Chromosome manipulation: a systematic approach toward understanding human chromosome structure and function.

The prospect of manipulating human chromosomes in an experimentally tractable system has been an attractive, but elusive one ever since Murray and Szostak introduced the concept of an artificial chromosome constructed from isolated components a dozen years ago (1). Subsequent studies in yeast using the artificial chromosome system have provided numerous insights into the molecular nature of and mechanistic basis for functional chromosomal elements (2), have elucidated the behavior of yeast chromosomes during both mitosis and meiosis (3), and, as a not insignificant fringe benefit, provided the initial tools for cloning of large DNA molecules as part of the Human Genome Project (4). Attempts to carry out similar analyses in mammalian cells have been frustrated by considerable size and engineering differences between yeast and mammalian chromosomes. Both yeast and mammalian chromosomes are linear and appear to utilize the same set of basic functional elements-telomeres, centromeres, and origins of replication; however, whereas the largest chromosome in the yeast Saccharomyces cerevisiae is <3 Mb in size and the chromosomes of the fission yeast Schizosaccharomycespombe range in size from -3 to 5.7 Mb, human chromosomes contain from -50 to 250 Mb of DNA. From an engineering perspective, these larger chromosomes appear to be harder to move around during the cell cycle as well. Yeast chromosomes require binding of only a single microtubule to ensure productive interactions with the mitotic spindle and proper segregation, whereas mammalian chromosomes bind dozens of microtubules. Lastly and probably most importantly, the size and complexity of human and other mammalian chromosomes notwithstanding, those interested in studying the organization and mechanics of mammalian chromosomes have thus far lacked the powerful genetic systems available to yeast and, increasingly, to Drosophila cytologists. Experimental approaches for manipulating mammalian chromosomes are, therefore, sorely needed, and, in that light, the report of Heller et al. (5) in this issue of the Proceedings is a welcome step forward. They report the latest in a series of studies to use the method of chromosome truncation to generate mitotically stable "mini-chromosomes" derived from the human Y chromosome, usually 50-75 Mb in size. The resulting minichromosomes range in size from -3.5 to 9 Mb and thus are approaching a size possibly capable of being shuttled between mammalian cells and yeast for further manipulation.