Effect of methyl DNA adducts on 3'-5' exonuclease activity of human TREX1

Three-prime repair exonuclease 1 (TREX1) is a 3'-5' exonuclease that plays an important role in clearing cytoplasmic DNA. Additionally, TREX1 is translocated to the nucleus after DNA damage and assists in DNA repair. In this work, we evaluated the activity of TREX1 in the context of the removal of methyl DNA adducts. We observed that TREX1 was less efficient at degrading methyl methanesulfonate (MMS)-treated methylated DNA compared with normal DNA. Two methyl DNA adducts, N1-methyladenine and N3-methylcytosine, were found to block TREX1 exonuclease activity. To understand the mechanism of limited TREX1-mediated degradation of MMS-damaged DNA, stem-loop substrates containing solitary methyl adducts were prepared. We found that when the solitary methyl adducts were present at the 3'-terminal single-stranded overhang, it prevented degradation by TREX1. However, TREX1 could efficiently process internally located duplex DNA methyl adducts when the 3'-terminal of the scissile strand was damage-free. Broadly, these observations suggest that TREX1 may be capable of resecting methyl adducts containing DNA, but it might be less proficient of removing 3'-terminal DNA methyl adducts.