A congenital disorder of deglycosylation: Biochemical characterization of N-glycanase 1 deficiency in patient fibroblasts

N-glycanase 1, encoded by NGLY1, catalyzes the deglycosylation of misfolded N-linked glycoproteins retrotranslocated into the cytosol. We identified 9 cases with mutations in NGLY1. The patients show developmental delay, seizures, peripheral neuropathy, abnormal liver function, and alacrima (absence of tears). The mutations in NGLY1 resulted in the absence of N-glycanase 1 protein in patient-derived fibroblasts. Applying a recently established cellular deglycosylation-dependent Venus fluorescence assay, we found that patient fibroblasts had dramatically reduced fluorescence, indicating a pronounced reduction in N-glycanase enzymatic activity. Using this assay, we could find no evidence of other related activities. Our findings reveal that NGLY1 mutations destroy both N-glycanase 1 protein and enzymatic