Kinetochore proteins, peripheral location of chromosomes and nuclear budding: another look at the genesis of aneuploidy

Origin of nuclear buds and micronuclei in normal and folate-deprived human lymphocytes

Micronuclei are formed from chromosomes and chromosomal fragments that lag behind in anaphase and are left outside daughter nuclei in telophase. They may also be derived from broken anaphase bridges. Nuclear buds, micronucleus-like bodies attached to the nucleus by a thin nucleoplasmic connection, have been proposed to be generated similarly to micronuclei during nuclear division or in S-phase as a stage in the extrusion of extra DNA, possibly giving rise to micronuclei. To better understand these phenomena, we have characterized the contents of 894 nuclear buds and 1392 micronuclei in normal and folate-deprived 9-day cultures of human lymphocytes using fluorescence in situ hybridization with pancentromeric and pantelomeric DNA probes. Such information has not earlier been available for human primary cells. Surprisingly, there appears to be no previous data on the occurrence of telomeres in micronuclei (or buds) of normal human cells in general. Our results suggest that nuclear buds and micronuclei have partly different mechanistic origin. Interstitial DNA without centromere or telomere label was clearly more prevalent in nuclear buds (43%) than in micronuclei (13%). DNA with only telomere label or with both centromere and telomere label was more frequent in micronuclei (62% and 22%, respectively) than in nuclear buds (44% and 10%, respectively). Folate deprivation especially increased the frequency of nuclear buds and micronuclei harboring telomeric DNA and nuclear buds harboring interstitial DNA but also buds and micronuclei with both centromeric and telomeric DNA. According to the model we propose, that micronuclei in binucleate lymphocytes primarily derive from lagging chromosomes and terminal acentric fragments during mitosis. Most nuclear buds, however, are suggested to originate from interstitial or terminal acentric fragments, possibly representing nuclear membrane entrapment of DNA that has been left in cytoplasm after nuclear division or excess DNA that is being extruded from the nucleus.

Comparative analysis of colchicine induced micronuclei in different cell types in vitro

In the present study we analyzed the induction of micronuclei (MN) by colchicine, at different treatment times, in four histogenetically different cell cultures: human skin fibroblasts (HSF), bovine skin fibroblasts (BSF), bovine bladder fibroblasts (BBF) and human skin epithelial cells (HK), developed and characterized in our laboratory. The frequencies of dead cells, nuclear budding and mitotic index were also evaluated. The HSF and BSF cell lines showed similar frequencies of micronucleated cells (4.7% and 4.9%, respectively) at 96 h of treatment time. The BBF cell line showed the lowest frequency of micronucleated cells (2.6%) and HK did not show any MN. The studied cell lines differed in their responses to colchicine. The data revealed the relevance of utilization of other end-points as growth curves, dead cells, mitotic index and other nuclear alterations for accurate analysis of the effect of agents that disturb cell cycle or are cytotoxic.

Impact of rearrangements on function and position of chromosomes in the interphase nucleus and on human genetic disorders

A synthesis of numerous published data and my own observations reveal that chromatin structure in interphase is functional, dynamic and complex. I hypothesize that: (1) chromosome regions organize nuclear structures and thus their own environment (address themselves in sites and condensation patterns most appropriate for their functional state in the particular cell); (2) chromosome rearrangement could alter nuclear architecture and thus function; and (3) these ideas can explain the contribution of chromosome rearrangements, even in a balanced form, to human pathologic conditions.

Significance of aneuploidy

Aneuploidy is a state of abnormal and highly variable DNA and chromosome content found in both hereditary disorders and human malignancy. For two decades flow cytometry has allowed a wide-ranging survey of aneuploidy in clinicopathological series. Although up to 75 per cent of all tumours analysed display aneuploidy, its value as a clinical marker of biological aggressiveness is still uncertain. New technologies promise to reveal more precisely the genetic and subchromosomal changes that constitute aneuploidy and contribute to the malignant phenotype in human tumours.

Cytogenetic changes in primary, immortalized and malignant mammalian cells

Some chromosomes in transformed rat cells and somatic cell hybrids fail to display the presence of kinetochore proteins as detected by antikinetochore antibodies. Such chromosomes (K- chromosomes) may constitute a novel mechanism for the genesis of aneuploidy. We have analyzed primary, immortalized and malignant mammalian cells for the presence of kinetochore proteins and micronuclei. Our results suggest a correlation of the K- chromosome and micronucleus frequency with the variability in chromosome number. Upon in situ hybridization with the minor satellite and alpha satellite sequences some K- chromosomes showed a signal. This indicates that the observed lack of kinetochores is not necessarily due to a lack of centromeric DNA. We conclude that dislocated K- chromosomes may become incorporated into micronuclei which are prone to loss. Such events would be associated with the generation of aneuploidy.