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Is ex vivo adenovirus mediated gene transfer a therapeutic option for the treatment of corneal diseases?
  1. T Ritter1,
  2. N Gong2,
  3. U Pleyer2
  1. 1Institute of Medical Immunology, Charité–University Medicine Berlin, Germany, and Regenerative Medicine Institute, National University of Ireland, Galway, Ireland
  2. 2Department of Ophthalmology, Charité–University Medicine Berlin, Germany
  1. Correspondence to: Thomas Ritter PhD, Institute of Medical Immunology, Charité–University Medicine Berlin, Campus Mitte, Monbijoustrasse 2a, 10117 Berlin, Germany; thomas.rittercharite.de

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Improvements in vectors, promoters, and transgenes have to be accomplished before gene therapy could be considered as an option in cornea gene therapy

Currently there are almost 1000 clinical trials for the treatment of cancer, inherited monogenic diseases, and cardiovascular diseases.1 Viral vectors have been used extensively to transduce human cells or tissues, mostly retroviruses and adenoviruses representing the majority because of their transduction efficiency compared to other gene transfer techniques (liposomes, gene gun, electroporation). However, further progress in gene medicine has been compromised by severe problems that arose in clinical trials using recombinant adenoviruses or retroviruses. Severe side effects resulted either from “overdosing” of adenoviral particles resulting in fulminant inflammation or from retroviral insertion into the genomic DNA leading to deregulated proliferation of transduced cells.

The question arises whether there could be a therapeutic option for gene therapy in the treatment of non-life threatening diseases? If there is a consensus that this could be useful, there are some important prerequisites: the gene therapeutic vehicle has to be safe, it should be specifically expressed only in the target cells/tissues without inducing undesired effects, and the transgene should be expressed for extended periods of time. Although this will be difficult to achieve there are some clinical settings where gene therapy could be a therapeutic option. Transplantation of allogeneic cells or tissues is considered as the final option for end stage organ disease; however, in most cases it is not a life threatening disease. The advantage of gene therapeutic application in this setting is that the organ could be transduced with the vector during storage or/and perfusion.2 Another very interesting use would be the genetic manipulation of corneal grafts before transplantation. Corneal transplantation is the most common transplantation of allogeneic tissue.3,4 In contrast with other organs the cornea can be stored up to 1 month without significant loss of function, which allows extensive quality testing and MHC typing for subsequent transplantation. This renders the cornea an ideal target for genetic manipulation. Indeed, several groups have explored the potential of gene therapy on the cornea, focusing on the transduction of the endothelium as a critical target during transplant rejection.5–10 Most of these studies have been investigating the potential of recombinant adenovirus as a gene therapy vehicle because of its high transduction efficiency of endothelial cells but not of stroma and epithelial cells. In this issue of BJO, Jessup and colleagues (p 658) expand our knowledge on the in vitro adenovirus mediated gene transfer to the human cornea. They show that human corneas can be efficiently transduced with recombinant adenovirus following up to 28 days in cold storage. Moreover, secreted proteins can be found in the culture medium at high doses over prolonged periods of time. It is obvious that increasing the dose of adenovirus particles per cornea increases the transduction efficiency; however, one has to carefully optimise the dosage since very high doses of adenoviral vectors seem to reduce endothelial cell densities. Moreover, the efficiency of transduction seems to vary greatly resulting in corneas almost resistant against adenoviral transduction (<2% transduction). The reason for this phenomenon is unclear but has been observed in animal models of ex vivo gene transfer of cultured corneas as well (T Ritter, unpublished observation). The reason for the endothelial cell loss after high dose adenovirus transduction is not clear but adenoviral proteins expressed at low levels in transduced cells or toxic adenoviral capsid proteins following cell entry of adenovirus particles may trigger apoptotic processes and contribute to accelerated cell loss. These problems could be partially circumvented by using third generation adenoviral vectors that do not express adenoviral genes. An important prerequisite for successful gene therapy is based on efficient transgene and protein expression in/from transduced cells. It is generally accepted that adenovirus mediated gene transfer does not lead to long term gene expression because of the episomal status of the adenovirus upon transduction, which is gradually lost during cell divisions. However, since human corneal endothelial cells have lost their capacity to divide (in contrast with rodent corneal endothelial cells) this might not be a severe problem. Moreover, long term gene expression might not be necessary in the transplant situation since short term gene expression might be sufficient to induce immunomodulation.2 In addition, long term expression observed in vitro does not necessarily result in long term expression in vivo. First evidence for that has been previously reported showing that transgene expression in vivo is shut off after a few days upon adenovirus mediated gene transfer in vitro.5 Finally, results obtained so far in rodent models using ex vivo adenovirus mediated gene transfer for the prevention of corneal graft rejection have not been very successful,7,11–13 except one study using an interleukin-10 expressing adenoviral vector in a sheep model of cornea transplantation.8 Whether the low success rate of ex vivo adenovirus mediated gene transfer is a result of the specific environment of the eye leading to loss of transgene expression has to be studied in more detail. Improvements in vectors (low immunogenic14), promoters (other than viral promoters), and transgenes (it is still not clear which is the best therapeutic gene) have to be accomplished before gene therapy could be considered as an option in cornea gene therapy.

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Improvements in vectors, promoters, and transgenes have to be accomplished before gene therapy could be considered as an option in cornea gene therapy

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