Studies of multipolar mitoses in euploid tissue cultures. II. Somatic segregation of the sex chromosomes

Occurrence and possible consequences of multipolar mitoses in primary cultures of adult rat hepatocytes

Proliferating primary cultures of adult rat hepatocytes are characterized by the occurrence of multipolar mitoses, and chromosome loss resulting in the formation of micronuclei at telophase. The percentage of multipolar mitotic figures was determined to be 12.76 +/- 7.9%, 80% of which were tripolar. Multipolar mitotic stages showed a high incidence of chromosome loss, increasing from meta- (61.7 +/- 16.6%) to telophase (72.1 +/- 19.3%). Regular bipolar mitotic figures on the other hand also showed chromosome loss, however, to a lesser degree and decreasing from meta- (49.5 +/- 10.4%) to telophase (34.9 +/- 7.9%). The incidence of chromosome loss even in regular mitotic figures is very high compared to other cells and appears to depend on another special feature of hepatocytes: they remain flat and well attached during mitosis, so that shearing forces could be responsible for the separation of chromosomes from the mitotic spindle. Additionally this morphology creates a situation allowing for a maximal interaction of mitotic spindles of binucleated cells, leading to the high rate of multipolar mitoses observed. Both multipolar mitoses and chromosome loss could also explain the consecutive detachment of hepatocytes reported for proliferating primary cultures, since the aneuploid daughter cells generated can be expected to be non-viable in most cases and eventually detach.

Somatic cell fusion as a source of genetic rearrangement leading to metastatic variants

Tumor cell populations displaying metastatic properties often have higher gene dosage than their less malignant progenitor tumors, as shown by increased ploidy levels, chromosome duplication and gene amplification. The acquisition by tumor cells of high chromosome numbers may be due to endoreduplication or somatic hybridization either between tumor cells or between tumor and host cells. All such mechanisms increase genetic variability and instability in tumor cells since they trigger a polyploidization-segregation cycle. Among the wide variety of segregants which may emerge from high-ploidy cells, variants with increased malignancy can be positively selected in vivo. Evidence for in vivo fusion of tumor and normal host cells has been reported in different tumor systems. However the attainment by tumor-host hybrids of a higher degree of malignancy has only been observed following substantial chromosome segregation. The involvement of a cell of bone marrow origin as preferential host partner in the fusion process has been proved both by studies on tumor-host hybrids in bone marrow radiation chimeras and in vitro hybridization experiments between non-metastatic tumors and normal lymphoreticular cells which have led to the establishment of metastatic variants. Several different segregational mechanisms may bring about homozygosity or hemizygosity of recessive alleles in tumor-host hybrids, leading to their expression. The marked chromosome dynamics of tumor-host hybrids are also responsible for extensive chromosome rearrangements. At the molecular level these may represent mechanisms causing altered oncogene activity. The activation of new oncogenes by transposition or amplification as well as the amplification of previously activated oncogenes are the mechanisms most likely to be responsible for transition from low to high malignancy, occurring through ploidy changes, such as those produced by somatic mating.

Euploid somatic recombinants with two active X or XY (1)Y(I) chromosomes isolated from cultured male Indian Muntjac cells after HVJ virus fusion, and their use for gene assignment

Four diploid somatic recombinants were isolated from hybrids either between or within two diploid cell lines of a male Indian muntjac after HVJ virus-mediated cell fusion. Both parental lines had a normal male karyotype, 7,X,Y1,Y2, in which the largest autosomal pair was heteromorphic with respect to the size of the secondary constriction (1h+/1h-), C bands, and nucleolar organizers. Of the four recombinants, three showed a 6,XX,1h+/1h+ or 1h+/1h- karyotype, the remaining one a 7,XY1Y2,1h+/1h+. No late-replicating X chromosome was found in the XX recombinants, although it was demonstrated in the natural XX line, suggesting the presence of two active X chromosomes in the former. The G6PD, PGK, and HPRT activities were proportional to the number of active X chromosomes present in all cell types examined, indicating their X-linkage, whereas the same level of activities obtained for LDH and 6PGD indicated their autosomal linkage.

Late DNA replicaion of X chromosomes in female and pseudofemale cells of Microtus agrestis

The late replication pattern of the short arms of the X chromosomes of Microtus agrestis was studied in female cells and in cells with 2 X chromosomes of male origin by means of the BUdR-Giemsa technique and of 3H-thymidine labelling. The light absorption of Giemsa stained chromosome sections which were unifilarly substituted with BUdR (labelled), was found to be 59.2% of that of unlabelled chromosomes. In female cells, asynchrony of DNA replication of both X chromosomes indicated the presence of facultative heterochromatin in the X2 and euchromatin in the X1. In the male cells only euchromatic X chromosomes were observed in diploid XX and XO cells as well as in triploid XXY, XX and XO cells. The results show that inactivation of an X chromosone in vitro, in cells with more than one originally active X chromosome does not occur even after a culture duration of several years.