The specificity of chromosome A2 involvement in DMBA-induced rat sarcomas

Cancer research in rat models

Rat has been the major model species used in several biomedical fields, notably in drug development and toxicology, including carcinogenicity testing. Rat is also a useful model in basic cancer research. Several rat models of monogenic (Mendelian) human hereditary cancers are available. Some were obtained spontaneously, while others were generated either by mutagenesis of tumor suppressor genes or by transgenesis of activated oncogenes (transgenesis can be performed efficiently in the rat). In addition, among the hundreds of inbred rat strains that have been isolated, some are highly susceptible or resistant to certain types of cancer, and these divergent phenotypes were shown to be polygenic. Numerous quantitative trait loci (QTLs) controlling cancer susceptibility/resistance have been defined in linkage analyses, and several of these QTLs were physically demonstrated in congenic strains. These studies led, in particular, to rapid translation to the human, with the identification of loci controlling susceptibility to a form of multiple endocrine neoplasia (monogenic trait) and to breast cancer (polygenic disease). The biology of cancer resistance has also been analyzed, and in some (but not all) cases, it was linked to regression of preneoplasic lesions. Rat tumors have been the subject of various types of analyses, and these studies led to important conclusions, including that tumors can be classified on the basis of the identity of the inducing agent, thereby suggesting that analyses of human tumors may be valuable in determining retrospectively the role of specific carcinogens in the formation of human cancers, and of human breast cancer in particular.

Absence of Ras mutations in rat DMBA-induced mammary tumors

Animal cancer models reduce genetic background heterogeneity and thus, may facilitate identification and analysis of specific genetic aberrations in tumor cells. Rat and human mammary glands have high similarity in physiology and show comparable hormone responsiveness. Thus, spontaneous and carcinogen (e.g., NMU and DMBA)-induced rat mammary models are valuable tools for genetic studies of breast cancer. In NMU-induced rat mammary tumors, activating mutations in Hras codon 12 have frequently been reported and are supposed to contribute to the mammary carcinogenic process. Involvement of Ras mutations in DMBA-induced tumors is less clear. In the present study we investigated the mutation status of the three Ras genes, Hras, Kras, and Nras, in DMBA-induced rat mammary tumors. We examined codons 12, 13, and 61 of all three genes for mutations in 71 tumors using direct sequencing method that in experimental conditions is sensitive enough to detect single nucleotide mutations even when present in only 25% of the test sample. No activating Ras gene mutation was found. Thus, in contrast to NMU-induced rat mammary tumor, tumorigenesis in DMBA-induced rat mammary tumors seems to be independent on activating mutations in the Ras genes. Our finding suggests that the genetic pathways selected in mammary tumor development are influenced by and perhaps dependent on the identity of the inducing agent, again emphasizing the importance of tumor etiology on the genetic changes in the tumor cells.

Genomewide assessment of genetic alterations in DMBA-induced rat sarcomas: cytogenetic, CGH, and allelotype analyses reveal recurrent DNA copy number changes in rat chromosomes 1, 2, 4, and 7

Rat sarcomas, induced by subcutaneous injections of 7,12-dimethylbenz[a]anthracene (DMBA), were studied with the objective of identifying critical chromosome regions associated with tumorigenesis. We employed three genomewide screening techniques-cytogenetics, CGH, and allelotyping-in 19 DMBA-induced sarcomas in F1 (BN/Han x LE/Mol) rats. The most conspicuous finding in the cytogenetic analysis was a high incidence of trisomy for rat chromosome 2 (RNO2). Signs of gene amplification (hsr) were also seen in several tumors. The CGH analysis revealed that gains in copy number were much more common than losses. The gains mostly affected RNO2 (10/19), RNO12q (7/19), and RNO19q (5/19), as well as the proximal part of RNO4 (8/19) and the distal part of RNO7 (7/19). Reduction in copy number was seen in RNO17 (2/19). For the allelotyping, we used 318 polymorphic microsatellite marker loci covering the entire genome. We identified regions of allelic imbalance affecting most of the rat chromosomes. The highest incidences of recurrent allelic imbalance were observed at loci in certain regions in RNO1, 2, 4, and 7 and at lower incidences in parts of RNO12, 16, 18, and 19. The combined results suggested that genetic alterations detected in RNO2 and RNO12 usually corresponded to complete or partial trisomy, whereas those in RNO1 and RNO7 seemed to involve regional deletions and/or gains. Furthermore, we could confirm that copy number gains occur proximally in RNO4, where a previous study showed amplification of the Met oncogene in a subset of these tumors.

