Novel cytogenetic expression of gene amplification in actinomycin D-resistant somatic cell hybrids: transfer of resistance by centric chromatin bodies

Drug-specific rearrangements of chromosome 12 in hydroxyurea-resistant mouse SEWA cells: support for chromosomal breakage model of gene amplification

In order to investigate whether specific, nonrandom chromosome rearrangements were involved in the induction of hydroxyurea (HU) resistance in mouse SEWA cells, we undertook detailed cytogenetic analyses of three independently selected lines during the long-term treatment with HU. We found that cells with trisomy 12 had selective advantage during early steps of HU treatment. Subsequently, numerous rearrangements of chromosome 12 took place in each of the HU-resistant cell lines. More specifically, the proximal end of chromosome 12 (band A3) was frequently involved in breaks and fusions generating multicentric marker chromosomes. In situ hybridization showed that the functional Rrm2 gene was located in this particular region of chromosome 12. Furthermore, amplification and rearrangements of the structural gene Rrm2 were detected both at the chromosomal and at the molecular level. As discussed, the results of the cytogenetic analyses support the chromosomal breakage model of gene amplification.

Satellite DNA sequences flank amplified DHFR domains in marker chromosomes of mouse fibrosarcoma cells

This study centers on marker chromosomes carrying expanded chromosomal regions which were observed in two independent derivatives of the AA12 murine fibrosarcoma line, the 10(-3) M MTX-res H2 and the 5 x 10(-7) M MTX-res E. Previous characterization of the marker chromosomes of MTX-res variants showed their common derivation from a marker chromosome (m) of the parental line, endowed with two interstitial C-bands. Cytogenetic evidence pointed to one C-band of m as the site involved in the chromosomal rearrangements leading to the HSR/ASR chromosomes. ISH of a 3H-labeled satellite DNA probe allowed satellite sequences flanking the HSR/ASR in the marker chromosomes, where the C-band was no longer visible, to be detected. FISH experiments using biotinylated DHFR and satellite DNA probes showed that the respective target sequences are contiguous in new marker chromosomes. They also allowed inter- and intrachromosomal rearrangements to be seen at DHFR amplicons and satellite sequences. Double-color FISH using digoxygenated satellite DNA and biotinylated pDHFR7 showed that in a marker chromosome from the H2 cell line the two target sequences are not only adjacent, but closer than 3 Mb, as indicated by overlapping of the different fluorescence signals given by the two probes. Another marker chromosome in the E variant was shown to display a mixed ladder structure consisting of a head-to-head tandem of irregularly-sized satellite DNA blocks, with two symmetrical interspersed DHFR clusters.

Genome reorganization in Nicotiana asymmetric somatic hybrids analysed by in situ hybridization

In situ hybridization was used to examine genome reorganization in asymmetric somatic hybrids between Nicotiana plumbaginifolia and Nicotiana sylvestris obtained by fusion of gamma-irradiated protoplasts from one of the parents (donor) with non-irradiated protoplasts from the other (recipient). Probing with biotinylated total genomic DNA from either the donor or the recipient species unequivocally identified genetic material from both parents in 31 regenerant plants, each originating from a different nuclear hybrid colony. This method, termed genomic in situ hybridization (GISH), allowed intergenomic translocations containing chromosome segments from both species to be recognized in four regenerants. A probe homologous to the consensus sequence of the Arabidopsis thaliana telomeric repeat (5'-TTTAGGG-3')n, identified telomeres on all chromosomes, including 'mini-chromosomes' originating from the irradiated donor genome. Genomic in situ hybridization to plant chromosomes provides a rapid and reliable means of screening for recombinant genotypes in asymmetric somatic hybrids. Used in combination with other DNA probes, it also contributes to a greater understanding of the events responsible for genomic recovery and restabilization following genetic manipulation in vitro.

Regularities of karyotypic evolution during stepwise amplification of genes determining drug resistance

Analysis of chromosomal alterations during stepwise development of mdr1, dhfr, or CAD gene amplifications in a large number of independently selected Djungarian hamster DM-15 and murine P388 sublines revealed typical patterns of karyotypic evolution, specific for multiplication of each of these genes in each cell type. Some principal similarities of karyotypic evolution were noted in at least two different systems. They include: (i) appearance at the first selection step of a new chromosomal arm bearing the resident gene copy followed at the next selection steps by the formation in these specific chromosomal arms of amplified DNA tandem arrays; (ii) translocations of amplified DNA from its initial site to other, also non-random, chromosomal sites; and (iii) emergence in the cell variants with high degrees of gene amplification of multiple extra-chromosomal elements. The most prominent distinctions among the systems were as follows: (i) different structures, evidently containing amplified DNAs, appeared at the initial steps of amplification of different genes--additional heterogeneously staining regions in specific chromosomal segments in the case of amplification of dhfr or CAD genes in DM-15 cells, and mini-chromosomes in the case of mdr1 gene amplification in both DM-15 and P380 cells; (ii) distinct patterns of location of the amplified mdr1 gene copies are characteristic of Djungarian hamster DM-15 and murine P388 cell derivatives after subsequent steps of selection--at the site of resident gene localization or in some other, also non-random, chromosomal sites in DM-15 sublines, and predominantly extra-chromosomal in P388 sublines. We propose that different mechanisms are responsible for the initial steps of amplification of dhfr and CAD genes on the one hand and the mdr1 gene on the other: non-equal sister-chromatid exchanges and autonomous replication of the extra-chromosomal elements. It seems, however, that both mechanisms may be involved in further rounds of amplification of each of these three genes.

