Making melanoma cells immortal comes down to previously unknown gene mutation synergies

For a cell to become cancerous, it must cross the threshold of immortality—that is, it must transform into an infinitely dividing machine. Many cells like those in the skin cancer melanoma are thought to be immortalized due to mutations in the gene TERT. But scientists have puzzled over why the same TERT mutations seen in patients aren’t enough to immortalize cells in a test tube.

Thanks to the right mix of science and serendipity, researchers at the University of Pittsburgh School of Medicine found at least one missing link: mutations in another gene that encodes for a protein called TPP1. Coupled with TERT mutations, the TPP1 mutations dramatically extend the length of melanoma cells’ telomeres—the protective ends of chromosomes that keep them from degrading. The team reported their discovery on Nov. 10 in a paper published in Science.

“We just added another link in the chain as we try to understand the process of cancer,” Jonathan Alder, Ph.D., assistant professor in the institution’s Division of Pulmonary, Allergy and Critical Care Medicine, told Fierce Biotech Research. “There have been lots of great studies before ours that illuminated part of the pathway as you go from a normal cell to cancer, and this is just shining the flashlight a little bit farther and moving it along.”

The signal to study TPP1 came from some test results that landed right in front of Alder’s nose—literally. His lab normally researches diseases involving short telomeres such as cystic fibrosis. Patients with short telomere syndromes acquire the same TERT promoter mutations found in melanoma, which protect them from premature telomere shortening. In healthy individuals, such a mutation would put their cells on the road to cancer.  

“But if you have a premature aging syndrome, these TERT promoter mutations allow you to keep going,” Alder explained.

One day, a new graduate student in his lab, Pattra Chon-On, M.D., met with Alder to discuss the mutations she’d found during an analysis on the promoter region of TPP1 in melanoma cells. Alder's next meeting was with a genetic counselor who was reviewing sequencing data from patients with short telomere syndromes. When Chon-On left, Alder noticed that the TPP1 mutations she found in her research were the very same ones the patients had.

“I looked down at my desk and I said, ‘Wait a minute, we’ve made a huge mistake—somebody’s mixed up samples,’” he recalled. “Then I realized what it was: Patients with short telomeres were acquiring the same mutations that we saw in these cancers.”

While the team members knew from the beginning that they were onto something, they still needed to fill in the blanks. To that end, they conducted a series of cell studies to see how the TPP1 mutations affected telomere length.

In the first experiment, the scientists put TPP1 in a cell line that already expressed telomerase. After seeing that TPP1 lengthened these cells’ telomeres, they added it to a second set of cells grown from human skin to see whether they’d get the same results. Unlike the first set, these cells didn’t already express telomerase.

But when they added TPP1, nothing happened. The cells stopped dividing at roughly the same time as they have would under normal conditions. Adding TERT alone or TPP1 and TERT together caused the cells to become immortal. 

Initially, “the experiment was a bit of a disappointment,” Alder recalled. “We thought maybe there’d be a tiny bit of telomerase in these cells and that when we added TPP1, they’d grow for longer, but we saw nothing.”

Then they decided to check the telomere length. As it turned out, TERT and TPP1 had been working together.

“When we’d put TPP1 in the cells, nothing happened to the telomeres. When we put TERT in, the telomeres got longer,” Alder said. “But when we put both in, wow, the telomeres got way longer.”

Further CRISPR-Cas9 studies inducing the TPP1 mutations in cells that had the TERT mutations gave the same results: The telomeres grew far longer, and the cells were immortalized. 

What’s going on, exactly? The team isn’t completely sure, but it has a hypothesis.

“When you get a TERT promotor mutation, you express a little bit more telomerase, so the cells can grow for longer, but they’re not immortal,” Alder said. “But when you get this second hit—when you get the TPP1 promoter mutation, where you’re making a little too much telomerase and a little too much TPP1—that’s enough to immortalize them.”

Experiments are already underway to bear this hypothesis out in the lab, Alder added. The researchers hope their findings will eventually translate to improved treatment targets for melanoma.