Leading geneticist Alan Lambowitz, Ph.D.,
introduces introns, a breakthrough that may make gene therapy safe and
effective agains HIV.
The momentous and widely heralded decoding
of a "rough draft" of the human genome, announced in June, may well
improve prospects for using gene therapy to treat human disease, including
HIV. Up until now, though, successes have not only been rare, but one
experiment actually proved lethal: Last year, a teenage boy died from
an allergic reaction to a virus used to introduce genes into his body
to treat a non-life-threatening metabolic problem. The good news is
that recent work by Alan Lambowitz, PhD, director of the Institute for
Cellular and Molecular Genetics at the University of Texas at Austin,
and his colleagues may mark a turning point. One of the key problems
in gene therapy has been how to safely transport the therapeutic genes
into the right places in quantities large enough to make a difference.
By using gene components called introns rather than potentially problematic
viruses to insert the genetic material, Lambowitz's group has hit on
a method that may overcome the failures and dangers of earlier approaches.
Reporter Maia Szalavitz recently interviewed Lambowitz about the exciting
possible applications of his team's work for treating HIV disease.
POZ: What are introns, and why
are they important to your new method of gene therapy?
Alan Lambowitz: In higher organisms, a
gene consists of information sequences, called exons, separated
by intervening sequences, called introns. Up to half of the genome
consists of these introns. We don't know why these genomic parasites
are there. But we do know they are harmless: When the DNA that the introns
are in is used to make protein, they are transcribed and splice themselves
out. All that remains is the information necessary to make the protein.
Ironically, Group II introns, the type we're working with, may be ancestors
of retroviruses like HIV. But when retroviruses insert themselves into
the genome, they -- unlike introns -- don't splice themselves
out and thus can cause harm.
In our recent work, we found a way to use a Group
II intron to disrupt the gene used to produce CCR5, a receptor protein
on the CD4 cell that allows HIV to enter and infect the cell. In the
lab, we inserted into human cells both an intron and a copy of a CCR5
gene carried on a plasmid (a piece of DNA that carries genes
from place to place). In this artificial situation, we demonstrated
that the intron could find and attack its target -- in this case, the
But it remains to be seen if the intron will
be able to insert itself into the actual chromosome (a more complex
structure composed of two strands of DNA). We are working on doing this
What would this mean if it worked?
If we could disrupt CCR5 in chromosomes, we could
generate a population of HIV-resistant CD4 cells. In principle, this
could arrest the progression of disease, because while you would still
have some infected cells, you could generate new resistant cells and
repopulate the immune system with them. Conceivably, this could work
permanently. And you wouldn't need to use a virus to get the genes into
the cells -- you could micro-inject the introns into cells and then
infuse them into the patient. Introns insert at precisely designated
sites, so you can use them to introduce a new gene or disrupt existing
You have also used introns against HIV itself.
Yes. You could, in principle, use introns to
target any part of HIV. We are exploring applications that would target
virus that has already integrated itself into human cells. The idea
is to use introns to introduce an additional component that is active
against HIV. It would essentially use HIV against itself: The HIV would
be infused with the new introns and something called a ribozyme, which
can destroy the virus' ability to reproduce. We have done this in plasmids,
but again, we have to try it in chromosomes.
There are some barriers, but in nature, there
is a very similar process. And if it works in chromosomes, you could
treat virtually any genetic disease, cancer and a variety of DNA viruses
(such as human papilloma-virus, hepatitis B and herpes viruses) -- all
could be targeted by introns.
What are the dangers of this approach? Although
people with defective CCR5 seem healthy, we really don't know whether
disrupting it might be dangerous or whether introns themselves might
cause side effects.
It's hard to assess potential dangers because
we're not yet at the stage of doing gene therapy. In general, this mode
of gene delivery would be safer because it is done on individual cells
outside the patient and because it doesn't use viruses.
Where has your funding come from, and where
do you see this approach going from here?
Introns are pretty esoteric, and we didn't expect
them to lead to practical applications. So up until now our funding
has mainly been basic-science grants from the National Institutes of
Health. Recently, a company was formed to provide patent protection
and pursue commercial applications. We really do think that introns
have potentially wide uses -- although I can't put a timetable on it.
But it's a sophisticated, clever mechanism and the possibilities are