Isolation and characterization of 5-fluorouracil-resistant mutants of Chinese hamster ovary cells deficient in the activities of orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase

Advances and challenges in fluoropyrimidine pharmacogenomics and pharmacogenetics

In cancer pharmacogenetics (the study of how variability in a single or set of known genes influences drug response) and pharmacogenomics (the study of variability on a genome-wide scale), one of the most important fields of research focuses on the fluoropyrimdines (FPs) and, in particular, 5-fluorouracil (5-FU). After over 40 years of use, FPs remain one of the most commonly used cancer chemotherapy agents and their application includes a wide spectrum of cancer types. FPs also continue to be the baseline component for many new regimens with novel molecular-targeted agents that are being rapidly introduced. Hence, it would seem appropriate that pharmacogenetic/genomic models for optimizing cancer patient management would involve indicators of FP response. In this article, the current trends in FP pharmacogenetics and pharmacogenomics are reviewed based on the advances made to date and the challenges faced in realizing their full potential.

UMP synthase activity expressed in deficient hamster cells by separate transferase and decarboxylase proteins or by linker-deleted bifunctional protein

Segments of the human UMP synthase cDNA coding for the orotate phosphoribosyl transferase (OPRT) and orotidylate decarboxylase (ODC) domains of the bifunctional protein UMP synthase were produced by polymerase chain reaction techniques and cloned into a eukaryotic expression vector. The separate OPRT and ODC vectors, along with a selectable marker, were cotransfected into UMP synthase-deficient hamster cells (Urd-C) that require exogeneous uridine for growth. Transfected Urd-C cells surviving selection in media without added uridine were isolated and designated transferase decarboxylase Urd-C (TDU). All of the selected colonies contained DNA corresponding to the OPRT and ODC expression vectors. Two cell lines (TDU3 and TDU5) integrated many more copies of the OPRT and ODC vectors into their genomes compared to the other TDU lines. A 28.6-kDa ODC protein band and a 24.4-kDa OPRT band were detected on western blots with UMP synthase-specific polyclonal antiserum. The OPRT activity of the TDU lines was up to 8.7 times the OPRT activity of control CHL cells, and the ODC activity was up to 12.5 times control levels. Both OPRT and ODC activities in the monofunctional proteins were less heat stable than in the bifunctional UMP synthase protein. The monofunctional OPRT protein was less stable than the ODC protein at 45 degrees C. Growth of transfected cells in 6-azauridine resulted in striking increases in activity and temperature stability for the monofunctional ODC protein. A UMP synthase bifunctional protein was constructed with a deletion of the suspected linker region joining the two catalytic domains. The linker-deleted UMP synthase showed no significant change in either OPRT or ODC activity or temperature stability. The increased stability of the bifunctional protein may be a factor in its evolutionary selection in mammalian cells.

Cloning of breakpoints in 3q21 associated with hematologic malignancy

Rearrangements of band 3q21 have been well documented in leukemia. To analyze the region involved, we have isolated a normal genomic P1 clone that spans the 3q21 breakpoints derived from two leukemia patients carrying t(3;3)(q21;q26). Both breakpoints are contained within a 50-kb NotI restriction fragment, but are not identical. They are separated by 9-11 kb and may disrupt genes associated with either the proximal or distal NotI sites. We further show that one patient carried a small insertion of material from chromosome 2 in the 3q- derivative. The P1 clone will allow isolation of candidate genes for these breakpoints and investigations into the clustering of other 3q21 leukemia breakpoints.

Characterization of trimethoprim- and sulphisoxazole-resistant Chlamydia trachomatis

Trimethoprim and sulphisoxazole were used as selective agents in culture to isolate, by a stepwise procedure, a series of Chlamydia trachomatis L2 populations resistant to the cytotoxic effects of the drugs. Two trimethoprim-resistant populations, L2TriR-60 and L2TriR-100, and one sulphonamide-resistant population, L2SulfR-100, were characterized in more detail. In addition to being resistant to trimethoprim, L2TriR-60 was cross-resistant to methotrexate, sensitive to sulphisoxazole and displayed a ribonucleotide auxotrophy similar to that of its parental wild type, C. trachomatis L2. Surprisingly, L2TriR-100 and L2SulfR-100 appeared phenotypically identical. Both mutants were highly resistant to trimethoprim, sulphisoxazole, and methotrexate. In contrast to wild-type C. trachomatis L2, these populations were sensitive to 5-fluorouracil. L2TriR-100 and L2SulfR-100 were incapable of taking pyrimidine ribonucleotides from the host cell and no longer synthesized thymidine nucleotides de novo. The pyrimidine requirement of these mutants was met by salvaging host-cell uracil and thymidine, a property which can account for their drug-resistant characteristics. L2TriR-100 and L2SulfR-100 could also salvage adenine and guanine. Using L2TriR-100 as a starting stock, a mutant population resistant to the cytotoxic effects of trimethoprim and 5-fluorouracil (L2Tri/5-FU) was selected. L2Tri/5-FU was resistant to 5-fluorouracil because it had regained the capacity to take pyrimidine ribonucleotides from the host cell.

