High-Throughput Screening Identifies Kinase Inhibitors That Increase Dual Adeno-Associated Viral Vector Transduction In Vitro and in Mouse Retina

Genome editing without nucleases confers proliferative advantage to edited hepatocytes and corrects Wilson disease

Application of classic liver-directed gene replacement strategies is limited in genetic diseases characterized by liver injury due to hepatocyte proliferation, resulting in decline of therapeutic transgene expression and potential genotoxic risk. Wilson disease (WD) is a life-threatening autosomal disorder of copper homeostasis caused by pathogenic variants in copper transporter ATP7B and characterized by toxic copper accumulation, resulting in severe liver and brain diseases. Genome editing holds promise for the treatment of WD; nevertheless, to rescue copper homeostasis, ATP7B function must be restored in at least 25% of the hepatocytes, which surpasses by far genome-editing correction rates. We applied a liver-directed, nuclease-free genome editing approach, based on adeno-associated viral vector-mediated (AAV-mediated) targeted integration of a promoterless mini-ATP7B cDNA into the albumin (Alb) locus. Administration of AAV-Alb-mini-ATP7B in 2 WD mouse models resulted in extensive liver repopulation by genome-edited hepatocytes holding a proliferative advantage over nonedited ones, and ameliorated liver injury and copper metabolism. Furthermore, combination of genome editing with a copper chelator, currently used for WD treatment, achieved greater disease improvement compared with chelation therapy alone. Nuclease-free genome editing provided therapeutic efficacy and may represent a safer and longer-lasting alternative to classic gene replacement strategies for WD.

Extending AAV Packaging Cargo through Dual Co-Transduction: Efficient Protein Trans-Splicing at Low Vector Doses

Adeno-associated viral (AAV) vectors represent one of the leading platforms for gene delivery. Nevertheless, their small packaging capacity restricts their use for diseases requiring large-gene delivery. To overcome this, dual-AAV vector systems that rely on protein trans-splicing were developed, with the split-intein Npu DnaE among the most-used. However, the reconstitution efficiency of Npu DnaE is still insufficient, requiring higher vector doses. In this work, two split-inteins, Cfa and Gp41-1, with reportedly superior trans-splicing were evaluated in comparison with Npu DnaE by transient transfections and dual-AAV in vitro co-transductions. Both Cfa and Gp41-1 split-inteins enabled reconstitution rates that were over two-fold higher than Npu DnaE and 100% of protein reconstitution. The impact of different vector preparation qualities in split-intein performances was also evaluated in co-transduction assays. Higher-quality preparations increased split-inteins' performances by three-fold when compared to low-quality preparations (60-75% vs. 20-30% full particles, respectively). Low-quality vector preparations were observed to limit split-gene reconstitutions by inhibiting co-transduction. We show that combining superior split-inteins with higher-quality vector preparations allowed vector doses to be decreased while maintaining high trans-splicing rates. These results show the potential of more-efficient protein-trans-splicing strategies in dual-AAV vector co-transduction, allowing the extension of its use to the delivery of larger therapeutic genes.

Full-length ATP7B reconstituted through protein trans-splicing corrects Wilson disease in mice

Wilson disease (WD) is a genetic disorder of copper homeostasis, caused by deficiency of the copper transporter ATP7B. Gene therapy with recombinant adeno-associated vectors (AAV) holds promises for WD treatment. However, the full-length human ATP7B gene exceeds the limited AAV cargo capacity, hampering the applicability of AAV in this disease context. To overcome this limitation, we designed a dual AAV vector approach using split intein technology. Split inteins catalyze seamless ligation of two separate polypeptides in a highly specific manner. We selected a DnaE intein from Nostoc punctiforme (Npu) that recognizes a specific tripeptide in the human ATP7B coding sequence. We generated two AAVs expressing either the 5′-half of a codon-optimized human ATP7B cDNA followed by the N-terminal Npu DnaE intein or the C-terminal Npu DnaE intein followed by the 3′-half of ATP7B cDNA, under the control of a liver-specific promoter. Intravenous co-injection of the two vectors in wild-type and Atp7b^−/− mice resulted in efficient reconstitution of full-length ATP7B protein in the liver. Moreover, Atp7b^−/− mice treated with intein-ATP7B vectors were protected from liver damage and showed improvements in copper homeostasis. Taken together, these data demonstrate the efficacy of split intein technology to drive the reconstitution of full-length human ATP7B and to rescue copper-mediated liver damage in Atp7b^−/− mice, paving the way to the development of a new gene therapy approach for WD.

