Flow Cytometry in Formamide Treated Cells

The use of formamide for the study in flow cytometry of cell cycle phases, by DNA content measurement in human cancer cell lines, was recently published. In this manuscript we verify the possibility of extending the procedure to simultaneous analysis of other parameters. The results obtained, here reported, show that the treatment of samples by formamide is compatible with the simultaneous detection of DNA content and surface phenotypes, with quantification of replicating DNA and with measurement of cells with fractional content of DNA. For each of these three applications we have adapted the procedure in order to gain simple, reproducible and above all advantageous protocols. Regarding the simultaneous analysis of DNA content and phenotyping the use of formamide achieves optimal DNA stoichiometric staining (C.V.<3; G2/G1 ratio = 2 ± 0.05) and sufficient maintenance of physical parameters and membrane fluorescence. In the study of duplicating DNA labeled with click chemistry, our procedure eliminates paraformaldehyde (PFA) fixation improving the DNA stoichiometric staining and allows the use of 7-aminoactinomycin D (7-AAD) preserving the Alexa Fluor 488 quantum efficiency. Concerning the detection of cells with fractional content of DNA, permeabilization and fixation by formamide gives the advantage of resolve on linear scale sub-G1 cells from debris and to allow optimal sample recovery (>90%) which is essential in the study of cell necrobiology. Cells treatment by formamide, suitably modified for different applications, can be used to prepare cell samples for flow cytometry analyses that go far beyond of stoichiometric staining of DNA.