Electrophoretic shift mutants in Chinese hamster ovary cells: evidence for genetic diploidy

Azacytidine-induced reactivation of a DNA repair gene in Chinese hamster ovary cells

Six X-ray-sensitive (xrs) strains of the CHO-K1 cell line were shown to revert at a very high frequency after treatment with 5-azacytidine. This suggested that there was a methylated xrs+ gene in these strains which was structurally intact, but not expressed. The xrs strains did not complement one another, and the locus was autosomally located. In view of the frequency of their isolation and their somewhat different phenotypes, we propose that the xrs strains are mutants derived from an active wild-type gene. However, there is in addition a methylated silent gene present in the genome. Azacytidine treatment reactivated this gene. We present a model for the functional hemizygosity of mammalian cell lines, which is based on the inactivation of genes by de novo hypermethylation. In contrast to results with xrs strains, other repair-defective lines were found not to be reverted by azacytidine.

Azacytidine-induced reactivation of a DNA repair gene in Chinese hamster ovary cells

Six X-ray-sensitive (xrs) strains of the CHO-K1 cell line were shown to revert at a very high frequency after treatment with 5-azacytidine. This suggested that there was a methylated xrs+ gene in these strains which was structurally intact, but not expressed. The xrs strains did not complement one another, and the locus was autosomally located. In view of the frequency of their isolation and their somewhat different phenotypes, we propose that the xrs strains are mutants derived from an active wild-type gene. However, there is in addition a methylated silent gene present in the genome. Azacytidine treatment reactivated this gene. We present a model for the functional hemizygosity of mammalian cell lines, which is based on the inactivation of genes by de novo hypermethylation. In contrast to results with xrs strains, other repair-defective lines were found not to be reverted by azacytidine.

Functional hemizygosity for the MDH2 locus in Chinese hamster ovary cells

Isolation of electrophoretic mobility shift mutants for a large number of enzyme loci in CHO cells has allowed the identification of many genes which are functionally hemizygous. To gain further insight into the nature of hemizygosity in CHO cells and the mechanisms by which it has arisen, we are attempting to determine whether hemizygous gene loci are clustered in a few localized chromosomal regions in CHO or are more generally distributed throughout the genome. Isozyme analysis of a series of CHO electrophoretic mobility shift mutants for MDH2 (malate dehydrogenase 2, EC 1.1.1.37) revealed that this locus is functionally hemizygous in CHO cells, but the locus could not be mapped by conventional approaches because of the similar electrophoretic mobilities of Chinese hamster and mouse MDH2 isozymes. Construction of intraspecific CHO X CHO hybrids using electrophoretic mobility shift mutants with secondary, selectable drug-resistance markers allowed us to determine that MDH2 is not closely linked to any previously mapped hemizygous marker loci in CHO, but is linked to alleles for two dizygous gene loci, PGM3 and APRT, on CHO chromosome Z7. A possible genetic basis for hemizygosity of the MDH2 locus in CHO cells is discussed.

Linkage of the MBG locus to another functionally hemizygous gene locus (IDH2) on chromosome Z3 in Chinese hamster ovary cells

To gain insight into the nature of hemizygosity in Chinese hamster ovary (CHO) cells and the mechanisms by which it has arisen, we are attempting to map and determine linkage relationships for as many hemizygous loci as possible. In this study, we have shown by segregation analysis of intraspecific somatic cell hybrids that the hemizygous gene locus associated with resistance to methylglyoxalbisguanyl hydrazone (MBG) in CHO cells is linked to the hemizygous IDH2 locus on chromosome Z3. Nine of the ten autosomal hemizygous gene loci that have been mapped to date in CHO cells are clustered on three chromosomes, with five such markers on chromosome 2, two on chromosome 8, and now two on the Z3 chromosome. With the mapping of MBG to the Z3 chromosome, selectable drug resistance markers are now available on eight different CHO chromosomes.

