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Molecular analysis of O6-substituted guanine-induced mutagenesis of ras oncogenes

G Mitra1, G T Pauly, R Kumar

  • 1Developmental Oncology, National Cancer Institute-Frederick Cancer Research Facility, MD 21701.

Proceedings of the National Academy of Sciences of the United States of America
|November 1, 1989
PubMed

Insights

Chemically synthesized O6-substituted guanines, including O6-methylguanine and O6-benzylguanine, were incorporated into the Ha-ras protooncogene to assess mutagenicity. Both compounds were mutagenic, with O6-methylguanine inducing more mutations than O6-benzylguanine.

Area of Science:

  • Molecular Biology
  • Genetics
  • Carcinogenesis

Background:

  • The Ha-ras protooncogene is crucial in cell signaling and cancer development.
  • Understanding the mutagenicity of DNA adducts is vital for cancer research.
  • Specific guanine modifications can lead to oncogene activation.

Purpose of the Study:

  • To evaluate the mutagenicity of O6-methylguanine and O6-benzylguanine within the Ha-ras gene.
  • To investigate the mutation mechanisms induced by these O6-substituted guanines.
  • To explore the role of guanine residue accessibility in carcinogen-induced mutations.

Main Methods:

  • Design of a specialized Ha-ras/thymidine kinase (TK) cassette for incorporating synthetic DNA adducts.
  • Introduction of O6-methylguanine and O6-benzylguanine into the 12th codon of the rat Ha-ras protooncogene.
  • Monitoring mutations by assessing the transformation of Rat4 TK- cells by modified Ha-ras DNA.

Main Results:

  • Both O6-methylguanine and O6-benzylguanine were found to be significantly mutagenic.
  • O6-methylguanine induced a higher percentage of transformed colonies compared to O6-benzylguanine.
  • O6-methylguanine exclusively caused G-to-A transitions, while O6-benzylguanine induced G-to-A transitions, G-to-C, and G-to-T transversions.

Conclusions:

  • O6-substituted guanines are potent mutagens within the Ha-ras gene.
  • The distinct mutation profiles suggest different mutagenic mechanisms for O6-methylguanine and O6-benzylguanine.
  • Guanine residue accessibility may influence Ha-ras oncogene activation by methylating carcinogens.

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