2003; baxter - Supplements - Haematologica

120T. VandenDriessche et al.ed to further explore the full potential of lentiviralvectors for hemophilia A gene therapy.HC-Ad vectorsAdenoviral vectors can transduce dividing andnon-dividing cells and are by far the most efficientvectors for hepatic gene delivery. 35 The adenoviralvector genome remains episomal, implyingthat the risk of neoplastic transformation dueto insertional mutagenesis is low. Whereas early-generationadenoviral vectors still containmost viral genes which contribute to inflammatoryresponses, toxicity and short-term transgeneexpression, HC-Ad vectors retain only the necessarycis-acting elements that are required for generatinginfectious vector particles during vectorproduction and the transgene of interest. 35 HC-Ad vectors expressing B-domain deleted humanor canine FVIII from different liver-specific promoterswere injected intravenously into hemophilicFVIII-deficient mice which resulted inphysiologic levels of canine or human FVIII (VandenDriesscheet al, unpublished observations).These results underscore the potential usefulnessof HC-Ad for hemophilia A gene therapy. However,the induction of neutralizing antibodiesdirected against the human or canine xenoproteinsprecluded stable phenotypic correction.ConclusionsThe ideal gene therapy vector for hemophilia Ais not yet available but significant progress hasbeen made in further improving viral vectortechnology for FVIII gene delivery. Proof-of-concepthas recently been established demonstratingthat hemophilia A could be cured by genetherapy in a clinically relevant animal model thatmimics the cognate human disease. 21 This wasaccomplished by intravenous injection of oncoretroviralvectors expressing FVIII into neonatalhemophilic mice. Although these findings mayultimately pave the way towards potential genetherapy for pediatric hemophilia A, clinical trialsin children can only commence once safety hasbeen established in adults or perhaps in childrensuffering from a lethal disease, for which notreatment is currently available. The limitationof onco-retroviral vector mediated gene transferin dividing hepatocytes, could be overcome byusing either lentiviral or HC-Ad vectors, whichresulted in efficient hepatic gene transfer in adultmice. Using HC-Ad vectors, physiologic FVIIIexpression levels could be achieved in hemophilicmice. Follow-up studies in hemophilic mice anddogs are required to further evaluate the potentialof these vectors for gene therapy.The induction of inhibitory antibodies followinggene therapy may be related to several confoundingvariables including the type of vectorused, the purity of the vector preparation, thepromoter used to drive FVIII expression, the siteof administration, the transduced cell types(APC) or the underlying genetic defect and otherhost factors. Whether gene therapy wouldincrease or decrease the likelihood of inhibitorformation compared to protein replacementtherapy is one of the important questions thatstill needs to be addressed. Continuous productionof high levels FVIII in situ following genetherapy may actually induce immune tolerancereminiscent of current immune tolerizationstrategies by repeated high dose clotting factoradministration. Alternatively, gene transfer tonaive patients could evoke the same or even amore potent immune response to FVIII. A majorconcern is that the use of viral vectors expressingcoagulation factors or that impurities in the vectorpreparations may provide immunologicaldanger signals that may facilitate inhibitor formation.36 In addition, gene transfer may result inthe presentation of endogenously synthesizedFVIII–derived peptides in the context of MHCclass I molecules potentially resulting in CTLresponses that could eliminate the FVIII-engineeredtarget cells. The secreted FVIII protein thatis produced in vivo may also be presented in thecontext of MHC class II as in the case of infusedclotting factors. It is not clear whether gene therapycould break tolerance in patients that are tolerantto FVIII and whether inhibitors can be suppressedonce they occur following gene therapy.Since many adult hemophilia patients have aninfectious or inflammatory disease, complexinteractions can influence the therapeutic efficacyof the gene therapy procedure or the propensityfor inhibitor formation and caution is warrantednot to exacerbate these underlying conditions.Despite the tremendous progress in the fieldover the past few years many questions remainlargely unexplored. Extensive gene therapy studiesin preclinical hemophilia models are neededto anticipate the possible outcome in patientsand to increase the overall efficiency of the variousgene therapy strategies while further improvingtheir safety. The development of hemophiliagene therapy will undoubtedly continue to contributeto a better understanding of vector-hostinteractions that will benefit the entire field ofgene therapy. The results from the preclinicalstudies in hemophilic animal models indicatethat the simultaneous development of differentstrategies is likely to bring a permanent cure forhemophilia one step closer to reality.AcknowledgmentsThe authors wish to acknowledge Dr. Schiedner,Dr. Kochanek, Dr. Follenzi, Dr. Naldini, Dr. Berneman,Dr. Ory, Dr. Mulligan, Dr. Saint-Remy, Dr.Kazazian, Dr. Lillicrap, Dr. De Geest Mr. Thorrez,Mrs. Vanslembrouck, Ms. Gillijns, Mrs. Vanderhaegen,Mr. Lenjou Mrs. Johnston and Mrs. Hertelfor their valuable contributions.References1. Chuah MK, Collen D, VandenDriessche T. Gene therapyfor hemophilia: hopes and hurdles. Crit Rev OncolHematol 1998; 28:153-71.haematologica vol. 88(supplement n. 12):september 2003