Karyotypic evolution of cells in culture: a new concept

The Chapter summarizes peculiarities of karyotypic variability during establishment and long-term cultivation of permanent cell lines. A new concept on pathways of karyotypic evolution of cells in culture is put forward. A detailed description is presented of the author's original approach of cytogenetic analysis of cell lines provided for a principally new characteristic of the cell line: its generalized reconstructed karyotype (GRK). Its use as a criterion to evaluate authenticity, purity, and stability of cell lines is discussed. Based on analysis of the GRK, two stages of karyotype evolution of cell lines are revealed: establishment and stabilization, different in karyotypic variability of the cell population and in peculiarities of clone selection. Comparison of peculiarities of karyotypic variability of leukemic and tumor cells both in vitro and in vivo was made, and general regularities of their karyotypic evolution have been established, such as nonrandom changes in the number and structure of chromosomes and deletion of one of the sex chromosomes, as well as regularities characteristic only of cells in culture in most human and animal cell lines (at least 85%) of disomy on all autosomes. The rest of the cell lines, 15%, are characterized by either partial or total monosomies on certain autosomes during long-term cultivation. Three main compensatory mechanisms of maintaining viability of cell lines that have lost genetic material are discussed: polyploidization of the initial cell clone, amplification of oncogenes (predominantly of mys family), and extracopying of whole autosomes or of their fragments.

Specific chromosomal defects associated with ultraviolet radiation-induced cutaneous tumors in Monodelphis domestica (Marsupialia, mammalia)

Cytogenetic analysis of eight ultraviolet radiation (UVR)-induced cutaneous tumors and one spontaneously transformed fibroblast cell line of Monodelphis domestica showed that two of the tumor cell lines were of murine origin and that the remaining six marsupial tumor cell lines had hyperdiploid stemline numbers ranging from 21 to 31. Each tumor cell line showed structural and numerical abnormalities. The single transformed fibroblast cell line also showed a hyperdiploid chromosome number with structural and numerical defects. All M. domestica tumor cell lines and the fibroblast line showed a common structural abnormality: deletion of the short arm (p) of a chromosome 1. In some cell lines, the short arm of chromosome 1 was replaced by a translocation with the X chromosome. We suggest, based on the Giemsa-banding homology of chromosome 1p in M. domestica and human chromosome 6q involved in melanomas, that marsupial chromosome 1p may harbor tumor suppressor gene(s) that are associated with UVR-induced cutaneous tumors.

Karyotype instability and altered differentiation of rat sarcoma cells after retroviral infection

The karyotypic and phenotypic stability of cultured rat fibrosarcoma cells was challenged by infection with Moloney murine sarcoma virus (MoMuSV). After transformation, the spindle-like morphology of the parental HH-16 cl.2/1 cells had altered to a rounded phenotype, which was maintained in tumors produced by inoculating transformed cells into congenic animals. In contrast to the parental cells, transformed cells lacked cables of cytokeratins 14-16 and 19 and showed reduction of the mesenchymal marker protein vimentin. Additionally, the morphologically altered cell clones tf-1 to tf-3 had lost growth arrest in the presence of dexamethasone. The DNA of the transformed cells contained between four and six randomly integrated proviral copies. Karyotypic alterations were manifested by reduction of morphologically intact chromosomes in the MoMuSV-transformed cells together with increase of structural aberrations. Three additional markers were identified in the virus-transformed cell clones. Karyotypic instability induced by MoMuSV infection appeared closely related to reduction of the cellular differentiation status, although only cells of clone tf-1 had increased metastatic potential.

