Artificial chromosomes could be the future of gene therapy as it allows large amounts of DNA to be introduced into a cell without disrupting the existing genome. Researchers in Tottori University in Japan took an artificial chromosome containing the gene to correct a serious disease, put it in a stem cell, and transplanted it into a body. They are part of Japans 21st century COE (centers of excellence) program. Mitsuo Oshimura's team has now proved that the concept works by correcting a genetic defect in mouse stem cells. "It's a significant step forward," says Bruce Bunnell, who heads a competing group at Tulane University in New Orleans.
Artificial chromosomes can carry large amounts of DNA, even multiple genes, but don’t insert themselves into the existing genome. Instead, the artificial chromosomes sit inside the cell’s nucleus expressing their genes alongside the original genome.
Previously researchers at Chromos Molecular Systems of Burnaby in British Columbia, Canada, used artificial chromosomes to add an extra gene to cells grown in the lab, and showed that the gene functioned when the cells were transplanted into mice (New Scientist, 19 June 2004, p 10). Now, Oshimura has actually corrected a genetic defect in stem cells.
Oshimura’s team worked with stem cells from the testes of newborn mice in which the p53 gene had been knocked out – p53 makes a protein that prevents tumour growth. Adding an artificial chromosome carrying a copy of p53 restored production of the protein in the stem cells, and activated another gene that is normally controlled by p53.
At the same meeting, Chromos announced that its researchers have inserted artificial chromosomes into human embryonic stem cells. Company vice-president Harry Lebedur claims that Chromos’s chromosomes have the advantage that they can be more easily purified from the cell cultures in which they are grown and can be transferred to stem cells with greater efficiency. “Oshimura’s work, combined with ours, is paving the way for using artificial chromosomes as vectors for gene therapy,” he says.
There is much work to do before the technique is used in the clinic. For example, the chromosomes must be shown to be stable in stem cells over long periods.