Two Genomes Cause Double Trouble in Cell

Link Between Tetraploidy and Cancer Supports Hundred-year-old Theory

Photo by Graham Ramsay Dividing cells do not separate when chromosomes do not split evenly, which may set the stage for cancer, say a pair of papers by (clockwise from left) David Pellman, Madhavi Bandi, Randy King, and Masayuki Nitta.

The equation behind most cell division is simple: one cell cleaves itself into two. In preparation, the parent cell duplicates its chromosomes so when it divvies up its contents, each daughter cell receives a full set, or 46 chromosomes per cell in people.

Sometimes, though, cells cannot do the math, sending 45 chromosomes to one daughter cell and 47 to the other, for example. When that happens, new research shows, the daughter cells, which are hanging on to each other by little more than a thread, spontaneously fuse back together into one cell with 92 chromosomes contained in two mismatched nuclei.

More often than not, researchers observed, these cells with double genomes do not divide, a potentially protective maneuver. But combined with other genetic abnormalities—such as a missing tumor suppressor gene—they can more easily turn malignant in mice, a related study reports.

The two papers on a cause and an effect of cells with quadruplet chromosomes, known as tetraploids, are published in the Oct. 13 Nature. The findings are believed to be the first direct evidence backing a century-old idea that cell division failure alone may set the stage for some cancers.

Most of us probably walk around with cells containing multiple genome copies, said Randy King, HMS assistant professor of cell biology and senior author of one of the papers, although no one knows much about how they arise or their consequences. Some cells are meant to have many copies of the genome, such as the giant blood cells called megakaryocytes whose cytoplasms fragment into the tiny genome-free platelet cells. In other cells, it happens by stress, such as endothelial cells lining hypertensive arteries or liver cells coping with oxidative damage and aging. King’s paper shows it may happen more often than anyone thought.

“We think the cell is trying to compensate,” King said. “It is as if the cell is postponing the consequences of having abnormal numbers of chromosomes. It may be saying, OK, I made an irreversible mistake, and instead of propagating, I’ll convert to another state where I don’t pose as much of a risk.”

But tetraploidy may have a serious downside. “Forming a tetraploid cell can be a pathway for generating a tumor,” said David Pellman, HMS associate professor of pediatrics at the Dana–Farber Cancer Institute and Children’s Hospital Boston and senior author of the other paper. “Cancer is not the only possible outcome of tetraploidy. It probably depends on the cell type and genetic context.” In people, tetraploid cells have been observed in many early-stage cancers, most thoroughly documented in the premalignant condition called Barrett’s esophagus.

An Extra Genome
Cells can become tetraploids with just a single chromosome misdirection, reports the paper by King and Qinghua Shi, a former postdoctoral fellow who now heads his own lab in China.

With another goal in mind, Shi began by adding up the natural rate
of chromosome segregation mistakes in cancerous and noncancerous human cell lines. He immediately became sidetracked when he noticed a striking increase in mismatched chromosomes in cells with two nuclei compared to their standard single-nucleus counterparts.

Using a microscope housed inside an incubator, Shi observed live cells during long-term imaging experiments. Time-lapse movies show that segregation errors seem inextricably linked to cytokinesis, the final cut that normally separates the two daughter cells. The cells proceed through the usual stages of mitosis until they are virtually independent except for a thin cytoplasmic bridge. In dividing cells with mismatched chromosomes, the bridge widens hours later, and the cells reunite. (See movie.)

Image courtesy of Randy King

The genome shuffle. Cells divide properly when chromosomes are evenly split between the two new cells (top left). But when chromosomes are segregated incorrectly, the dividing cells fuse back together into a cell with a double genome contained in two mismatched nuclei (top right). Once cells become tetraploid, according to this model from Randy King, they may be able to divide their extra genome evenly (bottom left), but they are more prone to further errors, especially if they lack the p53 tumor suppressor gene (bottom right).

In the study, more than half of the noncancerous cells with double nuclei stopped dividing. The rest of them attempted to even up their chromosomes in the next cell division, ending up with daughter cells each containing a single nucleus with matching double genomes. In contrast, all the tetraploid cancer cells divided to produce even more chromosomally unbalanced cells (see figure). The cancer cells also had a defect in the p53 tumor suppressor gene.

“People have known for years that cancer cells have abnormal numbers of chromosomes,” King said. “But it is not known how they get that way. In the past, the idea was that chromosome missegregation directly produces aneuploid cells. Now we have to consider binucleated tetraploid cells as important transient intermediates in the process.”

The Cancer Connection
This is where the story of tetraploidy and cancer is picked up by findings from Pellman’s lab. Tetraploid breast cells with a mutant p53—but not their normal diploid counterparts—spontaneously cause cancer in mice, according to experiments led by postdoctoral fellow Takeshi Fujiwara, who now runs his own lab in Japan, and graduate student Madhavi Bandi.

Pellman’s lab had been investigating how altered chromosome content, including tetraploidy, affects genetic instability in a yeast model. Lurking in the background was an intriguing suggestion made a century ago by Theodor Boveri, a German scientist, that genomic instability resulting from whole genome doublings (tetraploidy) could play an important role in cancer.

Boveri discovered the centrosome, an organelle that organizes the mitotic spindle and lines up the chromosomes just before they split. In a 1914 treatise, he suggested that too many centrosomes, perhaps caused by cytokinesis failure, could trigger more abnormal cell division and ultimately cancer. In the same paper, he also anticipated tumor suppressors and oncogenes, the clonal nature of cancer, and the link between inflammation and cancer, Pellman said.

Confirming Boveri’s prediction, Pellman’s team found an unusually high number of chromosome losses, gains, and rearrangements in tetraploid mouse breast epithelial cells that lacked p53, whose tumor-suppressing function is missing in half of human cancers. In culture dishes, the tetraploid cells turned malignant after exposure to a cancer-causing chemical, but not their diploid counterparts.

Most strikingly, even without a carcinogen, one quarter of the tetraploid cells grew into tumors when transplanted into the skin of nude mice, compared to none of the diploid control cells. Upon close examination, the cancers contained an eight- to 30-fold amplification of a chromosome region containing matrix metalloproteinase genes, whose overexpression is linked to breast cancer in people and animal models.

Now that they know mistakes in cell division can set the stage for cancer, the researchers want to learn exactly how a tetraploid cell can turn malignant.

For the last year, the Pellman and King labs have held joint group meetings to share data and ideas about their complementary research. King wants to understand more about how cytokinesis is regulated in cells that make chromosome errors during mitosis. Pellman is looking to identify the alterations in cell division and the specific genes essential to the cancer-causing activity of tetraploid cells in hopes of finding new therapeutic targets.

—Carol Cruzan Morton

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