Researchers have found a way to deliver gene-activating molecules called transcription factors into specific tissues of a living animal for the first time. The approach, which many have written off as too technically challenging, prevented a form of liver damage in mice—though it has many more technical hurdles to clear before it can be used in other tissues, or in people.
“This is a piece of very exciting work,” says Hsian-Rong Tseng, a chemist at the University of California (UC), Los Angeles. “It’s going to bring the field of transcription factor delivery to a different place.”
Our cells produce more than a thousand unique transcription factors, each of which binds to a specific region of DNA to prompt a gene’s transcription: the creation from DNA of an RNA template for a new protein. Changing the activity of these factors could, for example, amp up the production of proteins that suppress tumors or reduce inflammation, and even reprogram adult cells into immature cells or new cell types. And unlike other gene therapy approaches that permanently introduce DNA to boost protein production, transcription factors break down and leave no lasting effect on the genome.
But cells have ways of shutting out or destroying relatively big proteins such as transcription factors when they are delivered from the outside rather than being made inside the cell, says Niren Murthy, a bioengineer at UC Berkeley. If a transcription factor gets into a cell at all, it will end up digested within a waste disposal organelle called the lysosome and will never make it to where it can turn on a gene—the nucleus. “That is still a big unsolved problem,” he says. And among proteins, transcription factors are particularly sensitive to chemical modification, he adds. Binding them to useful molecules that could interact with cell receptors or penetrate membranes often means changing their chemical structures so much that they no longer do their jobs.
In 2011, Tseng’s group found a new way to deliver cumbersome transcription factors into cells without significantly changing their chemical structure. By attaching a transcription factor to a loop of DNA that contains the same sequence it is meant to recognize in the cell, and then wrapping it in a positively charged nanoparticle that can penetrate the cell membrane, they were able to deliver the proteins into human cells in a dish.
In the new study, Murthy and colleagues built on this basic binding concept to attach a transcription factor to a chunk of DNA, but they used that DNA as a scaffold to hold several other molecules that come in handy on the transcription factor’s journey. This new complex, which they named a DNA assembled recombinant transcription factor (DART), includes two chemical chains that can disrupt a cell’s lysosome membrane. These are capped with sugar molecules that prevent them from working until the DART gets trapped in the highly acidic contents of the organelle. These sugars also initially aim the DART specifically at liver cells by interacting with receptors on their surfaces.
For the first prototype DART, the group chose a well-studied transcription factor called Nrf2, which regulates several anti-inflammatory and antioxidant genes. “In animal models, it has literally been able to protect against every known inflammatory disease, ranging from Alzheimer’s to liver disease to atherosclerosis,” Murthy says. To test the DART’s ability to send this protein to the liver, the researchers first injected mice with a high dose of the pain reliever acetaminophen that would be expected to cause liver damage. An hour later, they injected Nrf2-bearing DARTs.
They found that the DARTs were taken up primarily in the liver and increased the expression of three genes known to protect against oxidative stress. Liver samples from the DART-treated mice closely resembled those of healthy mice, while the livers of untreated mice showed significant damage, the group reports online today in Nature Materials.
Those results are “quite dramatic,” says José Manautou, a toxicologist at the University of Connecticut, Storrs, and the treatment could potentially be valuable for patients with acetaminophen overdose. But those patients usually arrive in the hospital a day after ingestion, not an hour, he says. Murthy’s team is now working on studies to see if the treatment can actually reverse existing damage, including the effects of chronic liver disease.
Others are much more interested in seeing DARTs work on other tissues. “It’s relatively easy to get things taken up by liver cells,” says David Frank, an oncologist at the Dana-Farber Cancer Institute in Boston. “That’s always kind of step one in any new delivery technology.” But he sees exciting potential in cancer, for example, if a DART could incorporate a mutated version of a transcription factor that would shut down genes that promote tumor growth.
Murthy’s group doesn’t have immediate plans to work on targeting other tissues. The sugar they use in this study serves a handy dual purpose of disrupting membranes and targeting the liver. “In theory, you could do the same things with peptides and antibodies and things like that,” he says, “but the chemistry will get much more complicated.”
Tseng, who has continued to work on his own transcription factor delivery method to reprogram adult cells into stem cells, is enthusiastic about the DART strategy. “We have been struggling trying to find out a good way of applying this transcription factor delivery story,” he says. “I couldn’t think of anything better at this stage.” But he warns that the proteins themselves are very expensive, which puts additional pressure on the use of transcription factor therapies. “If they work well, then I think cost may not be the issue,” he says, “but you need to find out the key applications, beyond liver damage.”