The nucleolus, virus infection, and trisomy in man

On the origin of the maternal age effect in trisomy 21 Down syndrome: the Oocyte Mosaicism Selection model

We have recently documented that trisomy 21 mosaicism is common in human foetal ovaries. On the basis of this observation we propose that the maternal age effect in Down syndrome (DS) is caused by the differential behaviour of trisomy 21 in relation to disomy 21 oocytes during development from foetal life until ovulation in adulthood. In particular, we suggest that trisomy 21 oocytes, lagging behind those that are disomic, may escape the timed pruning of the seven million in foetal life to the 300–400 finally selected for ovulation. The net effect of this preferential elimination will be an accumulation of trisomy 21 oocytes in the ovarian reserve of older women. We here highlight the implications of this Oocyte Mosaicism Selection (OMS) model with respect to the prevalent view that the maternal age effect is complex, dependent on many different biological and environmental factors. We examine conclusions drawn from recent large-scale studies in families, tracing DNA markers along the length of chromosome 21q between parents and DS children, in comparison to the OMS model. We conclude that these family linkage data are equally compatible with the maternal age effect originating from the accumulation of trisomy 21 oocytes with advancing maternal age. One relatively straightforward way to get to grips with what is actually going on in this regard would be to compare incidence of trisomy 21 oocytes (and their pairing configurations) in foetal ovaries with that in oocytes at the meiosis I stage from adult women.

Chromosomal aberrations, sister-chromatid exchanges, cell-cycle kinetics and satellite associations in human lymphocyte cultures exposed to vanadium pentoxide

Treatment of human lymphocytes with vanadium pentoxide (V2O5) was not found to increase the frequency of structural chromosomal aberrations (CA) and sister-chromatid exchanges (SCE). However, V2O5 significantly increased polyploid cell frequency; also, the mitotic index (MI) was significantly decreased, and the average generation time (AGT) was significantly increased. Finally, the frequency of cells with satellite associations (SA), the frequency of SA per cell, and the frequency of chromosomes associated increased in treated cultures.

Nucleolus, nucleolar chromosomes, and nucleolus-associated chromatin from early diplotene to dictyotene in the human oocyte

The shape, relationships, relative DNA content, and nucleolar activity of the short arm of acrocentric bivalents were studied in human oocytes from early diplotene to dictyotene. At the beginning of diplotene, the short arms of the previously paired chromosomes were again separated and displayed the same morphological features as in mitotic prophase chromosomes. They were connected only with the nucleolus. In situ hybridization and silver staining showed that the nucleolar organizer regions (NORs) were located in the peripheral region of the nucleolus. Tritiated-uridine incorporation was active. At birth, the relationships of the acrocentric short arms showed increasing complexity. The chromosomes ended in nucleolus-associated chromatin blocks of irregular shape, containing large quantities of DNA as demonstrated by intense binding of 3H-actinomycin D. The number of chromosomes converging on these chromatin blocks exceeded the number of acrocentrics, suggesting that heterochromatic regions of other chromosomes were associated with the short arm of acrocentrics. In the electron microscope, the NORs were represented by fibrillar centers located on the periphery of the nucleolus and consistently connected with the blocks of dense chromatin. These relationships remained unchanged in the primordial oocyte in the adult ovary. Persistence of 3H-uridine uptake showed that the oocyte was not at a "resting" stage. The possible cytogenetic consequences of these observations are discussed.

Chemically induced aneuploidy in mammalian cells in culture

Our objectives were to assess whether there exist useful aneuploidy tests in vitro, to identify chemicals that showed potential for mitotic aneuploidy induction, and to recommend some features of suitable protocols for such testing. From over 100 papers we selected 24 for review. The acceptable studies examined hyperdiploidy at metaphase, had concurrent negative controls with low background rates of hyperdiploidy, used a fixation time sufficient for cells to complete more than one cell cycle after treatment and had multiple dose levels with at least 100 cells scored per point. We judged that 12 compounds were positive, 7 inconclusive, and 4 negative with the reservation that 2 of the 4 compounds had not been tested up to toxic doses. Many of the positive compounds are also known to cause structural chromosome aberrations. We separately reviewed qualitative reports of 'C-mitotic' effects, anaphase lagging, multipolar mitoses, or altered DNA content, since these effects may sometimes by associated with aneuploidy induction. No well-validated in vitro aneuploidy assay exists, and much research is required to develop tests, perhaps using chromosome counts, DNA content, or effects on cell organelles necessary for mitosis. In test protocol development we should carefully consider choice of cell sample size, use of in vitro metabolic activation systems, and selection of doses, especially with regard to the problem of whether cytotoxic concentrations should be used.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

