Measurement of Sister Chromatid Exchanges at Very Low Bromodeoxyuridine Substitution Levels Using a Monoclonal Antibody in Chinese Hamster Ovary Cells

A monoclonal antibody to bromodeoxyuridine (BrdUrd) incorporated into DNA allowed visualization of sister chromatid exchanges (SCE) when as little as 0.6% of the thymine in a single DNA strand has been substituted. Measurement of the SCE frequency as a function of BrdUrd substitution in a normal Chinese hamster ovary cell line showed a plateau of six SCEs per cell for substitution levels up to at least 20%. A clear elevation in frequencies was noted at 60% substitution. However, in the mutant line EM9, previously shown to have a highly elevated frequency of SCE, the level of exchanges declined continuously as the percentage of BrdUrd substitution decreased. At 0.6% substitution, the frequency of SCE was still 4-fold higher than that of the parental cells. The antibody procedure described here should be useful in evaluating the extent to which SCEs induced by mutagenic agents result from interactions between the DNA damage caused by the agent and the BrdUrd routinely used for measuring SCE.