Linkage of the MBG locus to another functionally hemizygous gene locus (IDH2) on chromosome Z3 in Chinese hamster ovary cells

To gain insight into the nature of hemizygosity in Chinese hamster ovary (CHO) cells and the mechanisms by which it has arisen, we are attempting to map and determine linkage relationships for as many hemizygous loci as possible. In this study, we have shown by segregation analysis of intraspecific somatic cell hybrids that the hemizygous gene locus associated with resistance to methylglyoxalbisguanyl hydrazone (MBG) in CHO cells is linked to the hemizygous IDH2 locus on chromosome Z3. Nine of the ten autosomal hemizygous gene loci that have been mapped to date in CHO cells are clustered on three chromosomes, with five such markers on chromosome 2, two on chromosome 8, and now two on the Z3 chromosome. With the mapping of MBG to the Z3 chromosome, selectable drug resistance markers are now available on eight different CHO chromosomes.

Chromosomal rearrangements and gene expression in CHO cells: mapping of alleles for eight enzyme loci on CHO chromosomes Z3, Z4, Z5, and Z7

Analysis of CHO electrophoretic mobility shift mutants for six enzyme loci ( LDHA , GAA, IDH2 , ME1, PGM3, and MPI) that have been previously mapped to Chinese hamsters chromosomes 3 and 4 indicated that each of these loci, with the exception of IDH2 , are functionally dizygous in CHO. Segregation analysis of CHO X mouse somatic cell hybrids allowed regional gene mapping assignments for a total of eight Chinese hamster chromosome 3- or 4-derived marker loci (the above six, plus APRT and PKM2) to CHO chromosomes Z3 , Z4 , Z5 , and Z7 . For seven of these enzyme loci (all but IDH2 ), two alleles are expressed in CHO cells, each segregating with a different Z-group chromosome. These gene mapping assignments confirm genetically that CHO chromosomes Z3 , Z4 , Z5 , and Z7 are, in fact, derived from Chinese hamster chromosomes 3 and 4, and provide insight into the effects of chromosomal rearrangements on gene expression and hemizygosity in CHO cells.

Genetic effects of chromosomal rearrangements in Chinese hamster ovary cells: expression and chromosomal assignment of TK, GALK, ACP1, ADA, and ITPA loci

Polyethylene glycol-mediated fusion of Chinese hamster ovary (CHO) cells with mouse Cl1D cells produced interspecific somatic cell hybrids which slowly segregated CHO chromosomes. Cytogenetic and isozyme analysis of HAT- and bromodeoxyuridine-selected hybrid subclones and of members of a hybrid clone panel retaining different combinations of CHO chromosomes enabled provisional assignments of the following enzyme loci to CHO chromosomes: TK, GALK, and ACP1 to chromosome 7; TK and GALK to chromosome Z13; ACP1, ADA, and ITPA to chromosome Z8; and ADA and ITPA to chromosome Z9. These genetic markers reflect the origin of each of these Z group chromosomes and indicate the functional activity of alleles located on rearranged chromosomes. Identification of diploid electrophoretic shift mutations for ADA and ITPA was consistent with those observations. Assignment of the functional TK locus in TK+/- CHO-AT3-2 cells indicated that gene deletion may be responsible for TK hemizygosity in this subline.

Genetic effects of chromosomal rearrangements in Chinese hamster ovary cells: expression and chromosomal assignment of TK, GALK, ACP1, ADA, and ITPA loci

Polyethylene glycol-mediated fusion of Chinese hamster ovary (CHO) cells with mouse Cl1D cells produced interspecific somatic cell hybrids which slowly segregated CHO chromosomes. Cytogenetic and isozyme analysis of HAT- and bromodeoxyuridine-selected hybrid subclones and of members of a hybrid clone panel retaining different combinations of CHO chromosomes enabled provisional assignments of the following enzyme loci to CHO chromosomes: TK, GALK, and ACP1 to chromosome 7; TK and GALK to chromosome Z13; ACP1, ADA, and ITPA to chromosome Z8; and ADA and ITPA to chromosome Z9. These genetic markers reflect the origin of each of these Z group chromosomes and indicate the functional activity of alleles located on rearranged chromosomes. Identification of diploid electrophoretic shift mutations for ADA and ITPA was consistent with those observations. Assignment of the functional TK locus in TK+/- CHO-AT3-2 cells indicated that gene deletion may be responsible for TK hemizygosity in this subline.

