Murine mucopolysaccharidosis type VII: The impact of therapies on the clinical course and pathology in a murine model of lysosomal storage disease

Murine mucopolysaccharidosis type VII (MPS VII) is a lysosomal storage disease caused by a recessively inherited deficiency of the lysosomal enzyme β-glucuronidase. Affected mice have clinical, biochemical and pathological findings similar to those seen in humans with MPS VII (Sly syndrome), including growth retardation, facial dysmorphism, deafness, behavioural deficits and widespread glycosaminoglycan storage in lysosomes in the viscera, skeleton and brain. This mouse model is a useful tool for the evaluation of the effectiveness and experimental therapies for the MPS disorders. Syngeneic bone marrow transplantation performed in newborn MPS VII animals - before clinical evidence of disease is pronounced - prolongs life, improves hearing and bone growth, and prevents lysosomal storage in many sites, but does not correct the central nervous system disease. Enzyme therapy with β-glucuronidase from the first days of life does reduce lysosomal storage in the brain in murine MPS VII. The enzyme-replaced mice also have reduced visceral lysosomal storage, impressive normalization of their phenotype and an improved life span. The effectiveness of gene therapy for the treatment of lysosomal storage disease has also been tested using the MPS VII model. When transplanted into MPS VII mice, syngeneic haematopoietic stem cells or mouse skin fibroblasts infected with retrovirus expressing β-glucuronidase decreased storage, but only in the liver and spleen. Injection of an adenovirus vector expressing β-glucuronidase into the vitreous of the MPS VII mice reduced storage in the retinal pigment epithelium and corneal endothelium. Intravenous administration of the adenovirus vector transduced with the β-glucuronidase gene reduced liver and spleen storage and, when instilled into the cerebral ventricles, this viral vector caused β-glucuronidase production in epithelial cells lining the ventricles. Recently, retroviral vector-corrected MPS VII fibroblasts secreting high levels of β-glucuronidase were engrafted directly into the brains of adult MPS VII mice with resultant reduction in storage in neurons and glia adjacent to the grafts. Future efforts aimed at prolonging expression of the β-glucuronidase gene by viral vectors and more precisely directing the therapeutic effect to the skeleton and brain will be important in optimizing treatments for murine MPS VII and extending the results of such therapies to humans with MPS.