169. Expression Studies of Ornithine Transcarbamylase in Liver Using Minicircle Vectors for Gene Therapy: Flag-Tagging of a Mitochondrial Enzyme and Stable Expression Under Control of an Endogenous Otc Promoter-Enhancer

Abstract

Ornithine transcarbamylase deficiency (OTCD) is the most common inherited defect of the urea cycle resulting in severe hyperammonemia and death if left untreated. We are aiming at correcting OTCD using naked-DNA minicircle (MC) vector mediated gene therapy in the spfash mouse model with low residual OTC activity. A critical parameter is delivery to periportal hepatocytes where the urea cycle is located because of metabolic zonation in the liver. To distinguish between MC-born and endogenous OTC enzyme, we generated an expression cassette with an internally Flag-tagged OTC enzyme. Internal epitope tagging is required because of the N-terminal mitochondrial import sequence. A corresponding Flag-tag sequence was introduced C-terminally of the mitochondrial import signal. The tagged protein performed similar to its non-tagged OTC enzyme upon hydrodynamic tail vein injection for liver targeting in spfash mice, indicating that the tag neither interferes with the mitochondrial import nor the formation of a functional OTC trimer. Furthermore we generated an endogenous Otc promoter-enhancer construct, termed PmO1, for potential specific or “natural” OTC transgene expression. According to others, the promoter (672 bp) sequence derived from mouse Otc was not sufficient for liver specificity (Veres et al, J Biol Chem 261: 7588-7591, 1986) and requires a corresponding enhancer sequence (Nishiyori et al, J Biol Chem 269: 1323-1331, 1994). An enhancer sequence (232 bp) originating from rat Otc was used that contains several binding sites for liver-selective transcription factors (Murakami et al, Mol Cell Biol 10: 1180-1191, 1990). The resulting MC-vector expressing OTC from this PmO1 promoter-enhancer construct was delivered to spfash mice via hydrodynamic tail vein injection, resulting in similar enzymatic activity in whole liver extracts compared to wild-type mice. Vectors designed to express Flag-tagged OTC driven from a natural Otc promoter might be useful to study biodistribution and/or long-term stable expression after MC-based gene therapy for OTCD.