Key mechanism for maintaining appropriate telomere length identified

The length of telomeres that protect the ends of our chromosomes must be strictly regulated. Those that are too long predispose to cancer, and those that are too short lose their protective capacity, leading to telomere disorders with serious health consequences.

Our cells prevent this excessive shortening by adding telomeric DNA to the ends of chromosomes. The Rockefeller researchers showed that this process is mediated by two enzymes: telomerase and the CST-Polα/primase complex. After determining how telomerase is recruited, scientists were left with a fundamental question: how does CST-Polα/primase reach the telomere?

Now, a new study published in Cell demonstrates that CST is recruited to the telomere end and regulated by subtle chemical modifications to POT1, a Shelterin complex protein involved in telomere maintenance and implicated in cancer risk. The findings provide new insights into how human telomeres function at the molecular level, with implications for many diseases and disorders.

“After the discovery of telomerase, it took decades to understand how it reaches the telomere. Now, just months after discovering that CST-Polα/primase is the second essential enzyme required for telomere maintenance, we understand the details of how it works,” explains Titia de Lange, Professor Léon Hess. “In addition, we found out how this process is regulated.”

Recruitment and regulation of the CST

Telomeres have two different types of strands, G-rich and C-rich. Scientists have long known how telomerase maintains the length of the G-rich strand, but only recently has it been recognized that the same problem also exists for the C-rich strand. A recent study from Lange's laboratory identified the CST-Polα/primase complex as the main regulator responsible for maintaining this strand intact.

It remained to be seen how CST and its associated enzyme Polα-primase move toward telomeres to facilitate C-strand maintenance throughout replication cycles. Sarah Cai, a Ph.D. candidate at Rockefeller, began studying this piece of the telomere puzzle.

Building on a decade of Lange lab's preparatory work on CST, Cai added cryo-EM to the techniques used in this study while co-advised by Rockefeller's Thomas Walz.

“The interdisciplinary nature of the study is one of the most exciting aspects,” says Cai. “This was a very successful dual-lab effort, using many different technologies.” Walz, whose research focuses on cryo-EM, noted how Cai incorporated AlphaFold, a deep learning algorithm capable of predicting the unique 3D structures of proteins, into his work.

Using the combined power of biochemistry, structural biology and cell biology, the team finally confirmed that CST is recruited to telomeres by the POT1 protein. Once CST–Polα/primase is in place, the addition and removal of phosphate groups from POT1 appears to function as an on/off switch that coordinates the final steps of telomere replication.

Phosphorylated POT1 ensures that CST-Polα/primase remains inactive until telomerase has finished its work, after which dephosphorylation of POT1 activates CST-Polα/primase to add the finishing touch to the telomere.

Telomere disorders and cancer

Next, the team will look for specific enzymes that attach and remove phosphates during this process, controlling the POT1 on/off switch and determining their role in regulating CST-Polα/primase recruitment and activity. .

A better understanding of how CST is recruited to the telomere cannot be achieved quickly enough for patients with telomere disorders, such as Coats plus syndrome, a serious multiorgan disease characterized by abnormalities in the eyes. , brain, bones and gastrointestinal tract.

“For a long time, we didn't know why slight alterations in a single amino acid would cause such a devastating disease,” says Cai. “We now have a better idea of ​​how these mutations affect the recruitment of this essential telomere maintenance machine and lead to Coats plus syndrome.”

The findings will also impact their cancer research. Lange's lab has spent decades studying how telomere shortening contributes to tumor suppression and genome instability in cancer, and the present research could ultimately help answer questions that are at the heart of development of tumors.

“Everything that is essential for regulating telomere length could also be essential for cancer prevention,” says de Lange. “This is a major goal of our laboratory and one of the reasons why we will look more closely at the interaction between CST-Polα/primase and telomerase in the future.”