Gene therapy could save mice from heart attacks — humans could be next

The new research is significant for demonstrating the capability to control gene activity in response to injury, confining it to a specific part of the tissue and within a defined time period as opposed to being constantly active in the entire organ.

In their research, scientists employed a segment of fish DNA called TREE (tissue regeneration enhancer element). TREEs are a family of gene enhancers included in the genome that are in charge of detecting damage and activating the genes involved in repair so that regeneration may take place in a specific location.

As the healing process is over, these enhancers can also turn off gene activity. Along with zebrafish, these regulatory components have also been found in fruit flies, worms, and mice.

"We probably have them too"

"We probably have them too," said Ken Poss, Ph.D., the James B. Duke Distinguished Professor of Regenerative Biology in the Duke School of Medicine, who discovered heart regeneration in zebrafish two decades ago and has been studying them since. "But it's just easier for us to find them in zebrafish and ask if they work in mammals."

These 1,000 nucleotide-long enhancer sequences are packed with recognition sites where various factors and stimuli can bind and alter gene activity. "We don’t fully understand how they do this and what they’re truly responding to," Poss said.

He further said that different cells in an animal have different types of enhancers as well. “Some of them are responsive in multiple tissues -- those are the ones we use here. But when we profile regenerating spinal cord or fins in fish, we get different sequences.” The human genome may contain tens of thousands of these enhancers, he added.

Zebrafish genomes

Researchers incorporated different zebrafish TREEs into the genomes of embryonic mice and used a visual marker to signify gene activation. They discovered that around half of the enhancers functioned as anticipated and turned tissue blue when and where they sensed tissue damage in the transgenic mammals.

The next question was whether they could employ adeno-associated virus, a common gene therapy method for delivering gene sequences into cells, to incorporate the enhancer elements into an adult mouse. All tissues were exposed to the virus' enhanced DNA, but the TREEs had to become active only in response to damage.

It was demonstrated through a series of studies on mouse heart attack models that viruses containing a TREE could be infused a week before the damage, and the enhancer would then activate when it detected damage.

However, scientists discovered that giving it to the animal a day or two after the heart attack also had positive results.

“All three TREEs that we tested could be effective if delivered one day or sometimes longer after the injury -- they could still target expression to the injury,” Poss said. “Is this method of delivering a TREE and a gene going to allow us to deliver a molecular cargo to the right place at the right time? We found that it does in mice.”

Testing on human-like hearts

Pigs, who have far bigger hearts and more human-like heart rates, were also virally given a TREE and a fluorescent marker gene. Again, the marker only lit up at the location of the injury when viruses were injected into the pig hearts through the coronary arteries either before or after a heart attack.

Then, to test if this system could truly repair damage as opposed to merely detecting damage and activating a gene that lights up tissue, scientists gave a hyperactivated variant of YAP, a potent tissue growth gene that is linked to cancer. The main concern was whether this "very strong hammer" that may cause cell division to go rampant could be lassoed into just operating at the proper time and location.

They used a mutated YAP controlled by a TREE to test if the muscle grows safely after a heart attack in mice. “The TREE turned on a mutated YAP for a few weeks, just in the injury site, and then it naturally shut down expression,” Poss said.

“You really wouldn’t want to express YAP at full blast, that can cause problems like excessive growth, but what we found is that we could direct it,” Poss said. “The whole animal gets the gene therapy, but the YAP cargo only gets expressed at measurable levels when and where you injure the heart,” Poss said.

“We think we can use these methods to control genes in a certain time and certain space, and that includes shutting them off.”

The next step for the researchers will be to better understand what molecules bind to the enhancers, what controls their functions, and where they are found in the human genome, along with strengthening their targeting capabilities.

The study was published in the journal Cell Stem Cell.