Mechanisms of nondisjunction: facts and perspectives

An ordered segregation requires distinct processes of differentiation within the germ cell for recognition and segregation of homologous chromosomes/chromatids. These include synchronous maturation of the nucleus and cytoplasm, chromosome pairing and assembly at the metaphase plate, and movement within the spindle; all of them may be under direct/indirect regulatory control by the surrounding somatic compartments. Interference, e.g., by hormonal alterations at any of these steps, may alter the normal program of differentiation, thus increasing the risk of chromosomal malsegregation. Hence, many different causative mechanisms may exist which basically, nevertheless, act via (a) nonsegregation, (b) chance segregation, both during meiosis 1 and 2, or (c) presegregation during meiosis 1. In our animal models of the Djungarian hamster and the NMRI/Han mouse strain, we are analyzing the mechanisms of nondisjunction and presegregation during meiosis 1 in oocytes, as well as during aging of the females, by the application of hormones (gonadotrophins and steroids) and specific microtubular inhibitors (colchicine and methylbenzimidazolcarbamate). We suggest that chromosomes play a relatively passive role, although chromosomal properties, e.g., length, chiasma number, NORs, and position within the spindle, may provide an individual risk for each bivalent to be affected by nondisjunction. Failures in the endocrine control of follicular and germ cell maturation are considered to be primary causes for nondisjunction in young and aging oocytes. The understanding of the differentiation processes resulting in the maturation of follicles "at risk" may provide us with the tool to prevent the generation of aneuploidy in man.

The genetic basis of non-disjunction: increased incidence of hyperploidy in oocytes from F1 hybrid mice

Oocytes from parental mice strains NMRI/Han, C57/bl and Balb/c and from F1 hybrid lines were analysed for aneuploidy due to non-disjunction after gonadotropin-stimulated ovulation. No hyperploid oocytes were present in five of the strains studied. F1 hybrids from crosses of NMRI/HanxC57/bl did ovulate, however, a significantly increased number of hyperploid oocytes, although females from their parental strains show a rather low incidence of non-disjunction. The evidence for a genetic basis for non-disjunction is assessed and possible causative factors are discussed.

Association of ribosomal genes in the fibrillar center of the nucleolus: a factor influencing translocation and nondisjunction in the human meiotic oocyte

Prophase I meiosis was studied in the human oocyte obtained from 16- to 24-week-old fetuses. Electron microscopy and silver stainihg showed that, at pachytene, the ribosomal genes belonging to several chromosomes are gathered in the same nucleolar fibrillar center, where they are embedded in an argyrophilic protein. The nucleolus showed spontaneous segregation of its components due to temporary inactivation of the ribosomal genes. The fibrillar center, separated from the other nucleolar components, was penetrated as midpachytene by chromatin fibers containing rDNA emanating from one to three nucleolar bivalents. Thus, the ribosomal genes from 4-12 chromatids are temporarily juxtaposed inside the same structure. Such a structural arrangement is completely different from that observed in the pachytene-stage mouse oocyte, where two independent and active nucleoli, each displaying its own fibrillar center, were formed on the bivalents containing paired ribosomal genes. These different structural patterns are correlated with the high frequency of nondisjunction in the human oocyte and the relative infrequency of such in the mouse oocyte. The pattern observed in the human oocyte may be a cause of translocations.

Parental origin of triploidy and D and G trisomy in spontaneous abortions

A cytogenetic study of 15 triploid spontaneous abortuses, 12 trisomic abortuses, and their parents was carried out using the Q-banding technique. Polymorphic regions in abortus chromosomes were compared to corresponding regions of parental chromosomes to determine the origin of the extra chromosomes. Using marker chromosomes it was found that 7 triploids and only one trisomy were informative. Three triploids arose by failure to shed the second polar body during oogenesis. One triploid arose by dispermy. The other three triploids inherited an extra haploid set of chromosomes from the father, either by dispermy or by failure of the second meiotic division during spermatogenesis. The only informative trisomy, a trisomy 22, inherited the extra 22 from the mother. Chromosome polymorphisms are useful in determining the parental origin of extra chromosomes in certain cases.

