Coexpression of Formylglycine-Generating Enzyme Is Essential for Synthesis and Secretion of Functional Arylsulfatase A in a Mouse Model of Metachromatic Leukodystrophy

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Abstract
Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder involving inherited deficiency of arylsulfatase A (ASA). The disease is characterized by progressive demyelination and widespread deposition of sulfatide in both the central and peripheral nervous systems. Direct injection of viral vector through the blood–brain barrier is a possible gene therapy approach to MLD. However, to treat all brain cells, it is essential to secrete a sufficient amount of functional ASA from limited numbers of transduced cells. In the present study, we tested the utility of formylglycine-generating enzyme (FGE) for overexpression of functional ASA. FGE is a posttranslational modifying enzyme essential for activating multiple forms of sulfatases including ASA. COS-7 cells were transfected with ASA- and FGE-expressing plasmids. ASA activity was increased up to 20-fold in cell lysates and 70-fold in conditioned medium by coexpression of FGE. Intravenous injection of the expression plasmids into MLD knockout mice by a hydrodynamics-based procedure resulted in a significant synergistic increase in ASA activity both in liver and serum. Blot hybridization analysis of FGE mRNA demonstrated that the expression of endogenous FGE was particularly low in human brain. Our results suggest, on the basis of cross-correction of ASA deficiency, that coexpression of FGE is essential for gene therapy of MLD.