Time-resolved fluorimetric probing of DNA structure in irradiated human lymphocytes

Abstract

An in situ technique has been developed that detects genomic conformational changes in irradiated human cells. Cells are treated on ice with detergent, mild alkali and ethidium bromide (EB) and the resulting intact nuclei are examined using kinetic spectrofluorimetry. In the nuclei of unirradiated lymphocytes the fluorescence decay profile is tri-exponential with a long-lived component (∼23 ns) attributable to EB intercalated within double-stranded DNA, an intermediate life-time component (∼6 ns) indicative of a loosely bound DNA biomolecular-EB complex, and a short-lived component (∼2 ns) corresponding to unbound EB. Irradiated fresh human lymphocytes show three similar components but their relative contributions are changed. Results from a typical donor, show that after 1 Gy the intermediate component decreased with a concomitant increase in the long-lived component while the short-lived component remained essentially unchanged. Fresh whole blood from healthy donors was irradiated at doses of 0.1–1 Gy, and the samples analyzed with or without post-irradiation incubation at 37 °C for 24 h prior to lymphocyte extraction. For doses of 1.0 Gy in the absence of incubation there is good agreement between multiple samples of the same individual, or among the six donors, as compared with the results from irradiated isolated lymphocytes. Whole blood incubation was unreliable but results from one individual at 0.1 and 1.0 Gy were similar to those observed without incubation. Fluorescence lifetime analysis can detect DNA structural/topological damage in irradiated human lymphoid cells, and it may have potential application to in vivo bio-dosimetry and bio-monitoring.