Chromatin Bridges, not Micronuclei, Activate cGAS after Drug-induced Mitotic Errors in Human Cells

Abstract

Mitotic errors can activate cGAS and induce type-I interferon (IFN) signaling. Current models propose that chromosome segregation errors generate micronuclei whose rupture activates cGAS. We used a panel of anti-mitotic drugs to perturb mitosis in fibroblasts and measured abnormal nuclear morphologies, cGAS localization and IFN signaling in the subsequent interphase. Micronuclei consistently recruited cGAS without activating it. Instead, IFN signaling correlated with formation of cGAS-coated chromatin bridges that were selectively generated by microtubule stabilizers and MPS1 inhibitors. cGAS activation by chromatin bridges was suppressed by drugs that prevented cytokinesis. We confirmed cGAS activation by chromatin bridges in cancer lines that are unable to secrete IFN by measuring paracrine transfer of cGAMP to fibroblasts. We propose that cGAS is selectively activated by self-chromatin when it is stretched in chromatin bridges. Immunosurveillance of cells that fail mitosis, and anti-tumor actions of taxanes and MPS1 inhibitors, may depend on this effect.

Nuclear cGAS: guard or prisoner?

“…Other possible mechanisms differentiating cGAS‐activating potential of micronuclear chromatin vs intact‐nuclei chromatin would be potential aberrant chromatin structures in micronuclei recognized by cGAS, or yet to be identified mechanisms or factors for cGAS inhibition present in nuclei but absent from micronuclei. Nevertheless, there is so far no direct biochemical evidence that aberrant chromatin in micronuclei cannot just recruit, but also activate cGAS—recent work in fact implicates co‐occurring chromatin bridges, rather than micronuclei, as the actual source of cGAS activation after drug‐induced mitotic errors (preprint: Flynn et al , 2021). Another example of cytosolic chromatin activating cGAS is neutrophil extracellular traps (NETs), which are released by neutrophils during a specific form of regulated cell death called NETosis.…”

Mitotic disassembly and reassembly of nuclear pore complexes

“…Consistently, in mitotic cells cGAS is not activated by exogenous DNA [107][108][109], although prolonged mitotic arrest can lead to a slow activation of cGAS, followed by IRF3-dependent apoptosis [40]. At late stages of mitosis, persistent DNA bridges exposed to stretching forces during cytokinesis or traction by cell movement can also potently activate cGAS [39].…”

“…Whereas most forms of cytosolic DNA are recognized as non-self and are subject to cellular defense pathways of the innate immune response, mitotic chromosomes are usually inert (Box 2). Recent studies have, however, revealed that prolonged mitosis and mitotic errors can lead to activation of cyclic GMP-AMP synthase (cGAS) and induction of type I interferon signaling [39,40] (Box 2).…”

Karyotype Aberrations in Action: The Evolution of Cancer Genomes and the Tumor Microenvironment

“…CIN and micronucleus formation, however, do not always cause EMT or promote invasive behavior, even if cGAS-STING is active [257]. Similarly, micronucleus formation does not always lead to cGAS activation [258]. One study found that chromatin bridges, but not micronuclei originating from whole chromosomes, activated cGAS, resulting in the spread of inflammatory signaling from cancer cells to stromal cells (fibroblasts and monocytes) in a co-culture model [258].…”

“…Similarly, micronucleus formation does not always lead to cGAS activation [258]. One study found that chromatin bridges, but not micronuclei originating from whole chromosomes, activated cGAS, resulting in the spread of inflammatory signaling from cancer cells to stromal cells (fibroblasts and monocytes) in a co-culture model [258]. Therefore, while the effects of CIN and micronucleus formation on EMT and cGAS-STING activation appear contextdependent, both whole chromosome missegregation and chromatin bridges may induce a chronic inflammatory response that fuels tumor progression.…”