Characterization of Wild‐Type and Mutants of Recombinant Human GTP Cyclohydrolase I

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Abstract
: To explore the molecular etiology of two disorders caused by a defect in GTP cyclohydrolase I—hereditary progressive dystonia with marked diurnal flucturation (HPD), also known as dopa‐responsive dystonia (DRD), and autosomal recessive GTP cyclohydrolase I deficiency—we purified and analyzed recombinant human wild‐type and mutant GTP cyclohydrolase I proteins expressed in Escherichia coli. Mutant proteins showed very low enzyme activities, and some mutants were eluted at a delayed volume on gel filtration compared with the recombinant wild‐type. Next, we examined the GTP cyclohydrolase I protein amount by western blot analysis in phytohemagglutinin‐stimulated mononuclear blood cells from HPD/DRD patients. We found a great reduction in the amount of the enzyme protein not only in one patient who had a frameshift mutation, but also in an HPD/DRD patient who had a missense mutation. These results suggest that a dominant‐negative effect of chimeric protein composed of wild‐type and mutant subunits is unlikely as a cause of the reduced enzyme activity in HPD/DRD patients. We suggest that reduction of the amount of the enzyme protein, which is independent of the mutation type, could be a reason for the dominant inheritance in HPD/DRD.

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