Flow cytometric DNA-histogram analysis: non-stoichiometric fluorochrome binding and pseudoaneuploidy.

Detection of aneuploid subpopulations using flow cytometry requires stoichiometric binding of nucleic acid-specific fluorochromes onto DNA. It is shown that parameters like cell type specificity and differentiation stage, cell cycle stage, loss of DNA-integrity, cell preparation, and cytochemistry affect fluorochrome binding to DNA and give rise to the appearance of pseudo-aneuploid cell populations. Intercalating as well as non-intercalating fluorochromes show non-stoichiometric DNA-labelling in cell populations with identical DNA content, and pseudo-aneuploidy was found in flow cytometers equipped with either arc lamps or argon lasers. Pseudo-aneuploidy was never observed with intercalating and non-intercalating fluorochromes within identical specimens, consisting of cells of various differentiation states (e.g., bone marrow) or containing large numbers of dead cells. Therefore, fluorochromes exhibiting different base-pair specificities or steric binding modes should be applied to be sure of the correct interpretation of small levels of hypo- or hyper-diploidy (+/- 20 per cent).