Chromatin‐bound cGAS is an inhibitor of DNA repair and hence accelerates genome destabilization and cell death

DNA repair via homologous recombination (HR) is indispensable for genome integrity and cell survival but if unrestrained can result in undesired chromosomal rearrangements. The regulatory mechanisms of HR are not fully understood. Cyclic GMP‐AMP synthase (cGAS) is best known as a cytosolic innate immune sensor critical for the outcome of infections, inflammatory diseases, and cancer. Here, we report that cGAS is primarily a chromatin‐bound protein that inhibits DNA repair by HR, thereby accelerating genome destabilization, micronucleus generation, and cell death under conditions of genomic stress. This function is independent of the canonical STING‐dependent innate immune activation and is physiologically relevant for irradiation‐induced depletion of bone marrow cells in mice. Mechanistically, we demonstrate that inhibition of HR repair by cGAS is linked to its ability to self‐oligomerize, causing compaction of bound template dsDNA into a higher‐ordered state less amenable to strand invasion by RAD51‐coated ssDNA filaments. This previously unknown role of cGAS has implications for understanding its involvement in genome instability‐associated disorders including cancer. Synopsis cGMP‐AMP synthase (cGAS) is best known as a cytosolic DNA sensor activating STING‐dependent innate immune signaling pathways. Chromatin‐bound cGAS is here shown to have an independent function in impeding homologous recombination (HR) repair of nuclear DNA and facilitating genome destabilization. cGAS is constitutively present in the nucleus as a chromatin‐bound protein in various cell types. Chromatin‐bound cGAS restrains DNA repair and accelerates genome destabilization, generation of micronuclei, and cell death independently of STING signaling. cGAS deficiency protects against γ‐irradiation‐induced ablation of bone marrow cells in vivo . Nuclear cGAS restrains DNA repair by HR, although not being specifically recruited to double‐strand breaks. cGAS impedes HR repair by oligomerizing on and compacting bound dsDNA it into a higher‐order state less amenable to invasion by RAD51‐filaments. Graphical Abstract The cytosolic innate immune sensor cGAS has an unexpected STING‐independent function in compacting nuclear DNA and impeding RAD51‐mediated recombination repair.