PROJECT SUMMARY/ABSTRACT Mucolipidosis is a group of inherited conditions in which lysosomal enzymes do not properly traffic to the lysosomes, due to deficient activity of UDP-N-acetylglucosamine: lysosomal enzyme N-acetyl-glucosamine-1- phosphotransferase and subsequent failure to add mannose-6-phosphate residues to lysosomal enzymes. As a result, lysosomes accumulate a variety of substrates that cannot be properly degraded, causing progressive physical and neurological disease and early death. Meanwhile, soluble lysosomal enzymes, lacking mannose- 6-phosphate, can be found in the bloodstream in abnormally high amounts. There is no available treatment. Here, using a null mouse model of mucolipidosis type II (also known as “I-cell disease”), we propose to study gene therapy with a novel S1S3 phosphotransferase for correction of this disorder. We will employ an adeno- associated viral vector, AAV-9, that has the ability to effect widespread transduction of systemic organs and brain when administered intravenously to mice. The experiments are designed to address the following regarding the amenability of mucolipidosis type II to treatment with S1S3 phosphotransferase: 1) biodistribution of phosphotransferase expression to relevant organs including brain, 2) restoration of mannose-6- phosphorylation of lysosomal hydrolases, 3) improvement in lysosomal storage, and the distribution of this effect, 4) improvement in phenotype, including growth and behavior, and 5) lack of apparent toxicity, including mortality, in treated mice. The project combines complementary expertise in glycosylation and trafficking of lysosomal proteins, therapy development for lysosomal diseases, and characterization of the mucolipidosis-II mouse including behavior and pathology.