Chromosomal instability (CIN) in cancer cells has been reported to activate the cGAS-STING innate
immunity pathway via micronuclei formation, thus affecting tumor immunity and tumor progression.
However, converse effects of the cGAS/STING pathway per se on CIN have not yet been investigated.
I addressed this issue using knockdown and add-back of each component of the
cGAS/STING/TBK1/IRF3 pathway, and monitored the extent of CIN by measuring micronuclei
formation after release from nocodazole-induced mitotic arrest. Interestingly, knockdown of cGAS
(cyclic GMP-AMP synthase) along with induction of mitotic arrest in HeLa and U2OS cancer cells
clearly resulted in an increased micronucleus phenotype and chromosome missegregation. Knockdown
of STING (stimulator of interferon genes), TBK1 (TANK binding kinase-1), or IRF3 (interferon
regulatory factor-3) also increased micronuclei formation. Moreover, transfection of cGAMP, the
product of cGAS enzymatic activity, as well as add-back of cGAS WT (but not catalytic-dead mutant
cGAS), or WT or constitutively active STING (but not an inactive STING mutant) rescued the
micronuclei phenotype, demonstrating that all components of the cGAS/STING/TBK1/IRF3 pathway
play a role in preventing CIN. Moreover, p21 levels were decreased in cGAS-, STING-, TBK1- and
IRF3-knockdown cells in association with precocious G2/M transition and an enhanced micronuclei
phenotype. Overexpression of p21 or inhibition of CDK1 in cGAS-depleted cells reduced micronuclei
formation and abrogated precocious G2/M transition, indicating that the decrease in p21 and
subsequent precocious G2/M transition is the main mechanism underlying the induction of CIN by
defective cGAS/STING signaling.