O-Alkylation in DNA does not correlate with the formation of chromosome breakage events in D. melanogaster

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Postmeiotic cell stage of repair-proficient ring-X (RX) males were treated with methyl methane-sulfonate (MMS), ethyl methanesulfonate (EMS), diethylnitrosamine (DEN) or ethylnitrosourea (ENU) and then mated to either repair-defective ( mei-9 L1) or to repair-cor-competent females ( mei-9 +). Absence of the mei-9 + function resulted in a hypermutability effect to all alkylating agents (AAs) when they were assayed for their ability to induced chromosomal aberrations (chromosome loss; CL), irrespective of marked differences in distribution of DNA adducts brought about by these AAs. This picture is different from that described previously for the induction of point mutations (Vogel et al., 1985a). There, evidence was presented indicating that reduction in DNA excision repair does not affect point mutation induction (recessive lethals) by those AAs most efficient in ring-oxygen alkylation such as ENU, DEN, N-enthyl- N′-nitro- N-nitrosoguanidine (ENNG), and isopropyl methanesulfonate (iPMS): the order of hypermutability of AAs with mei-9 L relative to mei-9 + was MMS > MNU > DMN = EMS > iPMS = ENU = DEN = ENNG. When the percentage of lethal mutations induced in mei-9 L1 females were plotted against those determined for mei-9 + females, straight lines of following slopes were obtained: MMS = 7.6, NU = 5.4, DMN = 2.4, EMS = 2.4, and iPMS = ENU = DEN = ENNG = 1. Those findings, together with the recent observation that AAs do not split into two groups when assayed for their ability to cause CL, point to the involvement of different DNA alkylation products in ENU- and DEN-induced chromosome loss vs. that of point mutations. It is concluded that with ENU and DEN chromosomal loss results from N-alkylation products whereas point mutations (SLRL) are the consequence of interactions with oxygen-sites in DNA. Thus, as a consequence of a very dominating role of O-ethylguanine (and possibly O 4-alkylation of thymine), N-alkylation in DNA does not contribute measurably to mutation induction in the case of ENU-type mutagens while O-alkylation, very clearly, does not show a positive correlation with the formation of chromosome breakage events in Drosophila. Conversely, it appeared that with MMS-type mutagens (MMS; dimethyl sulfate, DMS; trimethyl phosphate, TMP), alkylation products such as 7-methylguanine and 3-methyladenine, if unrepaired or misrepaired, are potentially mutagenic lesions causing both mutations and chromosomal aberrations.