Gene therapy for some diseases, including Duchenne muscular dystrophy (DMD), can be tricky because the needed gene is often too large to fit inside the viral vector used for delivery. Researchers at the University of Rochester have now taken advantage of a basic cell biology discovery to deliver therapeutic muscular-dystrophy-related genes to mouse cells in two pieces, which the cells can then stitch together into a functional gene.
The technique, called Stitch RNA or StitchR, successfully delivered functional dysferlin and dystrophin muscle proteins to mice, restoring the levels of each protein to normal. The results were published in Science on Nov. 14.
The work was done in partnership with CANbridge Pharmaceuticals, a rare-disease biotech based in China with a U.S. headquarters in Massachusetts.
The StitchR DMD therapy was licensed from the University of Rochester by Scriptr Global, a company co-founded by senior author of the paper Douglas Anderson, Ph.D. Scriptr has since sub-licensed the rights to the treatment to CANbridge, Anderson told Fierce Biotech in an interview.
“There's hundreds or thousands of different monogenic diseases occurring from large genes that could be targeted this way,” said Anderson, a cell biologist at the University of Rochester School of Medicine and Dentistry. Some diseases Anderson and colleagues are looking at next include cystic fibrosis, hemophilia and different vision and hearing disorders.
StitchR takes advantage of the cell’s natural ability to repair RNA that has been cut by a ribozyme. Ribozymes are themselves made of RNA but can perform biochemical functions similar to enzymes. RNA cut by a ribozyme has a distinct chemical signature that cells can recognize and repair by reconnecting the fragments.
After discovering that cells can naturally repair RNA cut by ribozymes several years ago, Anderson's team turned to finding therapeutic uses for the ability, the researcher said.
In the StitchR gene therapy, two AAV viral vectors are packaged with half of the gene coding for either dysferlin or dystrophin, two large muscle proteins that malfunction in different forms of muscular dystrophy. Each genetic sequence contains part of the final gene attached to a ribozyme sequence. Once delivered to a muscle cell, the gene is transcribed into mRNA and the ribozyme activates, cutting itself off of the strand and leaving behind the chemical footprint the cell recognizes.
With both halves in the same cell, the repair machinery springs into gear and stitches the two strands together into one mRNA, which is then translated into a functional version of the protein. The RNA repair enzyme RtcB ligase is involved in the process, Anderson said, but the exact details of the repair pathway remain unknown.
Gene therapy techniques for DMD, including Sarepta Therapeutics' approved treatment Elevidys, deliver a truncated version of the dystrophin gene to cells, called micro-dystrophin, because of the protein’s immense size. This comes with the possibility of accidentally missing an important functional part of the protein.
“We were able to encode something that has all of the known functional domains required for dystrophin expression,” Anderson said. And for dysferlin, he added, StitchR is able to deliver a full-length copy of the gene. “Other techniques are not able to do that.”
Therapies for DMD have proven vexing for many biotechs. Sarepta’s Elevidys was placed on two clinical holds before eventually gaining approval, and, over the summer, the drug’s accelerated approval was expanded even though it failed a phase 3 clinical trial.
Elevidys has since surged in sales, generating $181 million in the third quarter alone. Earlier this month, Sarepta scrapped another DMD treatment—a peptide oligomer—due to safety concerns.
Meanwhile, Pfizer ditched its DMD gene therapy candidate in July after it, too, failed a phase 3 trial, which led to multiple rounds of layoffs.
Further, Dyne Therapeutics saw its stock tumble and three executives resign after its DMD drug showed serious side effects in a phase 1/2 trial.