Figure 5. Simultaneous repair of two closely spaced MNU-induced lesions may lead to a DSB.

Such a situation occurs when an N-alkyl lesion located within ≈500 nt of an O⁶mG lesion is processed simultaneously (‘Lesion Arrangement at-risk’). Note that the mismatch repair (MMR) excision track can occur on either strand as described for noncanonical MMR (Peña-Diaz et al., 2012). Reaction of N-methyl-N-nitrosourea (MNU) with double-stranded DNA induces N-alkylation adducts, mostly 7mG and 3mA shown as * and O-alkylation adducts (O⁶mG), at a ratio of 10:1 approximately. Step 1: a base excision repair (BER) event is initiated at an N-alkyl adduct, creating a nick. Step 2: concomitantly, an MMR event takes place, in the opposite strand, at a nearby O⁶mG:C site. Step 3: the MMR machinery extends the nick into a several hundred nt-long gap by means of Exo1 action. Step 4: the two independently initiated repair events lead to a double-strand break (DSB), if the MMR gap reaches the BER-initiated nick before resealing.