RNAi as Antiviral Therapy: The HIV-1 Case - RNA Interference from Biology to Therapeutics - page 228

ornithine transcarbamylase deficiency and received an adenovirus vector encoding
the wild-type gene. This patient died 4 days later of a massive immune response,
most likely triggered by the high viral vector load [ 181 ]. SCID patients received a
gamma-retroviral gene transfer with the wild-type interleukin 2 gene in a trial that
started in 2000. Although this procedure improved the condition of all patients, a
true success, two patients developed leukemia due to clonal lymphocyte prolifera-
tion [ 182 ]. In both cases, the retroviral vector was integrated near the LMO2 proto-
oncogene, leading to enhanced expression of the LMO2 protein that has a crucial
role in hematopoietic development [ 183 ]. Such insertional oncogenesis was
described for a few more patients. After nearly 10 years of follow-up, gene therapy
seems highly successful in correcting the immunodeficiency associated with SCID
[ 184 ]. Hematopoietic stem-cell transplantation will remain the standard therapy, but
gene therapy may be an option in the absence of donors with a compatible HLA-
type [ 185 ] .
The retroviral vectors have gradually been replaced by lentiviral vectors that are
considered more safe because they tend to integrate in genes distant from the pro-
moter region. In addition, all transcriptional enhancer motifs have been removed
from the so-called third-generation lentiviral vectors “self-inactivating” design
[ 186 ]. Experiments with a lentiviral vector and hematopoietic stem cells in tumor-
prone mice did not, in contrast to the retroviral vector, show signs of insertional
oncogenesis [ 187 ]. Other safety and regulatory issues concerning lentiviral vectors
are addressed in a comprehensive review based on the expertise gained in the first
lentiviral trial [ 188 ]. The power of the lentiviral vector system to safely transduce
CD34 + hematopoietic precursor cells ex vivo was recently demonstrated in a gene
therapy trial for children with adrenoleukodystrophy [ 60 ] .

11.6 Future Perspectives

An overview of ongoing gene therapy trials for treatment of HIV-1 infection has
been published [ 189 ]. Thus far, these clinical trials have failed to demonstrate a
therapeutic benefit. One of the bottlenecks is effective gene delivery to a clinically
relevant number of cells, in particular in the early studies that treated T cells or
hematopoietic stem cells with retroviral vectors [ 59 ]. The use of lentiviral vectors
allows a much higher transduction efficiency of a variety of cell types.
The first clinical trial with a lentiviral vector was, in fact, directed against HIV-1
by expression of an extended antisense transcript against the viral RNA genome.
Persistent in vivo expression of the therapeutic antisense molecule was docu-
mented by the VirXsys company [ 123 ]. In addition, vector integration sites in
blood cells revealed a preference for gene-rich regions, which is typical for a len-
tivirus, and no signs of insertional oncogenesis were observed. Another anti-HIV
gene therapy trial that uses a triple RNA payload (ribozyme, decoy, shRNA) was
performed at the City of Hope by the team of John Rossi. AIDS patients under-
going autologous transplantation for lymphoma were treated with gene-modified