Biomedical Engineering Reference
In-Depth Information
types: the entry of any specifi c retrovirus is dependent upon the existence of an appropriate viral
receptor on the surface of a target cell. As the identity of most retroviral receptors remains un-
known, it remains diffi cult to predict the entire range of cell types any retrovirus is likely to infect
during a gene therapy protocol. Integration and expression of the exogenous gene in cells other
than target cells could result in physiological complications.
An additional drawback with regard to retroviral-based vectors is the propensity of the trans-
ferred gene to integrate randomly into the chromosomes of the recipient cells. Integration of the
transferred DNA in the middle of a gene whose product plays a critical role in the cell could ir-
revocably damage cellular function. For example, disruption of a central metabolic enzyme could
cause cell death, and disruption of a tumour suppresser gene could give rise to cellular transfor-
mation. In addition, integration of the proviral nucleic acid to sites adjacent to quiescent cellular
proto-oncogenes could result in their activation.
The theoretical complications posed by random chromosomal integration became a medical
reality in 2002, when two children who had received retroviral-based gene therapy 2 years pre-
viously developed a leukaemic-like condition. The initial clinical trial aimed to treat X-linked
severe combined immunodefi ciency (SCID-X1), a hereditary disorder in which T-lymphocytes
and NK cells in particular do not develop, due to a mutation in the gene coding for the
c cytokine
receptor subunit. The clinical consequence is near abolition of a functional immune system.
The trial entailed retroviral-mediated
ex vivo
transduction of haematopoietic stem cells from
10 young SCID-X1 sufferers, with subsequent re-infusion of the treated cells. A marked and pro-
longed clinical response in which the condition was essentially reversed was observed in 9 out of
the 10 patients. The prolonged response was likely due to the transduction of pluripotent progenitor
cells with self-renewal capacity (Chapter 10). However, the two youngest patients (1 and 3 months
old at the time of treatment) developed uncontrolled proliferation of mature T-lymphocytes 30
months and 34 months after gene therapy respectively.
It has subsequently been shown that this leukaemia-like condition was triggered by proviral integra-
tion at a site near the LM02 proto-oncogene promoter, leading to gene activation. This development
resulted in an initial ban on further retroviral-based gene therapy trials in some world regions, and the
proportion of trials undertaken subsequently using retroviral-based systems has dropped signifi cantly.
γ
14.3.2 Adenoviral and additional viral-based vectors
A number of additional viral types may also prove useful as vectors in the practice of gene therapy.
Chief amongst these are the adenoviruses. Adeno-associated virus, the herpes virus, and a number
of other viruses, are also being considered (Table 14.2).
Adenoviruses are relatively large, non-enveloped structures, housing double-stranded DNA as
their genetic material. Their genome is much larger (approximately 35 kb) and more complex than
those of retroviruses. In most instances, only a small fraction of this genome is removed when
constructing an adenovirus-based vector. Upon cellular infection, adenoviral DNA becomes local-
ized in the nucleus, but does not integrate into the host cell DNA. Usually, infection by wild-type
adenoviruses is associated with, at worst, mild clinical symptoms in humans.
As potential vectors for gene therapy, adenoviruses display a number of both advantages and
disadvantages (Table 14.3), and they have been used in over 300 gene therapy trials to date. Their
major advantage relates to their ability to infect non-dividing cells effi ciently and the usually
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