Hepatic gene transfer with adeno-associated viral (AAV) vectors shows much promise for the treatment of the X-linked bleeding disorder hemophilia B in multiple clinical trials. In an effort to further innovate this approach and to introduce alternative vector designs with potentially superior features into clinical development, we recently built a vector platform based on AAV serotype 3 because of its superior tropism for human hepatocytes. A vector genome with serotype-matched inverted terminal repeats expressing hyperactive human coagulation factor IX (FIX)-Padua was designed for clinical use that is optimized for translation using hepatocyte-specific codon-usage bias and is depleted of immune stimulatory CpG motifs. Here, this vector genome was packaged into AAV3 (T492V + S663V) capsid for hepatic gene transfer in non-human primates. FIX activity within or near the normal range was obtained at a low vector dose of 5 x 10(11) vector genomes/kg. Pre-existing neutralizing antibodies, however, completely or partially blocked hepatic gene transfer at that dose. No CD8(+) T cell response against capsid was observed. Antibodies against the human FIX transgene product formed at a 10-fold higher vector dose, albeit hepatic gene transfer was remarkably consistent, and sustained FIX activity in the normal range was nonetheless achieved in two of three animals for the 3-month duration of the study. These results support the use of this vector at low vector doses for gene therapy of hemophilia B in humans.

In a recent Nature Genetics letter, entitled “Recurrent AAV2-related insertional mutagenesis in human hepatocellular carcinomas,” Nault and colleaguesdocument that of 193 patients with hepatocellular carcinoma (HCC), 11 contained an integrated genome sequence of the wild-type adeno-associated virus 2 (AAV2), and suggest that AAV2 is associated with oncogenic insertional mutagenesis in human HCC. Because AAV2 has long been known to be a nonpathogenic human parvovirus and, in fact, has been shown to possess antitumor activity, it is critical that the scientific and clinical implications of these studies be rigorously assessed to justify their conclusions. We have carefully analyzed the data presented by Nault and colleaguesand reached a conclusion that is at variance with that of the authors.