scholarly journals Accretion of Unstable Heterochromatin as the Origin of Double Minute Chromosomes: Evidence from Bloom Syndrome.

CYTOLOGIA ◽  
2003 ◽  
Vol 68 (1) ◽  
pp. 75-82
Author(s):  
Jack R. Edelman ◽  
Yue J. Lin
Keyword(s):  
Bloom Syndrome ◽  
Double Minute ◽  
Double Minute Chromosomes

Induction of metallothionein-I mRNA in cultured cells by heavy metals and iodoacetate: evidence for gratuitous inducers

1984 ◽  
Vol 4 (3) ◽  
pp. 484-491
Author(s):  
D M Durnam ◽  
R D Palmiter
Keyword(s):  
Heavy Metals ◽  
Cultured Cells ◽  
Mrna Accumulation ◽  
Mouse Hepatocyte ◽  
Hepatocyte Cell Line ◽  
Regulatory Molecule ◽  
Heavy Metal Detoxification ◽  
Double Minute ◽  
Double Minute Chromosomes

A mouse hepatocyte cell line selected for growth in 80 microM CdSO4 (Cdr80 cells) was used to test the role of metallothioneins in heavy metal detoxification. The cadmium-resistant (Cdr80) cells have double minute chromosomes carrying amplified copies of the metallothionein-I gene and accumulate ca. 20-fold more metallothionein-I mRNA than unselected cadmium-sensitive (Cds) cells after optimal Cd stimulation. As a consequence, the amount of Cd which inhibits DNA synthesis by 50% is ca. 7.5-fold higher in Cdr80 cells than in Cds cells. Cds and Cdr80 cells were compared in terms of their resistance to other heavy metals. The results indicate that although Zn, Cu, Hg, Ag, Co, Ni, and Bi induce metallothionein-I mRNA accumulation in both Cdr80 and Cds cells, the Cdr80 cells show increased resistance to only a subset of these metals (Zn, Cu, Hg, and Bi). This suggests that not all metals which induce metallothionein mRNA are detoxified by metallothionein and argues against autoregulation of metallothionein genes. Metallothionein-I mRNA is also induced by iodoacetate, suggesting that the regulatory molecule has sensitive sulfhydryl groups.

Mathematical models of gene amplification with applications to cellular drug resistance and tumorigenicity.

Genetics ◽  
1990 ◽  
Vol 125 (3) ◽  
pp. 633-644
Author(s):  
M Kimmel ◽  
D E Axelrod
Keyword(s):  
Mathematical Models ◽  
Cell Lines ◽  
Gene Amplification ◽  
Dihydrofolate Reductase ◽  
Gene Copy Number ◽  
Gene Copy ◽  
Published Data ◽  
Reductase Gene ◽  
Double Minute ◽  
Double Minute Chromosomes

Abstract An increased number of copies of specific genes may offer an advantage to cells when they grow in restrictive conditions such as in the presence of toxic drugs, or in a tumor. Three mathematical models of gene amplification and deamplification are proposed to describe the kinetics of unstable phenotypes of cells with amplified genes. The models differ in details but all assume probabilistic mechanisms of increase and decrease in gene copy number per cell (gene amplification/deamplification). Analysis of the models indicates that a stable distribution of numbers of copies of genes per cell, observed experimentally, exists only if the probability of deamplification exceeds the probability of amplification. The models are fitted to published data on the loss of methotrexate resistance in cultured cell lines, due to the loss of amplified dihydrofolate reductase gene. For two mouse cell lines unstably resistant to methotrexate the probabilities of amplification and deamplification of the dihydrofolate reductase gene on double minute chromosomes are estimated to be approximately 2% and 10%, respectively. These probabilities are much higher than widely presumed. The models explain the gradual disappearance of the resistant phenotype when selective pressure is withdrawn, by postulating that the rate of deamplification exceeds the rate of amplification. Thus it is not necessary to invoke a growth advantage of nonresistant cells which has been the standard explanation. For another analogous process, the loss of double minute chromosomes containing the myc oncogene from SEWA tumor cells, the growth advantage model does seem to be superior to the amplification and deamplification model. In a more theoretical section of the paper, it is demonstrated that gene amplification/deamplification can result in reduction to homozygosity, such as is observed in some tumors. Other applications are discussed.

scholarly journals Relationship of amplified dihydrofolate reductase genes to double minute chromosomes in unstably resistant mouse fibroblast cell lines.

1981 ◽  
Vol 1 (12) ◽  
pp. 1077-1083 ◽  
Author(s):  
P C Brown ◽  
S M Beverley ◽  
R T Schimke
Keyword(s):  
Dihydrofolate Reductase ◽  
Deoxyribonucleic Acid ◽  
Fibroblast Cell ◽  
Mouse Fibroblast ◽  
Gene Sequences ◽  
Resistant Mouse ◽  
Methotrexate Resistance ◽  
Double Minute ◽  
Relationship Of ◽  
Double Minute Chromosomes

Murine 3T6 selected in increasing concentrations of methotrexate were unstable with respect to dihydrofolate reductase overproduction and methotrexate resistance when they are cultured in the absence of methotrexate. An analysis of the karyotypes of these resistant cells revealed the presence of numerous double minute chromosomes. We observed essentially identical kinetics of loss of dihydrofolate reductase gene sequences in total deoxyribonucleic acid and in deoxyribonucleic acid from fractions enriched in double minute chromosomes and in the numbers of double minute chromosomes per cell during reversion to methotrexate sensitivity, and this suggested that unstably amplified gene sequences were localized on double minute chromosomes. This conclusion ws also supported by an analysis of cell populations sorted according to dihydrofolate reductase enzyme contents, in which relative gene amplification and double minute chromosome content were related proportionally.

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