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 in real-time, a technique we call Real-time Convective Delivery (RCD). With this approach, we can tell quickly if something is going
wrong with the infusion. In addition, we have recently discovered a new way of delivering viral vectors to the cortex by taking advantage of
neuronal projections from the thalamus. Because the cortex is large in relationship to the thalamus, infusion of AAV into the thalamus may be an efficient way to distribute therapeutic genes to the cortex without performing much more invasive multiple infusions. We are currently planning a Phase 1 clinical trial to treat infants who have the most severe form of Niemann-Pick disease. Hopefully this same approach will be applicable to other kinds of
Lysosomal Storage Disorders.