Gene amplification accompanied by the loss of a chromosome containing the native allele and the appearance of the amplified DNA at a new chromosomal location

The organization of amplified DNA in mammalian cells in the form of inverted repeats rather than tandem repeats was first observed and studied in the 3B rat cell line. The structure and chromosomal location of the amplified inverted duplications in this cell line have been further analyzed by cloning, long-range mapping, and fluorescence in situ hybridization. The amplification unit is at least 450 kilobases in size and all of the amplicons are located in a single chromosomal location of approximately 10 or 11 megabases. No heterogeneity in either size or molecular structure is detected between the 3B amplicons, indicating that the 20- to 40-fold amplification occurred in a single event and not through a series of events, which would result in heterogeneity among the amplicons. Thus the amplification in 3B cells may reflect more closely the situation seen in tumors containing amplified oncogenes/protooncogenes than the amplifications present in cell lines after multiple selections with cytotoxic drugs. The progenitor Rat-2 cell line contains three alleles of the region of DNA that is amplified in 3B cells; two are located on the two normal homologues of rat chromosome 2 and the third is at the equivalent position on a marker chromosome, der(3)t(2;3). 3B cells contain only one of the two normal homologues of chromosome 2 in addition to chromosome der(3)t(2;3). All of the amplified DNA is located on a new marker chromosome, M2, whose amplified DNA region does not resemble chromosome 2. These results are consistent with the amplification model proposed by Passananti et al. [Passananti, C., Davies, B., Ford, M. & Fried, M. (1987) EMBO J. 6, 1697-1703], in which the excision from a chromosome of the DNA to be amplified results in the loss of rearrangement of that chromosome. In this model the excised DNA can be amplified extrachromosomally during a single S phase before becoming stabilized by integration into a chromosome, probably at a different location to that of its unamplified allele.

Chromosomal changes in cell lines from mouse tumors induced by nickel sulfide and methylcholanthrene

Rhabdomyosarcomas were induced in mice by intramuscular injections of crystalline nickel sulfide and 3-methylcholanthrene. At early passage, karyotypes were performed by G-banding for four nickel sulfide cell lines and for three 3-methylcholanthrene cell lines. Six cell lines were near-diploid and one nickel sulfide line was near-tetraploid. Three of the nickel sulfide cell lines were characterized by a rearranged marker chromosome which was present in a majority of the cells of each line. The rearrangements leading to the formation of marker chromosomes were different in each nickel sulfide cell line but involved chromosome 4 in two of the nickel sulfide cell lines. Extra copies of chromosome 15 were present in two nickel sulfide cell lines. Possible rearrangement and/or gene activation was examined for the c-mos oncogene on chromosome 4 and the c-myc oncogene on chromosome 15, but no alteration or activation was observed. None of the 3-methylcholanthrene cell lines contained rearranged marker chromosomes; however, one MCA cell line did contain large numbers of double minutes. In all cell lines, minichromosomes (small atypical acrocentric chromosomes) were observed that contained distinct centromeric regions but no other G-positive bands.

Nonrandom involvement of chromosome 4 in the progression of rat prostatic cancer

Owing to progression of the original spontaneous Dunning R-3327 rat prostatic cancer, a large series of transplantable prostatic tumors have been isolated that differ widely in their histological degree of differentiation, growth rate, androgen sensitivity, and metastatic ability. Using these parameters as criteria, the full spectrum of disease progression is represented within this Dunning system of rat prostatic cancers, ranging from slow-growing, well-differentiated, androgen-sensitive, nonmetastatic forms to fast-growing, anaplastic, androgen-independent, highly metastatic forms. Cytogenetic analysis of the two least progressionally advanced Dunning cancers (i.e., histologically well-differentiated, slow-growing, nonmetastatic variants) demonstrated no structural or numerical chromosomal aberration, suggesting that the initial development of prostatic cancer may not require detectable cytogenetic changes. In contrast, all 16 of the progressionally more advanced Dunning variants analyzed had a series of characteristic structural and/or numerical chromosomal aberrations that minimally involved chromosome 4. This nonrandom involvement of chromosome 4 was consistently observed regardless of whether the karyotype of the cancer was near-diploid or hyperaneuploid, suggesting that chromosome 4 aberrations are specifically involved in the progression of rat prostatic cancer. In addition, all four variants that were highly metastatic had, besides aberration of chromosome 4, structural aberrations involving chromosomes 1, 2, and 11. Of the 14 variants that did not have a high metastatic ability, only two had a similar aberrations involving chromosomes 1, 2, 4, and 11, suggesting that these specific chromosomal aberrations may be necessary, albeit not sufficient, for a high metastatic ability of rat prostatic cancers.

