Comment on Jiang et al, page 107
In this issue of Blood, Jiang and colleagues demonstrate long-term therapeutic efficacy in hemophilia A mice and dogs following liver-directed factor VIII gene therapy with AAV vectors. However, the efficacy advantage of alternative AAV serotypes 8 and 6 over type 2 in rodents does not appear to translate to higher species, which has important implications for gene therapy clinical trials.
Hemophilia has been considered an ideal trailblazer for gene therapy, since only a slight increase in clotting factor levels can convert a severe to a moderate phenotype. Though stable expression of factor IX (FIX) has been achieved by gene therapy in large-animal models and results from clinical trials are encouraging,1 gene therapy for hemophilia A has been more challenging. Jiang and colleagues showed that therapeutic FVIII levels at 2% to 5% of normal canine FVIII levels could be achieved by liver-directed gene therapy in 6 of 8 hemophilia A dogs for approximately 4 years with no spontaneous bleeds, suggesting a potential for a cure in humans. This was consistent with the absence of FVIII-specific inhibitory antibodies, though occasional transient inhibitors were noted. Nevertheless, relatively high vector doses were required, which justifies the continued development of improved adeno-associated virus (AAV)-FVIII vectors with stronger expression cassettes.
One of the hallmarks of this study is that different serotypes of AAV were compared in the canine model, notably AAV2, AAV6, and AAV8. These vectors differ only in their capsid, and display either Cap2, Cap6, or Cap8 on their surface. The use of alternative AAV serotypes has generally been proposed as a means to achieve higher clotting factor levels in patients, based on their superior hepatic transduction efficiencies in mice,2,3 particularly with AAV8. Indeed, the highest clotting factor levels could be obtained in FVIII-deficient mice using AAV8, followed by AAV6, which could be attributed to improved hepatic gene transfer efficiencies compared with AAV2 or AAV5. Jiang et al showed that in contrast to hemophilic mice, hepatic gene transfer efficiencies and FVIII levels were similar in the hemophilia A dogs, regardless of the serotype used. This is consistent with a recent study showing no transduction advantage of using AAV8 over AAV2 or AAV5 vectors in nonhuman primates.4 Similarly, coinjection of 2 AAV vectors based on serotype 8 or 9 and encoding either the FVIII heavy or light chain in hemophilia A dogs resulted in only modest FVIII expression levels.5 Assuming that the canine and primate models accurately predict the outcome in humans, these studies caution that the use of AAV8 may not necessarily result in higher clotting factor levels in patients. However, one advantage of using AAV8 over AAV2 is that it can be administered via peripheral vein injection rather than by a hepatic vein or artery without compromising transduction efficiency.4 Moreover, AAV8 may at least partially circumvent the inhibition of transduction by anti-AAV2 antibodies prevalent in humans.6
One of the key questions is whether converting serotypes from AAV2 to AAV8 would prevent cellular immune responses against transduced hepatocytes. Gene therapy in patients with hemophilia B using AAV2 resulted in FIX levels above 10%, but expression was transient and accompanied by acute liver toxicity.1 This was due to immune destruction of transduced hepatocytes by cytotoxic T cells (CTLs) specific for AAV2 capsids.1 Since the epitopes recognized by these CTLs are relatively conserved, serotype switching may not suffice to overcome this immune rejection. However, the more rapid turnover of AAV8 capsids, versus AAV2, may perhaps diminish this risk. Whether this also holds true in large animals would need to be ascertained. ▪