Long-term correction of ornithine transcarbamylase deficiency in Spf-Ash mice with a translationally optimized AAV vector

Ornithine transcarbamylase deficiency (OTCD) is an X-linked liver disorder caused by partial or total loss of OTC enzyme activity. It is characterized by elevated plasma ammonia, leading to neurological impairments, coma, and death in the most severe cases. OTCD is managed by combining dietary restrictions, essential amino acids, and ammonia scavengers. However, to date, liver transplantation provides the best therapeutic outcome. AAV-mediated gene-replacement therapy represents a promising curative strategy. Here, we generated an AAV2/8 vector expressing a codon-optimized human OTC cDNA by the α1-AAT liver-specific promoter. Unlike standard codon-optimization approaches, we performed multiple codon-optimization rounds via common algorithms and ortholog sequence analysis that significantly improved mRNA translatability and therapeutic efficacy. AAV8-hOTC-CO (codon optimized) vector injection into adult OTC(Spf-Ash) mice (5.0E11 vg/kg) mediated long-term complete correction of the phenotype. Adeno-Associated viral (AAV) vector treatment restored the physiological ammonia detoxification liver function, as indicated by urinary orotic acid normalization and by conferring full protection against an ammonia challenge. Removal of liver-specific transcription factor binding sites from the AAV backbone did not affect gene expression levels, with a potential improvement in safety. These results demonstrate that AAV8-hOTC-CO gene transfer is safe and results in sustained correction of OTCD in mice, supporting the translation of this approach to the clinic.