Fake Chromosomes, From Scratch

Scientists have built an artificial human chromosome using only known DNA regions. The chromosome, described in the May issue of Nature Biotechnology, is a step toward someday introducing therapeutic genes into the nuclei of human cells in the body.

For an artificial chromosome to work, it must include three DNA sequences. The specific sequence where DNA copying begins is essential, as are telomeres--specialized stretches that prevent the DNA molecule from becoming shorter each time it's copied. Finally, the chromosome must have a centromere, a DNA region necessary for the duplicated chromosome to be pulled into daughter cells during cell division. Trouble is, scientists have known only the precise location of the telomeres in human chromosomes.

A team led by Hiroshi Masumoto of Nagoya University in Japan inserted a certain type of alpha satellite DNA--repeats of a simple DNA sequence in the centromere region of human chromosome 21--into yeast artificial chromosomes (YACs). Each YAC has a centromere, telomeres, and a replication origin of yeast, and often serves as a host for cloning human DNA molecules. Next they added human telomere sequences and selectable marker genes to the YACs, which they injected into human cells in culture. To Masumoto's surprise, the experiment worked on the first try. "I couldn't help shouting in the lab at midnight," he says. He found that the chromosome was still there after 60 days worth of cell division.

The success "brings human artificial chromosomes another step closer to reality," says Huntington Willard, a geneticist at Case Western Reserve University School of Medicine in Ohio, whose research group last year constructed a stable human artificial chromosome using a synthetic centromere and unidentified DNA. Masumoto's group is now planning to try to introduce the artificial chromosomes into mice.