Virus uses 'Swiss army knife' protein to cause infection

Wednesday, 21 August 2013

Islamabad, Aug 22 (Newswire): In an advance in understanding Mother Nature's copy machines, motors, assembly lines and other biological nano-machines, scientists are describing how a multipurpose protein on the tail of a virus bores into bacteria like a drill bit, clears the shavings out of the hole and enlarges the hole.

They report on the "Swiss Army knife" protein, which enables the virus to pump its genetic material into and thus infect bacteria, in the Journal of the American Chemical Society.

Akio Kitao and colleagues focus on a group of viruses termed "bacteriophages," which literally means "bacteria eaters." These viruses infect bacteria like E. coli and usually make the bacteria dissolve. Infection involves injecting their own DNA or RNA into the bacteria, so that the viral genetic material takes over control of the bacteria.

The tools for doing so are among numerous invisible nanomachines -- so small that 50,000 would fit across the width of a human hair -- that work unnoticed in organisms ranging from microbes to people.

The scientists recreated intricate details of the protein's work as it helps the tail of the virus infect E. coli bacteria.

Their computer models show that the protein performs tasks in a regular sequence, starting with a screw-like motion as it begins to penetrate the outer membrane of E. coli. The protein acts as a cell-puncturing bit, a pipe to draw away membrane debris and a tool to enlarge the puncture hole, among other functions.

The infection process demonstrates "a case where a single-function protein acquired multiple chemical functions" as different parts of its structure come in contact with bacterial membrane proteins.

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Mutant gene identified that causes abnormal chromosome count, leading to cancer

Islamabad, Aug 22 (Newswire): Cells with too few or too many chromosomes have long been known to be a hallmark of cancer -- but the cause of this abnormal number of chromosomes has been little understood.

Now, in the Science, researchers at the Georgetown Lombardi Comprehensive Cancer Center, a part of Georgetown University Medical Center, have identified a gene that is commonly mutated in human cancers and have demonstrated its direct role in causing aneuploidy, an abnormal number of chromosomes.

The researchers found that 20 percent of the brain cancer (glioblastoma multiforme), skin cancer (malignant melanoma), and bone cancer (Ewing's sarcoma) samples they examined made no STAG2 protein, often due to a missing or mutated STAG2 gene. The STAG2 gene encodes a component of a protein structure known as the "cohesin complex" which regulates the separation of replicated chromosomes during cell division.

What this means is that if the STAG2 gene has been inactivated by a mutation, chances increase that a cell undergoing division will distribute an uneven number of chromosomes to the two new "daughter" cells being created. These cells, which now have too few or too many genes, are significantly more likely to develop into cancer.

"Scientists have long been searching for the genetic basis of aneuploidy in cancer cells, and our study provides substantial new insight into that process," says the study's senior investigator, cancer geneticist Todd Waldman, M.D., Ph.D., an associate professor at Georgetown Lombardi.

"In the cancers we studied, mutations in STAG2 appear to be a first step in the transformation of a normal cell into a cancer cell," he says. "We are now looking at whether STAG2 might be mutated in breast, colon, lung, and other common human cancers."

The study may also provide a new direction for cancer therapy, says the study's lead author, David Solomon, Ph.D., a student in the M.D./Ph.D. program at Georgetown University School of Medicine. "We are now attempting to identify a drug that specifically kills cancer cells with STAG2 mutations," says Solomon, who received his Ph.D. in May, 2010. "Such a drug would be of clinical benefit to the many patients whose tumors have inactivation of STAG2."

The study was a product of Solomon's work on a doctoral thesis. Waldman, who heads the M.D./Ph.D. program at Georgetown, mentored Solomon in his cancer genetics lab. Solomon's thesis was to search for genes that are mutated in glioblastoma multiforme, the most common and lethal form of brain cancer.

Using advanced gene chip technology, Solomon and Waldman identified regions of the human genome that were missing in aneuploid brain cancer cells. "One day, we found the gene STAG2 was deleted in one of our brain cancer samples," says Waldman. "It was easy to imagine that this gene might play a role in aneuploidy and that got us excited."

During the hunt for STAG2 mutations, Solomon recalls a "eureka" moment. "I had first identified mutations of STAG2 in a few brain tumors but was unsure of its importance as a broad spectrum cancer gene in tumor types other than brain. The day that I did a Western blot of 10 Ewing's sarcoma tumors, a bone tumor most common in adolescents, and found that 6 out of 10 Ewing's sarcoma tumors had mutations or deletions of STAG2," he says. "I knew then that STAG2 was indeed an important tumor suppressor gene in several tumor types. That day I wrote EUREKA! in my lab notebook."