Relationships between satellite association and the occurrence of non-disjunction in man

It has been reported that there is an increased incidence of Down's syndrome among the children of parents who have been exposed to ionizing radiations for radiodiagnostic or radiotherapeutic reasons. Work with Drosophila, mice and human lymphocytes has shown that irradiation with X- or gamma-rays induces aneuploidy, presumably by non-disjunction. It has been suggested that in man the frequency of satellite association (s.a.) of acrocentric chromosomes may be involved in the causation of chromosomal non-disjunction. In the present work the effects of radiation on s.a. have, therefore, been investigated. The frequency of s.a. between acrocentric chromosomes was determined after the exposure of human blood from normal and chromosomally abnormal individuals to various small doses of Co-60 gamma-rays. The criteria of Zang and Back were used for the evaluation of s.a. complexes. No effects of radiation on the frequency of s.a. were apparent within the dose range investigated. The same result was obtained when s.a. was evaluated using the silver-staining technique in which physical connections between the associating satellites may be observed and the association complexes evaluated directly. The effects of other radiation sources have also been investigated.

Origin of trisomies in human spontaneous abortions

Chromosome heteromorphisms of 34 trisomic abortuses and their parents were compared to determine the origin of the extra chromosome. Fourteen of the trisomies were maternal in origin, ten resulting from a first-meiotic-division error and four from either first- or second-meiotic-division errors. No paternally derived trisomy was identified.

Nucleoli of cultured human lymphocytes. II. Nuceolar fusion and its relation to acrocentric association

Nucleolar fusion and nucleolus formation occurred simultaneously, immediately after mitosis, in cultured human lymphocytes. Evidence is presented that in late telophase and post-telophase, the individual nucleolar organising site includes two components, represented in post-telophase by the nucleolus and its attached process; that fusion can be either directly between the nucleoli or via the processes; and that the latter type of fusion persists into metaphase as acrocentric associations, whilst the former is lost.

The effects of ageing on the meiotic chromosomes of male and female mice

The effects of age on the chiasma frequencies, chiasma position and numbers of univalents at MI in males and females of three strains of mouse were examined. Males showed a slight but non significant rise in chiasma frequency in age due to an increase in bivalents with two chiasmata at the expense of single chiasmata bivalents. In contrast, females exhibited a significant decrease in chiasma frequency with age due to the loss of two chiasma bivalents with a corresponding increase in single terminal chiasmata bivalents. In both males and females there was no significant increase in univalents with age in the strains studied. Of interest was the finding of a greater degree of contraction of the MI chromosomes in the oocytes of old relative to young females, a differential contraction that was independent of culture time. This finding is discussed with regard to the "production line theory" and non disjunction at Anaphase in other strains of mice.

Trisomy of chromosome 15 in spontaneous leukemia of AKR mice

Karyotypes of spontaneous thymomas of AKR mice were determined by trypsin-Giemsa banding methods. Trisomy of chromosome 15 occurred in 10 of 11 leukemic mice. Seven of the thymomas were predominantly trisomic for chromosome 15, one was trisomic for chromosome 12, and one exhibited multiple trisomies of chromosomes 3, 12, 15, and 17. Trisomy was not found in the norm-l AKR tissues examined.

Biochemical and cytogenetic studies on the nucleolus organizing regions (NOR) of man. II. A family with the 15/21 translocation

The amount of ribosomal DNA (rDNA) was determined quantitatively by RNA-DNA hybridization in the genomes of a mother and her daughter, both with the karyotype 45,XX,t(15q21q). The saturation values found were 0.030% (mother), 0.023% (daughter), and 0.022% for the husband and father of the daughter. A detailed cytogenetic analysis of the short arms of the acrocentric chromosomes of these probands allowed the biochemical results to be interpreted in terms of the size of the individual set of nucleolus organizing regions (NORs) present in each proband. The correlation existing between the biochemical and the cytogenetic findings shows that the amount of rDNA in the human genome is not primarily a function of the number of acrocentric chromosomes, but depends on the individual combination of variant NORs occurring in the human genome.

"Adaptation" and chromosome variability in excised roots of cereals

Heteroploidy, often coinciding with phenotypic or metabolic changes, was found in cereal root cultures. In some cases the changes were apparent after only 4 passages and affected the roots of bread wheat as well as those containing all or some of the wheat and rye genomes. It is unsafe to assume that cultured organised tissues are cytologically stable. The cause of the abnormality is not known but possible mechanisms are discussed.