Deirdre McMenamin

A dual AAV system enables the Cas9-mediated correction of a metabolic liver disease in newborn mice

Many genetic liver diseases in newborns cause repeated, often lethal, metabolic crises. Gene therapy using nonintegrating viruses such as adeno-associated virus (AAV) is not optimal in this setting because the nonintegrating genome is lost as developing hepatocytes proliferate. We reasoned that newborn liver may be an ideal setting for AAV-mediated gene correction using CRISPR-Cas9. Here we intravenously infuse two AAVs, one expressing Cas9 and the other expressing a guide RNA and the donor DNA, into newborn mice with a partial deficiency in the urea cycle disorder enzyme, ornithine transcarbamylase (OTC). This resulted in reversion of the mutation in 10% (6.7–20.1%) of hepatocytes and increased survival in mice challenged with a high-protein diet, which exacerbates disease. Gene correction in adult OTC-deficient mice was lower and accompanied by larger deletions that ablated residual expression from the endogenous OTC gene, leading to diminished protein tolerance and lethal hyperammonemia on a chow diet.

Hepatic Gene Transfer in Neonatal Mice by Adeno-Associated Virus Serotype 8 Vector

For genetic diseases that manifest at a young age with irreversible consequences, early treatment is critical and essential. Neonatal gene therapy has the advantages of achieving therapeutic effects before disease manifestation, a low vector requirement and high vector-to-cell ratio, and a relatively immature immune system. Therapeutic effects or long-term rescue of neonatal lethality have been demonstrated in several animal models. However, vigorous cell proliferation in the newborn stage is a significant challenge for nonintegrating vectors, such as adeno-associated viral (AAV) vector. Slightly delaying the injection age, and readministration at a later time, are two of the alternative strategies to solve this problem. In this study, we demonstrated robust and efficient hepatic gene transfer by self-complementary AAV8 vector in neonatal mice. However, transduction quickly decreased over a few weeks because of vector dilution caused by fast proliferation. Delaying the injection age improved sustained expression, although it also increased neutralizing antibody (NAb) responses to AAV capsid. This approach can be used to treat genetic diseases with slow progression. For genetic diseases with early onset and severe consequences, early treatment is essential. A second injection of vector of a different serotype at a later time may overcome preexisting NAb and achieve sustained therapeutic effects.

Comparative Study of Liver Gene Transfer With AAV Vectors Based on Natural and Engineered AAV Capsids

Vectors based on the clade E family member adeno-associated virus (AAV) serotype 8 have shown promise in patients with hemophilia B and have emerged as best in class for human liver gene therapies. We conducted a thorough evaluation of liver-directed gene therapy using vectors based on several natural and engineered capsids including the clade E AAVrh10 and the largely uncharacterized and phylogenically distinct AAV3B. Included in this study was a putatively superior hepatotropic capsid, AAVLK03, which is very similar to AAV3B. Vectors based on these capsids were benchmarked against AAV8 and AAV2 in a number of in vitro and in vivo model systems including C57BL/6 mice, immune-deficient mice that are partially repopulated with human hepatocytes, and nonhuman primates. Our studies in nonhuman primates and human hepatocytes demonstrated high level transduction of the clade E-derived vectors and equally high transduction with vectors based on AAV3B. In contrast to previous reports, AAVLK03 vectors are not superior to either AAV3B or AAV8. Vectors based on AAV3B should be considered for liver-directed gene therapy when administered following, or before, treatment with the serologically distinct clade E vectors.

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.

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.

