Satellite DNA in the three C-bands of an unusual mouse marker chromosome. A model of chromosomal evolution.

A marker chromosome in the stemline of a new murine cell line is described on the basis of different stainings and in situ hybridization. The marker was characterized originally by three C-bands, one from each centromeric region of the three chromosomes constituting the marker. In the course of stemline evolution, two of the C-bands have been lost and the marker has developed into a monocentric chromosome, phenotypically and functionally normal.

Further studies on the asymmetry of chromosome 15 duplication in trisomic leukemias of heterozygous origin: preferential status of the AKR chromosome.

Four combinations of translocation heterozygotes with cytogenetically distinct chromosomes 15 were used to investigate whether the T-cell leukemia-associated preferential duplication of the AKR-derived chromosome 15 (AKR-15) is determined by factors within this chromosome, or is due to genes within the AKR genotype, but outside chromosome 15. Two of the four combinations were also used to determine whether the AKR-15 duplication preference could be cancelled by MCF-viremia in permissive F1 hybrids. Chemically and virally induced 15-trisomic leukemias showed the same AKR-15 duplication preference, which was due to some autonomous property of AKR-15 itself. It was maintained in (C57BL 6;15 X C57BL) F1 leukemias, where 6;15 is the only AKR-derived chromosome propagated on the C57BL/background. In the (C57BL 6;15 X AKR) F1 hybrid cross where both chromosomes 15 are of AKR origin, duplication occurred at random. To approach the second question, MCF viremia was induced by neonatal virus inoculation into permissive (AKR 6;15 X B6Fv-In) F1 hosts. The preferential duplication status of the AKR-derived 6;15 remained unchanged.

High resolution banding analysis of the involvement of strain BALB/c- and AKR-derived chromosomes No. 15 in plasmacytoma-specific translocations.

Plasmacytomas were induced in (BALB/c X AKR 6;15) X BALB/c backcross mice where one of the BALB/c-derived chromosomes No. 15 was replaced by the AKR(6;15)-derived Robertsonian 6;15 chromosome. (BALB/c X AKR 6;15)F2 mice that were homozygous for Rb 6;15 were mated to BALB/c mice. Plasmacytomas were induced in the progeny by intraperitoneal injection of pristane. The cytogenetic marker permitted the distinctive identification of the two chromosome 15 homologues, including the distal segment involved in the plasmacytoma-specific translocations. 7 of the 10 plasmacytomas contained the typical t(12;15) translocation. The BALB/c-derived 15 chromosome served as the donor of the translocated segment in six of them. In the seventh, the Rb 6;15 chromosome of the AKR strain was the donor. The remaining three tumors contained the same type of intrachromosomal rearrangement. It arose by the pericentric inversion of the Rb 6;15 chromosome, leading to a variant plasmacytoma-associated rcpt (6;15) translocation. Unlike the usual 6;15 variant that arises by a reciprocal exchange between two separate chromosomes, it was generated by an exchange of the distal segments of a single chromosomal element. High resolution banding analysis of the tumors showed that all translocated breakpoints on chromosomes 15, 12, and 6 were identical with the previously described breakpoints characteristic for the typical 12;15 and the variant 6;15 translocation in murine plasmacytomas. It is known that the distal segment of chromosome 15 carries the c-myc oncogene (23). The PC-associated translocations cut across the 5'-exon of c-myc in the majority of the cases (24,26). The severed oncogene is transposed to the Ig-region on the recipient chromosome. Since the BALB/c strain is highly sensitive to PC-induction, we were interested to examine the question whether its chromosome 15 is preferred as the oncogene donor in AKR X BALB/c backcross mice that carry cytogenetically distinguishable 15 chromosomes. Our results show that this is not the case, since the same segment of the AKR-derived chromosome 15 could also serve in the same capacity. This is in contrast with T cell leukemogenesis where we have previously found that the trisomization-associated duplication of chromosome 15 occurred in a highly asymmetrical fashion, depending on the donor strain of No. 15 (9-11).

Unusual SCD in cancer cells: a phenomenon related to decreased BrdU incorporation?

The simultaneous presence of chromosome segments with and without sister chromatid differentiation (SCD) in the same metaphase was observed in a human melanoma cell line after BrdU incorporation for 48-72 hr. This phenomenon was related to the time of BrdU incorporation by the cells: i.e., it was not observed after exposure to BrdU for only 17-20 hr. It is proposed, therefore, that the unusual staining pattern is caused through reduced BrdU incorporation by the cells after completing the first division.

Chromosome 15 trisomy in spontaneous and carcinogen-induced murine lymphomas of B-cell origin.

G-banding analyses of 14 independently derived B-cell lymphomas showed the frequent occurrence of chromosome 15 trisomy. It was present in seven of nine spontaneous B-cell lymphomas, but in company with other trisomies, monosomies and marker chromosomes. In five carcinogen-induced primary B-cell leukemias, trisomy 15 was the dominating change. Taken together with the previously demonstrated importance of chromosome 15 trisomy for T-cell leukemogenesis and of the 12;15 translocation in plasmacytogenesis in the mouse, it appears likely that the distal part of chromosome 15 carries a cluster of genes, perhaps a supergene region, that may play an important role in the differentiation and/or the normal responsiveness of various lymphoreticular cell types to growth control.

Chromosome studies of two transplantable leukemias of BN mice.

Karyotypes of two transplantable murine ascites leukemias, LBN/a2 and LBN/b3, were studied with the use of the trypsin-Giemsa technique. The original tumors arose in adult female mice of strains BN/a and BN/b after prolonged antilymphocyte globulin administration. The karyotypes of both leukemias showed similar patterns. Both were hyperdiploid with modal chromosome numbers of 41 and 42 in LBN/a2 and LBN/b3, respectively. The cells consisted of an average of 37 normal chromosomes and 4--5 abnormal chromosomes. The most consistent karyotype deviation was monosomy of the X-chromosome and of several autosomes: no. 9, 14, and 7 in the LBN/a2 line and no. 7, 12, 14, and 9 in the LBN/b3 line. In most LBN/b3 cells and in a lower proportion of LBN/a2 cells, trisomy of chromosome no. 15 was found. With regard to the occurrence of abnormal marker chromosomes, both tumors exhibited great cell-to-cell variation. Two of the markers were common to both leukemia lines.

Antigenic studies on mouse lymphomas bearing the T190 translocation chromosome.

Heterozygous mice of the T190/tf translocation stock, having one superlong and one short marker chromosome, involving the H-2-bearing ninth linkage group, were typed by membrane fluorescence and cytotoxicity tests against a panel of H-2 antisera. Marker-positive segregants differed serologically from their marker-negative siblings. Lymphomas were induced by the Moloney virus in (A.CA x T190/tf)F1 mice. A combined serological and cytogenetic analysis showed that the tumors could be subdivided into two main classes, corresponding to marker-positive and marker-negative types. Each lymphoma was relatively stable with regard to karyologic and correlated antigenic characteristics during serial passage in (A.CA x T190/tf)F1 mice. This system opens new possibilities to study the genetic mechanisms of H-2 antigenic variation, demonstrated previously in heterozygous tumors.