Sustained correction of OTC deficiency in spf( ash) mice using optimized self-complementary AAV2/8 vectors

Ornithine transcarbamylase deficiency (OTCD) is the most common inborn error of urea synthesis. Complete OTCD can result in hyperammonemic coma in the neonatal period, which can rapidly become fatal. Current acute therapy involves dialysis; chronic therapy involves the stimulation of alternate nitrogen clearance pathways; and the only curative approach is liver transplantation. Adeno-associated virus (AAV) vector-based gene therapy would add to current treatment options provided the vector delivers high level and stable transgene expression in liver without dose-limiting toxicity. In this study, we employed an AAV2/8-based self-complementary (sc) vector expressing the murine OTC (mOTC) gene under a liver-specific thyroxine-binding globulin promoter and examined the therapeutic effects in a mouse model of OTCD, the spf (ash) mouse. Seven days after a single intravenous injection of vector, treated mice showed complete normalization of urinary orotic acid, a measure of OTC activity. We further improved vector efficacy by incorporating a Kozak or Kozak-like sequence into mOTC complementary DNA, which increased the OTC activity by five or twofold and achieved sustained correction of orotic aciduria for up to 7 months. Our results demonstrate that vector optimizations can significantly improve the efficacy of gene therapy.

Preclinical evaluation of a clinical candidate AAV8 vector for ornithine transcarbamylase (OTC) deficiency reveals functional enzyme from each persisting vector genome

Ornithine transcarbamylase deficiency (OTCD), the most common and severe urea cycle disorder, is an excellent model for developing liver-directed gene therapy. No curative therapy exists except for liver transplantation which is limited by available donors and carries significant risk of mortality and morbidity. Adeno-associated virus 8 (AAV8) has been shown to be the most efficient vector for liver-directed gene transfer and is currently being evaluated in a clinical trial for treating hemophilia B. In this study, we generated a clinical candidate vector for a proposed OTC gene therapy trial in humans based on a self-complementary AAV8 vector expressing codon-optimized human OTC (hOTCco) under the control of a liver-specific promoter. Codon-optimization dramatically improved the efficacy of OTC gene therapy. Supraphysiological expression levels and activity of hOTC were achieved in adult spf(ash) mice following a single intravenous injection of hOTCco vector. Vector doses as low as 1×10(10) genome copies (GC) achieved robust and sustained correction of the OTCD biomarker orotic aciduria and clinical protection against an ammonia challenge. Functional expression of hOTC in 40% of liver areas was found in mice treated with a low vector dose of 1×10(9) GC. We suggest that the clinical candidate vector we have developed has the potential to achieve therapeutic effects in OTCD patients.

Muscle-directed gene therapy for hemophilia B with more efficient and less immunogenic AAV vectors

BACKGROUND

Adeno-associated viral vector (AAV)-mediated and muscle-directed gene therapy is a safe and non-invasive approach to treatment of hemophilia B and other genetic diseases. However, low efficiency of transduction, inhibitor formation and high prevalence of pre-existing immunity to the AAV capsid in humans remain as main challenges for AAV2-based vectors using this strategy. Vectors packaged with AAV7, 8 and 9 serotypes have improved gene transfer efficiencies and may provide potential alternatives to overcome these problems.

OBJECTIVE

To compare the long-term expression of canine factor IX (cFIX) levels and anti-cFIX antibody responses following intramuscular injection of vectors packaged with AAV1, 2, 5, 7, 8 and 9 capsid in immunocompetent hemophilia B mice.

RESULTS

Highest expression was detected in mice injected with AAV2/8 vector (28% of normal), followed by AAV2/9 (15%) and AAV2/7 (10%). cFIX expression by AAV2/1 only ranged from 0 to 5% of normal levels. High incidences of anti-cFIX inhibitor (IgG) were detected in mice injected with AAV2 and 2/5 vectors, followed by AAV2/1. None of the mice treated with AAV2/7, 2/8 and 2/9 developed inhibitors or capsid T cells.

CONCLUSIONS

AAV7, 8 and 9 are more efficient and safer vectors for muscle-directed gene therapy with high levels of transgene expression and absence of inhibitor formation. The absence of antibody response to transgene by AAV7, 8 and 9 is independent of vector dose but may be due to the fact that these three serotypes are associated with high level distribution to, and transduction of, hepatocytes following i.m. injection.