Tuesday, August 2, 2011

Engineering HIV resistance

You may have heard in the news of people who are “immune” to HIV. This is because in order to infect cells, HIV needs to bind to a primary cell-surface receptor called CD4 and a co-receptor. HIV can use two kinds of co-receptors, CXCR4 and CCR5; X4 strains use only CXCR4, R5 strains use only CCR5 and X4R5 strains can use either one.  Some people are born with a mutation in the CCR5 receptor that makes it unrecognizable to HIV. This means that they’re not susceptible to infection from R5 viruses, but could still get HIV from X4 or X4R5 viruses.

Can we use a similar strategy to treat/prevent HIV infection? Maybe. There is a drug on the market (maraviroc) that blocks CCR5 so that HIV can’t recognize it. While this drug slows down infection, it usually comes back. First, because there is now a selective pressure for proliferation of any X4 or X4R5 viruses the patient might be harboring.  Second, because in some cases the virus mutates to recognize even the blocked receptor. (HIV has very little proofreading, so mutations accumulate very rapidly. One talk I went to estimated that every base is mutated to every other base every day in every patient.)

What if we could find a way to block both receptors at once in such a way that the virus couldn’t mutate to recognize them? Enter the zinc finger nucleases (ZFNs)!

ZFNs are engineered enzymes that use a zinc-finger domain to bind to a targeted DNA sequence and an endonuclease cleavage domain to cut the DNA at the binding site. The cleavage domains often need to function as dimers, so to edit DNA you need a pair of ZFNs that bind to adjacent regions. ZFNs can be used to edit genes, since after they cut DNA, it will be stuck back together in a process called non-homologous end joining (NHEJ) which will induce insertions or deletions that make the gene non-functional.

Scientists have tried this in cell cultures and found that they can indeed use ZFNs to edit both CCR5 and CXCR4 and these edited cells are resistant to HIV infection. Even more promisingly, this strategy also protects humanized mice which are immunodeficient mice that have human CD4+ T cells engrafted into them. When a portion of the T-cells are ZFN-edited, the mice show significant reduction in viral load and edited T-cells can be recovered, suggesting that they have a selective advantage.

Currently, clinical trials are underway. In these trials, a sample of CD+ T-cells is removed from the patient and edited with ZFNs and returned to the body. The hope is that these cells will create a virus-resistant reservoir that will preserve immune function. Of course, this therapy only applies to CD4+ T-cells and these cells won’t live forever, but extending immune function may prevent opportunistic infections and progression to AIDS in HIV infected patients.

ZFNs are already used for editing in a variety of research applications and scientists believe they show promise as a therapy for a variety of single-gene diseases.  Maybe ZFN editing will one day allow me to have permanently blue hair! I can only hope.

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