Researchers at Case Western Reserve University's School of Medicine and Athersys Inc. have created the first artificial human chromosomes. The synthetic chromosomes represent a breakthrough in medical research and provide scientists with a powerful new tool for the study of human genetics. Artificial chromosomes may also offer a new approach to gene therapy and the treatment of a broad range of genetic diseases. A report of the research was published in the April issue of Nature Genetics.
"This opens the door to a whole new avenue of research in chromosome biology and gene therapy," said Huntington F. Willard, senior author of the study and chairman of genetics at the School of Medicine and University Hospitals of Cleveland. "While it's been known since the early years of this century that chromosomes carry genes, until now the complexity and size of normal chromosomes has limited our ability to analyze their structure and function. The synthetic microchromosome system now allows us to perform detailed studies on the nature of chromosomes -- essentially the next phase of the Human Genome Project which is to move from just mapping genes to actually understanding how they work and influence human disease."
Normal chromosomes are modular structures consisting not only of hundreds or thousands of genes, but also of specialized elements that are believed to be important for chromosomal stability and function. Telomeres, which consist of DNA and protein, are located at the ends of chromosomes, protecting them from damage. Centromeres are specialized regions of DNA that are essential for the proper control of chromosome distribution during cell division. Human centromeres are believed to consist of large segments of highly repetitive DNA, called alpha satellite DNA, which is thought to play a significant role in centromeric function.
"Our successful creation of functional centromeres and incorporation of them into artificial chromosomes were the critical achievements enabling the stability and normal behavior of the chromosomes throughout the cell cycle," noted Willard. He added that past attempts at producing synthetic chromosomes have failed because they lacked proper centromeres, and thus could not persist through multiple cell divisions.
In this study, the research team created artificial chromosomes from normal human material. The researchers first synthesized arrays of alpha satellite DNA, then introduced the resulting centromeric material into human cells in conjunction with telomeres and genomic DNA. Inside the cells, the independent elements assembled to form miniature chromosomes, or synthetic microchromosomes, that were structurally similar to human chromosomes, but contained less genetic material. Analysis of the newly introduced artificial chromosomes demonstrated normal centromeric activity, genetic stability, and continued gene expression through repeated rounds of the cell cycle.
"Synthetic chromosomes have the potential to overcome a major stumbling block in gene therapy," said John J. Harrington, lead author of the study, a postdoctoral fellow at CWRU, and now vice president and director of research at Athersys. "The characteristic stability of our synthetic chromosomes enables, for the first time, the potential long-term expression of therapeutic proteins in target tissues of patients treated using gene therapy."
The synthetic microchromosome remains independent within the host cell and functions essentially as an accessory chromosome. In contrast, most gene therapy systems currently under development utilize viral vectors, which often require the integration of the therapeutic gene into an existing chromosome and thus can result in chromosomal damage or interference with normal gene expression. Viral vectors can also induce immune responses that limit therapeutic efficacy.
"We are already moving ahead to refine the system and begin building an efficient and versatile vehicle for the introduction and stable maintenance of therapeutic genes in human cells," remarked Gil Van Bokkelen, president and CEO of Athersys and an author of the study. "This is a technology that we are very excited about. It could ultimately provide treatments for a wide variety of genetic disorders."
In addition to Willard, Harrington, and Van Bokkelen, the authors on the paper are: Robert Mays of CWRU and Athersys, and Karen Gustashaw of CWRU. Athersys Inc. is a privately held Cleveland company engaged in the research and development of novel therapeutic and diagnostic products based in the areas of chromosome structure and function and T cell-mediated immune responses.
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Toni Searle, amf2@po.cwru.edu
Editor, "Campus News, CWRU
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Journal
Nature Genetics