Editor’s note: As you read in the press about advances in gene repair, look for advancement of knowledge about the technology. Once there are good vehicles for delivery of repair information to all cells, then the possibility of repair of the VHL gene is more likely.

Through the use of a novel technique termed "chimeraplasty", researchers have, for the first time, successfully repaired the genetic defect associated with Crigler-Najjar syndrome, a rare but devastating liver disease, in lab rats.

The finding may lead to gene therapies that cure other genetic diseases such as hemophilia, sickle cell anemia, [and von Hippel-Lindau disease,] according to the report published in the Proceedings of the

National Academy of Sciences.¹

Principal investigator Dr. Clifford Steer, of the University of Minnesota School of Medicine in Minneapolis described his team’s work in an interview with Reuters Health. "We did two things that were important: one, we developed a system whereby we could go in and literally rewrite the genetic sequence in a specific gene of interest; and two, we developed a technology that would do it in the hepatocyte, the type of liver cell that controls the major defect [of Crigler-Najjar syndrome]." The syndrome is characterized by an inability to properly metabolize bilirubin, a byproduct of normal red blood cell degradation. The genetic disease features jaundice and destructive changes in parts of the brain.

The researchers created a molecule called a chimeraplast, which is based on the structure of the defective gene and "targeted to that portion of the gene with the mutation," Steer explained. When the molecule is introduced into the cell, "it tricks the cell into thinking that there is a defect in its DNA sequence for that particular gene and by tricking the cell, the cell basically repairs [what it perceives as] its own defect."

What is being accomplished, in essence, is genetic repair without having to employ techniques that require "the introduction of new genes [via genetically modified viruses] to take the place of the [original defective] genes," as is the case in conventional gene therapy, according to Steer.

"All we are doing is repairing the genetic defect in a gene that is already there," he explained. "And when you think about it, what is really the best way to do gene therapy? It would be to go in to correct the defect so that the gene is in the right position, controlled and regulated by the [appropriate] regulatory elements that would normally control that gene."

An important aspect of this technology is that "once the genetic change is made, the repair is permanent," Steer said. And although this work was carried out in an animal model, the Gunn rat – which has a genetic defect similar, though not identical, to that seen in human Crigler-Najjar syndrome – Steer "feels very confident that it’s going to work in humans. It works in animal models, it works in plants, it works in bacteria, it works in any structure that has DNA in it."

The type of mutation corrected in the study involves an omission or deletion of one "letter" in a specific DNA genetic sequence. DNA is composed of basic units called nucleotides, which are designated by specific letters. These combine into sequences that "spell out" a genetic code. Changing one "letter" into another is easier than replacing a missing letter, Steer said, and in the human disease, changing rather than replacement is required to correct the defect. Consequently, "we hope that our results will be even more exciting in human beings than they were in the rodent model."

Together with his colleagues at the Albert Einstein College of Medicine in the Bronx, New York, Steer expects to submit a clinical trial application for the technique to the Food and Drug Administration early next year. "We’re going to involve 3 to 5 pediatric patients already identified with Crigler-Najjar, who live in the Amish Country in Pennsylvania," Steer told Reuters Health. He commented that the Amish have a very high frequency of this disorder and that these particular children all have the same genetic defect, a mutation at a single point in the genetic code.

The majority of genetic diseases in human beings are single-point mutations, Steer explained, "so we have a technology here that can be applied to many different types of disease. The technology is here, it’s here to stay, and it is very different from gene therapy."

The investigators are already developing a number of other animal models to look at potential clinical applications in disease such as Gaucher’s disease, hemophilia, thalessemia, and sickle cell anemia. Though sickle cell anemia’s defect originates in the bone marrow, Steer believes that the team will be able "to develop a delivery system for the chimeraplasts" that will direct them to the marrow, the site of the stem cells from which the defective red blood cells in sickle disease originate.

"It’s going to be more challenging than liver, only because it’s bone marrow and the progenitor cells are more difficult to deal with," he noted.

Steer emphasized that though the results achieved thus far are "very, very exciting," with potentially "broad-based application," much remains to be done. Further developing and refining the technology’s seemingly limitless possibilities will "keep the medical profession busy for many, many years," he predicts.

1. Proceedings of the National Academy of Sciences USA (1999) 96:10349-10354.

As printed in the VHL Family Forum 8:1, March 2000. For permission to reprint, please contact VHL Family Alliance, editor@vhl.org. Further information is available from the VHL Family Alliance, info@vhl.org.