Posted by: blog4globalhealth | 03/09/2011

SLICING HIV DNA FROM INFECTED HUMAN CELLS

How a tiny protein could have a big impact on HIV treatment

Great strides have been made in the development of drugs to manage HIV infection. Yet significant challenges to treatment remain – especially in resource-poor settings where HIV infection rates are among the highest in the world. Lifelong treatment is expensive, even when low-cost generic drugs are available. Additionally, if resistance arises, there is little opportunity for access to second-line therapy. Social, economic and political factors can also interrupt access to drugs at any time, putting patients at risk.

Imagine, instead, a cure. It goes without saying that a cure for HIV would have a dramatic impact on the lives of all those infected – particularly those who do not currently have access to the best treatment. A single course, reasonably priced and widely available, could replace lifelong drug regimens. Cured individuals would no longer be infectious, and problems of drug adherence and viral relapse would be eliminated.

To date, there has been little hope of a cure for HIV, but my research group is one of the many trying to change that trajectory. The HIV virus works by taking a steadfast grip on the cells of those infected. It makes its way into some of the longest-lived cells in the human body – memory T cells of the immune system – and weaves copies of its own DNA into the host’s DNA. None of the existing approaches are able to find the HIV needle within this chromosomal haystack.

Our research is built on the hope that one type of protein – called homing endonucleases – could not only find the HIV virus, but also snip inserted HIV DNA out of infected cells. If we are successful, such an approach could actually “cure” human cells of HIV infection.

Homing endonucleases do one thing, and they do it extremely well. They search through complex genomes for a very specific DNA sequence, and when they find it, they cut the DNA in two. In nature, homing endonucleases exist in single-celled organisms. Their primary job is to cut a targeted DNA sequence and then insert a copy of their own DNA into the resulting DNA gap.

In human cells, the process of DNA repair works differently. Rather than inserting material into a DNA gap, the broken ends of the DNA get rejoined. During this repair process, some of the DNA surrounding the break site is lost, and the broken DNA is ultimately destroyed by the cell’s repair machinery.

So, what would happen if homing endonuclease was able to find and cleave sequences from a retrovirus like HIV that is hiding within the infected cell’s chromosomes? In one experiment, we challenged an existing homing endonuclease to sniff out a virus we had specifically created to be susceptible to the natural enzyme. This test showed that the homing endonuclease was able to delete parts of the viral DNA large enough to eliminate viral protein expression. Not every last trace of the virus was gone, but all that was left was a harmless shell. What’s more, the cells came through the treatment unharmed. In other words, the cell had been cured of the virus.

Unfortunately, in nature homing endonucleases don’t recognize HIV sequences. Now that we know the protein can eliminate a virus from a human cell, we face the challenge of redesigning these proteins in such a way that they recognize HIV itself.

With funding support from the Bill & Melinda Gates Foundation’s Grand Challenges in Global Health initiative, we are now working on generating such proteins. Using X-ray crystallography, a laboratory technique that can reveal the precise structure of a protein, we are looking to visualize homing endonucleases bound to their natural DNA targets. Once we know what they look like bound to their natural targets, we can then use powerful computer modeling to predict structural modifications that will allow the proteins to bind to HIV sequences. The third step will then be making those modifications – and a homing endonuclease that recognizes HIV DNA.

At first glance, the project seems like a classic big-budget scientific undertaking. Indeed, work like this takes significant resources and the combined expertise of scientists from a wide variety of fields. However, the ultimate motivation for this project lies where the end product could have its greatest impact – in developing countries. Lifelong antiviral therapy will continue to be a challenge in global health, and despite our best efforts, lives continue to be lost to HIV because sick people cannot access these drugs on a consistent basis.

While an HIV cure is still just a dream, the field has now progressed to a point that we can at least discuss possible ways to achieve it. Turning the dream into reality will take resources and the combined expertise of scientists from a range of fields. It will take innovative thinking and persistence. We must remain focused on the impact the end product could have. We must keep this vision in our minds as we tackle the difficult work ahead.

LEARN MORE

Read the other blogs in the Grand Challenges in Global Health Series

Bill Gates, Bill & Melinda Gates Foundation, Great Ideas From Unexpected Places
Chris Wilson, Global Health Discovery Program, Bill & Melinda Gates Foundation,Engaging the Best Minds to Tackle Global Health Challenges
Dan Feldheim, University of Colorado, Boulder, The Golden Treatment: Staying One Step Ahead of Drug Resistance
Dr. Szabolcs Márka, Columbia University, From Black Holes to Malaria


Dr. Keith Jerome, MD, PhD is an Associate Professor of Laboratory Medicine at the University of Washington, and an Associate Member of the Fred Hutchinson Cancer Research Center.


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