Amplification of a cloned Chinese hamster dihydrofolate reductase gene after transfer into a dihydrofolate reductase-deficient cell line

Chinese hamster ovary cell line DXB-11: chromosomal instability and karyotype heterogeneity

BACKGROUND

Chinese hamster ovary cell lines, also known as CHO cells, represent a large family of related, yet quite different, cell lines which are metabolic mutants derived from the original cell line, CHO-ori. Dihydrofolate reductase-deficient DXB-11 cell line, one of the first CHO derivatives, serves as the host cell line for the production of therapeutic proteins. It is generally assumed that DXB-11 is identical to DUKX or CHO-DUK cell lines, but, to our knowledge, DXB-11 karyotype has not been described yet.

RESULTS

Using differential staining approaches (G-, C-banding and Ag-staining), we presented DXB-11 karyotype and revealed that karyotypes of DXB-11 and CHO-DUK cells have a number of differences. Although the number of chromosomes is equal-20 in each cell line-DXB-11 has normal chromosomes of the 1st and 5th pairs as well as an intact chromosome 8. Besides, in DXB-11 line, chromosome der(Z9) includes the material of chromosomes X and 6, whereas in CHO-DUK it results from the translocation of chromosomes 1 and 6. Ag-positive nucleolar organizer regions were revealed in the long arms of chromosome del(4)(q11q12) and both chromosome 5 homologues, as well as in the short arms of chromosomes 8 and add(8)(q11). Only 19 from 112 (16.96%) DXB-11 cells display identical chromosome complement accepted as the main structural variant of karyotype. The karyotype heterogeneity of all the rest of cells (93, 83.04%) occurs due to clonal and nonclonal additional structural rearrangements of chromosomes. Estimation of the frequency of chromosome involvement in these rearrangements allowed us to reveal that chromosomes 9, der(X)t(X;3;4), del(2)(p21p23), del(2)(q11q22) /Z2, der(4) /Z7, add(6)(p11) /Z8 are the most stable, whereas mar2, probably der(10), is the most unstable chromosome. A comparative analysis of our own and literary data on CHO karyotypes allowed to designate conservative chromosomes, both normal and rearranged, that remain unchanged in different CHO cell lines, as well as variable chromosomes that determine the individuality of karyotypes of CHO derivatives.

CONCLUSION

DXB-11and CHO-DUK cell lines differ in karyotypes. The revealed differential instability of DXB-11 chromosomes is likely not incidental and results in karyotype heterogeneity of cell population.

Using cell engineering and omic tools for the improvement of cell culture processes

Significant strides have been made in mammalian cell based biopharmaceutical process and cell line development over the past years. With several established mammalian host cell lines and expression systems, optimization of selection systems to reduce development times and improvement of glycosylation patterns are only some of the advances being made to improve cell culture processes. In this article, the advances pertaining to cell line development and cell engineering strategies are discussed. An overview of the cell engineering strategies to enhance cellular characteristics by genetic manipulation are illustrated, focusing on the use of genomics and proteomics tools and their application in such endeavors. Included in this review are some of the early studies using the 'omic' technique to understand cellular mechanisms of product synthesis and secretion, apoptosis, cell proliferation and the influence of the physicochemical environment. The article highlights the significance of integrating genomics and proteomics data with the vast amounts of bioprocess data for improved analysis of the biological pathways involved. Further improvements of the techniques and methodologies used are needed but ultimately, the new cell engineering strategies should provide great insight into the regulatory networks within the cell in a bioprocess environment and how to manipulate them to increase overall productivity.

Fine structure mapping of DNA repair within a 100 kb genomic region in Chinese hamster ovary cells

We have investigated at a high level of resolution the repair of cyclobutane pyrimidine dimers (CPD) in a large amplified genomic region in Chinese hamster ovary B11 cells. We found strand selective repair in DNA fragments within two active genes, DHFR and an unknown gene adjacent to DHFR. These genes generate divergent transcripts from the same promoter region; their transcribed strands were virtually free of CPD within 24 h after irradiation with 10 j/m2 of ultraviolet light (254nm), while their non-transcribed strands were poorly repaired. We also examined the repair of CPD in three DNA fragments within a 50 kb region downstream of DHFR, in which two origins of replication flanking a matrix attachment site have been characterized from independently derived cell lines with amplified DHFR domains; repair of CPD in this non-transcribed region was similarly poor in both DNA strands. Transcription-coupled repair of CPD in the DHFR gene exhibited the same proficiency throughout the transcription unit: analysis of the efficiency of removal of CPD over time revealed no differences between repair in the 5' and the 3' ends of the DHFR gene. Implications for mechanisms of transcription-coupled repair are discussed.

Cytological, flow cytometric, and molecular analysis of the rapid evolution of mammalian chromosomes containing highly amplified DNA sequences

Transfection of a mouse dihydrofolate reductase (DHFR) cDNA contained in a plasmid "expression vector" into DHFR deficient Chinese hamster cells, followed by progressive selection of cells in increasing concentrations of methotrexate (MTX), leads to marked amplification of the exogenous DHFR sequences in the recipient hamster cells. This gene amplification is evident at the cytological level, in the form of homogeneously staining chromosomal regions (HSRs), at a gene expression level, in the form of fluorescein-methotrexate binding, and at the DNA level. Flow sorting, based on variable fluorescein-MTX binding, or direct cellular cloning, followed by chromosome analysis, revealed intercellular heterogeneity of HSRs in size and distribution. This suggested that there was a rapid evolution of HSRs in MTX-resistant transfectants. Chromosomal analysis of HSR evolution in situ, by examining individual colonies presumably derived from one or a few cells, underscored this impression of chromosome structural fluidity. Rates of HSR change in excess of 0.01 per cell division, increased by low doses of the recombinogen, mitomycin C, were detected. The Chinese hamster DHFR transfectants described should be amenable to detailed, coordinate cytological and molecular characterization. Such an analysis should contribute to an understanding of processes such as homologous recombination in mediating HSR evolution in mammalian chromosomes.

Quantitative genetic analysis of tumor progression

Metastasis and resistance to chemotherapy are common features of progressed cancers. With respect to the latter phenotype, it is thought that during tumor growth drug-resistant cells arise spontaneously at rates characteristic of the genetic alterations involved. On application of chemotherapy, such variant tumor cells are more likely to survive, and they may eventually dominate, resulting in a non-responsive malignancy. Aspects of this model have been confirmed in a number of experimental systems and in patients. In contrast to our understanding of drug resistance, steps involved in the progression to metastatic spread of tumor cells are much less well-understood. In this review we describe methodologies of quantitative genetic analysis with reference to development of drug resistance. We then describe attempts by ourselves and others to use a similar approach to investigate metastatic properties. Based on these studies, we have proposed the quantitative 'dynamic heterogeneity' model of tumor metastasis, which is presented here. Using an 'experimental' metastasis assay and Luria-Delbruck fluctuation analysis, we determined that in murine KHT fibrosarcoma and B16 melanoma lines, 'metastatic' variants with a distinct phenotype are generated at high rates. These variants are relatively unstable resulting in a dynamic equilibrium between generation and loss of metastatic variants. The metastatic ability of such a tumor population is thus dependent on the frequency of a subpopulation of metastatic variants which are turning over rapidly. This dynamic heterogeneity model is able to quantitatively provide a unifying explanation for a wide range of observations concerning tumor heterogeneity and clonal instability. Genetic mechanisms involving rapid rates have been characterized in drug-resistant variants. We speculate that similar processes may be involved in different aspects of tumor progression such as those resulting in metastasis.