J Genet Syndr Gene Ther Gene Therapy for Hemophilia ISSN:2157-7412 JGSGT an open access journal Editorial Open Access Genetic Syndromes & Gene Therapy Sarkar et al. J Genet Syndr Gene Ther 2012, S:1 http://dx.doi.org/10.4172/2157-7412.S1-e001 As this special issue of the Journal of Genetic Syndromes and Gene herapy illustrates, gene therapists are closer than ever to a cure for the X-linked bleeding disorder hemophilia. Hemophilia A and B are caused by deiciency of functional coagulation of factor VIII (FVIII) or factor IX (FIX), respectively. Patients with severe hemophilia typically require frequent intravenous infusions of recombinant or plasma-derived clotting factor protein [1]. his type of therapy is not eicient enough to prevent bleeding complications and tissue damage unless given in prophylactic manner, which requires even more frequent infusions. A major complication of treatment is formation of inhibitory antibodies, which occurs in a subset of patients and further complicates treatment. Another major challenge for recombinant protein therapy is the high cost, which can be more than $300,000/year in factor products. hus, hemophilia has been viewed as an ideal candidate for gene therapy, which holds the promise of rendering the patient’s own cells into a factory for continued production of functional coagulation factor. Recent Clinical Success with AAV Gene Transfer While several early gene therapy strategies appeared promising in laboratory animals, these treatments largely failed to provide a sustained clinical efect [2]. However, the team of scientists who pioneered clinical gene transfer for hemophilia using adeno-associated virus (AAV) vectors persevered through these setbacks to change the outlook [3- 5]. An international team of investigators (including scientists at the University College London, St. Jude’s Children’s Hospital, and the original groups at he Children’s Hospital of Philadelphia and Stanford University) has now reported a groundbreaking study, in which liver-directed AAV gene transfer resulted in sustained therapeutic FIX expression in several patients with hemophilia B [6]. his result provides a great stimulus to propel the exciting ield of gene therapy for hemophilia forward. here are several advantages of using recombinant AAV, which makes it a desirable therapeutic vector for a number of diseases [7,8]. AAV vectors are derived from a non-pathogenic virus and have a good safety proile. hey are eicient in transferring genes in vivo to a number of cell types, which has resulted in many years of transgene expression in several large animal studies and in humans [9]. hese characteristics of AAV make it a very beneicial vector for hemophilia A and B gene therapy. he aforementioned landmark study by Nathwani et al. reported the signiicant inding that a single intravenous administration of an AAV vector could direct expression of therapeutic FIX levels for at least 22 months, thereby eliminating the need for frequent factor infusions [6]. In this Phase I/II dose-escalation study, a total of six patients were treated with serotype 8 vector, which has strong in vivo tropism for hepatocytes [10]. While all patients received a therapeutic beneit, FIX levels were vector dose dependent [6]. Four of the six patients stopped conventional treatment altogether, and the other two have extended the time period between prophylactic injections. From a inancial perspective, treatment of one hemophilic patient costs can add up to $20 million for a lifetime of treatment. Utilizing gene therapy, this cost could be greatly reduced to roughly $30,000 per year since the use of factor concentrate would be highly reduced or removed. Future challenges for the protocol include pre-existing immunity to AAV (because of natural infection with the wild-type virus) in the form of neutralizing antibodies (that are present in some patients and block gene transfer) and in the form of memory CD8 + T cells to the viral capsid. Upon reactivation, these may destroy transduced hepatocytes. Multiple potential solutions are being investigated, ranging from vector engineering to transient immune suppression. For example, elimination of surface exposed tyrosine residues can improve efective of AAV vectors in gene transfer, thereby allowing for a reduction in vector dose and thus in lower capsid antigen doses [8]. Remaining Challenges for AAV-Based Gene herapy for Hemophilia Future use of the protocol in pediatric patients with hemophilia is complicated by the predominant episomal nature of the AAV genome, which on the one hand reduces the risk for insertional mutagenesis while on the other hand vector genomes are lost over time in a growing liver. However, a recent paper showed that genome editing with zinc inger nucleases ofers the exciting possibility of inserting the AAV vector genome into speciic places of the host genome, thereby creating a stable form [11]. hus far, clinical trials with AAV vectors have focused on hemophilia B, i.e. FIX deiciency. FVIII is a larger protein and therefore more diicult to package in vectors and express at therapeutic levels. his makes clinical success in hemophilia A more challenging. To remedy this, bioengineering FVIII has been a priority [12]. Bioengineering approaches include manipulating DNA or RNA via codon optimization and manipulation at the protein level such as altering the FVIII amino acid sequence for improved expression and secretion or to reduce the size of the protein. Combined with other vector engineering strategies, these eforts can also help the use of AAV vectors, which have limited gene-packaging capacity. However, one must proceed with caution since the risk of immune responses against these new molecules needs to be evaluated. Alternative Viral Vectors for Hemophilia Gene herapy Another important aspect to assure future success of gene therapy for hemophilia is that a number of promising alternative approaches are in the pipeline, utilizing a variety of vectors and target cells for transfer of the therapeutic clotting factor genes. For example, lentiviral vectors (LV) may circumvent pre-immunity to the vector and can transduce both dividing and non-diving cells [13]. Also, LV have a larger *Corresponding author: Roland W. Herzog, Professor, Department of Pediatrics, Microbiology and Molecular Genetics, University of Florida, USA, Tel: +352-273- 8113; Fax: +352-273-8342; E-mail: rherzog@ul.edu Received February 06, 2012; Accepted February 08, 2012; Published February 10, 2012 Citation: Sarkar D, Perrin GQ, Zolotukhin S, Srivastava A, Herzog RW (2012) Gene Therapists Determined to Stop the Bleeding! J Genet Syndr Gene Ther S1:e001. doi:10.4172/2157-7412.S1-e001 Copyright: © 2012 Sarkar D, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Gene Therapists Determined to Stop the Bleeding! Debalina Sarkar, George Q. Perrin, Sergei Zolotukhin, Arun Srivastava and Roland W. Herzog* Division of Cellular and Molecular Therapy, Department of Pediatrics, University of Florida, Gainesville, FL 32610, USA