Summary: Researchers at the University of Pittsburgh report new insights into telomere biology that clarify how oxidative damage affects telomerase activity and could influence cancer, aging and inflammation.
Source: University of Pittsburgh.
University of Pittsburgh scientists have revealed new molecular details about telomeres — the protective DNA “caps” at chromosome ends — and how oxidative damage to DNA influences telomere maintenance. The study appears in the journal Nature Structural & Molecular Biology.
Telomeres are repetitive DNA sequences that shorten each time a cell divides, progressively shrinking as organisms age. When telomeres become critically short, cells stop dividing, reducing tissue regeneration capacity and contributing to age-related disorders, explains Patricia Opresko, Ph.D., associate professor of Environmental and Occupational Health at Pitt and lead author of the study. In many cancers, by contrast, the enzyme telomerase is upregulated, restoring telomere length and enabling unlimited cell division.
Because telomere length and integrity influence cancer, inflammation and aging, understanding how telomerase is regulated is important for both promoting healthy tissue maintenance and selectively targeting cancer cells. “These findings can help guide new approaches to preserve telomeres in healthy cells and, conversely, to deplete telomeres selectively in cancer cells,” said Dr. Opresko.
Oxidative stress, a condition in which reactive molecules called free radicals accumulate, is known to accelerate telomere shortening. Free radicals damage telomeric DNA and the nucleotide precursors used by telomerase to extend telomeres. Oxidative stress is implicated in inflammation, cancer and the aging process because damage from free radicals accumulates over time.
The team set out to determine precisely how oxidative damage alters telomere maintenance. They investigated two possibilities: direct oxidation of the telomeric DNA sequence itself, and oxidation of the nucleotide building blocks (dNTPs) that telomerase uses during extension.
Contrary to their initial expectation, the researchers found that telomerase can extend telomeres even when the telomeric DNA carries oxidative lesions. In fact, certain forms of damage to the telomeric sequence can destabilize higher-order DNA structures at the chromosome end and actually promote telomerase activity.
However, when the oxidized lesion occurs in the available nucleotide pool — specifically as the oxidized precursor 8-oxodGTP — telomerase can incorporate that damaged nucleotide onto the telomere but then fails to extend further. In other words, incorporation of an oxidized dNTP is mutagenic and terminates further telomere elongation. This mechanism offers a clear explanation for how oxidative stress can accelerate telomere shortening by interfering with the building blocks telomerase needs, rather than solely by damaging telomeric DNA directly.
The work identifies a previously underappreciated route by which oxidative damage regulates telomerase: oxidation of the dNTP pool. The researchers observed that depletion of cellular enzymes that remove oxidized nucleotides worsens telomere dysfunction and promotes cell death in telomerase-positive cancer cells that already have shortened telomeres. This suggests that manipulating nucleotide oxidation pathways could represent a therapeutic strategy for cancer, by selectively inhibiting telomerase activity in tumor cells.
Conversely, the observation that preexisting oxidative lesions within telomeric DNA can promote telomerase access and activity highlights the complexity of how oxidative stress impacts telomere dynamics and underscores that the biological outcome depends on where and how the oxidative damage arises.
To investigate telomere-specific effects in living cells, Dr. Opresko and colleagues plan to use a novel photosensitizer developed at Carnegie Mellon University that generates oxidative damage selectively at telomeres. That technology will allow the team to observe how telomere damage is processed and how cells respond when damage is confined to chromosome ends.
Funding: This research was supported by multiple National Institutes of Health grants (including R01ES022944, R21AG045545, R21ES025606, 1DP2GM105453 and CA148629), an American Cancer Society grant (RSG-12-066-01-DMC), and the Abraham A. Mitchell Distinguished Investigator fund. The study also used core facilities supported in part by NIH grant P30CA047904.
Collaborators: Coauthors and collaborators included Elise Fouquerel, Justin Lormand and Arindam Bose (University of Pittsburgh); Hui-Ting Lee and Sua Myong (Johns Hopkins University); Grace Kim (University of Illinois); Jianfeng Li and Robert Sobol (University of South Alabama); and Bret Freudenthal (University of Kansas Medical Center).
Source: Arvind Suresh — University of Pittsburgh. Image: public domain. Original research: “Oxidative guanine base damage regulates human telomerase activity,” published in Nature Structural & Molecular Biology (Elise Fouquerel et al., November 7, 2016; doi: 10.1038/nsmb.3319).
The study examines how the common oxidative lesion 8-oxo-7,8-dihydro-2′-deoxyguanine (8-oxoG) affects human telomerase. When the oxidized form 8-oxodGTP is present in the nucleotide pool, telomerase may incorporate it into telomeres, producing mutations and terminating further elongation. Depleting MTH1, an enzyme that removes oxidized dNTPs, increases telomere dysfunction and cell death in telomerase-positive cancer cells with short telomeres. By contrast, if 8-oxoG is already present within the telomeric DNA sequence, it can destabilize G-quadruplex structures and promote telomerase activity. Thus, the origin of oxidative guanine damage — whether by insertion of oxidized nucleotides or direct reaction with free radicals — determines whether telomerase is inhibited or stimulated, and thereby influences cellular outcomes.