Long‐Term Health Outcomes of Individuals With Pseudodeficiency Alleles in <scp> <i>IDUA</i> </scp> May Inform Newborn Screening Practices for Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I (MPS I), a lysosomal disorder caused by variants in IDUA, was added to the Recommended Uniform Screening Panel for newborn screening in 2016. Positive screening results for MPS I are commonly due to variants known as "pseudodeficiency alleles," which decrease in vitro alpha-L-iduronidase enzyme activity but are thought to provide sufficient in vivo activity. Despite the historic assumption that these variants are biologically benign, the possibility that they could give rise to complex, multigenic, or attenuated phenotypes has not been systemically evaluated in adults. We completed a retrospective matched cohort study using a hospital-based biorepository with data from 65,309 participants, we identified 1803 individuals harboring homozygous IDUA pseudodeficiency alleles. Using electronic medical records (EMR), we compared the prevalence of features of MPS I in participants with homozygous pseudodeficiency alleles to a cohort of matched control participants. We found no clinically relevant significant differences between cases and controls nor genotype-phenotype associations across four alleles. These findings provide empiric support that adults with homozygous IDUA pseudodeficiency alleles are unlikely to develop mild symptoms of disease compared with controls. This study provides a proof-of-concept model for other nonclassical disease variants related to other inherited metabolic disorders, which is necessary as newborn screening expands.