Mucopolysaccharidosis type VII in the developing mouse fetus.

Mucopolysaccharidosis type VII (MPS VII) is a lysosomal storage disease caused by a deficiency of beta-glucuronidase (1). MPS VII is a fatal, progressive degenerative disorder, and a number of patients die of hydrops fetalis. Thus an approach to treating this disease may be by transplantation or gene therapy in utero. A mouse model of MPS VII has been studied extensively but the disease in affected fetal mice has not been characterized, which is essential for evaluation of therapeutic efficacy. Fetal and newborn mice affected with MPS VII were examined for lysosomal enzyme activities and for the presence of typical storage lesions in comparison to normal and carrier littermates. No beta-glucuronidase enzymatic activity was detected in any of the tissues of affected mice, indicating that transplacental transfer of beta-glucuronidase from the dam did not occur. Lesions were not detected in affected fetuses of 13.5 d gestational age on light or electron microscopy. Vacuolation in cells, typical of lysosomal accumulation of substrate, was first seen in a small number of cells of the reticulo-endothelial system in 15.5 d gestational age livers and in 18.5 d gestational age brains. Storage lesions were not seen consistently in endothelial and Kupffer cells of fetal livers until 18.5 d gestational age and in brains until birth. The results suggest that treatment of affected mice performed at 13.5 d gestational age may be effective in forestalling disease manifestations.