Metabolism and mechanisms of action of 9-(tetrahydro-2-furyl)-6-mercaptopurine in Chinese hamster ovary cells

The mechanisms of action of 9-(tetrahydro-2-furyl)-6-mercaptopurine (THFMP) have been studied in Chinese hamster ovary (CHO) cells in tissue culture. THFMP is relatively unstable in physiological buffers, being facilely converted to 6-mercaptopurine (6-MP) even in the absence of cells. Consequently, THFMP undergoes metabolic conversions characteristic of 6-MP, namely formation of 6-thioIMP and incorporation into DNA as 6-thio-guanine (6-TG) nucleotide. A number of purines are capable of preventing the toxicity of THFMP in wild-type cells in a manner similar to that of 6-MP. However, exogenous purines and pyrimidines did not prevent the toxicity of THFMP to cells deficient in the enzyme, hypoxanthine-guanine phosphoribosyltransferase (EF 2.4.2.8; HGPRTase). Cells lacking HGPRTase were 20--40-fold resistant to 6-TC and 6-MP but were only 2--4-fold resistant to THFMP. Furthermore, the time-course for killing CHO cells deficient in HGPRTase was different from that in wild-type cells containing the enzyme. There was no apparent effect of THFMP on the utilization of precursors for DNA, RNA or protein synthesis in the enzyme-deficient mutant cell line. The results suggest that THFMP is converted non-enzymatically to 6-MP and shares its mechanisms of action in wild-type cells containing HGPRTase, i.e., inhibition of de novo purine biosynthesis and incorporation into DNA as 6-TC nucleotide. However, the mechanism of action of THFMP in cells lacking HGPRTase is probably unique and is presently unknown.

Structural and functional hemi- and dizygous Chinese hamster chromosome 2 gene loci in CHO cells

Fourteen independent somatic cell hybrid clones made between diphtheria toxin (DT)-resistant mouse Cl1D cells and DT-sensitive Chinese hamster ovary (CHO) cells slowly segregated CHO chromosome. Concordant segregation analysis of electrophoretically resolvable Chinese hamster chromosome 2 gene products and CHO chromosomes 2 and Z2 (having q1-q24 deletion) in DT-selected and control hybrid subclones was conducted. Analysis revealed that loci for DT sensitivity and galactose-1-phosphate uridyltransferase could be regionally assigned to Chinese hamster chromosome 2q1-24 and were physically hemizygous in CHO cells. Enolase (ENO1), 6 phosphogluconate dehydrogenase (PGD), and phosphoglucomutase 1 (PGM1) were located outside the q1-24 region on Chinese hamster chromosome 2 and were dizygous in CHO cells. Functional dizygosity of ENO1, PGD, and PGM1 in CHO cells, as determined by the isolation of diploid heterozygous electrophoretic shift mutants following UV and EMS exposure, confirmed their location outside the Z2 deletion and indicated that the deletion did not result in the inactivation of adjacent loci. These results are discussed in relation to current theories on the basis for high frequency of drug-resistant autosomal recessive mutants in CHO cells and conservation of mammalian autosomal linkage groups.

Confirmational, provisional, and/or regional assignment of 15 enzyme loci onto Chinese hamster autosomes 1, 2, and 7

PEG-mediated fusion between mouse Cl1d cells and primary Chinese hamster spleen cells produced interspecific hybrids which slowly and nonrandomly segregated Chinese hamster chromosomes. Cytogenetic and isozyme analysis (31 loci) of HAT and BrdU selected hybrid clones and subclones and of members of a hybrid clone panel retaining different combinations of Chinese hamster chromosomes enabled provisional assignment of the following enzyme loci on Chinese hamster chromosomes: thymidine kinase, galactokinase, and acid phosphatase-1 to chromosome 7; galactose-1-phosphate uridyltransferase to chromosome 2; and adenosine kinase, esterase D, glutathione reductase, glyoxalase, nucleoside phosphorylase, peptidases B and S, and phosphoglucomutase (PGM) 2 to chromosome 1. Assignments of PGM1, 6-phosphogluconate dehydrogenase, and enolase 1 to chromosome 2 were confirmed, and a chromosome 2 deletion (q23-q33) enabled the provisional assignment of PGM1 to that region. The assignments provide markers for the study of the genetic consequences of chromosomal rearrangements in Chinese hamster cell lines and support the concept of conservation of mammalian autosomal linkage groups.

