Blood Cancer Stopped In Mice By Shortening The Ends Of Chromosomes
More research into immortality vs. cancer.
A Johns Hopkins team has stopped a form of blood cancer in its tracks in mice by engineering and inactivating an enzyme, telomerase, thereby shortening the ends of chromosomes, called telomeres.
"Normally, when telomeres get critically short, the cell commits suicide as a means of protecting the body," says Carol Greider, Ph.D., the Daniel Nathans chair of molecular biology and genetics at Johns Hopkins. Her study, appearing online at Cancer Cell, uncovers an alternate response where cells simply - and permanently - stop growing, a process known as senescence.
In an unusual set of experiments, the research team first mated mice with nonoperating telomerase to mice carrying a mutation that predisposed them to Burkitt's lymphoma, a rare but aggressive cancer of white blood cells. Telomerase helps maintain the caps or ends of chromosomes called telomeres, which shrink each time a cell divides and eventually - when the chromosomes get too short - force the cell to essentially commit suicide. Such cell death is natural, and when it fails to happen, the result may be unbridled cell growth, or cancer.
The first generation pups born to these mice contained no telomerase and very long telomeres. These mice all developed lymphomas by the time they were 7 months old. The researchers then continued breeding the mice to see what would happen in later generations. By the fifth generation, the researchers discovered that the mice had short telomeres and stopped developing lymphomas.
When the researchers blocked the suicide machinery in these fifth-generation mice, they were very surprised to find that the mice still remained cancer free.
"We were confused as to what was going on; we thought for sure that blocking the cells' ability to commit suicide would lead to the cancer's returning," says Greider. A closer look showed microtumors in the mice's lymph nodes that had begun the road to cancer, but stopped, falling instead into a state of senescence.
"They don't die, they don't divide, they just sit there in permanent rest," says Greider. Greider, who won the Lasker Award in 2006 for her discovery of telomerase, says further study of the road to senescence should suggest new ways of preventing or treating cancer by interfering safely with telomerase and the cell-suicide system.
The research was funded by the National Institutes of Health. Authors on the paper are David Feldser and Carol Greider, both of Johns Hopkins.
'Swiss Army Knife' Protein Plays Unexpected Role Protecting Chromosome Tips; Possible Tie To Metastatic Cancer
ScienceDaily (Aug. 16, 2009) -- A protein specialist that opens the genomic door for DNA repair and gene expression also turns out to be a multi-tasking workhorse that protects the tips of chromosomes and dabbles in a protein-destruction complex, a team lead by researchers at The University of Texas M. D. Anderson Cancer Center reports in the Aug. 13 edition of Molecular Cell.
"Instead of being a really important tool dedicated just to regulation of gene transcription, Gcn5 is more like a Swiss Army knife that performs different functions depending on what needs to be done in the cell," said senior author Sharon Dent, Ph.D., professor in M. D. Anderson's Department of Biochemistry and Molecular Biology.
The researchers document a chain of events that starts with depletion of Gcn5, which leads to decreased activity by another protein that protects yet a third protein from destruction. That last protein, TRF1, protects telomeres, dense structures at the end of chromosomes which, like the compressed plastic tips on the ends of a shoelace, keep the chromosome ends intact.
Variation in the gene that expresses the middle protein in this model, ubiquitin specific protease 22 (USP22), is part of an 11-gene signature associated with highly metastatic cancers and poor prognosis, the authors note.
"Our results indicate that the Gcn5 complex regulates not just gene transcription but also protein stability," Dent said. "They also suggest that the role of USP22 in highly aggressive cancers might be due to these new functions."
Telltale telomere damage
Chromosomes are made of DNA that is tightly intertwined with proteins called histones to form chromatin. Chromatin is a condensed structure that forms a natural barrier inhibiting access to DNA. Gcn5 was previously known for its role in a complex of proteins that loosens chromatin to allow access to DNA by the cell's DNA repair machinery and by transcription factors that launch the process of gene expression.
"Years ago a student in my lab found that mice deficient in Gcn5 died early during embryonic development," Dent said. "The reason they died, in part, was that telomeres were fusing together. There was no reason to think Gcn5 would have anything at all to do with telomeres, so these fusions were quite puzzling."
Clues for protein's role in metastatic cancer
While Dent and colleagues attacked the problem, a research group elsewhere discovered that USP22 is active in the same protein complex in which Gcn5 operates. USP22 protects proteins by pealing off ubiquitin molecules that attach to the proteins and mark them for destruction by the proteasome complex.
A literature review showed that TRF1 carries a ubiquitin mark that makes it vulnerable to degradation by the proteasome.
"TRF1 normally resides at the telomeres and tells the cell that this is a normal chromosome end and you should leave it alone," Dent said. "If you don't have enough TRF1, the cell now thinks these chromosomal ends are abnormal and tries to fix them when they shouldn't be fixed."
