(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.
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
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 tether.
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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