Genetic polymorphisms and gene expression variations at enzyme loci in chinese hamster cell lines

A series of Chinese hamster cell lines commonly used in somatic cell genetics research was examined for variations in either expression or electrophoretic mobility of 43 enzyme gene products by starch gel electrophoresis followed by histochemical staining. Stable variations in the qualitative expressions of the creatine kinase B and adenylate kinase 1 (AK1) loci were detected among the cell lines and verified in subclones. All ten cell lines examined were deficient in the expression of the lactate dehydrogenase B locus. Polymorphisms were detected for the adenosine deaminase (ADA) and AK2 loci among the cell lines and shown to exist in a series of Chinese hamsters. Electrophoretic banding patterns from tissues, sublines, and subclones of the same genetic source and Hardy-Weinberg distribution of phenotypes from different genetic sources confirmed that the polymorphisms were based on its inheritance of two codominant autosomal ADA and AK2 alleles that specified electrophoretically variant enzymes.

Molecular relationships between closely related strains and species of nematodes

Electrophoretic comparisons have been made for 24 enzymes in the Bergerac and Bristol strains of Caenorhabditis elegans and the related species, Caenorhabditis briggsae. No variation was detected between the two strains of C. elegans. In contrast, the two species, C. elegans and C. briggsae exhibited electrophoretic differences in 22 of 24 enzymes. A consensus 5S rRNA sequence was determined for C. elegans and found to be identical to that from C. briggsae. By analogy with other species with relatively well established fossil records it can be inferred that the time of divergence between the two nematode species is probably in the tens of millions of years. The limited anatomical evolution during a time period in which proteins undergo extensive changes supports the hypothesis that anatomical evolution is not dependent on overall protein changes.

Immuno-overlay: a method for identification of hepatoma cell colonies that secrete albumin

A screening technique was developed for the identification of clones of hepatoma cells that secrete albumin. The technique employs the overlay of a 1% agarose solution containing antiserum to albumin onto clones of hepatoma cells. A distinct immunoprecipitation complex is formed in the immuno-overlay that corresponds directly to the position of each secreting clone. Clones deficient in albumin secretion do not form an immunoprecipitate. Thus comparison of the immuno-overlay and the cell colonies results in identification of variant clones as well as those capable of secretion. Biochemical characterization of the region of agarose overlay from secreting and nonsecreting clones demonstrates the specificity of the method and its potential for selection of colonies that are secreting other hepatic or cellular proteins.

Electrophoretic patterns of aminoacylase-1 (ACY-1) isozymes in vertebrate cells and histochemical procedure for detecting ACY-1 activity

A histochemical procedure has been developed for staining aminoacylase-1 (ACY-1) isozymes after electrophoresis on cellulose acetate membranes. N-Formyl-L-methionine and N-acetyl-L-methionine were excellent enzyme substrates in the staining reaction. The ACY-1 isozymes from tissue culture cells of several vertebrate species showed distinguishable electrophoretic patterns. The ACY-1 isozymes in extracts of mouse, human, owl monkey, and African green monkey kidney cells each had electrophoretic mobilities different from those of peptidases S, A, and C from the same cells. Exept for African green monkey kidney (CV-1) cells, the animal species expressed a single anodally migrating ACY-1 band. Human-mouse somatic cell hybrids containing the short arm of human chromosome 3 expressed three ACY-1 bands, as would be predicted from the dimeric structure of the enzyme. CV-1 cells expressed two (or three) ACY-1 bands, suggesting the possibility that CV-1 cells contained two alleles at a single locus or two genetic loci for ACY-1.

Isolation of Chinese hamster cell mutants deficient in dihydrofolate reductase activity

Mutants of Chinese hamster ovary cells lacking dihydrofolate reductase (tetrahydrofolate dehydrogenase, 7,8-dihydrofolate:NADP+ oxidoreductase; EC 1.5.1.3) activity were isolated after mutagenesis and exposure to high-specific-activity [3H]deoxyuridine as a selective agent. Fully deficient mutants could not be isolated starting with wild-type cells, but could readily be selected from a putative heterozygote that contains half of the wild-type level of dihydrofolate reductase activity. The heterozygote itself was selected from wild-type cells by using [3H]deoxyuridine together with methotrexate to reduce intracellular dihydrofolate reductase activity. Fully deficient mutants require glycine, a purine, and thymidine for growth; this phenotype is recessive to wild type in cell hybrids. Revertants have been isolated, one of which produces a heat-labile dihydrofolate reductase activity. These mutants may be useful for metabolic studies relating to cancer chemotherapy and for fine-structure genetic mapping of mutations by using available molecular probes for this gene.

