Gene therapy for hemophilia: what does the future hold?

Abstract

Recent phase I/II adeno-associated viral vector-mediated gene therapy clinical trials have reported remarkable success in ameliorating disease phenotype in hemophilia A and B. These trials, which highlight the challenges overcome through decades of preclinical and first in human clinical studies, have generated considerable excitement for patients and caregivers alike. Optimization of vector and transgene expression has significantly improved the ability to achieve therapeutic factor levels in these subjects. Long-term follow-up studies will guide standardization of the approach with respect to the combination of serotype, promoter, dose, and manufacturing processes and inform safety for inclusion of young patients. Certain limitations preclude universal applicability of gene therapy, including transient liver transaminase elevations due to the immune responses to vector capsids or as yet undefined mechanisms, underlying liver disease from iatrogenic viral hepatitis, and neutralizing antibodies to clotting factors. Integrating vectors show promising preclinical results, but manufacturing and safety concerns still remain. The prospect of gene editing for correction of the underlying mutation is on the horizon with considerable potential. Herein, we review the advances and limitations that have resulted in these recent successful clinical trials and outline avenues that will allow for broader applicability of gene therapy.

Keywords: adeno-associated virus; gene therapy; hemophilia; lentivirus.

Figure 1.

Overview of adeno-associated virus (AAV) mediated liver-directed gene therapy for hemophilia. The wildtype AAV genome consists of two inverted tandem repeat (ITR) regions flanking the rep (replication) and cap (capsid) genes. These genes are replaced by a tissue-specific promoter with enhancer, intron, and transgene of interest in the recombinant (r)AAV vector genome, which is packaged into capsids and injected into subjects via a peripheral venous infusion. Once infused, rAAV vector can be neutralized by pre-existing antibodies in a serotype-specific manner or transduce hepatocytes where the capsid is degraded and the genetic material maintained as an episome in the nucleus to produce the transgene product. Capsid peptides can be presented on the surface of hepatocytes to CD8+ T cells, thought to lead to a cellular immune response coinciding with loss of transgene and rise in liver transaminases in some clinical trials. Modifications in the transgene, serotype, infusion of empty capsids, and production process may all affect efficacy. Options to bypass the pre-existing humoral response or liver disease are listed. Additional hurdles to general application of liver-directed AAV gene therapy include inhibitors to factors VIII and IX as well as infusion in young patients. FVIII, factor VIII; FIX, factor IX; CpG, cytosine-guanine residues.

Figure 2.

Modeling of capsid-triggered cellular immune response, liver transaminase, and factor level correlation in published adeno-associated virus (AAV) liver-directed hemophilia gene therapy clinical trials. Correlation between alanine aminotransferase (ALT, ), factor activity () and T-cell enzyme-linked immunospot (ELISPOT). (a) The observation of a rise in liver alanine aminotransferase (ALT) with coincident decline in transgene expression is thought to be associated with a cellular immune response against the AAV capsid as confirmed by positive ELISPOT (bottom panel). This effect can be rescued with a short course of corticosteroids (prednisone). (b) In two recent trials, a rise in ALT does not always correlate with loss of factor activity or positive T-cell ELISPOT. Although steroids are used in these trials, ALT response is not uniform in all cases. PBMC, peripheral blood mononuclear cells; SFU, spot-forming units