Direct and indirect non-disjunction in the origin of trisomy in cultured human lymphocytes

The aim of the present work was to investigate the processes involved in the origin of trisomic karyotypes, i.e. co-migration of sister chromatids (mitotic non-disjunction, MND) and recovery of micronuclei (MN) originating from lagging chromosomes/chromatids at anaphase (mitotic indirect non-disjunction, MIND), and to evaluate their relative contribution to aneuploidy in human lymphocytes mitotically activated in vitro. Therefore, phytohaemagglutinin-stimulated human lymphocytes from one donor were treated with 10 and 25 nM colchicine and analysed through two cell cycles by means of both molecular (FISH with centromeric DNA probes specific for chromosomes 7 and 11) and classical cytogenetic techniques. The following events were analysed: (i) chromosome/chromatid Loss (a MN-generating event) in M-1 bipolar ana-telophases; (ii) MN recovery in M2+ prophases; (iii) non-disjunction and loss of chromosomes 7 and 11 by FISH analysis in cytochalasin B-induced binucleate cells; (iv) spontaneous frequency of trisomic cells by chromosome counting and FISH analysis in M-1 c-metaphases; (v) induced frequency of trisomic cells by chromosome counting and FISH analysis in M-2 c-metaphases. Our results indicate that MND plays a major role compared with MIND in the origin of trisomic karyotypes, being similar to 4- to 5-fold higher in colchicine-treated cells. Moreover, remarkable reductions in the observed frequencies of trisomic cells were recorded in comparison with the expected ones, with an observed/ expected frequency ratio of trisomic M-2 c-metaphases ranging between 1/3 and 1/6.