Journal Article

<i>O</i><sup>6</sup>-Ethylguanine and <i>O</i><sup>6</sup>-benzylguanine incorporated site-specifically in codon 12 of the rat H-<i>ras</i> gene induce semi-targeted as well as targeted mutations in Rat4 cells

Ronald E. Bishop, Gary T. Pauly and Robert C. Moschel

in Carcinogenesis

Volume 17, issue 4, pages 849-856
Published in print April 1996 | ISSN: 0143-3334
Published online April 1996 | e-ISSN: 1460-2180 | DOI: http://dx.doi.org/10.1093/carcin/17.4.849
O6-Ethylguanine and
                        O6-benzylguanine incorporated site-specifically
                    in codon 12 of the rat H-ras gene induce semi-targeted as well
                    as targeted mutations in Rat4 cells

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To examine the miscoding properties of modified guanine residues bearing increasingly bulky O6-substituents, Rat4 cells, grown in the presence of O6-benzylguanine to deplete the DNA repair protein O6-alkylguanine-DNA alkyltrans-ferase, were transfected with plasmids carrying H-ras genes in which O6-methyl, O6-ethyl- and O6-benzylguanine were substituted for the first, second or both the first and second guanine residues of codon 12 (GGA). DNA from isolated transformed colonies was amplified by PCR and directly sequenced by high-temperature manual and automated methods. The results show that O6-ethylguanine and O6-benzylguanine induced semi-targeted as well as targeted mutations, in contrast to O6-methylguanine, which induced only targeted mutations. When incorporated in place of the first guanine of H-ras codon 12, the targeted mutations induced by all these modified guanines were exclusively G↑A transitions. When incorporated at the second position of codon 12, O6-benzylguanine induced G↑A, G↑T and G↑C mutations. O6-Ethylguanine at the second position induced chiefly G↑A transitions, and O6-methylguanine induced G↑A transitions exclusively. Semi-targeted mutations were strictly G↑A at the base 3‘ to a position 1 adduct or 5’ to a position 2 adduct. The mechanism for induction of targeted mutations probably involves decreasing preference for thymidine incorporation opposite an O6-modified guanine as the size of the O6-substituent increases, while the mechanism for non-targeted mutations may be related to abasic site formation or to translesion synthesis which might be made error-prone by obstructive DNA lesions in this context.

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Subjects: Clinical Cytogenetics and Molecular Genetics

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