M Chuah

Preclinical and clinical gene therapy for haemophilia

Summary.u2002 The goal of all haemophilia therapy is to prevent bleeding and its associated complications. Replacement by factor concentrates can only ever be suboptimum, and efforts are being made to correct the genetic cause of the disorder. Haemophilia is an ideal candidate for gene therapy, as it is caused by mutations in a single gene. A number of vectors have been used in an attempt to obtain therapeutic levels of factor VIII and factor IX in animal models, with some success. A number of phase 1 clinical trials have been conducted, and, although connection of the bleeding disorder was neither complete nor long‐lasting, they do offer hope for a permanent gene‐therapy cure for the disease.

Hyperactive PiggyBac Transposons for Sustained and Robust Liver-targeted Gene Therapy

The development of robust nonviral vectors could facilitate clinical gene therapy applications and may overcome some of the immune complications of viral vectors. Nevertheless, most nonviral gene deliver approaches typically yield only transient and/or low gene expression. To address these caveats, we have explored piggyBac transposons to correct hemophilia B by liver-directed factor IX (FIX) gene therapy in hemophilic mice. To achieve this, we combined the use of: (i) a hyperactive codon-optimized piggyBac transposase, (ii) a computationally enhanced liver-specific promoter, (iii) a hyperfunctional codon-optimized FIX transgene (FIX R338L Padua), and (iv) a modification of the transposon terminal repeats. This combination strategy resulted in a robust 400-fold improvement in vector performance in hepatocytes, yielding stable supraphysiologic human FIX activity (>1 year). Liver-specific expression resulted in the induction of FIX-specific immune tolerance. Remarkably, only very low transposon/transposase doses were required to cure the bleeding diathesis. Similarly, PB transposons could be used to express supraphysiologic factor VIII levels using low transposon/transposase doses. PB transposition did not induce tumors in a sensitive hepatocellular carcinoma-prone mouse model. These results underscore the potency and relative safety of the latest generation PB transposons, which constitutes a versatile platform for stable and robust secretion of therapeutic proteins.

Preclinical and clinical progress in hemophilia gene therapy.

Purpose of reviewHemophilia A and B are attractive target diseases for gene therapy, as stable expression of coagulation factor VIII and IX may correct the bleeding diathesis. This review focuses on the recent progress in preclinical and clinical studies in gene therapy for hemophilia A and B. Recent findingsHepatic gene delivery using vectors derived from adeno-associated virus (AAV) resulted in therapeutic but transient functional clotting factor IX (FIX) expression levels in severe hemophilia B patients. Although T-cell-mediated immune responses eliminated the transduced hepatocytes, transient immunosuppression may potentially overcome this limitation. Alternatively, vectors are being developed that result in higher FIX expression levels at lower vector doses. Lentiviral vectors are being explored for in-vivo hepatic gene delivery and for ex-vivo transduction of hematopoietic stem cells. This resulted in stable correction of the bleeding diathesis in hemophilic mice. Finally, nonviral vectors derived from transposons result in sustained clotting-factor expression in rodent models. Translational studies in large animal models are required to move these new approaches forward into the clinic. SummaryNew insights from clinical trials and advances in preclinical studies may ultimately pave the way toward a cure in patients suffering from hemophilia.

Haemophilia gene therapy: From trailblazer to gamechanger

Haemophilia is an attractive disease target for gene therapy that fostered the development of the field at large. The delivery of the clotting factor genes into the patients’ cells could be accomplished using different types of gene delivery vehicles or vectors. Adeno‐associated viral vectors (AAV) and lentiviral vectors represent some of the most promising gene delivery technologies that allow for a relatively efficient delivery of the therapeutic FVIII and FIX transgenes into the relevant target cells. To reduce the risks associated with insertional mutagenesis due to random vector integration, gene‐editing approaches have also been considered based primarily on zinc finger nuclease (ZFN) and CRISPR/Cas. However, comprehensive analysis of off‐target effects is still required. It is particularly encouraging that relatively stable therapeutic FVIII or FIX expression levels were reached in severe haemophilia patients in recent clinical trials after liver‐directed AAV gene therapy. This success could be ascribed in part to improvements in vector design. In particular, clotting factor levels could be increased by codon optimization of coagulation factor transgenes. Alternatively, incorporation of a hyperactive gain‐of‐function R338L mutation (FIX Padua) in the FIX gene improved the overall efficacy. However, some patients still show transient liver toxicity, especially at high vector doses, possibly due to inflammatory immune responses, requiring the need for transient immunosuppression. The exact immune mechanisms are not fully understood, but may at least in some patients involve an AAV‐capsid specific T cell response. Moreover, there is a need to identify the key factors that contribute to the interpatient variability in therapeutic efficacy and safety after gene therapy.