360. Genotype/Phenotype Correlation of Cellular Function and AAV-Mediated Gene Delivery to Treat Xeroderma Pigmentosum - Cockayne Syndrome (XP - CS)

Abstract

Cockayne Syndrome (CS) is a rare, autosomal recessive, neurodegenerative disorder characterized by deficiencies which all contribute to an overall phenotype of premature aging. The underlying cause is a defect in genes involving DNA repair mechanisms. This includes CSA and CSB as well as several genes associated with Xeroderma Pigmentosum (XP). Specifically, Xeroderma Pigmentosum Group G (XPG) is a disorder with two distinct clinical presentations: photosensitivity alone (XP) and photosensitivity with neurodegeneration (XP-CS).XPG's role as an endonuclease in nucleotide excision repair following UV exposure is well-described and explains the photosensitivity phenotype, yet a separate function for XPG that explains the neurological/early aging deficit that occurs in some patients remains obscure. Other CS proteins known to be involved in nuclear DNA repair have been shown to also function as free radical scavengers in mitochondria. As growing bodies of data illustrate the importance of mitochondria in aging, neuronal cell development and maintenance, and tissue repair, we hypothesize that XPG is trafficked to mitochondria where it could play an important role in mitochondrial function via free radical regulation. Such a deficiency could explain the neurodegeneration and multi-system early aging phenotype observed in CS patients. Our preliminary data (expansion rates, oxygen consumption, ATP generation, free radical sensitivity, Metronidazole [a drug that causes liver failure in CS patients] sensitivity) support this hypothesis through demonstration of decreased viability and mitochondrial function in fibroblasts derived from XPG patients displaying the XP-CS phenotype but not in those from patients displaying the XP phenotype alone. Mitochondrial isolations are being performed to confirm the presence of XPG in these organelles, as well as to better evaluate mitochondrial function.These characterizations will be used as outcome measures to determine the efficacy of AAV mediated gene therapy for XP-CS. A functional CMV-XPG plasmid has been cloned, and is being packaged into Adeno-associated virus (AAV). Further analyses in XP and XP-CS patient induced pluripotent stem cells differentiated into neurons and an XPG mouse model which closely replicates the human phenotype will yield useful information regarding the dual roles of this protein and provide data to support translation of gene therapy for CS. If successful, this will be the first therapeutic measure to demonstrate correction of the debilitating effects of XP-CS.