Scientists develop alternative to gene therapy

Posted: July 2, 2012 at 1:10 am

ScienceDaily (July 1, 2012) Scientists at The Scripps Research Institute have discovered a surprisingly simple and safe method to disrupt specific genes within cells. The scientists highlighted the medical potential of the new technique by demonstrating its use as a safer alternative to an experimental gene therapy against HIV infection.

"We showed that we can modify the genomes of cells without the troubles that have long been linked to traditional gene therapy techniques," said the study's senior author Carlos F. Barbas III, who is the Janet and Keith Kellogg II Professor of Molecular Biology and Chemistry at The Scripps Research Institute.

The new technique, reported in Nature Methods on July 1, 2012, employs zinc finger nuclease (ZFN) proteins, which can bind and cut DNA at precisely defined locations in the genome. ZFNs are coming into widespread use in scientific experiments and potential disease treatments, but typically are delivered into cells using potentially risky gene therapy methods.

The Scripps Research scientists simply added ZFN proteins directly to cells in a lab dish and found that the proteins crossed into the cells and performed their gene-cutting functions with high efficiency and minimal collateral damage.

"This work removes a major bottleneck in the efficient use of ZFN proteins as a gene therapy tool in humans," said Michael K. Reddy, who oversees transcription mechanism grants at the National Institutes of Health's (NIH) National Institute of General Medical Sciences, which helped fund the work, along with an NIH Director's Pioneer Award.

Questioning Assumptions

ZFNs, invented in the mid-1990s, are artificial constructs made of two types of protein: a "zinc-finger" structure that can be designed to bind to a specific short DNA sequence, and a nuclease enzyme that will cut DNA at that binding site in a way that cells can't repair easily. The original technology to make designer zinc finger proteins that are used to direct nucleases to their target genes was first invented by Barbas in the early 1990s.

Scientists had assumed that ZFN proteins cannot cross cell membranes, so the standard ZFN delivery method has been a gene-therapy technique employing a relatively harmless virus to carry a designer ZFN gene into cells. Once inside, the ZFN gene starts producing ZFN proteins, which seek and destroy their target gene within the cellular DNA.

One risk of the gene-therapy approach is that viral DNA -- even if the virus is not a retrovirus -- may end up being incorporated randomly into cellular DNA, disrupting a valuable gene such as a tumor-suppressor gene. Another risk with this delivery method is that ZFN genes will end up producing too many ZFN proteins, resulting in a high number of "off-target" DNA cuts. "The viral delivery approach involves a lot of off-target damage," said Barbas.

In the new study, Barbas and his colleagues set out to find a safer ZFN delivery method that didn't involve the introduction of viruses or other genetic material into cells. They experimented initially with ZFN proteins that carry extra protein segments to help them penetrate cell membranes, but found these modified ZFNs hard to produce in useful quantities. Eventually, the scientists recognized that the zinc-finger segments of ordinary ZFNs have properties that might enable the proteins to get through cell membranes on their own.

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Scientists develop alternative to gene therapy

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