Smaller tetraploid cells drive more aggressive cancer

Megan Sweet slices tumors.

A normal day in the lab finds the Virginia Tech graduate student with hands deep inside a refrigerated metal box, pulling a mounted mouse-grown tumor incrementally closer to a razor-sharp blade.

"It's all about fine tuning and making sure it's going to be an even slice," said Sweet, who studies biological sciences.

Finally, the blade meets the pinkie nail-sized nub of tissue and slices through it with a rhythmic chunk, chunk, chunk, chunk.

This is the hardest and most time-consuming part. But it's also kind of meditative."

Megan Sweet, Virginia Tech graduate student 

The tumor slices are so thin, they're translucent. Sweet gingerly dabs one onto a glass slide. Later, she will stain specific cellular structures to better see the tumor's architecture beneath a high-powered microscope. 

Slice, stain, stare, compare - again.

These precise steps, repeated day after day, yield small but important moments of clarity within the still murky question of why some cancers are worse than others.

In this practice, Virginia Tech biologists are uncovering how tumors progress and evolve, with findings published on May 25 in the Proceedings of the National Academy of Sciences and earlier this year in Cancer Research.

Replicating errors

Most normal cells in your body are diploid, meaning they have two copies of each chromosome - one set from each parent.

To stay healthy, a diploid cell divides to make more diploid cells. But occasionally, a dividing cell makes a mistake, which throws off the chromosome numbers. And then, like an error at a printing press, that mistake is replicated and starts to accumulate.

This is one of the ways diseases like cancer can form.

Working with cell biologist Daniela Cimini, Sweet and graduate student Mat Bloomfield have spent the past five years analyzing cells with abnormal chromosome numbers - one of the proverbial smoking guns of cancer.

To create the right environment for their studies, Cimini's research group members forced diploid cancer cells to duplicate their chromosomes but skip division. This resulted in what's called a tetraploid cell, which has four complete sets of chromosomes.

These double-stuffed cells aren't just a lab phenomenon. If tetraploidization occurs in the real world during human tumor development, it's not good news - it's associated with cancer progression and poor prognosis.

Tetraploid malignancy

But why do tetraploid cells make things so much worse?

To answer this, Sweet and Bloomfield compared tumors formed from diploid cancer cells with tumors formed from tetraploid cancer cells. They saw that the number of tetraploid cells actually diminished during tumor formation in mice, and yet tumor mass ballooned fast and large.

In a first-of-its-kind discovery, they found that this growth was driven by the recruitment of stromal cells - non-cancerous connective tissue cells that provide structural support.

"The presence of even a small fraction of these tetraploid cells can promote the recruitment of extra non-cancerous cells that support further tumor progression," Sweet said.

Cell size may predict tumor potential 

The second investigation initially targeted the physiology of tetraploid cells. However, when Bloomfield made human-derived cancer cells tetraploid and isolated single-cell clones, he noticed something unexpected: the cells from the first few clones differed in size.

They anticipated all the clones to be two times larger than regular diploid cells because of the extra material crammed inside - but some were 25 to 30 percent smaller than expected.

And the smaller clones happened to be more tumorigenic than the large clones.

"The smaller clones are more aggressive," Bloomfield said. "They grow faster, are more invasive, and more tolerant of common anti-cancer and stress-inducing drugs."

Later experiments in mice showed that tumors with smaller tetraploid cells often increased more rapidly. Moreover, results did not depend on cancer cell type - they saw the same behavior in colorectal and breast cancer.

But what about in humans?

Using data from the Cancer Genome Atlas, a database with thousands of annotated patient samples, the team found smaller tetraploid cells from several cancer types were indeed associated with worse prognosis and lower survival rates.

"We already knew that tetraploidy can make cells more tumorigenic, but now we know that if you incorporate the size of the cells, it can be more predictive of tumorigenic potential," Cimini said.

Next steps include a deeper investigation into the mechanisms behind these findings and continued analysis of human cancer data.

And in the meantime, Sweet will keep slicing.