174. Liver Fibrosis in Aged OTC-KO Heterozygotes and Successful Correction by AAV8-Mediated Gene Therapy

Ornithine transcarbamylase deficiency (OTCD) is an X-linked disorder of the urea cycle. Affected males are at risk of life threatening elevation of ammonia that can lead to irreversible cognitive impairment, coma and death. Female carriers show variable clinical manifestations depending on the level of X chromosome inactivation skewing. Histological analyses on liver samples from patients with urea cycle disorders found diffuse microvesicular steatosis, nuclear glycogen, and variable portal-to-portal bridging fibrosis. The OTC knockout (KO) model generated in our laboratory through the deletion of exons 2-3 closely mimics the severe neonatal onset form of OTCD in humans. Neonatal male OTC KO pups have elevated plasma ammonia levels due to the absence of OTC expression in the liver, and they inevitably die within 24 hours after birth. Heterozygous females breed normally, have normal plasma ammonia levels, reduced liver OTC enzyme activity, elevated urine orotic acid levels, and in some cases lower body weight compared to wild type (WT) littermates. Sirius red staining on liver samples from heterozygous mice of different ages (6, 12, and 18 months old) showed liver fibrosis in aged (18-month old) OTC-KO heterozygous female mice, similar to a liver sample from a 11-year-old OTCD patient. We then tested if gene therapy could prevent the liver fibrosis. Two-month old OTC-KO heterozygous received a single tail vein injection of a self-complementary AAV8 vector encoding a codon-optimized human OTC gene (hOTCco) at 1×10^10, 3×10^10, and 1×10^11 vector genome copies per mouse. One week following vector treatment, mice in all three vector dose groups had normal urine orotic acid levels which were maintained throughout the study (16 months). Liver samples were harvested from 18 month old treated mice for pathology analysis and compared to age-matched untreated heterozygous mice and WT littermates. All treated mice showed normal liver histology similar to WT, in contrast to the untreated heterozygous animals which had fibrosis throughout the liver. In conclusion, a single injection of AAV8sc-hOTCco vector can prevent liver fibrosis in OTC-KO heterozygous and has great potential for correction of liver fibrosis in OTCD patients.