Molecular analysis of mutations induced in human cells by N‐ethyl‐N‐nitrosourea

Mutational activation of cellular proto‐oncogenes is an important event in the pathogenesis of chemically induced tumors. We have used the on P‐tk shuttle vector, pHET, to analyze the types of DNA sequence changes induced after treating mammalian cells with the carcinogen N‐ethyl‐N‐nitrosourea (ENU). This shuttle vector contains the putative replication origin of the Epstein‐Barr virus (EBV) and is stably maintained as a plasmid in EBV‐transformed human lymphoblastoid cells. Populations of plasmid‐bearing cells were treated with ENU, and plasmid DNA was isolated approximately 7–8 population doublings after treatment for analysis of mutations induced at the herpes simplex virus type 1 thymidine kinase (HSV‐tk) target gene. After ENU treatment, frequencies of four of the six possible base substitution mutations significantly increased. Transition mutations were the most common sequence change: 48% of the 46 mutants sequenced were GC→Ã transitions and 17% were AT→GC transitions. In addition, the number of AT→TA (20%) and AT→CG (9%) transversion mutations significantly increased after ENU treatment. Based on the comparison of mutations induced by ENU in human cells with the types of base pair changes previously reported for other alkylating agents, we propose that the O²‐ethylthymine adduct may be a significant premutagenic lesion in mammalian cells, capable of resulting in AT base pair transversion mutations. Studies from other laboratories have demonstrated the importance of AT→TA transversion mutations in the activation of cellular proto‐oncogenes by ENU.