Journal of IiME Volume 1 Issue 2 www.investinme.org Gene therapy for mitochondrial dysfunctions using optimized mRNA transport to the mitochondrial surface (continued) A. Introduction Mitochondria play a central role in intermediary metabolism, energy production, ion homeostasis and apoptosis 1. Impairment of mitochondrial function is the key pathogenic factor in a growing number of human diseases. Indeed, primary defects in mitochondrial oxidative phosphorylation (OXPHOS) function are implicated in over 130 diseases 2. Their clinical presentations range from fatal infantile disease to adult muscle weakness and/or nervous system dysfunction. Moreover, mitochondrial impairment can lead to tumor formation and probably play a role in the aging process 3 . Mitochondrial OXPHOS disorders are far more common than was previously anticipated. Recent epidemiological studies have shown that their prevalence is at least one in 5000, making this group of diseases probably the most frequent form of metabolic disorders 4. Approximately 300 mitochondrial DNA (mtDNA) alterations have been identified as the genetic cause of mitochondrial diseases, one-third of which are located in coding genes for OXPHOS proteins 5. Despite, more than 70% of human degenerative diseases involving mitochondrial deficiencies remain unravelled at the molecular level; since they are caused by mutations in nuclear-encoded mitochondrial proteins. Hence, only 56 nuclear genes encoding mitochondrial proteins underly clinical mitochondrial disorders 6. The main obstacle encountered for the identification of disease causing genes is that at least half of the 1500 estimated mitochondrial proteins 7 is not yet discovered; indeed, up until today only 807 are ascribed to the most extensive database of human mitochondrial proteins (http://www.mitop.de:8080/mitop2 ), 8. The understanding of the pathogenesis of mitochondrial diseases has improved considerably in the last decade. Nevertheless, the most disappointing area is the lack of efficient treatment for patients with mitochondrial diseases. Indeed, they are still treated with vitamin and cofactor mixtures, harmless but largely inadequate and inefficient. Ocular involvement is a prevalent feature in mitochondrial diseases, indeed retina cells contain a large number of mitochondria, reflecting their high requirements for OXPHOS 9. Moreover, mitochondrial impairment may contribute to changes in macular function observed in aging and age-related macular dystrophy 10. Leber Hereditary Optic Neuropathy (LHON) and Dominant Optic Atrophy (DOA) are both non-syndromic optic neuropathies with a mitochondrial etiology. LHON is associated with point mutations in the mitochondrial genome. The majority of DOA patients harbor mutations in the nuclear-encoded protein OPA1 which is targeted to mitochondria. In both disorders the retinal ganglion cells (RGCs) are specific cellular targets of the degenerative process 11. Neurogenic muscle weakness, Ataxia, Retinitis Pigmentosa (NARP) syndrome is due to a point mutation in the mitochondrial ATP6 gene. The most common ocular feature associated with the mutation is retinal Invest in ME Charity Nr 1114035 dystrophy, with a substantial variability in rod and cone photoreceptor manifestations 9. As for other visual impairments or mitochondrial disorders, no efficient therapies are available at the present time and current understanding of the cellular and molecular mechanisms underlying retinal cell death due to mitochondrial dysfunction is still quite limited. Remarkably, the eye has a combination of features that make it ideally suited as a target organ for gene therapy. The highly compartmentalized anatomy of the eye facilitates accurate delivery of vectors at target sites within the globe especially at the vicinity of retinal cells, which minimizes systemic dissemination and unwanted systemic effects. The blood retinal barrier and the retinal pigment epithelium (RPE), anatomically protect a wide-spread diffusion of the vectors to the systemic circulation. These barriers also provide a beneficial effect in protecting the retina from the immune response 12. Retinal function can be easily monitored with non-invasive and quantitative tests such as ophthalmoscopy, electroretinogram (ERG), optical coherence tomography (OCT), and visual evoked potentials (VEP). Moreover, appropriate animal models resembling human retinal abnormalities are available for the development of experimental therapies. Notable successes have been achieved by gene replacement strategies in some of these models. For instance, the Swedish Briand dog is a model for a null mutation in the RPE65 gene. This gene encodes an RPE-specific visual cycle isomerase involved in the synthesis of 11-cis retinal. Mutations in the RPE65 gene are responsible of Leber’s Congenital Amaurosis (LCA), representing a group of severe earlyonset retinal dystrophies 13. The fact that there are close similarities between human and Briand dogs, in terms of the clinical characteristics of the disease allowed the evaluation of gene replacement therapy. Thus, three independents groups have now reported the restoration of vision in these dogs by the use of recombinant AAV vector-mediated delivery of the RPE65 gene 14, 15, 16. These recent advances have enabled the development of proposals for clinical trials of gene therapy for ocular diseases. In May 2007, the first patient, out of 12, has been treated with the rAAV2-RPE65 vector (Dr. R. Ali, College University, London) at the ophthalmologic hospital of Moorfields in London. This is the first step of phase I/II doseescalation clinical trial for this severe early-onset retinal degeneration. Dr. F. Rolling (INSERM U 649, Nantes) will conduct a clinical trial in 2009. Our main objective is to develop in the near future a gene therapy that could be both preventive and curative for retinal dystrophies due to mitochondrial dysfunctions. In this purpose we were mostly interested in the LHON disease. LHON was the first maternally inherited disease to be associated with point mutations in mtDNA and is now considered the most (continued on page 23) Page 22/72

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