(From Nature Biotechnology press release.) Each cell in our bodies contains 46 chromosomes. But one day far in the future, doctors may give us an extra miniature chromosome containing therapeutic genes to treat disease. Researchers are currently elucidating the molecular makeup of human chromosomes in an attempt to learn the fundamental elements required to make a functional artificial chromosome that could be used to deliver very large (or ultimately whole sets of) therapeutic genes (e.g., the dystrophin gene in muscular dystrophy). In the May issue of Nature Biotechnology, Hiroshi Masumoto and his colleagues at Nagoya University describe how, for the first time, they constructed an artificial chromosome from defined elements using insights gleaned from studies of human chromosome structure. They show these minichromosomes are stable in human cells without requiring selective pressure, are not lost upon cell division, and do not acquire DNA from the natural complement of human chromosomes.

Scientists now know that sites of DNA replication, specialized sequences at the ends of chromosomes (telomeres), and unique sequences in the middle of chromosomes (centromeres) are all required for correct chromosome function. Previous work has shown that it is possible to mix all of these elements into cells and allow the cell's own machinery to rearrange them into artificial chromosomes, but uncharacterized fragments of human DNA were always needed, making the exact composition of the resultant minichromosomes uncertain.

Hiroshi Masumoto and colleagues set out to construct a human artificial chromosome from a defined set of components that includes a segment of the centromere known as α-satellite DNA without incorporating undefined pieces of the genome. Using a yeast artificial chromosome as a starting point, they constructed a minichromosome containing α-satellite DNA from human chromosome 21, human telomere DNA and drug resistance markers (to allow the selection of cells containing reassembled chromosomes). Using solely these elements, they found that the cell's own machinery was able to rearrange, amplify, and kick out DNA without incorporating unknown fragments. The resultant minichromosomes-no larger than a tenth of the smallest natural human chromosome-were stable at one copy per cell without requiring selective pressure and did not acquire host DNA sequences. The ability to create chromosomes de novo from defined genetic elements makes the dream of creating human chromosomes for gene therapy look a little more like reality.—June Kinoshita

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  1. . Construction of YAC-based mammalian artificial chromosomes. Nat Biotechnol. 1998 May;16(5):431-9. PubMed.