Gene therapy for monogenic liver diseases: clinical successes, current challenges and future prospects

Abstract

Over the last decade, pioneering liver-directed gene therapy trials for haemophilia B have achieved sustained clinical improvement after a single systemic injection of adeno-associated virus (AAV) derived vectors encoding the human factor IX cDNA. These trials demonstrate the potential of AAV technology to provide long-lasting clinical benefit in the treatment of monogenic liver disorders. Indeed, with more than ten ongoing or planned clinical trials for haemophilia A and B and dozens of trials planned for other inherited genetic/metabolic liver diseases, clinical translation is expanding rapidly. Gene therapy is likely to become an option for routine care of a subset of severe inherited genetic/metabolic liver diseases in the relatively near term. In this review, we aim to summarise the milestones in the development of gene therapy, present the different vector tools and their clinical applications for liver-directed gene therapy. AAV-derived vectors are emerging as the leading candidates for clinical translation of gene delivery to the liver. Therefore, we focus on clinical applications of AAV vectors in providing the most recent update on clinical outcomes of completed and ongoing gene therapy trials and comment on the current challenges that the field is facing for large-scale clinical translation. There is clearly an urgent need for more efficient therapies in many severe monogenic liver disorders, which will require careful risk-benefit analysis for each indication, especially in paediatrics.

Fig. 1

The triad to consider for successful gene therapy

Fig. 2

Synthesis of an AAV vector. (a) Initially, the single-stranded proviral DNA is excised to remove Rep and Cap genes from different wild type AAV serotypes. The transgene expression cassette containing the promoter, the transgene and various regulatory elements is cloned between the 2 ITRs, which are the only wild type AAV sequences retained. (b) For vector synthesis, triple transfection of three plasmids is performed in a packaging cell with proviral plasmid encoding the recombinant viral genome, a plasmid containing Rep and Cap and a helper plasmid. “Pseudotyped” AAV vectors contain ITRs from a specific AAV serotype (usually AAV2) and a Cap gene encoding viral proteins (VP1, 2 and 3) from a different serotype (e.g AAV8) in order to provide organ-specific transduction of the recombinant AAV vector named AAV2/8. AAV: adeno-associated virus; Adv: adenovirus; ITR: inverted terminal repeat

Fig. 3

AAV vector uptake, in-cell processing and initiation of the immune response. Fenestrated endothelium of hepatic sinusoids allows the AAV vector to freely reach the hepatocyte. Once reaching the target cell, the vector binds an extracellular receptor and co-receptor specific to the capsid motifs. After an uptake by endocytosis, the vector is trafficked in the cytoplasm in early then late endosome. Acidification of the endosome modifies the capsid conformation. After endosomal escape, the AAV vector enters the nucleus via the nuclear pore complex. Capsid uncoating and release of the proviral DNA precede the synthesis of the 2nd strand of DNA. The viral genome then persists either as a non-integrated single- or double-stranded episome (99%) or (small percentage) integrates into the host genome (1%). Expression of the transgene is followed by synthesis of the protein of interest. Cell-mediated immune responses are initiated by the degradation of capsid or the transgene product (protein) in the proteasome and presentation at the surface of the transduced cell via the major histocompatibility complex I. CD8⁺ T cells recognise the antigen at the cell surface and initiate the immune cascade. Neutralising antibodies bind to the vector in the bloodstream and impair or prevent successful transduction of the organ target. MHC1: major histocompatibility complex I

Fig. 4

Species-related differences in transduction of the hepatic lobule by AAV vector compared with metabolic zonation for ammonia clearance: example with AAV2/8 vector