20 January 2014

Cancer and alternative telomere lengthening: new data

The factors implementing the program are determined
alternative telomere elongation in cancer cells

Scientists have linked the "sloppy" assembly of telomeres with unregulated proliferation of tumor cellsLifeSciencesToday based on the materials of the Salk Institute for Biological Studies:

Salk scientists identify factors that trigger ALT-enactive cancer cell growthWith all the typical diversity, cancerous tumors have a common feature – unlimited cell division.

To a large extent, this unregulated growth is due to the fact that tumor cells can restore the protective ends of their chromosomes, consisting of repeating DNA and protein sequences. As a rule, cell division stops as soon as these structures, called telomeres, "wear out" and become too short. But cancer cells continue to divide using one of two strategies for their recovery.

One of the strategies implemented in 90 percent of cancers is based on enhancing the synthesis of the telomerase enzyme, which lengthens telomeres. The strategy used by the remaining 10-15 percent of cancers, called Alternative Lengthening of Telomeres (ALT), is less studied. Until now, biologists' explanation of the phenomenon of alternative telomere elongation has essentially been reduced to the thesis: cancer cells can repair long, albeit "sloppy" looking, telomeres without telomerase. However, exactly how this is done remained unclear.

Professor of Molecular and Cellular Biology at the Salk Institute for Biological Studies Jan Karlseider and his laboratory staff report on the first experimental induction of ALT-telomere repair program in human cells. This discovery, published in the journal Nature Structural and Molecular Biology, presents factors that are drivers of ALT-dependent cancer cell growth as potential pharmacological targets (O'Sullivan et al., Rapid induction of alternative lengthening of telomeres by depletion of the histone chaperone ASF1).

Professor Karlseider is studying how the biochemistry of telomeres affects the development of cancer and the aging process.

"Telomerase targeting has long been considered a potential cancer treatment method," the scientist explains, noting that anti–telomerase drugs against several types of cancer are already undergoing clinical trials (phase II). "But studies in mice show that when telomerase is suppressed, cells can activate an alternative telomere lengthening program. This makes it absolutely necessary to develop ways to block ALT."

To find out exactly how cells turn on the alternative telomere lengthening program, the researchers experimentally removed two proteins, ASF1a and ASF1b, in normal lung cells and in cancer cells whose immortality is due to telomerase. ASF1 chaperones are molecular "accompaniments" of histone proteins, the complex of which with DNA is a structural unit of the chromosome – the nucleosome.

A model of the nucleosome, an elementary unit of DNA packaging in a cell:
the DNA strand is wrapped around a complex of histone proteins (Visuals Unlimited/Corbis)

In ASF1-deficient cells, as actually expected, given the loss of histone chaperone, scientists observed a relative deficiency of nucleosomes in the telomere region. At the same time, cancer cells deprived of ASF1 continued to proliferate unhindered, although telomerase synthesis was turned off in them, which could mean only one thing: tumor cells are able to use the second strategy of telomere elongation.

But the most interesting thing was shown by microscopy: the nuclei of cells deprived of ASF1 contained aggregates of telomeric DNA, known as PML-corpuscles. PML-corpuscles, so named because they were first detected in tumor cells with promyelocytic leukemia, are a characteristic feature of ALT-dependent types of cancer.

"The large-scale formation of PML bodies in normal cells was a surprise," comments the first author of the article Roddy O'Sullivan, a postdoctoral fellow in Professor Karlseider's laboratory. "This was the first clue that the alternative telomere lengthening program is induced by the loss of ASF1 proteins."

To confirm that the cells include ALT, the scientists used all possible means. As a decisive argument, Karlseider Laboratory postdoctoral fellow, co-author of the study, Nosica Arnoult resorted to fluorescence microscopy. Her colleagues embedded a fluorescent label in the telomere section of one of the chromosomes, and she tracked where this label turned out to be over time, using the FISH method, which allows visualizing the whole chromosome. She found that the loss of ASF1 initiated a kind of intracellular "ping-pong": cells multiplied the label and transferred it from chromosome to chromosome, building disorganized, but "serviceable" telomeres.

According to Dr. O'Sullivan, this technically complex experiment is very informative. "The exchange of telomeric DNA between chromosomes is the gold standard for detecting alternative telomere elongation," he explains. "When we saw it, we immediately realized that the loss of ASF1 is an ALT inducer."

Until now, cancer scientists have been of the opinion that telomeric DNA exchange in ALT can be explained by mutations in genes that limit the recombination process. (Returning to the ping pong metaphor, these mutations would cause the cells to lose control of the racket.) But Professor Karlseider is sure that the results of their study simply leave no room for discussion – the molecular starting point of alternative telomere elongation is no longer a matter of dispute.

"Our work shows that suppression of ASF1 causes alternative telomere elongation due to disruption of nucleosome assembly," he says. "Thus, the recombination we observed in ALT–dependent cells is a consequence, not a cause."

Dr. O'Sullivan came to Professor Karlseider's laboratory seven years ago to find out whether DNA-histone interactions in nucleosomes alter telomere functions. Soon he will start working at the Hillman Cancer Center University of Pittsburgh, satisfied that he was able to show that the aberrant chromosomal structure underlies the growth of a significant part of human cancers.

"This work illustrates a basic concept," the scientist comments, "namely, disruption of the order in how cells pack DNA into nucleosomes causes harm."

The development of ALT inhibitors is at a very early stage, and this study will help pharmacologists in creating and evaluating the effectiveness of anti-cancer substances targeted by the ALT/ASF1 pathway.

"Now that we have a controlled way to activate this pathway, we can test any gene potentially capable of being an inhibitor," concludes Professor Karlseider.

Portal "Eternal youth" http://vechnayamolodost.ru20.01.2014

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