Scientists from the NTU School of Biological Sciences, led by Professor Lars Nordenskiöld, recently published their study in The Embo Journal on an essential protein, telomere repeat binding factor 2 (TRF2), that plays a critical role in stabilising and maintaining the structures of telomeres that organize and protect the DNA at the ends of our chromosomes.
This research builds on their previous study on the novel columnar structure of telomeres, which was published in 2022 in Nature. Their new paper provides a more detailed understanding into how telomeres and their proteins work, allowing for potential future developments in cancer treatment and to address issues with our ageing process.
Back row (L-R): Aghil Soman, Wahyu Surya
Front row (L-R): Lars Nordenskiöld, Sook Yi Wong
Telomeres
Our DNA is tightly packed into chromosomes, with approximately 2 metres of DNA strands packed into a space less than 5 to 10 micrometres (1/1000 of a millimetre) in our cell nuclei. The ends of these chromosomes are protected by regions of repetitive DNA sequences known as telomeres.
Their repetitive sequence structures, along with specific protein complexes, allow telomeres to act as protective caps by preventing the ends of chromosomes from being mistaken by the cell’s DNA repair mechanisms and avoiding unwanted repair responses.
However, when chromosomes divide during cell division, the telomeres are unable to be fully replicated by the DNA replication mechanisms. This results in a small portion of telomeric DNA sequences being lost each time during cell division, with the telomeres getting shorter and shorter until the chromosome is unable to continue replicating itself without risking further damage, leading to apoptosis – programmed cell death. Scientists believe that this shortening of our telomeres is one of the contributors to our ageing process.
Notably, certain cells such as embryonic stem cells and germ cells have active telomerase, an enzyme that counteracts the shortening of telomeres by adding DNA sequences to maintain the telomeres. The enzyme effectively gives these cell lines “immortality.”
However, cancerous cells have been shown to hijack this system to achieve “immortality,” aiding in their unwanted proliferation. Thus, characterisation of telomeres and their mechanisms is of utmost importance for our understanding of cancers and health issues associated with ageing.
The importance of telomere repeat-binding factor 2 (TRF2)
TRF2 is one of the important proteins that make up the shelterin complex, a group of proteins essential to the telomeres’ protective qualities. The shelterin complex binds to strands of telomere DNA during the formation of the telomere loop (T-loop), which is when single-strands of telomere DNA fold back and pair with other double-strands of telomere DNA, creating a looped structure that protect the ends of the chromosome.
While scientists know how TRF2 interacts with telomeric DNA, little is known about how the protein interacts with telomeric chromatin, which encompasses the entire complex of telomeric DNA and the proteins involved, including the histones around which the telomeric DNA are tightly wound and packaged.
Previous studies were focused on studying the interactions between TRF2 and nucleosome arrays whose DNA templates were non-telomeric in nature. As such, these studies were unable to fully uncover the relationship between TRF2 and telomeric chromatin.
Motivated by this, as well as their recent discovery of the unique columnar structure of telomeric chromatin, the team from NTU decided to investigate the binding of TRF2 to in-vitro (i.e., in a test tube) human telomeric chromatin fibres.
An important discovery
Besides separating the samples at high-speed revolutions via analytical ultracentrifugation and employing the use of cryo-electron microscopy – where samples are quickly frozen before examination – and negative stain electron microscopy – where samples are prepared with a heavy metal salt to ease visualisation – the team also used magnetic tweezers to help in their efforts.
The magnetic tweezers technique uses tiny magnetic beads that can attach to biomolecules, and using a magnetic field, scientists can pull and manipulate single biomolecules and examine their mechanical properties and stability.
Their studies revealed that TRF2 does interact with telomeric chromatin and promotes the assembly of the columnar structure. Thus, TRF2 induces the compaction of telomeric chromatin into its novel columnar structure, which leads to a more stable telomere and demonstrates the protein’s importance in maintaining a healthy telomere.
What comes next?
With their newfound knowledge that TRF2 is important to maintaining healthy and stable telomeres, the team will next embark on establishing the molecular details of these TRF2-telomeric chromatin interactions via high-resolution structures, which are highly detailed, three-dimensional representations of these TRF2-telomeric chromatin complexes.
By continuing to develop a greater understanding of the mechanics of telomeres, scientists may be able to manipulate telomeres in the future. It may one day lead to the development of cancer treatments that can target the telomeres of cancer cells.
These treatments may prevent cancer cells from having strong, ‘immortal’ telomeres and cause them to experience apoptosis. There may be future treatments that do the opposite and strengthen telomeres, which can help in slowing down the ageing process or help to mitigate the health problems that come with ageing.