Foxton, R.; (2007) Gene transfer as a potential treatment for tetrahydrobiopterin deficient states. Doctoral thesis , University of London.

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Abstract

Tetrahydrobiopterin (BH4) is an essential cofactor for dopamine (DA), noradrenaline (NA), serotonin and nitric oxide (NO) synthesis in the brain. Inborn errors of BH4 metabolism including GTP cyclohydrolase 1 (GTP-CH) deficiency are debilitating diseases in which BH4, DA, 5-HT and NO metabolism are impaired. Current treatment for these disorders is typically monoamine replacement +/- BH4. Whilst correction of the primary defect is the ideal, BH4 treatment is problematic as it is expensive and inefficacious. One approach to treat BH4 disorders is to use gene therapy as a more permanent, effective alternative. In this thesis the potential of gene therapy in an animal model of partial BH4 deficiency, the hph-1 mouse, was examined. These mice show many neurochemical similarities associated with BH4 deficient states, including impaired BH4 (-69%), DA (-14%), NA (-23%), serotonin turnover (-55%) and NO metabolites in the brain. In cultured astrocytes from hph-1 mice BH4 was significantly lower than wild type (-53%), and produced less BH4 (- 89%) and NO metabolites (-64%) when stimulated with lipopolysaccharide (LPS) plus interferon-gamma (IFN-gamma), stimuli that increase GTP-CH and iNOS expression. When hph-1 astrocytes were infected with a recombinant adenovirus encoding human GTP cyclohydrolase (AdGCH), concentration-dependent increases in BH4 levels were observed, with just 1 virus particle per 10 cells resulting in 50-fold increases in BH4. AdGCH can upregulate the impaired NO production observed in hph-1 astrocytes following stimulation with LPS + IFN-y, although only if BH4 was increased prior to stimulation. Examination of the molecular mechanisms behind the impaired NO production in LPS + IFN-y stimulated cells revealed that iNOS dimerisation is attenuated in hph-1 astrocytes when compared wild type (-84%), and could be increased to wild type levels when cells were pre-treated with AdGCH. Analysis of total iNOS protein expression revealed no difference between wild type and hph-1. These results raise the possibility that gene therapy could be used as a corrective solution for tetrahydrobiopterin deficient states.

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