Regional mapping of the gene encoding dihydroorotate dehydrogenase, an enzyme involved in UMP synthesis, electron transport, and superoxide generation, to human chromosome region 16q22

De novo UMP synthesis is a critical metabolic pathway for nucleic acid synthesis and for a variety of metabolic pathways. The pathway is a target for many widely used cancer chemotherapy agents, several of which are pyrimidine analogs. Humans and cattle have been described with mutations in UMP synthesis that lead to serious inborn errors of metabolism. Dihydroorotate dehydrogenase (EC 1.3.3.1) (DHODH) carries out the fourth committed step in the pathway and may also be important for mitochondrial electron transport and oxygen radical metabolism. We report here that the gene encoding this enzyme in humans is located in the chromosomal region 16q22. With the mapping of DHODH, the mapping of all the steps of UMP synthesis is complete. All three genes involved map to different human chromosomes. This information is important in consideration of regulation of UMP synthesis in mammals, including humans.

Resistance to pyrazofurin and 6-azauridine in normal MC3T3-E1 murine osteoblasts

Stable variants resistant to pyrazofurin (PF) and 6-azauridine (AZUrd) were serially selected in increasing drug concentrations from an MC3T3-E1 nontumorigenic murine osteoblastic cell line. Monophosphates of both AZUrd and PF competitively inhibit orotidine-5'-monophosphate decarboxylase (ODCase) activity of the UMP synthase multifunctional enzyme. When compared to the wild type cells, the AZUrdr and PFr lines were 3000- and 10,000-fold more resistant, respectively. Flow cytometry indicated tetraploidy in wild type cells and a reduction of DNA content in both resistant cell lines. DNA dot blot analysis showed no amplification of the gene coding for UMP synthase in either AZUrdr or PFr cells. Measurements of UMP synthase showed a 6-fold higher activity in AZUrdr cells and no significant difference in PFr cells as compared to wild type. Sensitivity to 5-fluorouracil was increased in the AZUrdr line as opposed to PFr and normal cell lines, indicating an increased orotate phosphoribosyltransferase activity in the AZUrdr cells. In comparison to wild type cells, PFr cells were 100-fold resistant to 6-methylmercaptopurine riboside, suggesting a lack of adenosine kinase activity. The control and AZUrdr cells showed equal sensitivity to 5-fluorouridine, thus indicating unchanged uridine kinase levels. While PFr cells were not cross-resistant to AZUrd, the AZUrdr cells were cross-resistant to PF. These results indicate the possibility of an altered ODCase active site. Although amplification of unrelated sequences cannot be excluded, our findings show that bone tetraploid, nontumorigenic cells acquire drug resistance through mechanisms other than the amplification of a target gene and that this resistance is accompanied by the partial loss of a chromosomal complement.

A reversible selection system for UMP synthase gene amplification and deamplification

The bifunctional enzyme UMP synthase provides a unique reversible selection system whereby cells that have amplified the UMP synthase gene can be isolated from a wild-type population and cells that have deleted the extra genes can be selected from a population with amplified copies of the gene. UMP synthase catalyzes the conversion of orotic acid to orotidine 5'-monophosphate (OMP) and then OMP to UMP. In the amplification step, Chinese hamster lung cells are selected for resistance to pyrazofurin and 6-azauridine, two inhibitors of the orotidine 5'-decarboxylase activity that converts OMP to UMP. The resistant cells have increased levels of both activities of UMP synthase as a result of a stable amplification of the UMP synthase gene. The deamplification step depends on 5-fluorouracil (5FU), which is converted to its monophosphate form by the orotate phosphoribosyltransferase activity of UMP synthase. Thus cells with increases in this activity are more sensitive to 5FU cytotoxicity, permitting single-step selection of revertants that have lost their amplified UMP synthase genes. These 5FU-selected cells are similar to the parental cell line in their level of UMP synthase activity and number of UMP synthase gene copies. Reselection in increasing concentrations of pyrazofurin and 6-azauridine allows one to isolate cells that have reamplified the UMP synthase gene. The ability to cycle cells of a single lineage through states of amplification and deamplification will facilitate study of the gene amplification process and the factors that influence the composition and stability of amplified regions.

