SEX-CHROMOSOME LOSS AND NONDISJUNCTION IN WOMEN - ANALYSIS OF CHROMOSOMAL SEGREGATION IN BINUCLEATED LYMPHOCYTES

Chromosomal lagging and non-disjunction are the main mechanisms of chr omosomal malsegregation at mitosis. To date, the relative importance o f these two events in the genesis of spontaneous or induced aneuploidy has not been fully elucidated. A methodology based on in situ hybridi zation with centromeric probes in binucleated lymphocytes was previous ly developed to provide some insight into this matter. With this metho d, both chromosomal loss and non-disjunction can be simultaneously det ected by following the distribution of specific chromosomes in the nuc lei and micronuclei of binucleated cells. In this study, this approach was used for studying the role of chromosomal loss and non-disjunctio n in the age-related malsegregation of sex chromosomes in females. For this purpose, cultures of cytokinesis-blocked lymphocytes were establ ished from 12 healthy women ranging in age from 25 to 56. The occurren ce of malsegregation of X chromosomes in vitro was estimated in binucl eated cells that contained four signals, which originates from the div ision of normal disomic cells. In this cell population, the frequencie s of X chromosome loss and non-disjunction ranged from 0% to 1.69% (me an 0.75%), and from 0.20% to 1.33% (mean 0.57%), respectively. This in dicates that both events contribute to malsegregation of X chromosomes in vitro. Moreover, a small but not negligible fraction of binucleate d cells with two or six copies of the X chromosome was noticed in all donors. These cells, which are thought to arise from parental monosomi c and trisomic types, may indicate the malsegregation of X chromosomes in vivo. The frequency of X chromosome an euploidy both in vivo and i n vitro significantly correlated with the age of donors. Analysis of c hromosomal distribution in unbalanced cells demonstrated that both X h omologues were frequently involved. The frequency of such multiple eve nts (0.17%) was far greater than that expected by mere chance, indicat ing a tendency to multiple malsegregation events in the cell populatio n investigated. Finally, parallel analysis of the segregation of chrom osomes X and 1 in five of the donors confirmed the greater (about tenf old) susceptibility of X chromosomes to malsegregate compared with aut osomes.