Recommendations, evaluation and validation of a semi-automated, fluorescent-basedscoring protocol for micronucleus testing in human cells.

Micronucleus (MN) induction is an established cytogenetic end point for evaluating structural and numerical chromosomal alterations in genotoxicity testing. A semi-automated scoring protocol for the assessment of MN preparations from human cell lines and a 3D skin cell model has been developed and validated. Following exposure to a range of test agents, slides were stained with 4'-6-diamidino-2-phenylindole (DAPI) and scanned by use of the MicroNuc module of metafer 4, after the development of a modified classifier for selecting MN in binucleate cells. A common difficulty observed with automated systems is an artefactual output of high false positives, in the case of the metafer system this is mainly due to the loss of cytoplasmic boundaries during slide preparation. Slide quality is paramount to obtain accurate results. We show here that to avoid elevated artefactual-positive MN outputs, diffuse cell density and low-intensity nuclear staining are critical. Comparisons between visual (Giemsa stained) and automated (DAPI stained) MN frequencies and dose-response curves were highly correlated (R (2) = 0.70 for hydrogen peroxide, R (2) = 0.98 for menadione, R (2) = 0.99 for mitomycin C, R (2) = 0.89 for potassium bromate and R (2) = 0.68 for quantum dots), indicating the system is adequate to produce biologically relevant and reliable results. Metafer offers many advantages over conventional scoring including increased output and statistical power, and reduced scoring subjectivity, labour and costs. Further, the metafer system is easily adaptable for use with a range of different cells, both suspension and adherent human cell lines. Awareness of the points raised here reduces the automatic positive errors flagged and drastically reduces slide scoring time, making metafer an ideal candidate for genotoxic biomonitoring and population studies and regulatory genotoxic testing.