PRINS localization of centromeres and telomeres in micronuclei indicates that in mouse splenocytes chromatid non-disjunction is a major mechanism of aneuploidy.

Primed In Situ DNA Synthesis (PRINS) of telomeric and centromeric (minor satellite DNA) sequences has been applied together with the cytokinesis block micronucleus (MN) assay in mouse splenocytes, with the aim of understanding the mechanism of origin of spontaneous and induced MN. Splenocyte cultures were treated in vitro either with the clastogenic agent mitomycin C or with the aneugenic compound colcemid. The relative proportions of MN carrying 1 to 4 telomeric signals were in agreement with the known mechanism of action of the chemicals tested, i.e., an higher number of MN with less than 4 telomeres were found in MMC-than in colcemid-treated cultures. No MN lacking the telomeric sequences (0 spot) were found, indicating that the observed distributions should not be affected by false-negative data. Furthermore, all MN carrying a single telomere were negative for the centromere, thus indicating that this class represents true chromosome acentric fragments. Finally, MN with 4 telomeric spots always carried the centromeric sequence, as expected on the hypothesis that these MN correspond to whole chromosomes. With respect to centromere-positive MN, more than one half carried 4 telomeric signals (whole chromosomes), and only 1/4 or less showed 2 telomeric signals (probably corresponding to a single chromatid). This difference was statistically significant, either in untreated cultures or in cultures exposed to mitomycin C or colcemid. On the whole, these data indicate that non-disjunction followed by whole chromosome loss (with the production of two daughter monosomic nuclei) may be the main mechanism of malsegregation leading to MN formation.