DID Novel DNA-Sensing Pathway Found in Human Cells, Absent in Mice

Researchers at The University of Washington have discovered a unique novel DNA-sensing the pathway that launches an antiviral response to foreign genetic material in human cells.

Triggered by an enzyme called DNA protein kinase (DNA-PK), and the newly invented pathway is independent of cGAS-STING pathway until now considered as the most regulator of mammalian innate immune responses to DNA and it is missing or inactive in mouse cells. The finding raises a question about the promise of therapies that focus on cGAS-STING for immune modulation, researchers report in Science Immunology.

“It seems like a DNA-sensing a pathway that been completely overlooked—probably because much of the research has been done used murine systems,” says Christian Holm, who researches cGAS-STING at Aarhus University and wasn’t involved within the study. Previous work on antiviral responses has focused exclusively on cGAS-STING, he adds. “Now this comes along and says there’s this other pathway . . . that seems to be completely independent of STING and may be very important.”

It makes perfect sense to have another DNA-sensing mechanism.—Alexiane Decout, EPFL    

First described in 2013, the cGAS-STING pathway plays a critical role within the cell’s innate response to viral infection. Upon detecting cytosolic DNA (usually a tell-tale sign of viral entry), the cGAS enzyme binds to the transmembrane protein STING to trigger the assembly of interferons and other antiviral responses.

The pathway has become a well-liked target in drug development, with researchers trying to harness STING’s activity for cancer immunotherapy or to calm it in autoimmune diseases within which innate immune responses are overactive.

The University of Washington’s Dan Stetson tells Scientist that he and his team were studying with the effects of tumor-promoting viruses on cGAS-STING once they stumbled across the novel pathway. Graduate Katelyn Burleigh generated human cell lines lacking STING, he says, and located that they are still produced interferons when transfected with foreign DNA. Further assays using various human cell types and chemical inhibitors indicated that DNA-PK, an enzyme known for its role in detecting and responding to DNA damages within the nucleus were sensing that foreign DNA within the cell cytoplasm and launching its own, STING-independent response.

It’s not the primary time DNA-PK has been implicated in antiviral defenses. The University of Cambridge’s Geoffrey Smith and Brian Ferguson declared in 2012 that DNA-PK in mouse and human cells will promote interferon production in response to transfection with foreign DNA. However, that study concluded that DNA-PK probably triggered the response through STING, not independently of it.

“It’s nice to check that another group has found a crucial role for DNA-PK in sensing foreign DNA,” Smith tells The Scientist, adding that the Washington team’s paper presents The data concerned "considers STING-independent new pathway.

He notes that assays the team carried out using DNA-PK inhibitors perceived to influence antiviral responses differently depending on cell type—a result that Stetson says might need to do with interactions between the DNA-PK and gCAS-STING pathways within the various cell lines the team used. In some cases, “the two pathways may antagonize each other,” Stetson writes an email to The Scientist. “It is something we are interested in pursuing.”

Examining other mammalian cell lines, Stetson’s team founded evidence of the novel DNA-PK pathway in non-human primate cells and in rat cells. However, researchers were unable to identify the pathway in mouse cells, where most preclinical research on cGAS-STING therapy the research was conducted.

Alexiane Decout, a research scientist studying STING at EPFL in Switzerland who saw a preprint of the paper on bioRxiv, last year says she’s unsurprised by the finding because previous studies have already shown variations between human and mouse antiviral responses. “The mouse cGAS-STING pathway is far more easily activated then the human one,” she says as a result of that pathway’s lower activation in human cells, “it makes perfect sense [that those cells] have another DNA-sensing mechanism.”

This seems to be a largely neglected DNA-sensing pathway, Christian Holm, University of Aarhus.

The presence of this second the mechanism could have implications for efforts to modulate innate immune responses in patients with an autoimmune disorder. Stetson and colleagues propose in their paper that drugs designed to dampen cGAS-STING activity might be got to be paired with DNA-PK inhibitors (many of these are in clinical development) to be effective although, given an additional role in DNA repair, it is not clear how feasible such inhibition would be.

There also are potential applications for the novel DNA-sensing pathway in immunotherapy, notes Leticia Corrales, who has patent applications on STING-targeting compounds for cancer treatment and works at the pharmaceutical company Boehringer Ingelheim in Vienna.

She says that improving the innate immune response through DNA-PK may help to promote antitumor immunity. However, “first we have to verify that in humans this pathway has relevancy for antitumor immune responses,” she says, adding that researchers would need to be “very cautious about the toxicity aspects of using this approach.”

Stetson, who incorporates a patent-pending on DNA-PK modulation for disease treatment, says that his team is now exploring whether the newly discovered pathway promotes immunity to viral infection, and how the DNA-PK enzyme manages its dual roles of promoting repair of DNA damage within the nucleus and triggering responses to foreign DNA within the cytoplasm. 

For therapeutic purposes, “The long-term goal should be worked out a way to manipulate that,” he says, “and deliberately turn into what normally would be repaired into an extremely potent innate immune response.”

Previously demonstrated that the viral oncogenes of the DNA tumor viruses are potent antagonists of the cGAS-STING DNA sensing pathway, here, as per the report unexpected finding that the E1A oncogene of human adenovirus 5 blocks two distinct intracellular DNA sensing pathways in human cells: the well-known cGAS-STING pathway and a second, STING-independent DNA sensing pathway (SIDSP). Human cells have a second intracellular DNA pathway of sensing with implications for host defense, autoimmunity, and antitumor immunity.