pathways converging into Htt toxicity also suggests that an ideal HD therapy would be a multipronged approach that takes both these actions into consideration.