Biology Reference
In-Depth Information
11.3
HIV-1 Inhibition by Knockdown of Critical
Cellular Cofactors
A theoretical advantage of targeting host cell proteins that are important for HIV-1
replication is the reduced chance of escape by viral mutation. Silencing of several
of these cofactors resulted in HIV-1 inhibition: nuclear factor kappa B [
76
] , CD4
[
73,
129
] , CXCR4 [
129-
132
] , DDX3 [
133
] , LEDGF/p75 [
134
] , CCR5 [
129,
131,
135
], and stable expression of shRNA against several cofactors could inhibit HIV-1
replication in vitro up to 2 months [
136
]. CCR5 is a critical receptor for HIV-1 entry
and is one of the most promising cofactor targets. First, individuals with the delta-32
mutation in the CCR5 gene do not express the receptor and are not susceptible to
HIV-1 infection. Second, this gene inactivation does not cause any health problems,
except for an increased risk for infection with the West Nile virus [
137
] . Third,
potent shRNA targeting the mRNA for this host cell factor have already been
described [
129,
135
]. Fourth, the therapeutic potential of downregulation of CCR5
is supported by the cure of an HIV-1 infected patient who had leukemia in addition
to AIDS. This patient received a bone marrow transplantation of a matching donor
who was homozygous for the 32-bp deletion in the CCR5 gene. Surprisingly, HIV-1
has remained undetectable in the patient's plasma for at least 600 days post transfu-
sion [
138
], and the patient has recently been declared “cured” of HIV-1 [
139
] .
Viral escape is theoretically possible in the CCR5 case. CCR5-tropic viruses are
generally responsible for HIV-1 transmission, although the virus can also use the
alternative CXCR4 receptor. Downregulation of the CCR5 receptor will potentially
set the stage for selection of CXCR4-tropic HIV-1 variants, but this evolutionary
route was not observed in the Berlin patient. The same virus escape route was dis-
cussed when CCR5-blocking drugs were developed, and such receptor-switch
escape routes have indeed been witnessed in patients treated with the CCR5-
antagonist maraviroc [
140,
141
]. In general, the concept that targeting of a cellular
cofactor prevents viral escape should be verified experimentally [
136
] .
Many cellular targets cannot be considered for silencing because they are essen-
tial for cell metabolism and the host. For instance, the alternative HIV-1 coreceptor
CXCR4 is required for homing of hematopoietic stem cells to the bone marrow and
subsequent T cell differentiation [
142
]. Although HIV-1 replication has been studied
extensively, many details of the viral replication cycle remain elusive, including
many of the involved cellular cofactors. Recently, three high-throughput RNAi gene
knockdown screens were published that identified numerous new candidate cofac-
tors [
143-
145
]. Although each of the screens reported hundreds of new candidates,
overlap between them was surprisingly small with only three proteins: MED7,
MED8, and RELA. A number of reasons for this enormous variation in experimental
results have been discussed (reviewed in [
146,
147
]). For instance, the three studies
used rather different experimental setups, including different cell lines (293T versus
HeLa cells) and lab-adapted viral strains versus viral vectors. A meta-analysis of
these RNAi screens confirmed that experimental variation contributed to the many
discrepancies between the screens [
148
]. Although the overlap between individual
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