Specific Chromosomal Alterations Research Articles

Assignment of RAS proto-oncogenes in Chinese hamsters: implications for mammalian gene linkage conservation and neoplasia.

HRAS and KRAS are the cellular homologs of the oncogenic transforming genes found in the Harvey strain of murine sarcoma virus and the Kirsten murine sarcoma virus, respectively. Phyla as diverse as insects, birds, and mammals possess distinct HRAS and KRAS sequences, suggesting that these genes are essential to metazoa. In this report, we used a clone panel of Chinese hamster X mouse C11D somatic cell hybrids segregating hamster chromosomes to map those genes. Southern filter hybridization analyses of the hybrids revealed that hamster HRAS and KRAS gene sequences are on chromosomes 3 and 8, respectively. These gene assignments are consistent with the conservation of autosomal gene linkage groups observed among hamsters, humans, and mice and may provide insight into specific chromosomal alterations that have been observed during the spontaneous neoplastic transformation of Chinese hamster fibroblasts in vitro.

Molecular Genetics of Human B Cell Neoplasia

Recent developments in the identification and characterization of viral genes capable of inducing tumors in experimental animals have been clearly providing major advances in the understanding of the molecular basis of the neoplastic process and of the cellular genes that may be involved in human cancer. The demonstration that chromosomal segments involved in the specific translocations contain the human homologues of viral oncogenes capable of inducing tumors in experimental animals has opened new avenues in the understanding of the role of specific chromosomal alterations in the development of human neoplastic diseases. The studies concerning the involvement of the c-myc oncogene in Burkitt lymphoma have indicated that a cellular proto-oncogene, c-myc, can be activated through chromosomal translocations. This activation results from close proximity to genetic elements within the three immunoglobulin loci capable of activating gene transcription in cis over considerable chromosomal distances in B cells. Specific chromosome rearrangements observed in malignant cells provide an opportunity to identify “oncogenes” that are involved in the malignant process, which would be undetectable otherwise.

Chromosomal markers in study of colon cancer.

Examination of a relatively small subset of information pertaining to colon tumors indicates that examples in each category (including predisposing markers of chromosomal instability, causally relevant markers, correlation between karyotypic atypia and biological malignancy, and some tumors with no detectable chromosome changes) can all be found. This discussion began with questions about the purposes that markers could serve: some of those questions with reference to colon tumors can be answered. For the most part, the presence of chromosomal aberrations can indeed distinguish between the normal and the tumor cell and, in this sense, help to define the tumor. Moreover, some specific chromosomal alterations appear to be preferentially associated with colon tumors. If the specificity can be better defined, then, like the other specific tumor-chromosome associations, those in colonic mucosa may help to identify and localize gene functions important in the growth, development, or regulation of this specialized tissue. In any event, chromosomal aberrations are of value, not only in confirming the diagnosis, but also by their associations with different behavioral patterns in both malignant and premalignant lesions. Their definition, therefore, may have predictive value for the individual patient. In particular, inability to find such alterations, while it is not evidence of a nontumorous state, in a known malignancy may at least contribute some information about the biology of that particular tumor.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: VII genic mechanisms

The expression of embryonic phenotypic entities induced by a variety of carcinogenic agents can be unified by proposing a process of deheterochromatization. This central process can occur from purely genetic recombinations through the phenomenon of “position effect” of certain new gene proximities near facultative heterochromatin. The same effect may occur through genetic recombinations of Mendelian traits exemplified by the induction of melanomas in platy-swordtail hybrids or by specific chromosomal alterations through mechanisms of translocations, etc. as seen in human leukemias. It is further proposed that some of these chromosomal alterations involve the “turning on” of sarc-like genes i.e. protein phosphokinases which in turn cause alterations in the chromatin construct of further heterochromatic areas thus leading to a cascade phenomenon as discussed previously. (1). Viral processes also would be categorized in this latter mode since they would introduce aberrant protein phosphokinase genes into the cell. An entirely different origin of dehetero-chromatization would be achieved by such agents as ethionine. Ethionine is known to react with adenosine triphosphate to produce S-adenosyl-ethionine. This compound interferes with normal methylation reactions and produces hypomethylated transfer ribonucleic acid and deoxyribonucleic acid. Hypomethylated deoxyribonucleic acid has been correlated with the activation of globin genes. The mechanism may include the same deheterochromatization process although the choice between hypomethylated proteins which may also exist in these states or hypomethylated deoxyribonucleic acid as being basic to this mechanism, cannot be made. This hypothesis will explain how heredity can be a factor similar to other carcinogenic events. Furthermore it may explain processes such as the synergism between viruses and carcinogens. This unified theory would predict that the position effect may cause carcinogenic events only if the protein phosphokinase gene is involved. It would also require that protein phosphokinase genes are redundant and occur near or at the specific chromosomal anomalies described thus far. Minor isologous transfer ribonucleic acids are probably the result of modifications of minor nucleotides and, at least for the methylated subset of these, amy involve embryonic genes (2). This process appears to present transcriptional control mechanisms with an anomaly such that the overall phenotype of the embryo becomes expressed which includes further (embryonic) features of control. Other chemicals may modify heterochromatin by intercalation causing a “pseudo-position effect” or chemicals such as the drug phenobarbital, a tumor promotor, may induce certain enzymic changes, such as protein methylation or hydroxylations that result in the same chromatin alterations alluded to previously. Still other chemicals, the mutagenic and alkylating agents, could cause alterations similar to those induced by anomalous enzymic bioalkylations. Steroids as carcinogens have already been described in terms of “groove distorters” (3). Such a process fits well with a process of deheterochromatization. Deoxyribonucleic acid alterations such as those that would occur during groove distortion should result in hierarchical structural chromatin changes. However, the theory does not predict several features of carcinogenesis such as latency and certain requirements of chronic administration of carcinogenic agents.

