CONSTITUTIVELY INCREASED MICRONUCLEI ARE PREDOMINANTLY CAUSED BY ACENTRIC FRAGMENTS

The frequency of kinetochore (centromere)-positive micronuclei (MN) was determined in 32 fibroblast cell lines. We tested 16 probands with spontaneously high MN levels (greater-than-or-equal-to 20 MN/500 cells (4%)) and 8 probands (controls) with low MN levels (less-than-or-equal-to 13 MN/500 cells (2. 6%)). To study whether the elevation of MN levels is due to increased chromosomal breakage we used the antikinetochore antibody fluorescent staining method. Probands with spontaneously high MN had kinetochore-positive MN increased by a factor 2.1 compared to the controls whereas the kinetochore-negative MN were increased by a factor 6.14. This shows that spontaneous elevation of MN is mainly caused by increased chromosomal breakage and only in a minor proportion by chromosome segregation errors as a consequence of spindle defects.