The role of small molecules in cell and gene therapy

Cell and gene therapies have achieved impressive results in the treatment of rare genetic diseases using gene corrected stem cells and haematological cancers using chimeric antigen receptor T cells. However, these two fields face significant challenges such as demonstrating long-term efficacy and safety, and achieving cost-effective, scalable manufacturing processes. The use of small molecules is a key approach to overcome these barriers and can benefit cell and gene therapies at multiple stages of their lifecycle. For example, small molecules can be used to optimise viral vector production during manufacturing or used in the clinic to enhance the resistance of T cell therapies to the immunosuppressive tumour microenvironment. Here, we review current uses of small molecules in cell and gene therapy and highlight opportunities for medicinal chemists to further consolidate the success of cell and gene therapies.

Hair Cell Transduction Efficiency of Single- and Dual-AAV Serotypes in Adult Murine Cochleae

Gene delivery is a key component for the treatment of genetic hearing loss. To date, a myriad of adeno-associated virus (AAV) serotypes and surgical approaches have been employed to deliver transgenes to cochlear hair cells, but the efficacy of dual transduction remains unclear. Herein, we investigated cellular tropism of single injections of AAV serotype 1 (AAV1), AAV2, AAV8, AAV9, and Anc80L65, and quantitated dual-vector co-transduction rates following co-injection of AAV2 and AAV9 vectors in adult murine cochlea. We used the combined round window membrane and canal fenestration (RWM+CF) injection technique for vector delivery. Single AAV2 injections were most robust and transduced 96.7% ± 1.1% of inner hair cells (IHCs) and 83.9% ± 2.0% of outer hair cells (OHCs) throughout the cochlea without causing hearing impairment or hair cell loss. Dual AAV2 injection co-transduced 96.9% ± 1.7% of IHCs and 65.6% ± 8.95% of OHCs. Together, RWM+CF-injected single or dual AAV2 provides the highest auditory hair cell transduction efficiency of the AAV serotypes we studied. These findings broaden the application of cochlear gene therapy targeting hair cells.

Adeno-Associated Viral Vectors as a Tool for Large Gene Delivery to the Retina

Gene therapy using adeno-associated viral (AAV) vectors currently represents the most promising approach for the treatment of many inherited retinal diseases (IRDs), given AAV's ability to efficiently deliver therapeutic genes to both photoreceptors and retinal pigment epithelium, and their excellent safety and efficacy profiles in humans. However, one of the main obstacles to widespread AAV application is their limited packaging capacity, which precludes their use from the treatment of IRDs which are caused by mutations in genes whose coding sequence exceeds 5 kb. Therefore, in recent years, considerable effort has been made to identify strategies to increase the transfer capacity of AAV vectors. This review will discuss these new developed strategies, highlighting the advancements as well as the limitations that the field has still to overcome to finally expand the applicability of AAV vectors to IRDs due to mutations in large genes.

Seeing the Light after 25 Years of Retinal Gene Therapy

The retina has been at the forefront of translational gene therapy. Proof-of-concept that gene therapy could restore vision in a large animal led to the initiation of the first successful clinical trials and, in turn, to the recent approval of the first gene therapy product for an ocular disease. As dozens of clinical trials of retinal gene therapy have begun, new challenges are identified, which include delivery of large genes, counteracting gain-of-function mutations, and safe and effective gene transfer to diseased retinas. Advancements in vector design, improvements of delivery routes, and selection of optimal timing for intervention will contribute to extend the initial success of retinal gene therapy to an increasing number of inherited blinding conditions.