Functional assessment of IDUA variants of uncertain significance identified by newborn screening

With the expansion of newborn screening efforts for MPS disorders, the number of identified variants of uncertain significance in IDUA continues to increase. To better define functional consequences of identified IDUA variants, we developed a HEK293-based expression platform that can be used to determine the relative specific activity of variant α-iduronidases by combining a fluorescence-based activity assay and semi-quantitative western blotting. We employed the current platform to characterize over thirty different IDUA variants, including known benign and pathogenic variants, as well as multiple variants of uncertain significance identified through newborn screening. This analysis allowed the stratification of variant enzymes based on their relative specific activity, and uncovered distinct effects of the different variants on enzyme folding, processing, and stability. While relative specific activity serves as a useful first-level test for enzyme function, our observations reinforce the need for secondary analyses of enzyme function to fully assess variant pathogenicity.