The production of micronuclei from chromosome aberrations in irradiated cultures of human lymphocytes.

A rapid assay for chromosomal damage would greatly speed studies of the mechanism by which chromosomal aberrations are formed. The characteristics of such an assay--micronuclei produced in cultured human lymphocytes--are given here, together with the evidence that the assay accurately measures X-ray-induced chromosomal damage. Micronuclei arise from chromosomal fragments that are not incorporated into daughter nuclei at mitosis because they lack a centromere. In our experiments the response of lymphocytes from different donors was very uniform and agreed well with what was expected from metaphase analysis of aberrations: (1) the increase in micronucleus frequency begins at the time of the first mitoses, 48 hours after the cultures are started, (2) the exponent of the dose response equation (y=kDn) was 1.2 for micronuclei. For one-hit aberrations n=1 whereas for two-hit aberrations n=2. Since two-hit aberrations predominate in these cultures, a value of n= approximately 1.8 was expected if no increase in mitotic delay or cell death occurred at higher doses, and n less than 1.8 if an increase occurred, (3) the frequency of micronuclei was decreased by a factor of about two when the dose was fractionated, as expected when most of the aberrations are two-hit. The rejoining time for four or five donors was between 30 and 60 minutes, (4) the X-ray-induced micronucleus frequency in cells from people with Down's syndrome (trisomy-21) was twice that of control donors as expected from metaphase analysis [22,23]. Since the micronucleus assay reflects the aberration frequencies so well and is so fast, it is suitable for a rapid assessment of chromosomal damage.