Chromosome specific DNA hybridization in suspension for flow cytometric detection of chimerism in bone marrow transplantation and leukemia.

Flow cytometry was used to measure the fluorescence intensity of nuclei that were subjected to fluorescent in situ hybridization in suspension with chromosome specific DNA probes. Paraformaldehyde-fixed nuclei were protein digested with trypsin and hybridized simultaneously with a biotin- and DIG labeled chromosome specific centromere probe. A number of probes were tested in the suspension hybridizations. The method yielded fluorescent hybridization signals that allow discrimination between Y chromosome positive and negative nuclei when analyzed by flow cytometry. The method is especially suited for analysis of bone marrow cells derived from patients who have received a sex-mismatched allogeneic bone marrow transplantation. Male leukemia cells with a trisomy for chromosome 8 were mixed with normal female cells and simultaneously hybridized in suspension with a DIG labeled probe specific for chromosome 8 and the biotin labeled Y chromosome probe. Y chromosome positive or negative nuclei were sorted onto microscope slides and subsequently classified as being leukemic or not by fluorescence microscopy, on the basis of the presence of a trisomy for chromosome 8. A 120-fold enrichment could be achieved when 300 Y positive nuclei were sorted from a mixture originally containing 0.5% leukemia cells. Given the specificity of the flow cytometry and FISH procedure, the combination of the two methods can reach a lower detection level of 1 per 250,000.