What was also intriguing is that the STAG2 gene is located on the X chromosome, Waldman says, adding, "this is only the second cancer-causing gene ever found on the human sex chromosome." Men have only a single copy of the X chromosome, and women have only one functional copy (their other copy is dormant due to a process known as X-inactivation). "Therefore, unlike most genes which require mutations of two copies for complete inactivation, inactivation of STAG2 requires only a single mutational event," He says. "This may help explain the unusually high prevalence of STAG2 mutations in cancer."

After identifying the mutations of STAG2, the researchers then used a technique known as "human somatic cell gene targeting" that makes it possible to correct mutant genes in their normal chromosome location. Using this technique, Waldman and Solomon were able to correct the STAG2 mutation in glioblastoma cells, essentially replacing a bad gene with a good one. "When the STAG2 mutations were corrected, the chromosome count stabilized," Solomon says. "This offers compelling evidence that the STAG2 mutation was the cause of aneuploidy in these cells."

Waldman says STAG2 likely functions as a "caretaker" tumor suppressor gene. "It's not like most tumor suppressor genes, which when mutated lead to either enhanced cell proliferation or decreased cell death. Instead it's a tumor suppressor gene with a different function -- maintaining normal chromosome number and structure."

This work was conducted in collaboration with investigators at the National Institutes of Health, University of Texas Southwestern Medical Center, Memorial Sloan-Kettering Cancer Center, and the University of California, San Francisco.

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Cancer stem cells made, not born: Experiments and modeling reveal how tumors maintain cellular balance

Islamabad, Aug 22 (Newswire): In cancer, tumors aren't uniform: they are more like complex societies, each with a unique balance of cancer cell types playing different roles.

Understanding this "social structure" of tumors is critical for treatment decisions in the clinic because different cell types may be sensitive to different drugs. A common theory is that tumors are a hierarchical society, in which all cancer cells descend from special self-renewing cancer stem cells. This view predicts that killing the cancer stem cells might suffice to wipe out a cancer.

New findings by scientists at the Broad Institute of MIT and Harvard and Whitehead Institute, however, point to a much more decentralized society, with cancer cells able to interconvert between different types. These results have potential implications for the treatment of tumors, in particular, that attacking cancer stem cells alone may not be enough to fight cancer.

The research, which appeared in the Cell, was led by Broad director and senior author Eric Lander and first author Piyush Gupta, a Whitehead Institute member and assistant professor of biology at MIT who conducted this work as a postdoctoral researcher in Lander's lab. The research combines experimental evidence and mathematical modeling to show how cancers maintain their unique cellular balance.

The common view is that tumors have cancer stem cells that behave like stem cells in normal development -- at the top of a hierarchy of cell types, giving rise to both more cancer stem cells and daughter cells of other types. "The notion is that the only way stem cells occur is by self-renewal. Our work says that analogy may be wrong," said Lander.

The new work suggests an alternative possibility: that cancer cells are not fixed at all, but that, at any given point in time, they exist in one of several phenotypic "states" and those states can interconvert. In comparison to the traditional one-way hierarchy of cancer stem cells, in this new alternate model, more differentiated non-stem cells can revert to being stem-like cells. "That's not a hierarchical society at all," Lander said. "Cells aren't born into a medieval guild; they can change jobs."

Working with cancer cell lines cultured in the laboratory, scientists had observed that just as solid cancers tend towards certain proportions of cell states, cell lines in vitro also settled into a balance, or equilibrium. "We wanted to understand how cancer cells stably maintain characteristic proportions of these different states for extended periods of time," said Gupta. A better understanding of the mechanisms controlling that equilibrium could give a clearer picture of the nature of cancer at a cellular level.

To characterize how cancer maintains cellular equilibrium, the researchers studied two different breast cancer cell lines and examined three different cell states that were similar to normal breast epithelial cell types, known as basal, luminal, and stem-like.

The team sorted the different cell types from each other and then grew their relatively pure populations for six days. Remarkably, each of the three populations quickly returned to the same equilibrium -- and populations of non-stem cells generated new stem-like cells. "Even when you sort relatively pure populations, you quickly get back the same balance," said Lander.

The return to equilibrium proportions happens so rapidly that it cannot be due to different growth rates of the different cell types, but must instead be due to cells changing their state.

The authors showed that the process can be modeled -- and accurately predicted -- using a mathematical tool called a Markov model, in which cells change their states independently of one another. Although the process is completely decentralized, it quickly returns to the same equilibrium.

Surprisingly, the model predicted that non-stem cells can convert into stem-like cells. The team then showed experimentally that stem-like cells indeed arise quickly in the sorted subpopulations. In contrast with the traditional one-way view of cancer stem cells, the findings provide strong evidence for the alternate view of cancer cells and cancer stem cells as flexible entities that can change state.

The model also provides a quantitative framework to help predict the effects of genetic perturbations, potential therapeutic interventions, or other external pressures on populations of cancer cells.

The researchers now have to test whether tumor cells growing in patients show those same properties, as well as to understand the cellular mechanisms by which cells can change their states.

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