A plant-based mutant huntingtin model-driven discovery of impaired expression of GTPCH and DHFR.

Pathophysiology associated with Huntington's disease (HD) has been studied extensively in various cell and animal models since the 1993 discovery of the mutant huntingtin (mHtt) with abnormally expanded polyglutamine (polyQ) tracts as the causative factor. However, the sequence of early pathophysiological events leading to HD still remains elusive. To gain new insights into the early polyQ-induced pathogenic events, we expressed Htt exon1 (Htt ex1 ) with a normal (21), or an extended (42 or 63) number of polyQ in tobacco plants. Here, we show that transgenic plants accumulated Htt ex1 proteins with corresponding polyQ tracts, and mHtt ex1 induced protein aggregation and affected plant growth, especially root and root hair development, in a polyQ length-dependent manner. Quantitative proteomic analysis of young roots from severely affected Htt ex1 Q63 and unaffected Htt ex1 Q21 plants showed that the most reduced protein by polyQ63 is a GTP cyclohydrolase I (GTPCH) along with many of its related one-carbon (C 1 ) metabolic pathway enzymes. GTPCH is a key enzyme involved in folate biosynthesis in plants and tetrahydrobiopterin (BH 4 ) biosynthesis in mammals. Validating studies in 4-week-old R6/2 HD mice expressing a mHtt ex1 showed reduced levels of GTPCH and dihydrofolate reductase (DHFR, a key folate utilization/alternate BH 4 biosynthesis enzyme), and impaired C 1 and BH 4 metabolism. Our findings from mHtt ex1 plants and mice reveal impaired expressions of GTPCH and DHFR and may contribute to a better understanding of mHtt-altered C 1 and BH 4 metabolism, and their roles in the pathogenesis of HD.