Bone marrow stem cell gene therapy of arylsulfatase A-deficient mice, using an arylsulfatase A mutant that is hypersecreted from retrovirally transduced donor-type cells

Arylsulfatase A (ASA)- deficient mice represent an animal model for the fat al lysosomal storage disease metachromatic leukodystrophy, which is charact erized by widespread intralysosomal deposition of sulfatide. Bone marrow st em cell gene therapy in mice, using a retroviral vector mediating expressio n of wild-type human ASA, has the potential to ameliorate the visceral path ology, but improves the prevailing brain disease and neurologic symptoms on ly marginally. One factor that influences the efficacy of bone marrow trans plantation therapy in lysosomal storage diseases is the secretion level of the therapeutic enzyme from donor-type cells. Here we test the potential of a hypersecreted glycosylation variant of ASA. Although this mutant lacks m annose 6-phosphate residues it is taken up by cells by a mannose 6-phosphat e receptor-independent pathway and causes partial metabolic correction of A SA-deficient mouse cells. Retrovirally mediated transfer of the mutant cDNA into ASA-deficient mice results in the sustained expression of the transge ne. Serum levels argue for an increased secretion of the glycosylation muta nt also in vivo. Tissue levels were reduced to 2% in liver and up to 40% in kidney compared with animals treated with the wild-type enzyme, indicating reduced endocytosis. Thus, the limited uptake of the variant enzyme outwei ghs the putative advantageous effect of improved supply. Although the mutan t enzyme is able to correct the metabolic defect partially, histological ex aminations did not reveal any reduction of sulfatide storage in treated ani mals. Surprisingly, analysis of neurologic symptoms indicated a significant improvement of the gait pattern.