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Columns::September 8, 2003
Smell of success: Chemists receive $2.5 million National Science Foundation grant to study aromatic compounds
Delaware expert on academic effectiveness named director of UGA institutional research
Karen Holbrook, former UGA provost, will deliver McBee Lecture
CURO apprentices participate in national issues forum on terrorism
Researchers test less lethal means to find contamination levels
Economics professor lucks out with state lottery research project
Update: Private Giving
Kudos
Faculty of Engineering member discusses role of ethics in research projects
Food for thought
Campus News
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| Edward Kipreos (right) with co-authors on the project Hui Feng (left), a graduate student when the paper was written (and now a post doc), and Fernando Santiago (center), a laboratory technician. (Photo by Peter Frey) |
Stable environment
Scientists discover gene that maintains genome stability
By Phil Williams
phil@franklin.uga.edu
A team of UGA cell biologists led by Edward Kipreos has discovered a gene that maintains genome stability by controlling the level of DNA replication.
The discovery, while still at the level of basic science, could have important implications in future studies of the genome instability present in cancer cells and, in particular, of the gene-amplification “events” that often trigger cancer.
“The replication of DNA is strictly regulated to occur only once per cell cycle,” says Kipreos. “We found that the loss of a gene named CUL-4 completely abolishes this regulation. In cells lacking CUL-4, the replication of DNA is continuously re-initiated during the same cell cycle to produce cells with greatly expanded levels of genomic DNA.”
The research was published in the journal Nature and was supported by a grant from the National Institutes of Health. Co-authors of the paper are graduate students Weiwei Zhong and Hui Feng and technician Fernando Santiago, all of the Kipreos lab.
Using the tiny worm called Caenorhabditis elegans (the common nematode found in soils all over the world) as a model organism, the team found that CUL-4 controls proper DNA replication by promoting the degradation of a protein, called
CDT-1, that is required for the initiation of DNA replication.
There are five genes in the cullin family, CUL-1 through CUL-5, and they are involved in the degradation of other cellular proteins. They have been known for less than a decade, but scientists know that the over-expression of the CUL-4 gene in humans plays a role in both breast and liver cancers. Knowledge of how CUL-4 controls DNA replication could thus open new areas of investigation for cancer researchers.
The current study expands on research by other scientists, including work from the Imperial Cancer Research Fund published in September 2001 in The EMBO Journal. Using yeast as a model organism, a team led by Nobel laureate Paul Nurse found that the limited expression of two “DNA replication licensing proteins,” CDT-1 and CDC-18, is crucial to ensure a single cycle of DNA replication.
The work by the Kipreos lab shows that CUL-4 controls the CDT-1 replication licensing protein by removing it from cells that are duplicating their DNA. When CDT-1 is not degraded in replicating cells, its presence causes the re-initiation of DNA replication.
The UGA team used a technique called RNA-mediated interference to probe the function of CUL-4. The basis of the technique involves injecting double-stranded RNA, or dsRNA, into an organism, thereby inactivating the gene corresponding to the dsRNA. In this way, the researchers inactivated CUL-4 and were able to study cell division in its absence.
What they discovered was a dramatic increase in the size of “blast” cells, which normally proliferate in the developing C. elegans larvae.
“We measured the amount of genomic DNA in the enlarged blast cells and we were amazed at the extent of the genomic DNA expansion, which can be up to 50 times the level of normal cells,” says Kipreos.
The researchers knew that three mechanisms could be generating the dramatic increase in genomic DNA levels. These mechanisms include failed mitosis; endoreplication, in which cells bypass cell division and enter the next cell cycle with doubled DNA; and re-replication, in which cells remain “stuck” in one cell cycle phase and continuously re-initiate DNA replication.
Using molecular techniques, Kipreos and his colleagues found that DNA was amplifying by re-replication in the cells lacking CUL-4, resulting in markedly increased genome sizes. CUL-4 is the first known example in which the loss of a single gene produces such completely unrestrained DNA replication.
This suggests that CUL-4 acts as a guardian of the genome, protecting it from unrestrained replication. A failure to limit DNA replication leads to the indiscriminate expansion of genomic DNA, which has catastrophic consequences for the survival of the organism.
The identification of CUL-4 as a major regulator of DNA replication levels will likely lead to investigations in mouse and human models and could open new studies for how DNA replication goes awry in numerous disease processes. |
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