237. Evaluation of Low-Level, Constitutive Expression of β-glucuronidase on the Clinical Manifestations of Mucopolysaccharidosis Type VII

Abstract

Mucopolysaccharidosis Type VII (MPS VII) is caused by a deficiency in the acid hydrolase, |[beta]|-glucuronidase (GUSB). In the absence of this enzyme, glycosaminoglycans (GAGs) cannot be fully degraded and accumulate within the lysosomes of many cell types. Biochemically, MPS VII is characterized by elevated GAG levels and secondary elevations in other lysosomal enzymes, such as |[alpha]|-galactosidase (|[alpha]|-gal). Clinical symptoms include hearing and vision loss, skeletal dysplasia, mental retardation, and shortened life span. A mouse model of MPS VII exhibits many of these abnormalities, as well as reproductive and immune defects, and has been used extensively to evaluate new therapies. Previous studies have suggested that constitutive, low levels of GUSB (0.5|[ndash]|5% of normal) are sufficient to significantly correct this disease. At these levels, the biochemical symptoms show considerable improvement. However, the main goal of therapy is to alleviate the clinical symptoms of MPS VII. The relationship between biochemical and clinical improvement has not been carefully examined. Previous studies have demonstrated that adeno-associated virus-based (AAV) vectors delivered at birth and expressing high levels of GUSB can vastly improve the clinical manifestations of MPS VII. In order to understand how lower levels of expression impact disease progression, we constructed an AAV vector that drives expression of human GUSB in a liver-specific manner via the control of the human |[alpha]|-1 antitrypsin promoter. MPS VII mice, identified at birth, were given 2|[times]|109 IU of AAV intravenously. AAV-treated MPS VII mice had 4% normal levels of GUSB in the liver and maintained serum levels at 2% normal. GUSB activities in the spleen, kidney, lung and brain were 0.5%, 0.1%, 0.3%, 0.7%, respectively. At these levels, significant reductions in both |[alpha]|-gal levels (liver: MPS VII- 244|[plusmn]|3 U/mg; AAV-treated- 91|[plusmn]|29 U/mg; spleen: MPS VII- 186|[plusmn]|5 U/mg; AAV-treated- 121|[plusmn]|60; brain: MPS VII- 113|[plusmn]|11 U/mg; AAV-treated- 52|[plusmn]|2 U/mg) and in GAG levels (liver: MPS VII- 14|[plusmn]|0.6 |[mu]|g/mg; AAV-treated- 2.3|[plusmn]|0.8 |[mu]|g/mg) were observed. However, the clinical impact of our therapy does not mirror the biochemical improvements. AAV-treated MPS VII mice show no significant increase in longevity and the hearing loss shows only a small but significant improvement in the middle of the hearing range (20 kHz: wild type- 28|[plusmn]|10 dB; MPS VII- 82|[plusmn]|8 dB; AAV-treated- 62|[plusmn]|9 dB, p < 0.0002). Matings between AAV-treated MPS VII mice frequently resulted in pregnancies (83%), but many of the dams have fatal complications during labor. Retinal deficits show the best response to the therapy (wild type- 862|[plusmn]|254 |[mu]|V; MPS VII- 254|[plusmn]|114 |[mu]|V; AAV-treated- 575|[plusmn]|37 |[mu]|V, p<0.0001). Our findings suggest that, while low levels of GUSB expression improve the biochemical symptoms of MPS VII, the clinical manifestations of this disease are more refractory to therapy. These results highlight the importance of correlating biochemical correction with clinical improvements when evaluating the efficacy of preclinical gene therapy experiments.