Homing In On HIV’s Chromosome Target:
AIDS Virus Inserts Into Human Genome Using a DNA-Associated Protein
Findings have implications for better-designed gene therapies
(Philadelphia, PA) - A human DNA-associated protein called
LEDGF is the first such molecule found to control the location of HIV
integration in human cells, according to a new study from researchers
at the University of Pennsylvania School of Medicine.
This study, published in this week’s early online edition of Nature
Medicine, describes the first clear target for modulating where viruses
insert into the human genome, which has implications for better design
of gene-therapy delivery. Retroviral vectors are often used to introduce
therapeutic genetic sequences into human chromosomes, such as in the delivery
of Factor VIII for hemophilia patients.
HIV integrates into active transcription units on chromosomes within the
nucleus of human cells. These units are sites that lead to efficient expression
of the viral genome. Most HIV-infected cells in a patient will have a
very short life span, a day or less. “We surmise that this strategy
helps the virus make hay while the sun is shining, as it were, producing
lots of viral copies during a short time, so that the virus can maximize
production of daughter virions,” says Frederic Bushman,
PhD, Professor of Microbiology at Penn.
This present study demonstrates the first piece of a mechanism that dictates
where HIV integration takes place. Previous studies at other institutions
showed that LEDGF binds tightly to HIV integrase, the enzyme that’s
important for the integration reaction. Now, Penn researchers showed in
this study that the way LEDGF binds to HIV integrase and to specific sites
on chromosomes suggests that HIV targets integration using a molecular
Retroviruses contain RNA in their particles. They enter a cell and convert
RNA into DNA by the enzyme reverse transcriptase and then integrate that
DNA copy into the DNA of the host, using the integrase enzyme. The new
viral particles are made by transcription of the viral genome, as with
any cellular genes. If the cell divides, the viral DNA is copied and inherited,
along with cellular human genes.
Bushman and his team made cells that were depleted of LEDGF and found
that integration was less frequent in transcription units and in genes
regulated by LEDGF. “This implies that LEDGF is part of the machinery
that helps dictate the placement of retroviral integration sites within
chromosomes,” says Bushman.
Bushman notes that finding that LEDGF is part of the cellular apparatus
necessary for HIV replication is important to the field of gene therapy.
Controlling where gene-therapy vehicles insert in the human genome could
help make the delivery of new therapeutic sequences safer. The new findings
about LEDGF suggest that engineered tethering interactions might some
day allow control over integration site selection during gene therapy.
According to Bushman, this finding is of particular importance in light
of recent cases where integration of gene-therapy vectors near cancer
genes contributed to the development of leukemia in gene-therapy patients.
“This is first example of a cellular factor that’s a clear
player in target site selection,” says Bushman. “This isn’t
engineering yet, but it’s a key piece of information on the way.”
This research was funded in part by the National Institutes of Health,
the J.B. Pendleton Charitable Trust, and the F.B. Burns Foundation. Other
co-authors in addition to Bushman are: Angela Ciuffi, Christian Hoffman,
and Jeremy Leipzig, all from Penn, as well as Manuel Llano and Eric Poeschla
from the Mayo Clinic College of Medicine, Rochester, Minn., and Paul Shinn
and Joseph R. Ecker from The Salk Institute, La Jolla, Calif.
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