Think of chromosomes as nature’s shoelaces. Built from DNA, these thread-like structures carry and ferry the genetic information necessary for life. To maintain genetic integrity, chromosomes possess protective structures located at their ends called telomeres. These telomeres are like the plastic tips of shoelaces, preventing the genetic thread from unraveling as cells continuously divide.

Integral to the telomere maintenance process is the enzyme telomerase, which replenishes the DNA sequences in the telomere structure. Also important to this process is a six-subunit protein complex called shelterin, which is responsible for recruiting and activating telomerase enzymes. 

“Telomerase, along with the maintenance of telomeres, is incredibly important for long-term cell proliferation,” said Associate Professor Lifeng Xu, Department of Microbiology and Molecular Genetics.

“If you have failure in telomerase function,” added Xu, “telomeres shorten and you get premature senescence of the stem cells or premature loss of viability in the stem cells.” 

In normal cells, telomeres naturally shorten as organisms age. But cancer cells are characterized by excessive telomerase activity, which enables them to divide and spread.

In a study recently published in the Proceedings of the National Academy of Sciences, Xu and her colleagues used CRISPR/Cas9-mediated genome editing to decouple the telomerase recruiting and activating functions of the shelterin protein complex in cancerous cells. The mutations ultimately produced cancer cells with telomeres so short that they were no longer viable. The research is a step towards understanding how to interrupt the development of cancerous cells.

A structural region conserved from yeast to humans

Scientists have long known that the human TPP1 protein—a subunit of the shelterin protein complex that recruits telomerase to chromosome ends—also stimulates its activity in a test tube. What they didn’t know was the specific region of TPP1 directly mediating this activity and whether the stimulation is important for the long-term cell viability. Addressing these questions could help researchers develop methods meant to disrupt the division and spread of dangerous cells.

Through protein structural analogies between TPP1 and the budding yeast (Saccharomyces cerevisiae) telomerase-associated protein Est3, Xu and her colleagues identified a region of TPP1 that, when mutated, decouples the telomerase recruitment and activation processes. 

“We identified this structurally conserved region of TPP1 called TELR,” said Xu. “Then, we mutated that region and when you mutate it, you affect how TPP1 regulates telomerase.”

Inhibiting cancer at the molecular level

During their experiments, Xu and her team generated colon cancer cells containing TELR mutations. While the mutations didn’t adversely affect telomerase recruitment, they did impair telomerase activation, resulting in colon cancer cells that lost most of their telomeres.

Ultimately, these cells were unable to proliferate, even though they could produce telomerase and recruit telomerase to chromosome ends. This means that telomerase activation is necessary to maintaining telomere length and long-term cell viability. The finding potentially opens up a new molecular avenue in the exploration of cancer therapeutics. 

“If we’re thinking of inhibiting telomerase activity in cancer, one well-recognized strategy could be to perturb telomerase recruitment,” said Xu. 

But Xu noted that cancer cells are constantly evolving new strategies to proliferate. During the experiments, a small subset of cancer cells survived and continued dividing.

“If we know how this activation step is regulated, then theoretically we can also perturb this process at a second place where the surviving cells haven’t adapted to it yet,” said Xu.“This is why it’s important to know the whole signaling pathway,” she added. “Then you can break the system at different points.” 

Xu and her colleagues plan to focus the next phase of their research on understanding the upstream events of the telomerase activation step, which, if identified, could help stop the proliferation of the targeted cancer cells.

Additional authors on the paper include Ranjodh Sandhu, Madhav Sharma and Derek Wei, all of the Department of Microbiology and Molecular Genetics in the College of Biological Sciences.