Chromosomal alterations in cell lines derived from mouse rhabdomyosarcomas induced by crystalline nickel sulfide

Prior studies have shown a preferential decondensation (or fragmentation) of the heterochromatic long arm of the X chromosome of Chinese hamster ovary cells when treated with carcinogenic crystalline NiS particles (crNiS). In this report, we show that the heterochromatic regions of mouse chromosomes are also more frequently involved in aberrations than euchromatic regions, although the heterochromatin in mouse cells is restricted to centromeric regions. We also present the karyotypic analyses of four cell lines derived from tumors induced by leg muscle injections of crystalline nickel sulfide which have been analyzed to determine whether heterochromatic chromosomal regions are preferentially altered in the transformed genotypes. Common to all cell lines was the presence of minichromosomes, which are acrocentric chromosomes smaller than chromosome 19, normally the smallest chromosome of the mouse karyotype. The minichromosomes were present in a majority of cells of each line although the morphology of this extra chromosome varied significantly among the cell lines. C-banding revealed the presence of centromeric DNA and thus these minichromosomes may be the result of chromosome breaks at or near the centromere. In three of the four lines a marker chromosome could be identified as a rearrangement between two chromosomes. In the fourth cell line a rearranged chromosome was present in only 15% of the cells and was not studied in detail. One of the three major marker chromosomes resulted from a centromeric fusion of chromosome 4 while another appeared to be an interchange involving the centromere of chromosome 2 and possibly the telomeric region of chromosome 17.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple common fragile sites are expressed in the genome of the laboratory rat

Splenic lymphocytes from Sprague Dawley and Fischer 344 rats were exposed to two chemicals known to induce common fragile site expression in man: fluorodeoxyuridine (in conjunction with the enhancing effects of caffeine) and aphidicolin. Of 39 sites that were significantly damaged in excess, 12 meet the criteria for fragility proposed in this investigation. Rat fragile sites appear to differ from those in man in that no common hierarchical frequency of expression is evident from the two methods of induction. In addition, a comparison of published cancer-specific chromosome breakpoints from a variety of rat tumors reveals little or no apparent concordance with the identified fragile sites. The rat is an animal model in which multiple common fragile sites can be induced and, as such, will be valuable for testing hypotheses concerning the biological basis of chromosomal fragility.

Molecular and cytogenetic studies on nucleolar cistrons (rDNA) in mouse leukemia cells

The gene dosage change of nucleolar cistrons (rDNA) in tumor cells has not been extensively studied. The present studies showed that increased dosage, as well as abnormal distribution of rDNA, was frequently associated with leukemia cells of SL/Ni and AKR mice. In normal SL cells, 37%, 39%, and 25% of rDNA was located in nucleolar organizer regions (NOR) of chromosomes #12, #18, and #19, respectively. Increase of rDNA/DNA was shown by hybridization on filter membranes in SL1, SL2, SL3, and M1 leukemia cells. Direct measurement of rDNA/DNA in G1 cells revealed an 11% increase in synchronized M1 cells. The increased rDNA dosage was explained by trisomy 12 in SL1 and SL2, the ectopic NOR of #9 in SL3, and the double t(X;19) marker chromosomes in M1. On the other hand, in normal AKR cells, 27%, 29%, and 45% of rDNA was assigned to NORs of chromosomes #15, #16, and #18, respectively. The relative rDNA distribution among NORs estimated by autoradiographic grain counting was suggested to be abnormal in AKR leukemia cells despite their normal karyotype; 36% rDNA was shown to be in chromosomes #15 and #16, respectively, by relative reduction in chromosome #18 in AKR1; the trisomy 15 explained the increased rDNA in AKR2; a relative increase was found in chromosome #15 in AKR3. These results were discussed with reference to the reported NOR involvement in chromosome translocation and amplification in tumor cells.

