GTMB 7 - Gene Therapy & Molecular Biology

George et al: Gene therapy for vascular diseasesrespect to the former, angiogenic gene therapy may besignificantly more therapeutic with respect to collateralvessel formation with a combination of therapeutic genesrather than single gene therapy strategies. With recentadvances in adenoviral vector technology [e.g. using"gutted" adenoviral vectors (Kochanek et al, 1996; Parkset al, 1996)] the cloning capacity required for such studiesis now available. Equally, the gutted adenoviral vectorsystems are less immunogenic in vivo and would allowlonger term overexpression of transgenes that in turn maypromote sustained angiogenic effects. It is known thatvascular cell uptake by these vectors (all based on serotype5 adenoviruses) is extremely poor in comparison to othercells, such as hepatocytes in the liver (Nicklin et al, 2001).Indeed, pre-clinical experiments have shown that localdelivery of adenoviruses serotype 5 vectors to thevasculature leads to virion dissemination, not only to theliver but also to testes and other organs posing additionalsafety concerns (Hiltunen et al, 2000; Baker, 2002).Given the limited ability of liposomes andadenoviruses to enable long-term gene expression, andgiven the poor in vivo performance of retroviruses, theAAV vectors are being developed. This virus is smallerthan the adenovirus and has a relatively low-capacity size.However, it allows for long-term gene expression (ie,months to years) with only minimal induction ofinflammation or antiviral immune responses. A betterunderstanding of the life cycle of this virus, along withimproved production techniques, has allowed investigatorsto conduct clinical trials with AAV in diseases such ashemophilia and cystic fibrosis (see http://www.wiley.co.uk/wileychi/genmed/clinical/). Preclinical data in miceinjected intramuscularly with an AAV-human alpha-1-antitrypsin (1AT) vector are encouraging (Xiao et al,1998; Phillips et al, 2002).To date, the major problem in gene therapy remainsthe relative inefficiency of current vectors. Currently, thisinefficiency, coupled with a relatively poor specificity ofmost vectors, requires the delivery of large doses ofvector. This is both expensive and more likely to lead toside effects. Pathophysiological questions still remainabout which and how many cells need to be transduced toobtain a clinical response. One new and very exciting areaof gene therapy that has not yet reached clinical trials isthe "gene correction" (Gamper et al, 2000; Metz et al,2002). It is possible to design oligonucleotides that bind toareas of single-nucleotide changes that are associated withabnormal functions and to catalyze corrections of thenucleotide errors. This concept clearly has beendemonstrated to work in cell cultures and in animalmodels, although the efficiency is still quite low. With thedevelopment of better oligonucleotides and improveddelivery methods, this approach will likely be tested firstin diseases such as hemophilia and 1AT.When it is considered that angiogenic gene therapyshould be highly localised due to potential side effects[including potentiation of atherosclerosis (Celletti et al,2001) and development of cancer (Lee et al, 2000)] othervector systems should now be considered. The choice ofpotential new vectors is broad and must be consideredwith caution and evaluated based on current knowledge ofexisting systems (de Nigris et al, 2003). Additionalevidence now suggests that the vast majority of AAVgenomes remain in a non-integrative capacity withininfected cells (Nakai et al, 2001; Schnepp et al, 2003)further supporting the safety of this vector system. Ofequal potential are adenoviral vectors originating fromdifferent serotypes. Previous pre-clinical data support ofthe notion that novel vector systems can be isolated for thecapacity to efficiently infect an individual tissue type(Havenga et al, 2001, 2002). For example, adenovirusesbased on serotype 16 have a high propensity to transduceboth endothelial cells and smooth muscle cells thanserotype 5 vectors (Havenga et al, 2001). Again, likeAAV-2, this may provide a system through which tooptimise gene delivery for defined gene therapeuticapplications. The use of cell selective promoters (tissuespecificexpression) to drive transgene expression will adda further level of selectivity to such systems. Thecombined use of vectors and immuno-suppressors may bealso reasonable.Gene therapy remains the key link between advancesin genetics and genomics and the translation of thisknowledge into useful outcomes for patients. Althoughprogress has been slower than hoped for, clear advancesare being made; gene therapy will probably find a numberof key therapeutic niches. Together, these modificationswill enhance the utility and safety of gene therapy astransition from pre-clinical to clinical gene therapyproceeds for the vascular system and its diseases.ReferencesAlesci S Chrousos GP, Pacak K (2002) Genomic medicine:exploring the basis of a new approach to endocrinehypertension. Annals of the New York Academy ofSciences 970, 177-92Alexander M.Y, Brosnan M, Hamilton C (2000) Gene transfer ofendothelial nitric oxide synthase but not Cu/Zn superoxidedismutase restores nitric oxide availability in the SHRSP.Cardiovasc Res 47, 609-617.Alexander M.Y, Brosnan M, Hamilton CA, Downie P, Devlin A,Dowell F, Martin W, Prentice H, O'Brien T, Dominiczak,A.F (1999) Gene transfer of endothelial nitric oxide synthaseimproves nitric oxide-dependent endothelial function in ahypertensive rat model. Cardiovasc Res 43, 798-807.Angelini G (1992) Saphenous vein graft failure: etiologicconsiderations and strategies for prevention. CurrentOpinion in Cardiology 7, 939-944.Asahara T, Chen D, Tsurumi Y, Kearney, M, Rossow, S, PasseriJ, Symes J.F, Isner J (1996) Accelerated restitution ofendothelial integrity and endothelium-dependent functionafter phVDGF165 transfer. Circulation 94, 3291-3302.Autieri M.V, Yue T, Ferstein G.Z, Ohlstein E (1995) Antisenseoligonucleotides to the p65 subunit of NF-kB inhibit humanvascular smooth muscle cell adherence and proliferation andprevent neointima formation in rat carotid arteries. BiochemBiophys Res Commun 213, 827-836.Baker A, Mehta D, George S, Angelini G (1997) Prevention ofvein graft failure: potential applications for gene therapyapy.Cardiovasc Res 35, 442-450.144