Biodistribution of AAV8 Vectors Expressing Human Low-Density Lipoprotein Receptor in a Mouse Model of Homozygous Familial Hypercholesterolemia

Recombinant adeno-associated viral vectors based on serotype 8 (AAV8) transduce liver with superior tropism following intravenous (IV) administration. Previous studies conducted by our lab demonstrated that AAV8-mediated transfer of the human low-density lipoprotein receptor (LDLR) gene driven by a strong liver-specific promoter (thyroxin-binding globulin [TBG]) leads to high level and persistent gene expression in the liver. The approach proved efficacious in reducing plasma cholesterol levels and resulted in the regression of atherosclerotic lesions in a murine model of homozygous familial hypercholesterolemia (hoFH). Prior to advancing this vector, called AAV8.TBG.hLDLR, to the clinic, we set out to investigate vector biodistribution in an hoFH mouse model following IV vector administration to assess the safety profile of this investigational agent. Although AAV genomes were present in all organs at all time points tested (up to 180 days), vector genomes were sequestered mainly in the liver, which contained levels of vector 3 logs higher than that found in other organs. In both sexes, the level of AAV genomes gradually declined and appeared to stabilize 90 days post vector administration in most organs although vector genomes remained high in liver. Vector loads in the circulating blood were high and close to those in liver at the early time point (day 3) but rapidly decreased to a level close to the limit of quantification of the assay. The results of this vector biodistribution study further support a proposed clinical trial to evaluate AAV8 gene therapy for hoFH patients.

Determining the Minimally Effective Dose of a Clinical Candidate AAV Vector in a Mouse Model of Crigler-Najjar Syndrome

Liver metabolism disorders are attractive targets for gene therapy, because low vector doses can reverse the buildup of toxic metabolites in the blood. Crigler-Najjar syndrome is an inherited disorder of bilirubin metabolism that is caused by the absence of uridine diphosphate glucuronosyl transferase 1A1 (UGT1A1) activity. This syndrome is characterized by hyperbilirubinemia and jaundice. Unfortunately, current phototherapy treatment is not effective long term. We intravenously injected phototherapy-rescued adult UGT1 knockout mice with 2.5 × 1010–2.5 × 1013 genome copies (GC)/kg of a clinical candidate vector, AAV8.TBG.hUGT1A1co, to study the treatment of disease compared to vehicle-only control mice. There were no apparent vector-related laboratory or clinical sequelae; the only abnormalities in clinical pathology were elevations in liver transaminases, primarily in male mice at the highest vector dose. Minimal to mild histopathological findings were present in control and vector-administered male mice. At vector doses greater than 2.5 × 1011 GC/kg, we observed a reversal of total bilirubin levels to wild-type levels. Based on a significant reduction in serum total bilirubin levels, we determined the minimally effective dose in this mouse model of Crigler-Najjar syndrome to be 2.5 × 1011 GC/kg.

Alternative Start Sites Downstream of Non-Sense Mutations Drive Antigen Presentation and Tolerance Induction to C-Terminal Epitopes

CTL responses to the transgene product remain an active area of concern for the gene therapy field. A patient’s underlying genetic mutation may influence the qualitative nature of these potentially destructive T cell responses. Individuals with a mutation that introduces a premature termination codon (PTC) that prevents synthesis of the full-length peptide are considered more likely to mount a transgene-specific T cell response because of a lack of immune tolerance to C-terminal epitopes as a consequence of absent endogenous Ag presentation. In this article, we demonstrate that a human ornithine transcarbamylase gene containing various PTC-inducing non-sense mutations is able to generate and present epitopes downstream of the termination codon. Generation of these epitopes occurs primarily from alternative translation start sites downstream of the stop codon. Furthermore, we show that expression of these genes from adeno-associated virus vectors in C57BL/6 mice is able to induce peripheral tolerance to epitopes downstream of the PTC. These results suggest that, despite the lack of full-length endogenous protein, patients with PTC-inducing non-sense mutations may still present T cell epitopes downstream of the premature termination site that may render the subject tolerant to wild-type transgene products.