Phenotypic and karyotypic transitions in the spontaneous transformation of a rat cell line

After 20-50 transfers, a rat myofibroblast line, Hmf-n, 'spontaneously' transforms to an established (immortalized) line of smaller, rapidly cycling fibroblastoid cells (tHmf-f). From these 1 degree transformants, colonies of larger, slower growing anchorage-independent (tHmf-e) cells of epithelioid phenotype emerge. Both transformants grow in low serum and low calcium media, but the tHmf-f cells are highly tumorigenic in nude mice, have diminished substrate adhesivity, and limited anchorage independence, whereas tHmf-e are less tumorigenic, firmly substrate adherent, and markedly anchorage independent. Most tHmf-f are trisomic; most tHmf-e transformants are hypodiploid, a third are tetraploid, and all have chromosomal abnormalities, but no trisomy. Hmf-n cells have polar stress fiber arrays terminating in vinculin adhesion plaques, colinear extracellular fibronectin matrices, and linear non-coincident deposits of fodrin. Microtubules (mt) and vimentin-intermediate filaments (IF) parallel the actin cables. Stress fibers of the tHmf-f are moderately reduced, their vinculin adhesion plaques and fibronectin matrices intact; fodrin is diffuse. Mts and IFs are normal and axial. Most epithelioid tHmf-e have no stress fibers, adhesion plaques, or extracellular fibronectin; instead, dense actin microfilament meshworks are attached to plasma membrane, as is fodrin. Mt and IF are radial. Both transformed phenotypes are stable over greater than 300 continuous passages. The differentiation-inducing agents DMSO, cyclic AMP, 5-azacytidine, and mezerein, were ineffective in normalizing shape or cytoskeleton of transformed Hmf, and butyrate was selectively toxic to 50% of tHmf-e. But hydrocortisone induced striking polarization, and increase in number, and alignment of stress fibers of both tHmf-f and tHmf-e. Growth, anchorage, cytoskeletal arrangements, and tumorigenic potential are not closely correlated in these stable, spontaneously transformed lines of distinct pheno- and karyotype originating from the same normal parental cell, suggesting independent acquisition of properties associated with transformation.

Cellular DNA regions involved in the induction of rat thymic lymphomas (Mlvi-1, Mlvi-2, Mlvi-3, and c-myc) represent independent loci as determined by their chromosomal map location in the rat

The induction of thymic lymphomas by Moloney murine leukemia virus in the rat is linked to provirus integration in at least four independent cellular DNA regions (Mlvi-1, Mlvi-2, Mlvi-3, and c-myc). Because sequences homologous to at least three of these regions (Mlvi-1, Mlvi-2, and c-myc) map to chromosome 15 in the mouse, the question was raised whether they are closely linked in the rat genome and whether provirus integration in any one of these regions affects the same functional domain in rat DNA. In this study, we identified the chromosomal map location of Mlvi-1, Mlvi-2, and Mlvi-3 in the rat by using mouse-rat somatic cell hybrids that lose the rat chromosomes. The results showed that Mlvi-1 maps similarly to c-myc to chromosome 7, and Mlvi-2 maps to chromosome 2. Mlvi-3 probably maps to chromosome 15. We conclude that Mlvi-1, Mlvi-2, and Mlvi-3 are separate and independent genetic loci. Although Mlvi-1 and c-myc map to the same chromosome, they are not related, as determined by hybridization and restriction endonuclease mapping. The chromosomal map location of Mlvi-1 to chromosome 7 and Mlvi-2 to chromosome 2 is interesting, since chromosomal aberrations involving these two chromosomes are reproducibly observed in rat neoplasias induced by a variety of agents.

Restricted number of chromosomal regions implicated in aetiology of human cancer and leukaemia

It has been known since the days of Boveri that neoplasia is associated with chromosomal aberration. The introduction, some 10 years ago, of chromosome banding techniques provided the impetus for the description of an immense number of such aberrations, and for the localization to individual chromosome bands of the breaks underlying the aberrations. Hypothetically, the breaks should comprise two essentially different kinds: primary breaks that are actively involved in the malignant development, and secondary breaks, coincidental to this process. In the search for a possible method to identify primary breaks in human cancer, I selected from the catalogue of chromosome aberrations now available those cases that had one single structural aberration as their sole deviation from normality. I report here that the breakpoints thus specified affect a surprisingly limited number of chromosomal regions, and conclude that these regions contain genes of prime importance to cancer development.

Acid phosphatase-producing androgen-independent subline of rat prostatic adenocarcinoma (Dunning R3327 tumor) in cell culture

Establishment of a cell line derived from the androgen-independent subline of rat prostatic adenocarcinoma (Dunning R3327 tumor) is reported. Cells of this line produced acid phosphatase. When the cultured cells were transplanted to Copenhagen rats, solid tumors were formed. Histologically, the tumor consisted of spindle-shaped, large and bizarre polygonal cells; this feature was almost identical to that of the original tumor. Chromosomes were in the triploid range with seven frequently appearing marker chromosomes.

