Intracellular Rescue of the Uroporphyrinogen III Synthase Activity in Enzymes Carrying the Hotspot Mutation C73R

Abstract

A single mutation (C73R) in the enzyme uroporphyrinogen III synthase (UROIIIS) is responsible for more than one-third of all of the reported cases of the rare autosomal disease congenital erythropoietic porphyria (CEP). CEP patients carrying this hotspot mutation develop a severe phenotype of the disease, including reduced life expectancy. Here, we have investigated the molecular basis for the functional deficit in the mutant enzyme both in vitro and in cellular systems. We show that a Cys in position 73 is not essential for the catalytic activity of the enzyme but its mutation to Arg speeds up the process of irreversible unfolding and aggregation. In the mammalian cell milieu, the mutant protein levels decrease to below the detection limit, whereas wild type UROIIIS can be detected easily. The disparate response is not produced by differences at the level of transcription, and the results with cultured cells and in vitro are consistent with a model where the protein becomes very unstable upon mutation and triggers a degradation mechanism via the proteasome. Mutant protein levels can be restored upon cell treatment with the proteasome inhibitor MG132. The intracellularly recovered C73R-UROIIIS protein shows enzymatic activity, paving the way for a new line of therapeutic intervention in CEP patients.