Regulation of gene expression is necessary to avoid possible adverse effects of gene therapy due to excess synthesis of transgene products. To reduce transgene expression, we developed a viral vector-mediated somatic regulation system using inducible Cre recombinase. A recombinant adeno-associated virus (AAV) vector expressing Cre recombinase fused to a mutated ligand-binding domain of the estrogen receptor alpha (CreER(T2)) was delivered along with AAV vectors expressing dopamine-synthesizing enzymes to rats of a Parkinson disease model. Treatment with 4-hydroxytamoxifen, a synthetic estrogen receptor modulator, activated Cre recombinase within the transduced neurons and induced selective excision of the tyrosine hydroxylase (TH) coding sequence flanked by loxP sites, leading to a reduction in transgene-mediated dopamine synthesis. Using this strategy, aromatic L-amino acid decarboxylase (AADC) activity was retained so that l-3,4-dihydroxyphenylalanine (L-dopa), a substrate for AADC, could be converted to dopamine in the striatum and the therapeutic effects of L-dopa preserved, even after reduction of TH expression in the case of dopamine overproduction. Our data demonstrate that viral vector-mediated inducible Cre recombinase can serve as an in vivo molecular switch, allowing spatial and temporal control of transgene expression, thereby potentially increasing the safety of gene therapy.

Background: Recombinant adeno-associated virus vectors based on serotype 2 (AAV-2) have become leading vehicles for gene therapy. Most humans in the general population have anti-AAV-2 antibodies as a result of naturally acquired infections. Pre-existing immunity to AAV-2 might affect the functional and safety consequences of AAV-2 vector-mediated gene transfer in clinical applications. Methods: An enzyme-linked immunosorbent assay (ELISA) method was developed using microwell plates coated with intact particles of recombinant AAV-2 vectors, and horseradish peroxidase-conjugated anti-human immunoglobulin G (HRP-IgG). Neutralizing antibody titres were analysed by assessing the ability of serum antibody to inhibit transduction into HEK293 cells of AAV vectors that express b-galactosidase. Results: Anti-AAV-2 antibodies were detected by ELISA in two of 20 healthy subjects. The positivity criterion (optical density .0.5) in ELISA corresponded to the cut-off value (320-fold dilution of serum) in the AAV-2 neutralization assay. Influences of interfering substances were not observed. Conclusion: This ELISA method may be useful for rapid screening of anti-AAV-2 neutralizing antibodies in candidates for gene therapy.

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