Melanoma's secret to cheating death has finally been revealed

Writing this week in Science, Jonathan Alder, Ph.D., and colleagues describe a combination of genetic changes that allows melanoma cells to dramatically extend their lifespan while fueling rapid tumor growth. The discovery could reshape how researchers understand melanoma and may point to new treatment strategies.

"We did something that was, in essence, obvious based on previous basic research and connected back to something that is happening in patients," said Alder, assistant professor in the Division of Pulmonary, Allergy and Critical Care Medicine at Pitt's School of Medicine.

Telomeres Help Control a Cell's Lifespan

Telomeres are protective caps located at the ends of chromosomes that help keep DNA from breaking down. Every time a healthy cell divides, its telomeres become a little shorter. Eventually, they shrink to the point where the cell can no longer divide.

Keeping telomeres at the proper length is critical for health. Telomeres that become too short can cause disorders linked to premature aging and early death. On the other hand, unusually long telomeres are often associated with cancer.

Scientists have long known that melanoma tumors contain exceptionally long telomeres, especially compared with many other types of cancer.

"There's some special link between melanoma and telomere maintenance," said Alder. "For a melanocyte to transform into cancer, one of the biggest hurdles is to immortalize itself. Once it can do that, it's well on its way to cancer."

The Missing Genetic Link Behind Melanoma

The enzyme telomerase lengthens telomeres, helping protect chromosomes and preventing cells from dying. In most healthy cells, telomerase remains inactive. Many cancers, however, activate the enzyme through mutations in the telomerase gene known as TERT, allowing cancer cells to keep dividing.

Melanoma is particularly dependent on this strategy. Roughly 75% of melanoma tumors carry TERT mutations that increase telomerase production and activity.

Yet there was a mystery. Even after researchers introduced TERT mutations into melanocytes, they still could not recreate the unusually long telomeres found in melanoma tumors. That suggested another important factor was missing.

Pattra Chun-on, M.D., an internist pursuing her Ph.D. in Alder's lab, set out to uncover that missing link. Drawing on her background in cancer biology and growing interest in telomeres, she investigated why TERT mutations alone were not enough.

"The fun part of this story is when Pattra joined my lab," Alder said. "She contacted me and told me that she was interested in studying cancer. I told her that I study short telomeres and not long telomeres. This went on until I realized that Pattra would never take 'no' for an answer."

TPP1 Completes the Puzzle

Earlier work from Alder's laboratory had identified frequent mutations in a telomere binding protein called TPP1 while analyzing cancer mutation databases.

Chun-on discovered that these TPP1 mutations closely resembled the TERT mutations. They occurred in the newly annotated promoter region of TPP1 and boosted production of the protein. That finding immediately caught Alder's attention because scientists had already shown that TPP1 enhances telomerase activity.

"Biochemists more than a decade before us showed that TPP1 increases the activity of telomerase in a test tube, but we never knew that this actually happened clinically," he said.

Chun-on, who is also enrolled in a Ph.D. program in the Department of Environmental and Occupational Health at Pitt's School of Public Health, then introduced the mutated forms of both TERT and TPP1 into cells. Working together, the two proteins produced the exceptionally long telomeres that characterize melanoma tumors.

The results revealed that TPP1 was the long sought missing factor, one that had been hidden in plain sight.

New Target for Future Melanoma Treatments

The findings offer a new explanation for how melanoma develops and survives. They also identify a cancer specific telomere maintenance system that could become a promising target for future therapies.

Additional authors of the study are Angela M. Hinchie, Agustin A. Gil Silva, Ph.D., Elizabeth Rush, Cindy Sander, Brittani K.N. Seynnaeve, M.D., M.S., John M. Kirkwood, M.D., all of Pitt, UPMC or both; Holly C. Beale, Ph.D., and Olena M. Vaske, Ph.D., both of the University of California, Santa Cruz; Carla J. Connelly, of Johns Hopkins University; and Carol W. Greider, Ph.D., of the University of California, Santa Cruz and Johns Hopkins University.

The research was supported by National Institutes of Health grants R35CA209974 and R01HL135062.