Loss of the DNA-binding nuclear-envelope protein Banf1 (barrier-to-autointegration factor 1, BAF) causes double-stranded self-DNA of both nuclear and mitochondrial origin to accumulate in the cytosol of resting cells, activating a basal cGAS-STING-IRF3-driven interferon-stimulated gene program that is notably independent of type I interferon and STAT1 signaling. The unresolved mechanistic problem is to explain how Banf1 accomplishes this protection: what the source and identity of the accumulating cytosolic DNA are and how it reaches the cytosol; whether and how Banf1 affects mitochondrial integrity and mitophagy; where cGAS encounters this DNA; why the resulting basal signaling is IRF3-dependent yet interferon-independent; and how Banf1 expression levels across cell types and physiological states set basal innate-immune tone. Later work has resolved parts of this picture — direct competition between BAF and cGAS on exposed nuclear DNA, chromatin-bridge stabilization, and regulation of cGAS protein abundance — but the homeostatic source-attribution, the mitochondrial/mitophagy arm, the interferon-independent wiring, and the cell-type survey remain open.

Background

The cyclic GMP-AMP synthase (cGAS)-STING pathway is the principal innate-immune sensor of double-stranded DNA in the cytoplasm: when cGAS binds cytosolic DNA it produces the second messenger 2'3'-cGAMP, which activates the adaptor protein STING, leading to activation of the transcription factors IRF3 and NF-κB and induction of type I interferons (IFNs) and interferon-stimulated genes (ISGs). Because host genomic and mitochondrial DNA are normally compartmentalized in the nucleus and mitochondria, mislocalized self-DNA can trigger the same pathway and cause autoinflammation; how resting cells avoid this while remaining poised to respond to pathogens is a fundamental question in innate immunity. Barrier-to-autointegration factor 1 (Banf1, also called BAF), encoded by BANF1, is a small highly conserved dimeric protein that binds double-stranded DNA without sequence specificity and bridges chromatin to LEM-domain proteins of the inner nuclear membrane; it has essential roles in nuclear-envelope reassembly after mitosis and in the repair of nuclear-envelope ruptures, and recessive BANF1 mutations cause the premature-aging disorder Nestor-Guillermo progeria syndrome. A genome-wide CRISPR-Cas9 screen by Ma and colleagues (2020) identified Banf1 as a negative regulator of basal cell-intrinsic immunity: in BV2 microglial cells and mouse embryonic fibroblasts, reducing Banf1 expression raised baseline levels of cytosolic double-stranded DNA from both nuclear and mitochondrial sources and increased basal ISG expression through a cGAS-STING-IRF3 axis that, unexpectedly, did not require type I IFN or STAT1 signaling. Gamma-H2AX (phosphorylated histone variant H2AX, a marker of canonical DNA-damage response) was not induced, suggesting that any chromatin fragments reaching the cytosol are too small to trigger DNA-damage signaling yet still able to engage cGAS. Ma et al. could not isolate a complete Banf1 null, and commercial anti-mouse cGAS antibodies gave uninterpretable background staining, leaving open where cGAS acts. In parallel, independent studies established that BAF can directly compete with cGAS for DNA binding on chromatin exposed upon acute nuclear-envelope rupture (Guey et al. 2020); that stretched chromatin bridges between missegregating chromosomes, rather than micronuclei, are the structures that activate cGAS after mitotic errors (Flynn et al. 2021); that lamin C, BAF, and cGAS co-accumulate at nuclear-envelope rupture sites (Kono et al. 2022); that BAF destabilizes cGAS protein levels and thereby promotes gammaherpesvirus reactivation from latency (Broussard et al. 2023); and that BAF stabilizes chromatin bridges so that they do not degenerate into cGAS-activating structures under mitotic perturbation (Chastant et al. 2026). Tumor-cell work further showed that BANF1 inhibition activates cGAS-STING-dependent antitumor immunity (Wang et al. 2023). What these perturbation-context results leave unexplained is the original homeostatic phenomenon of Ma et al.: the origin of the cytosolic self-DNA that accumulates at baseline in Banf1-deficient cells, the contribution and regulation of the mitochondrial arm, the site of cGAS engagement, the interferon-independent IRF3 wiring, and the dependence of basal ISG tone on Banf1 expression levels across cell types and states.

Problem Statement

Determine the mechanism(s) by which Banf1/BAF prevents the basal accumulation of cytosolic self-DNA and the consequent activation of the cGAS-STING-IRF3 axis in resting cells. A complete answer must address, as parts of one mechanistic account: (i) the origin, identity, and route of exit of the double-stranded DNA that accumulates in the cytosol when Banf1 is deficient — apportioning the contributions of nuclear chromatin-derived DNA and mitochondrial DNA under homeostatic (unperturbed, non-rupture, non-mitotic-insult) conditions; (ii) whether Banf1 directly or indirectly affects mitochondrial integrity, including mitophagy, and thereby limits cytosolic mitochondrial DNA and cGAS-STING activation; (iii) the subcellular site(s) at which cGAS encounters and is activated by this DNA in Banf1-deficient cells, established with validated reagents; (iv) why the resulting basal ISG program in BV2 microglial cells and MEFs is IRF3-dependent yet independent of type I IFN and STAT1 signaling, and whether this wiring holds in other cell types; and (v) how Banf1 expression levels in different cell types (immune versus nonimmune) and states (quiescent versus dividing) set basal ISG tone. Each part (i)-(v) is an independently refutable sub-problem: a finding that directly addresses any single part, whether it affirms or refutes that part, is a partial resolution standing on its own, whereas the task as a whole is resolved only when all five parts have been addressed. Proposed mechanisms must be reconciled with the established partial answers — direct competition between BAF and cGAS on exposed nuclear DNA, chromatin-bridge stabilization, and BAF-dependent control of cGAS protein abundance — and must be tested directly rather than inferred by analogy with acute nuclear-envelope rupture or drug-induced mitotic-error contexts. (Optimizing a CRISPR screen to recover negative regulators of IFNAR signaling is methodological and outside this problem's scope.)

References

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