Determination of the baseline sister chromatid exchange frequency in human and mouse peripheral lymphocytes using monoclonal antibodies and very low doses of bromodeoxyuridine.

We measured the frequency of sister chromatid exchanges (SCEs) in human and mouse peripheral lymphocytes using doses of bromodeoxyuridine (BrdU) ranging from 30 nM to 100 microM (human) and from 10 nM to 10 microM (mouse). Heparinized peripheral blood was obtained from five healthy nonsmokers and from six C57B1/6 male mice. The blood was stimulated with PHA (human) or lipopolysaccharide (LPS, mouse) and grown for the first of two cell cycles in BrdU. Metaphase chromosomes were denatured and exposed to a monoclonal antibody reactive to single-stranded DNA containing BrdU. A second antibody was used to label the first antibody with fluorescein, and propidium iodide was used as a counterstain. Second-division metaphases were thus differentially stained red to indicate DNA content and yellow-green to indicate the presence of BrdU. The results indicate that the baseline SCE frequency in human and mouse peripheral lymphocytes is 3.6 and 2.4 SCEs per cell per generation, and that in the human these frequencies are invariant at the lowest BrdU levels. This suggests that SCEs are an integral part of DNA replication, even in the absence of agents known to induce SCEs. The distribution of SCEs per chromosome was analyzed and found to be Poisson-distributed in all 24 murine cultures and in 25 of 36 human cultures. The distribution of SCEs per chromosome may be due to either species-specific chromosome packaging or to karyotypic differences between the species.