CHAPEL HILL--Using a promising new technique, University of North Carolina at Chapel Hill scientists have for the first time successfully inserted large circular plasmids - doughnut-shaped pieces of DNA containing healthy genes - into human cells and showed the genes functioned as if they belonged there. After more than a year, the genes continued to operate normally.
The work is important, researchers say, because it may offer a more effective method of transferring healthy genes into humans with various illnesses such as hemophilia, cystic fibrosis and sickle cell anemia and improve treatment.
"We have been able to do this with a piece of DNA five to 20 times the usual size scientists work with," said Dr. Jean Michel Vos, associate professor of biochemistry at the UNC-CH School of Medicine. "Although so far we have worked only in cultured human cells in the laboratory, we believe it could eventually work well directly in humans. It is exciting and very promising."
A report on the research appears in the September issue of Human Gene Therapy, a scientific journal. Besides Vos, authors are Drs. Eva Maria Westphal, postdoctoral fellow in medicine; Halina Sierakowska, postdoctoral fellow in biochemistry; and Ryszard Kole, associate professor of pharmacology; and technician Elisabeth Livanos, all of the UNC Lineberger Comprehensive Cancer Center.
Researchers transferred genes responsible for producing Beta-globin, one of two chief components of hemoglobin, the large molecule that carries oxygen in blood. Each time their cultured human cells divided, the genes replicated, or reproduced, as well and functioned for more than a year.
"You can think of it as an artificial chromosome," Vos said. "It does not become part of any of the 46 original chromosomes in each cell nucleus by inserting itself into them, but it is in addition to them."
In collaboration with Kole's laboratory, Vos and Westphal also showed the genes to be active, going from double-stranded DNA to single strands of RNA, the first step toward production of protein, which is the genes' purpose. DNA and RNA are like molecular blueprints for protein production.
Bacteria first are used, like little factories, to clone DNA outside the human cells because bacteria take only an hour to reproduce rather than the 24 hours human cells require.
The scientists then attach the DNA in circular form to a harmless part of the Epstein-Barr virus because of the virus' ability to replicate and maintain itself in the nucleus of human cells. The virus serves as a vector, or carrier, of the human genes, something like attaching a locomotive to freight cars to take them where they are needed.
By creating separate, artificial chromosomes, researchers no longer have to worry about genes they are transferring attaching randomly to parts of other chromosomes and then not working independently, Westphal said. The circular design promotes strength and stability because linear forms have a greater tendency to break inside the cell nucleus.
"I want to stress that this work is not a gene therapy cure for any disease yet, but I do think it is a major technical step forward," Vos said. "It moves the field from working with shrunken versions of human genes to entire functioning genes. We have shown the genes function and function for a long time."
Three years ago, Vos and colleagues published a paper in Nature Genetics suggesting it would be possible to build a circular artificial chromosome. In August they published another paper in Nature Biotechnology showing how to build artificial chromosomes in mouse cells, a step before doing it in whole animals.
The U.S. Department of Energy and the Bayer Corp. supported the research, along with help from faculty and staff at the Roosevelt Park Cancer Center in Buffalo, N.Y., the Murdoch Institute in Melbourne, Australia, and Glaxo-Wellcome in Research Triangle Park.
By David Williamson
The above story is based on materials provided by University Of North Carolina At Chapel Hill. Note: Materials may be edited for content and length.
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