| Popis: |
Metachromatic Leukodystrophy (MLD) is a demyelinating lysosomal storage disorder with an urgent medical need. We previously showed that transplantation of lentiviral vector- transduced Hematopoietic Stem Cells (HSC) targets therapeutic genes to the CNS and PNS and prevents functional and pathological manifestations of MLD in the mouse model, when applied at early pre-symptomatic stage (Biffi A. et al., JCI 2004). We now report that the HSC-based approach can also correct already established neurologic functional deficits and neuronal damage when applied to symptomatic animals. These new data, considering the poor or null outcome of hematopoietic stem cell transplant on symptomatic patients, provide a strong rationale for testing gene therapy in MLD patients. To this regard, we showed that the degree of efficacy of gene therapy is dependent on the levels of enzyme activity in HSC and in target organs, thus strengthening the concept that the major advantage of gene therapy might be related to the possibility to express the functional enzyme largely above normal donor's levels in HSC and in their progeny. As far as the mechanism of disease correction and the biodistribution of the functional enzyme are concerned, we demonstrated the role of gene corrected, ARSA over-expressing microglia as unique and efficacious source of bioavailable enzyme in the brain. Further, we provided a long-sought proof of the direct in vivo occurrence of active enzyme secretion by HSC- derived microglia and of enzyme transfer to other neural and non-neural cells in the brain. Interestingly, by establishing a sustained hepatic source of ARSA in chimeric mice harboring ARSA over-expressing hepatocytes from transgenic donors, we failed to deliver the enzyme to the CNS. These new findings highlight the role of microglia as exclusive source of bioavailable enzyme to the brain, and most importantly challenge the potential efficacy of newly developed enzyme replacement strategies for CNS targeting in lysosomal disorders. Finally, the detection of ARSA in the peripheral nervous system of treated mice underlines, together with correction of PNS neurophysiological and histopathological abnormalities, underlines the relevance of this approach in targeting not only the CNS, but also the widespread distributed PNS network, whose involvement is critical for patient's prognosis. |