Karyotype evolution in a transformed rat cerebral endothelial cell line

Primary cultures of rat microvascular endothelial cells were transformed, in vitro, by exposure to Rous sarcoma virus. Transformed cells were followed and evaluated cytogenetically through numerous passages. Highly specific karyotypic changes in karyotype (both structural and numerical) were documented. These changes became established and intimately involved in further "karyotypic evolution". The findings were reproducible, and when considered in the light of the literature suggest regular patterns of karyotypic change in rat tumors. The in vitro methodology utilized promises to be of practical value in the study of the early stages of malignancy.

Chromosome changes in rat embryo cell lines transformed by temperature-sensitive mutants and sheared DNA of herpes simplex virus

The chromosomes of six rat embryo cell lines transformed with herpes simplex virus (HSV) temperature-sensitive (ts) mutants were examined at different passages of in vitro cultivation. Two cell lines were predominantly diploid, one cell line was hyperdiploid, one cell line was pseudodiploid, and two cell lines were hypotetraploid. In near-diploid cell lines chromosome No. 9 was most frequently involved in chromosome changes. All three cell lines derived from tumors obtained after one transplantation of HSV-transformed cells into baby rats were pseudodiploid, but each had different marker chromosomes. Chromosome No. 15 was involved in the formation of two out of four marker chromosomes. Four cell lines derived from tumors developing after two and three transplantations were hypodiploid and showed large chromosome variation. The occurrence of 25 marker chromosomes in three tumor-derived cell lines resulted in gains in parts from chromosomes No. 2, 6, and 7. One marker chromosome had a homogeneously faintly stained region. Chromosomes No. 2, 3, 7, and 12 were more frequently involved in the formation of marker chromosomes. No chromosome change was found to be specifically associated with HSV-induced transformation of rat cells, but chromosome changes in tumor-derived cell lines may provide selective advantage for survival and autonomous growth in the host animal.

Specific chromosome changes associated with viral transformation of rat glial cells

Karyotypes of three malignant cell lines derived from Wistar and WKA/Mk fetal rat glioblasts, transformed by murine sarcoma virus (MSV-M-os) as well as those of four cell lines derived from C6 glioma cells of Wistar origin, retransformed by MSV-M-os, were analyzed in early culture passages. The C6 line had a modal number of 42 chromosomes with a normal male karyotype, and only a minor population of cells with 43 chromosomes. The modal chromosome number in every transformed glial cell line shifted from 42 to 43. The G-banding pattern revealed consistent chromosome abnormalities. Structural chromosome changes occurred in one chromosome No. 2 (2q-) and in one No. 4 (4q+). The cells with a 43 chromosome karyotype showed trisomy of chromosome No. 12 and its heteromorphism, a finding also confirmed by silver staining. Identical chromosome changes were found in transformed C6 cell lines. A further interesting feature was that all malignant cells had different distribution patterns of silver-stained nucleolar organizer regions (Ag-NORs) among particular chromosomes (Nos. 3, 11 and 12) from normal cells, showing an increased frequency of chromosome No. 12 with Ag-NORs. These results suggested that the gain and/or loss of specific segments involved in chromosomes Nos. 2, 4 and 12 contain(s) genes favorable to malignant transformation in rat glial cells.

Changes in surface morphology associated with ageing and carcinogen treatment of Chinese hamster lung cells

The relationship between ageing and transformation has been investigated by a serial study of the changes in cell-surface morphology as normal and carcinogen-treated cells progressed in culture. A progressive increase in the density of cell surface microvilli occurred in association with the adoption of a more rounded profile and concomitant increase in the rate of cell detachment. These changes occurred earlier after carcinogen treatment, which appeared to indicate a carcinogen-induced acceleration of ageing. The alterations have also been described as characteristic of the transformed state. The observations suggest that the expression of in vitro transformation may be the result of continuous selection from a population with genetic instability and variable morphology.

