Hemophilia Gene Therapy: A Holy Grail Found
Authors: Katherine P Ponder
Journal: Molecular Therapy (2011)

In medieval legend, the Holy Grail was the cup used by Jesus at the Last Supper. Allegedly brought to Britain by Joseph of Arimathea, it became the quest of many knights. In modern times, “holy grail” has been used to denote a greatly desired object or outcome whose pursuit borders on a sacred quest. Gene therapy for hemophilia has been one of our field's holy grails for more than two decades, and this much-sought-after application of gene transfer technology has finally been achieved in patients, as reported at the annual meeting of the American Society of Hematology (ASH) in December 2010. Hemophilia is a bleeding disorder that results from a deficiency in a coagulation factor, with an incidence of 1 in 5,000 males. Hemophilia can be treated with intravenous injection of the deficient protein on an as-needed basis or prophylactically two or three times a week, at a cost in the United States of $100,000 to $200,000 per patient per year. In developing countries, the high cost of factor precludes frequent use and patients therefore suffer from more severe bleeding. Hemophilia A, resulting from a deficiency of factor VIII (FVIII), is present in approximately 80% of patients; hemophilia B, resulting from deficiency of factor IX (FIX), is present in most of the remaining patients. The low level of the proteins required in blood to prevent bleeding (100 ng/ml, or 0.3 nmol/l, for the 330-kilodalton (kDa) FVIII protein, and 5000 ng/ml, or 100 nmol/l, for the 56-kDa FIX protein) led early to the notion that hemophilia might be an easy target for gene therapy. Although hemophilia A is the more common type of hemophilia, a larger number of gene transfer studies have tackled hemophilia B because of the ease of expression of FIX, the ability of its gene to fit into smaller viral vectors, and a lower incidence of antibody inhibitors that reduce the activity of this factor. Indeed, in 1990, the laboratories of Inder Verma and Savio Woo reported the expression of FIX in vitro, leading to the hope that successful gene therapy in patients would follow shortly thereafter (Axelrod, JH et al., Proc Natl Acad Sci USA 87: 5173–5177, 1990; Armentano, D et al., Proc Natl Acad Sci USA 87: 6141–6145, 1990). Although there have since been numerous reports of therapeutic expression in mice and dogs with a variety of vectors and approaches, success in humans was elusive until the recent results presented at ASH by Amit Nathwani, of the University College in London, who described a study performed in collaboration with Andrew Davidoff and Arthur Nienhuis of St. Jude Children's Research Hospital in Memphis, Tennessee, along with Edward Tuddenham at Royal Free Hospital in London, Katherine High at the Children's Hospital of Philadelphia, Mark Kay at Stanford University, and other investigators (Nathwani, A et al., abstract, Annual Meeting, American Society of Hematology, 2010). They reported on the use of an adenovirus-associated virus serotype 8 (AAV8) vector that contained a self-complementary gene encoding a codon-optimized human FIX gene under control of a liver-specific promoter that was injected intravenously in adults with hemophilia B. Two patients received a lower dose of AAV8 vector (2 × 1010 vector genomes per kilogram), and two patients received a higher dose (6 × 1010 vector genomes per kilogram). One of the lower-dose patients achieved 2% of normal FIX activity in the plasma and has not required factor supplements for nine months, whereas the other lower-dose patient did not achieve a therapeutic response. One higher-dose patient achieved 4% of normal FIX activity and has not required factor for 68 days; the other higher-dose patient achieved no therapeutic response. Low-titer neutralizing antibodies were detected in both of the patients who failed to show a therapeutic effect, and these antibodies may have blocked transduction. The cost of vector preparation and testing was about $30,000 per patient for the intermediate dose of AAV8 vector for this study, suggesting that this gene therapy approach will provide substantial cost savings to patients and insurance companies or governments, although additional costs of a clinical trial and economies of scale could affect this figure. AAV8, which was discovered by James Wilson at the University of Pennsylvania (Gao, GP et al., Proc Natl Acad Sci USA 99: 11854–11859, 2002), is very efficient at transducing liver cells, which were the target cells in the study by Nathwani and colleagues. Mark Kay had previously demonstrated that a major advantage of the AAV8 serotype appears to be a more rapid uncoating of the viral particle following transduction than that with AAV2 (Thomas, CE et al., J Virol 78: 3110–3122, 2004). Because of this, AAV8 capsid proteins disappear more rapidly than AAV2 capsid proteins, which probably reduces the time during which a cytotoxic T-lymphocyte response might eliminate transduced cells that present capsid epitopes. This difference may explain why this AAV8 trial was more successful than a similar trial performed by the High and Kay laboratories that made use of an AAV2 vector (Manno, CS et al., Nat Med 12: 342–347, 2006). Future goals of such studies are to demonstrate continued efficacy, to achieve transfer to patients with neutralizing antibodies, to ensure that this gene therapy approach is safe in the long term, to apply gene therapy to patients with the more common hemophilia A, and to ensure access to this potential holy grail in developing as well as developed countries.