New Finding on Chromosomes

Mammalian chromosomes evolve by breaking at specific sites rather than randomly as long thought, according to a new study published in the July 22, 2005, issue of Science.

An international team of scientists from France, Singapore, and the United States compared the organization of the chromosomes of eight mammalian species: human, mouse, rat, cow, pig, dog, cat, and horse. Using computer software to align and compare the mammals' genetic material, the team determined that chromosomes tend to break in the same places as species evolve, resulting in rearrangements of their DNA. Before this, the prevailing view was that such rearrangements occurred at random locations.

Chromosomal breakages, also called translocations, are important in terms of evolution. When they break in egg or sperm cells, opportunities arise for the rearrangement of DNA in the resulting offspring. Such inheritable rearrangements may be lethal or cause disease. However, in some cases, the breaks can lead to the production of new or altered proteins with the potential to benefit an organism. In addition, chromosomal translocations are known to contribute to the development or progression of many types of cancer.

The researchers noted that the chromosomal abnormalities most frequently associated with human cancer are far more likely to occur in or near the evolutionary breakage hotspots than are less-common types of cancer-associated abnormalities. They speculate that the rearrangements seen near breakage hotspots may activate genes that trigger cancer and/or genes that normally suppress cancer. One finding was that the regions immediately flanking the breakage hotspots contain more genes, on average, than the rest of the genome. Another finding was that the rate of chromosomal evolution among mammals dramatically accelerated following the extinction of the dinosaurs about 65 million years ago.

The research team, which included investigators from the U.S. National Human Genome Research Institute (NHGRI, Bethesda, MD, USA), was led by Harris A. Lewin, Ph.D., of the University of Illinois at Urbana-Champaign (USA) and William J. Murphy, Ph.D., of Texas A&M University in College Station (USA).

"The dog genome map generated by NHGRI researchers and their collaborators played a key role in these new analyses,” observed Eric D. Green, M.D., Ph.D., scientific director of NHGRI. "This study shows the tremendous power of using multi-species genome comparisons to understand evolutionary processes, including those with potential relevance to human disease.”





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