Newly formed chromosome-like structures in independent mouse P388 sublines with developed in vivo mdr1 gene amplification

Mouse leukemia P388 sublines that acquired the resistance to multiple drugs as a result of treatment in vivo with anthracyclines (rubomycin, ruboxyl) and/or vincristine were studied. The mdr gene amplification was found in all tested cell lines: in four of five sublines all three members of the mdr gene family showed increased copy numbers, and in one cell line, developed after treatment with ruboxyl, mdr1a and mdr1b genes were amplified to the same degree, whereas the mdr2 gene was not amplified at all. The levels of amplification of mdr genes varied in different cell lines from 30-fold to 50-fold. Unusual cytological manifestations--relatively large newly formed chromosomelike structures, were revealed in four of five long-term independent sublines. Some of these structures did not contained C blocks; the others, in contrast, were enriched by C-heterochromatin. In situ hybridization showed the presence of mdr genes in newly formed bodies. In the majority of cases, the formation of chromosomelike structures was preceded by the appearance of other, smaller size, structures: the so-called "small chromatin bodies" (minichromosomes) and/or homogeneously G-positive small ring chromosomes.

Structure, mitotic and meiotic behaviour, and stability of centromere-like elements devoid of chromosome arms in the fly Megaselia scalaris (Phoridae)

Minute elements detected in Megaselia scalaris (Phoridae, Diptera) lack chromosome arms but carry centromeres and possess kinetochore microtubules in mitosis as well as in meiosis. These centromere-like elements (CLEs) were present in two geographically independent strains of the fly. This indicates that their origin is not a recent event in the karyotype evolution of M. scalaris and that they are rather stable constituents of the karyotype. Most often, two CLEs were found in gonial and somatic mitosis. Spermatocytes contained one CLE. Two individuals examined deviated from this rule in that a metaphase spermatogonium showed three and an anaphase spermatogonium eight CLEs. These animals are believed to have been aneuploid relative to the CLEs. An analysis of spermatogonial division revealed that the CLEs behave like the centromeres of the regular chromosomes but seem to separate precociously, since they were closer to the spindle poles in late anaphase cells. Whereas the size of the CLEs was not significantly different between mitotic cells and secondary spermatocytes, the CLEs in primary spermatocytes were larger in volume by a factor of about 4.5 than those in mitosis and meiosis II. The additional material is interpreted as a glue that holds two CLEs together. This, in turn, is a prerequisite for orderly segregation. The function of the CLEs is not known. They are considered as B chromosomes reduced to the minimum required for segregation, the centromere.

Centromere structure and function in neoplasia

The mammalian centromere plays an essential role in maintenance of diploidy in the cell. It is therefore imperative that we understand the structure and function of the mammalian centromere in order to plan strategy to control the incidence of aneuploidy and resultant malformations of the nonneoplastic as well as neoplastic tissues. Even though considerable information is available about the structure and some functional aspects of centromeres of lower eukaryotes such as yeast, the structure of the mammalian centromere is still a matter of conjecture limited to an understanding of the base composition of the alphoid sequences putatively located in the centromeric DNA of higher apes. We do, however, have a better understanding of the structure and role of the kinetochore. In all eukaryotes analyzed so far, the centromeres in a given genome separate in a sequential manner dependent upon the time of replication of pericentric and centromeric DNA. Some chromosomes, generally found in neoplastic cells, that carry more than one centromere show premature separation of the accessory centromeres. These centromeres and the associated pericentric regions replicate their DNA in an earlier part of the S phase than those that show kinetochore activity; both, however, carry DNA of the same composition. The active centromeres in these chromosomes show kinetochore protein binding as detected by antikinetochore antibody; the inactive centromeres are usually devoid of these proteins. The double minutes in neoplastic cells also lack kinetochore proteins, perhaps due to a lack of any centromere. Some dicentric and multicentric chromosomes in cancer cells and transformed cell lines do not display premature centromere separation. In these chromosomes, all centromeric sites show kinetochore proteins and all centromeric regions replicate their DNA simultaneously. These chromosomes also exhibited meiotic-like behavior of some centromeres and show postanaphase separation of some centromeres, resulting in bridges. These bridges, upon breakage and rejoining of sister chromatids, generate new multicentric chromosomes. The resulting chromosomes also exhibit formation of compound kinetochores. Some of these phenomena are novel descriptions of the centromere behavior in cancer cells. This review also discusses the role of aberrant centromere separation in human biology, providing correlates between errors of centromere separation and neoplasia.