ASM is synthesized in the Endoplasmic Reticulum (ER) as a 75 kDa N-glycosylated precursor. During transit of the Golgi membrane network, enzyme that contains high mannose oligosaccharides is directed to lysosomes by attachment of mannose-6-phosphate residues, but a subset undergoes processing into Endoglycosidase-H-resistant molecules. These molecules are then diverted into a secretory pathway, ending up in the extracellular space or circulation. Both forms of the enzyme are optimally active at the normal lysosomal pH of 5.5 – 6.0 and are dependent on Zn++ for activity. However, some activity can be detected at neutral pH, suggesting that secreted ASM may be beneficial in gene therapy of Niemann-Pick disease.
Gene Therapy for Niemann-Pick
The broad aim of this project is to develop an efficient means to deliver to the human brain a genetic therapy to ameliorate the neurological deficits encountered in Type A Niemann-Pick disease (NPD). Armed with mouse efficacy data (
Passini et al. 2005), we are confident that an
that encodes human acidic
is likely to be effective in treating the disease in humans. A major challenge, however, is that very widespread expression of hASM will probably be required in order to achieve significant clinical improvement in humans. Efficacy data in knockout mice, although encouraging, does not really address the technical issues that we face in the very much larger human brain. Clinical efficacy will rely considerably upon the development of techniques to deliver gene therapy vectors to such sensitive and highly problematic regions as brainstem. Recently, we have developed a method of visualizing placement of infusion cannulas on MRI, and can actually follow infusion of
tagged with
in real-time, a technique we call Real-time Convective Delivery (RCD). In preliminary experiments, we found that these liposomes distribute very like AAV1. We hypothesize that a mixture of AAV2 containing the hASM
and GDL will permit real-time tracking of AAV-mediated gene therapy. This method would remove much guesswork from vector delivery. Accordingly, we plan to use MRI-guided delivery of AAV2-hASM in the development of a therapy for Niemann-Pick disease. We believe that our approach will form the basis of a major improvement in brain gene therapy in general, and more specifically in the treatment of neurological aspects of Lysosomal Storage Disorders. For more information on this family of rare but devastating genetic diseases, go to
Lysosomal Storage Disease on Wikipedia.