Genetic analysis of tumorigenesis: VI. Chromosome rearrangements in tumors derived from diploid premalignant Chinese hamster cells in nude mice

The chromosome constitution of CHEF/16 clones recovered from methylcellulose and of uncloned, tumor-derived CHEF/16 populations are compared. Nine of 11 clones recovered from methycellulose were initiated by diploid cells. Moreover, chromosomally diploid cells were still present in most CHEF/16 clones even after growth in anchorage-independent conditions. In contrast, none of the CHEF/16 cells recovered from tumors were diploid. Nonrandom chromosome changes were observed, but no specific chromosome alterations were consistently found in tumor-derived CHEF cells. Although CHEF/16 cells are uniformly tumorigenic in nude mice, each of 10 uncloned tumor-derived populations from inocula of 10(2), 10(4), and 10(6) CHEF/16 cells consisted of only 1-3 stemlines. Our results show that diploid CHEF/16 cells are premalignant and undergo karyotypic changes leading to successful and usually clonal establishment of tumors in nude mice.

Biochemical and karyological properties of cells resistant to the quinazoline antifolate, methasquin

Eleven methasquin-resistant sublines were selected in vitro from a Chinese hamster cell line and maintained at drug concentrations of 0.01–50.0 μg/ml. Dose-response data showed that increases in resistance ranged from 6- to 6500-fold and that all sublines were cross-resistant to methotrexate. Dihydrofolate reductase activity was elevated 3- to 165-fold in the 11 methasquin-resistant lines. Relationships between drug response and enzyme activity suggested that for 6 sublines increased dihydrofolate reductase may be the primary determinant of response, whereas additional factors such as quantitative change in other tetrahydrofolate enzymes may influence resistance to antifolate in the other 5. Karyotype analysis of conventionally stained metaphase cells revealed the presence of nonuniformly altered chromosomes. Strikingly long chromosome segments were found on 1 homologue of chromosome 2 or 4 as a consistent feature of most, but not all, of the 11 methasquin-resistant sublines. The similarity of these findings to those previously reported for methotrexate sublines and more recent results obtained by trypsin-Giemsa banding analysis provide evidence of specific chromosomal alteration associated with increased intracellular dihydrofolate reductase.

Absence of specific chromosome alterations in bovine lung fibroblasts exposed to Rous sarcoma virus.

AbstractBovine lung fibroblasts from individual embryos were exposed to three different strains of Rous sarcoma virus (Bryan, Engelbreth‐Holm, Schmidt‐Ruppin) in vitro. Chromosomal analysis early as well as late after infection, including cultures with advanced morphologic cell transformation, failed to disclose any aberrations exceeding the findings in control cultures grown in parallel under identical conditions. Simian virus 40, on the other hand, caused pronounced changes with widely scattered heteroploid counts and various rearrangements.

The production of specific chromosome alterations with deoxyribonuclease

Specific alterations in the structure of some banded regions of the giant salivary gland chromosomes of Drosophila melanogaster are produced with purified preparations of DNase. These alterations can be seen by phase microscopy when the same fixed preparation is observed before and after DNase treatment. The nature of these structural changes would suggest that DNA has more than one structural role in the giant chromosomes of Drosophila.