Biomedical Engineering Reference
In-Depth Information
were treated using retroviral gene transfer, developed T-cell leukemia. However, the
combination of vector insertion near the oncogene Lmo2 and a transgene (gamma
common chain) that is involved in cell development, were probably required for
tumor formation [15]. Furthermore, in this trial, few gene corrected cells had to
undergo many cell divisions to repopulate the entire immune system of the patients.
Because biopacemaker gene therapy does not require cell division of transduced
cells and because the therapeutic genes are not involved in cell proliferation, the risk
of tumor formation seems low in this case [6]. Lentivectors are therefore generally
believed to be a safe delivery platform. This is further supported by the fact that
insertional mutagenesis never has been identified in HIV positive patients. In addition,
heart tissue is composed of non-dividing cells, making them relatively resistant to
oncogenesis.
The use of lentiviral vectors in the ex vivo generation of biopacemaker cells, such as
the use of hMSCs or hESCs should also be considered. Both cell types are efficiently
transduced by these vectors and, in hESCs, stable genetic modification was even
combined with a preserved cardiogenic potential. In these experiments, a CAG
promoter was used to circumvent problems of gene silencing in these cells [24, 50, 54].
6 Vector Delivery Techniques
Currently available vector delivery techniques to target a focal region include direct
intramyocardial injection, subepicardial injection and epicardial administration [8].
Intramyocardial injection of viral vectors results in highly localized transgene activity.
When Ad vectors were injected in the apex of the left ventricle, transgene expression
was limited to a few millimeters surrounding the injection site [18]. Other injection
sites have also been proven to be reachable. Ad vectors were injected into the left
bundle branch [33] and into the left atrium (subepicardially) [36]. Although major
complications were not mentioned, site specificity of these approaches was not
analyzed here. Atrial injections might be most challenging, but particularly relevant,
since this is the site of the natural pacemaker. The thinness of the atrial wall, as
experienced during sub-epicardial plasmid injections in the right atrium of pigs,
showed both the difficulty and the feasibility of this approach [12].
A highly innovative epicardial administration of Ad vectors appeared perfectly suited
for atrial gene transfer. A gelatinous poloxamer matrix complexed to the virus was
painted on both atria after they were exposed by a median sternotomy followed by a
pericardium incision. A 100% transmural gene transfer was obtained in all atrial
regions, using dilute trypsin concentrations combined with the poloxamer matrix. This
elegant approach showed no evidence of reporter gene expression in the cardiac
ventricles, lungs, liver, spleen, kidney, gonads or skeletal muscle [21]. More specific
targeting of epicardially applied viral vectors may hold the greatest promise. This could
be reached with engineered viral vectors that specifically bind to receptors present on
atrial cardiomyocytes or by the use of magnetic carriers in combination with externally
applied magnetic fields [55] (this approach has similarities with recently introduced
methods of intracardiac catheter delivery using magnetic fields [48]).
