The innate immune system has evolved to detect pathogens through germlineencoded
pattern recognition receptors (PRRs). Nucleic acid sensors, a defined class
of PRRs, bind to microbial and viral nucleic acids and trigger the production of type
I interferons and cytokines which mediate the first line of host defense. In the past,
these sensors have been primarily studied in the context of infections. Yet, the DNA
sensor cyclic GMP-AMP synthase (cGAS) has been recently described to detect not
only microbial and viral derived dsDNA but also self-DNA in a sequenceindependent
manner. At steady state, the organization of nuclear self-DNA is tightly
controlled by a complex network of multiple factors that can become imbalanced
during genotoxic stress. Whether and how this stress can be sensed by cGAS in the
absence of infection are the main questions of this thesis. Observing the fate of
cGAS in space and time in the presence of several genotoxic insults including
chemotherapeutics allowed me to gain novel insights into processes regulating the
localization and activation of cGAS.

In this thesis, I demonstrate that cGAS localizes to both nuclear and cytosolic
compartments of the cell. We show that cGAS is inactive in the nucleus due to
binding to the acidic patch on H2A-H2B heterodimers. Upon genotoxic stress, cGAS
is removed from the nucleus in a manner that is dependent on histones and remains
inactive. Moreover, with time cGAS transcription is repressed resulting in a gradual
decrease of cGAS levels. On the other hand, cGAS-activating DNA substrates such
as cytosolic chromatin fragments accumulate in the cytosol of senescent cells
deficient in Lamin B1 when cGAS levels are already decreased. Along with the
occurrence of these chromatin fragments, cGAS mediates an inflammatory response
dependent on STING (Stimulator of interferon genes), referred to as the
senescence-associated secretory phenotype (SASP).

Overall, we conclude that acute genotoxic stress does not result in cGAS-STING
signaling, whereas the chronic DNA damage response, senescence, activates the
cGAS-STING pathway. Moreover, the results presented in this work suggest a new
role for cGAS-STING signaling during senescence beyond the recognition of
pathogens.