Purine base composition analysis of normal and tumor rat DNA

Chemical and viral induced rat tumors were analyzed for their purine base composition and compared to normal tissue DNA'S. The tumors were induced by 7,12-dimethylbenz[a]anthracene (DMBA), 20-methylcholanthrene (MC), 3,4-benzopyrene (BP), 1,2-dimethylhydrazine (DMH) and Rous sarcoma virus (RSV). Normal DNAs were extracted from colon, caecum, liver, spleen and embryo and used as reference standards for base composition of normal rat DNA. The composition of purines was obtained by spectrophotometric estimation of the total adenine and guanine (A/G) contents after depurination of the DNA with 66% formic acid at 30 degrees C for 18 h and passage over a cationic exchange resin. Statistical comparison of the A/G molar ratios in normal rat DNAs (1.271) to those of chemical-induced primary tumors (1.342) has shown a highly significant increase. No significant differences could be detected when the base composition of the normals were compared to transplanted tumors, whether chemically or virally induced. Possible explanations from a mutational point of view are discussed.

Analysis of karyotype variation following carcinogen treatment of Chinese hamster primary cell lines

Chinese hamster primary fibroblasts derived from several embryos were treated with the carcinogens benzo(a)pyrene, 7,12-dimethylbenz(a)anthracene or N-methyl-N'-nitro-N-nitrosoguanidine. Karyotype analysis, sister chromatid exchange frequency, evidence of transformation by growth in agar, cell morphology and reaction to cytocholasin B were tested at regular intervals over many culture passages. Carcinogen treatment was found to shorten the time period before onset of permanent karyotypically changed stem and side lines and in vitro transformation. Chromosomes X, 6 and 10 were more frequently involved in all cultures in these karyotype changes which were usually preceded by a period of chromosome variation. Spontaneous chromatid aberrations and aneuploidy increased in frequency with time in culture and generally appeared prior to the expression of transformation. No specific chromosomes were involved with the different carcinogens. There was no correlation between in vitro transformation and karyotype evolution and the criteria for transformation were present independently of one another. It is suggested that the lack of correlation between the parameters tested indicates that the expression of in vitro transformation is a result of selection for growth advantage from a cell population expressing an increasing degree of genetic instability and variation with time in culture.

Vulnerability of specific rat chromosomes to in vitro chemically induced damage

The treatment of rat embryo secondary cultures with DMBA or DMBA-3H for 5, 9, or 24 h resulted in chromosome damage consisting mainly of chromatid type aberrations. There was an increase in the percentage of labelled nuclei and metaphases with increasing length of exposure. In terms of incidence of chromatid lesions, the largest telocentric chromosome (No. 2) was the most susceptible of the autosomes. Banding pattern analysis demonstrated that the region associated with negative band 2q24 of the No. 2 chromosome had the highest number of lesions. An increased accumulation of DMBA-3H label occurred in approximately the same chromatid area of a small fraction of cells exposed for either 5 or 9 h prior to mitosis. The complete loss of DMBA-3H chromosomal labelling after DNAse treatment suggests that the visible grains represent carcinogen-bound DNA. After DMBA and BrdUrd, there was an increase in the number of sister chromatid exchanges compared to controls treated with BrdUrd only; the location of the exchange points on chromosome No. 2 was similar in samples treated with either DMBA and BrdUrd or BrdUrd alone. Additional experiments with thymidine-3H showed that the non-random chromatid lesions on chromosome No. 2 may result from endogenous radiation from the incorporated tritium. These studies demonstrate that a specific chromosome may be affected by diverse agents and that chromatid lesions frequently occur at the site of sister chromatid exchanges.

Non-random chromosome changes in acute myeloid leukemia. Chromosome banding examination of 30 cases at diagnosis

Bone-marrow chromosomes were examined with the G-banding technique in 30 patients with acute myeloid leukemia at the time of diagnosis. In 13 of the 30 patients (43%) only normal diploid bone-marrow cells were found, and no deviations from the normal banding pattern could be detected in these cells. In bone-marrow cells of 17 patients (57%), distinct chromosome abnormalities were found; in 10 of the patients only abnormal cells were observed, whereas in 7 of the patients the abnormal cells coexisted with normal diploid cells without any visible chromosome banding abnormality. The results of the detailed analysis of the karyotypic aberrations demonstrated that when chromosome aberrations occurred they were clearly non-random. All patients except two displayed trisomy 8,9 or 21 or monosomy 7. Analysis of cases of acute leukemia from other laboratories indicated that the same consistent pattern of chromosome involvement prevailed in them.