Putting it all together, the team hypothesized that USP22 protects telomeres by knocking ubiquitins off of TRF1, sparing it from destruction. "Our model is of a pathway in which depletion of Gcn5 reduces USP22 activity, causing greater TRF1 ubiquination, which leads to TRF1 destruction and that leads to telomere problems," Dent said.
In the Molecular Cell paper, the researchers show that depletion of Gcn5 leads to chromosomal fusion and damage in mouse embryonic cell lines and also reduces the level of TRF1. They then demonstrate that USP22 interacts with TRF1 and is required for that protein to remain stable. Additional experiments identified ubiquitin removal is the mechanism by which USP22 protects TRF1.
Dent and colleagues continue to look for new proteins and cellular processes that are impaired in cells lacking GCN5 or USP22. "There must be a reason why USP22 is over expressed in highly metastatic cancers and we are encouraged that we will be able to provide important clues for this process," Dent said.
Co-authors with Dent and first author Boyko S. Atanassov, Ph.D., are Yvonne Evrard, Ph.D., and Zhijing Zhang, Ph.D, all of M. D. Anderson's Department of Biochemistry and Molecular Biology, Program in Genes and Development, and Center for Cancer Epigenetics; Asha Multani, Ph.D., of M. D. Anderson's Department of Genetics; and Sandy Chang, Ph.D., of the departments of Genetics and Hematopathology; and L·szlo Tora, Ph.D., and Didier Devys, M.D., Ph.D., of the Institut de GÈnÈtique et de Biologie MolÈculaire et Cellulaire at the UniversitÈ Louis Pasteur de Strasbourg in France.
Funding was provided by grants from the National Institutes of Health and from the Agence Nationale de la Recherche, and the Foundation de la Recherche Medicale in France.
Longevity Tied to Genes That Preserve Tips of Chromosomes
As recently reported in ScienceDaily, a team led by researchers at Albert Einstein College of Medicine of Yeshiva University has found a clear link between living to 100 and inheriting a hyperactive version of an enzyme that rebuilds telomeres -- the tip ends of chromosomes.
Telomeres play crucial roles in aging, cancer and other biological processes. Their importance was recognized last month, when three scientists were awarded the 2009 Nobel Prize in Physiology and Medicine for determining the structure of telomeres and discovering how they protect chromosomes from degrading.
Telomeres are relatively short sections of specialized DNA that sit at the ends of all chromosomes. One of the Nobel Prize winners, Elizabeth Blackburn, Ph.D., of the University of California at San Francisco, has compared telomeres to the plastic tips at the ends of shoelaces that prevent the laces from unraveling.
Each time a cell divides, its telomeres erode slightly and become progressively shorter with each cell division. Eventually, telomeres become so short that their host cells stop dividing and lapse into a condition called cell senescence. As a result, vital tissues and important organs begin to fail and the classical signs of aging ensue.
In investigating the role of telomeres in aging, the Einstein researchers studied Ashkenazi Jews because they are a homogeneous population that was already well studied genetically. Three groups were enrolled: 86 very old -- but generally healthy -- Gil Atzmon, Ph.D.people (average age 97); 175 of their offspring; and 93 controls (offspring of parents who had lived a normal lifespan).
"Telomeres are one piece of the puzzle that accounts for why some people can live so long," says Gil Atzmon, Ph.D., assistant professor of medicine and of genetics at Einstein, Genetic Core Leader for The LonGenity Project at Einstein's Institute for Aging Research, and a lead author of the paper. "Our research was meant to answer two questions: Do people who live long lives tend to have long telomeres? And if so, could variations in their genes that code for telomerase account for their long telomeres?"
The answer to both questions was "yes."
"As we suspected, humans of exceptional longevity are better able to maintain the length of their telomeres," said Yousin Suh, Ph.D., associate professor of medicine and of genetics at Einstein and senior author of the paper. "And we found that they owe their longevity, at least in part, to advantageous variants of genes involved in telomere maintenance."
More specifically, the researchers found that participants who have lived to a very old age have inherited mutant genes that make their telomerase-making system extra active and able to maintain telomere length more effectively. For the most part, these people were spared age-related diseases such as cardiovascular disease and diabetes, which cause most deaths among elderly people.
"Telomeres are one piece of the puzzle that accounts for why some people can live so long."
"Our findings suggest that telomere length and variants of telomerase genes combine to help people live very long lives, perhaps by protecting them from the diseases of old age," says Dr. Suh. "We're now trying to understand the mechanism by which these genetic variants of telomerase maintain telomere length in centenarians. Ultimately, it may be possible to develop drugs that mimic the telomerase that our centenarians have been blessed with."