Coordinated activity of Y family TLS Polymerases and EXO1 protects non-cycling cells from cytotoxic lesions

In the human body, many cells are post-mitotic or terminally differentiated. Depending upon their specific tissue origin, different sources of bulky lesions (e.g. UV-light, chemotherapeutics, cigarette smoke, pollution) can compromise the stability of the genome of differentiated cells.  We demonstrated that DNA damage checkpoint response in UV-irradiated non-cycling cells requires a coordinated activity of Nucleotide Excision Repair (NER) and Exonuclease 1 (EXO1). EXO1 generates long tracts of ssDNA in response to bulky adducts at a subset of lesions, whose repair is not efficiently completed by canonical NER. The nature of the lesions requiring EXO1 activity is still unknown. We proposed that clusters of lesions affecting both DNA strand (closely opposing lesions, COLs), may be problematic for NER and may require EXO1 intervention.  Accordingly, we observed that Y-family TLS polymerases are recruited at NER- and EXO1-positive sites. Polymerase κ and ι recruitment is dependent upon EXO1 activity, while Polymerase η and REV1 recruitment is not. Interestingly, if TLS polymerases recruitment and their activity are inhibited, we observed a hyper-activation of DNA damage checkpoint and generation of DSBs, deriving from the uncontrolled processing of NER intermediates by EXO1. A balanced activity of TLS Pols and EXO1, downstream of NER, is crucial to prevent unscheduled cytotoxic lesions formation after removal of DNA bulky adducts. Molecular details about NER, TLS Pols and EXO1 action after bulky lesions induction from different sources will be explored in physiologically non-cycling cells.