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Improving gene therapy efficacy
Clinical trial results of a gene therapy for hemophilia B reported unexpectedly low success. Cabanes-Creus et al. may have found a reason for this result. They report that the viral vector commonly used for liver-directed gene therapy, adeno-associated virus 2 (AAV2), mutates during culture, resulting in the virus binding more tightly to heparan sulfate proteoglycans (HSPGs). In contrast, naturally occurring AAVs isolated from human liver biopsies did not bind HSPGs tightly and were more successful at infecting human liver cells. However, these naturally occurring AAVs cultured over time also mutated, resulting in greater HSPG binding and decreased liver infection. Thus, naturally occurring AAVs may be more effective as vectors for liver-targeting gene therapies.
Abstract
Recent clinical successes in gene therapy applications have intensified interest in using adeno-associated viruses (AAVs) as vectors for therapeutic gene delivery. Although prototypical AAV2 shows robust in vitro transduction of human hepatocyte–derived cell lines, it has not translated into an effective vector for liver-directed gene therapy in vivo. This is consistent with observations made in Fah−/−/Rag2−/−/Il2rg−/− (FRG) mice with humanized livers, showing that AAV2 functions poorly in this xenograft model. Here, we derived naturally hepatotropic AAV capsid sequences from primary human liver samples. We demonstrated that capsid mutations, likely acquired as an unintentional consequence of tissue culture propagation, attenuated the intrinsic human hepatic tropism of natural AAV2 and related human liver AAV isolates. These mutations resulted in amino acid changes that increased binding to heparan sulfate proteoglycan (HSPG), which has been regarded as the primary cellular receptor mediating AAV2 infection of human hepatocytes. Propagation of natural AAV variants in vitro showed tissue culture adaptation with resulting loss of tropism for human hepatocytes. In vivo readaptation of the prototypical AAV2 in FRG mice with a humanized liver resulted in restoration of the intrinsic hepatic tropism of AAV2 through decreased binding to HSPG. Our results challenge the notion that high affinity for HSPG is essential for AAV2 entry into human hepatocytes and suggest that natural AAV capsids of human liver origin are likely to be more effective for liver-targeted gene therapy applications than culture-adapted AAV2.
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