696. TLR9 Signaling Mediates Transgene Antibody Formation

Recent success in gene therapy clinical trials have shown great promise towards developing efficacious treatments, however antibody formation to the therapeutic protein resulting in treatment attenuation still remains a concern. Driven by the immunologic stimuli of vector administration, transgene expression can elicit a humoral response against “non-self” protein, as these antigens were not presented during the process of central tolerance induction. Previously, we have shown that TLR9 signaling is necessary to mount a CD8+ T cell response that reduces expression of non-secreted transgene product. Here, we show that TLR9 signaling is also involved in driving the activation of B cells to secrete transgene-specific neutralizing antibody. Delivery of the viral vector and the effect of TRL9, route of administration, and promoter on the immune response to transgene expression was studied using the human IDUA gene, which is mutated in the lysosomal storage disorder, mucopolysaccharidosis type I (MPS I). IV administration of C57Bl/6 mice with AAV9.hIDUA under the control of a universal CMV promoter resulted in the production of neutralizing IgG antibodies and loss of transgene activity measured in serum. Surprisingly, when administered to TLR9 knockout mice, antibody formation was very modest to not detectable and IDUA activity maintained at a steady state. Next, we administered AAV9 vectors with expression driven by different promoters, including the universal promoters, CMV and CB7, and the liver-restricted promoter, TBG. Mice that received vectors containing universal promoters demonstrated elevated IgG to hIDUA, while those with liver-specific expression did not. Co-administration of the TBG- and CMV-containing vectors did not inhibit IgG formation, demonstrating that expression in non-liver tissue drives the immune system to a humoral response, and overcomes the potential tolerogenic effect of liver-specific expression. In ongoing experiments, hIDUA constructs containing CpG motifs that are known to bind efficiently to TLR9 will be used to assess the impact of TLR9 signaling on transgene-specific neutralizing antibody production. Overall, our results suggest that TLR9 signaling is important for the development of anti-transgene antibodies.

481. CRISPR/Cas9-Mediated In Vivo Genome Editing to Correct the OTC spfash Mutation in Newborn Mice

Many genetic liver diseases, including OTC deficiency, present in newborns with repeated, often lethal, metabolic crises. Adeno-associated virus (AAV) neonatal gene therapy in this setting would require multiple vector administrations to maintain the therapeutic effects because the non-integrating genome is lost as developing hepatocytes proliferate. As such, we reasoned that newborn liver may be an ideal setting for AAV-mediated gene correction using CRISPR/Cas9, a powerful genome-editing tool consisting of the Cas9 nuclease and a single-guide RNA (sgRNA). We developed a strategy using an AAV vector with high liver tropism (AAV8) to correct the point mutation in newborn spfash mice using Cas9 enzyme from Staphylococcus aureus (SaCas9). An animal model of OTC deficiency, the male sparse fur ash (spfash) mouse, has a G-to-A point mutation at the donor splice site at the end of exon 4 of the OTC gene, which leads to abnormal splicing and a 20-fold reduction in OTC mRNA and protein.We developed a two-vector approach to incorporate all 3 components of CRISPR/Cas9 into AAV. Vector 1 expresses the SaCas9 gene from a liver-specific TBG promoter, while vector 2 contains both the sgRNA1 sequence expressed from a U6 promoter and the 1.8 kb donor OTC DNA sequence. Spfash pups were injected intravenously on postnatal day 2 with mixtures of vector 1 and vector 2 and subsequently evaluated for indel (insertion and deletion) formation and functional correction of the spfash mutation. Following gene correction (3 and 8 weeks), indels were detected by deep sequencing in 31% of OTC alleles, and HDR-based correction of the G-to-A mutation was observed in 10% of OTC alleles. Liver sections were analyzed by immunohistochemistry for OTC expression, showing 15% OTC-positive cells at 3 weeks and 13% at 8 weeks. Direct measurements of OTC enzyme activity from liver homogenates and OTC mRNA from total cellular RNA from liver revealed similarly high levels of correction in treated animals. We further assessed the impact of gene correction on the clinical manifestations of OTC deficiency by evaluating the tolerance of spfash mice to a one-week course of high-protein diet. Spfash mice treated with the gene-editing vectors had significantly lower plasma ammonia levels and showed a survival improvement as compared with untreated spfash mice. This study provides convincing evidence for efficacy in an authentic animal model of a lethal human metabolic disease following in vivo genome editing.