Marker segregation without chromosome loss at the emt locus in Chinese hamster cell hybrids

Segregation, defined as the reexpression of the recessive phenotype, has been examined in Chinese hamster cell hybrids heterozygous at the recessive emtr locus. Segregants were selected in emetine, and the role of chromosome loss in the segregation process was evaluated by detailed karyotype comparison of segregants with their hybrid parent. In emtr CHO x emt+ CHO hybrids (CHO is thought to be hemizygous at the emt locus), segregants were obtained at high frequency, and no consistent chromosome loss was found in the segregants. In hybrids made with Emtr CHO and wild-type lines other than CHO (CHW,CHL,V-79), where two wild-type alleles are thought to be present in the hybrid, segregants were obtained at much lower frequency, consistent with a two-step segregation process. These segregants revealed consistent loss of one chromosome 2 or deletion of a part of the long arm of a chromosome 2. Thus, one step in segregation seems to be chromosome loss while the other step must have a different mechanism, possibly the same mechanism that operates in the CHO x CHO hybrids. Two major conclusions can be drawn: (1) the emt gene maps to a hemizygous region of the long arm of a chromosome 2 in Chinese hamster, and (2) a segregation mechanism other than chromosome loss appears to operate with high efficiency in intraspecific hybrids.

Single-step selection of mouse FM3A cell mutants defective in thymidylate synthetase

A tritium-suicide method for isolating thymidine auxotrophic mutants is described. Mutagenized mouse FM3A cells were cultured in medium containing [3H] deoxyuridine. Most of the surviving clones examined showed a phenotype of absolute thymidine auxotrophy. This phenotype is very stable and was found to be genetically recessive in cell-cell hybridization experiments. The growth of these variant clones was not supported by various pyrimidine nucleosides other than thymidine. The activity of thymidylate synthetase in crude extracts of these clones was less than 1% of that of FM3A cells. These results strongly indicate that the thymidine auxotrophic phenotype resulted from a genetic defect in thymidylate synthetase.

Random segretation of multiple genetic markers from CHO-CHO hybrids: evidence for random distribution of functional hemizygosity in the genome

The linkage relationship between various recessive markers isolated in Chinese hamster ovary (CHO) cells has been investigated. For such studies, multiple recessive markers conferring resistance to various drugs, e.g., resistance to emetine (Emtr), thioguanine (Thgr), azaadenine (Azar), phytohemagglutinin (Phar), diphtheria toxin (Dipr), toyocamycin (Toyr), aminopterin (Amnr), and methylglyoxalbisguanyl hydrazone (Mbgr), were introduced into CHO lines by selecting successively for one drug at a time. Hybrids were constructed between the multiply marked lines and the sensitive cells, and segregation frequencies for the various markers, singly and in different pairs, were examined. Results of such studies show that of the recessive markers examined, none cosegregated from the hybrid cells. The independent segregation of the X-chromosome-linked Thgr and of the rest of the markers indicates that none of these other mutations are located on the X chromosome. These results also provide strong suggestive evidence that functional hemizygosity in CHO cells is not restricted to one or a few chromosomal regions, but rather appears to be widespread.

Molecular recombination and the repair of DNA double-strand breaks in CHO cells

Molecular recombination and the repair of DNA double-strand breaks (DSB) have been examined in the G-0 and S phase of the cell cycle using a temperature-sensitive CHO cell line to test i) if there are cell cycle restrictions on the repair of DSB's' ii) the extent to which molecular recombination can be induced between either sister chromatids or homologous chromosomes and iii) whether repair of DSB's involves recombination (3). Mitomycin C (1-2 micrograms/ml) or ionizing radiation (50 krad) followed by incubation resulted in molecular recombination (hybrid DNA) in S phase cells. Approximately 0.03 to 0.10% of the molecules (number average molecular weight: 5.6 x 10(6) Daltons after shearing) had hybrid regions for more than 75% of their length. However, no recombination was detected in G-0 cells. Since the repair of DSB was observed in both stages with more than 50% of the breaks repaired in 5 hours, it appears that DSB repair in G-0 cells does not involve recombination between homologous chromosomes. The possibility is not excluded that repair in G-0 cells involves only small regions (less than 4 x 10(6) Daltons).