Enzyme replacement improves ataxic gait and central nervous system histopathology in a mouse model of metachromatic leukodystrophy.

Inherited deficiencies of lysosomal hydrolases cause lysosomal storage diseases (LSDs) that are characterized by a progressive multisystemic pathology and premature death. Repeated intravenous injection of the active counterpart of the deficient enzyme, a treatment strategy called enzyme replacement therapy (ERT), evolved as a clinical option for several LSDs without central nervous system (CNS) involvement. To assess the efficacy of long-term ERT in metachromatic leukodystrophy (MLD), an LSD with prevailing nervous system disease, we treated immunotolerant arylsulfatase A (ASA) knockout mice with 52 doses of either 4 or 50 mg/kg recombinant human ASA (rhASA). ERT was tolerated without side effects and improved disease manifestations in a dose-dependent manner. Dosing of 4 mg/kg diminished sulfatide storage in kidney and peripheral nervous system (PNS) but not the CNS, whereas treatment with 50 mg/kg was also effective in the CNS in reducing storage in brain and spinal cord by 34 and 45%, respectively. Histological analyses revealed regional differences in sulfatide clearance. While 70% less storage profiles were detectable, for example, in the hippocampal fimbria, the histopathology of the brain stem was unchanged. Both enzyme doses normalized the ataxic gait of ASA knockout mice, demonstrating prevention of nervous system dysfunctions that dominate early stages of MLD.

Non-inhibitory antibodies impede lysosomal storage reduction during enzyme replacement therapy of a lysosomal storage disease.

Enzyme replacement therapy is a treatment option for several lysosomal storage disorders. We reported previously that treatment of a knockout mouse model of the sphingolipid storage disease metachromatic leukodystrophy (MLD) by intravenous injection of recombinant human arylsulfatase A (rhASA) reduces sulfatide storage and improves nervous system pathology and function. Here, we show that treated mice can develop anti-rhASA antibodies, which impede sulfatide clearance without inhibiting enzyme activity. The neutralizing effect of antibodies was reproduced in cell culture models of MLD by demonstrating that mouse immune serum reduces the ability of rhASA to clear sulfatide from cultured ASA-deficient Schwann and kidney cells. We show that reduced clearance is due to an antibody-mediated blockade of mannose 6-phosphate receptor-dependent enzyme uptake, retargeting of rhASA from sulfatide-storing cells to macrophages, intracellular misrouting of rhASA, and reduction of enzyme stability. Induction of immunotolerance to rhASA by transgenic expression of an active site mutant of human ASA restores sulfatide clearance in mice. The data indicate that the influence of non-inhibitory antibodies must be more intensively considered in evaluating the therapeutic efficacy of enzyme replacement in lysosomal storage disorders in general and in patients without cross-reacting material specifically.

Induction of tolerance to human arylsulfatase A in a mouse model of metachromatic leukodystrophy.

A deficiency of arylsulfatase A (ASA) causes metachromatic leukodystrophy (MLD), a lysosomal storage disorder characterized by accumulation of sulfatide, a severe neurological phenotype and early death. The efficacy of enzyme replacement therapy (ERT) has previously been determined in ASA knockout (ASA-/-) mice representing the only available animal model for MLD. Repeated intravenous injection of human ASA (hASA) improved the nervous system pathology and function, but also elicited a progressive humoral immune response leading to treatment resistance, anaphylactic reactions, and high mortality. In contrast to ASA-/- mice, most MLD patients express mutant hASA which may entail immunological tolerance to substituted wildtype hASA and thus protect from immunological complications. To test this notion, a cysteine-to-serine substitution was introduced into the active site of the hASA and the resulting inactive hASA-C69S variant was constitutively expressed in ASA-/- mice. Mice with sub-to supranormal levels of mutant hASA expression were analyzed. All mice, including those showing transgene expression below the limit of detection, were immunologically unresponsive to injected hASA. More than 100-fold overexpression did not induce an overt new phenotype except occasional intralysosomal deposition of minor amounts of glycogen in hepatocytes. Furthermore, long-term, low-dose ERT reduced sulfatide storage in peripheral tissues and the central nervous system indicating that high levels of extracellular mutant hASA do not prevent cellular uptake and lysosomal targeting of substituted wildtype hASA. Due to the tolerance to hASA and maintenance of the MLD-like phenotype, the novel transgenic strain may be particularly advantageous to assess the benefit and risk of long-term ERT.