Mapping of the gene encoding the multifunctional protein carrying out the first three steps of pyrimidine biosynthesis to human chromosome 2

The CAD gene encodes a trifunctional protein that carries the activities of the first three enzymes (carbamyl phosphate synthetase II, aspartate transcarbamylase, and dihydroorotase) of de novo pyrimidine biosynthesis. Genomic fragments of the human CAD gene have been obtained by screening a human genomic library in bacteriophage lambda using a Syrian hamster cDNA clone as a probe. These human genomic clones have been used to assign the CAD gene to human chromosome 2 using in situ hybridization to human metaphase chromosomes and Southern blot hybridization analysis of DNA isolated from a panel of Chinese hamster/human hybrid cells. In situ hybridization analysis has allowed further localization of this gene to the chromosomal region 2p21-p22.

Molecular cloning and nucleotide sequence for the complete coding region of human UMP synthase

The last two steps in the de novo biosynthesis of UMP are catalyzed by orotate phosphoribosyltransferase (OPRT; orotidine-5'-phosphate:pyrophosphate phosphoribosyltransferase; EC 2.4.2.10) and orotidine-5'-monophosphate decarboxylase (orotidine-5'-phosphate carboxy-lyase; EC 4.1.1.23). In mammals these two activities are found in a single, bifunctional protein called UMP synthase. A human T-lymphoblastic cell cDNA library constructed in lambda gt10 was screened with a UMP synthase-specific rat cDNA probe. Human UMP synthase cDNAs were isolated and then used to select UMP synthase gene fragments. The complete coding sequence of the mRNA for UMP synthase was determined by analysis of overlapping cDNA and genomic fragments. One of the cDNAs appears to have been synthesized from an incompletely or alternatively processed form of the UMP synthase mRNA. This cDNA lacks a poly(A) tail and has an extended 3'-nontranslated region that hybridizes with larger forms of the UMP synthase mRNA. The UMP synthase protein is composed of 480 amino acids with a molecular weight of 52,199. The two activities of UMP synthase reside in distinct domains encoded by the 3' and 5' halves of the mRNA. The COOH-terminal 258 amino acids of the human UMP synthase protein contain the orotidine-5'-monophosphate decarboxylase catalytic domain. This region is highly homologous to the mouse orotidine-5'-monophosphate decarboxylase sequence. The NH2-terminal 214 amino acids contain the OPRT domain. There is amino acid homology between this protein domain and specific regions of the Escherichia coli OPRT. The human OPRT domain also contains the putative catalytic site common to other human phosphoribosyltransferases.

Decreased host toxicity in vivo during chronic treatment with 5-flourouracil

Chronic weekly administration of FUra to CD8F1 female mice bearing spontaneous mammary tumors produced body weight loss during the first 2 weeks of treatment, which became less severe during subsequent weeks of therapy. To our knowledge, the development of such a decrease in FUra toxicity in vivo during chronic treatment with the drug has not been described previously, and a study of this phenomenon was therefore undertaken in tumor-free CD8F1 female mice. Weekly administration of FUra at 85 mg/kg resulted in toxicity expressed in body weight loss and in depressed peripheral WBC levels; however, the magnitude of these toxic effects decreased significantly by the 5th week of treatment. Pretreatment of normal mice with FUra for 7 weeks resulted in a dose-related shift in the LD50 of FUra administered as a subsequent challenge. Compared with an LD50 of 240 mg/kg for FUra in normal mice, the LD50 in mice pretreated with FUra at 50 or 85 mg/kg per week was found to be significantly elevated to 370 and 460 mg/kg, respectively. Pretreatment with FUra at 85 mg/kg for 7 weeks did not alter the activity of the enzymes responsible for the activation of FUra, namely uridine kinase or orotate phosphoribosyltransferase, in the intestinal epithelium or bone marrow, but it did decrease the 24-h urinary excretion of intact [3H]FUra by almost 40% (P less than 0.01). In addition, the FUra pretreatment schedule resulted in a 31% (P = 0.14) increase in the activity of dihydrouracil dehydrogenase in the liver. These results suggest that increased degradation of FUra can be induced by chronic treatment with the drug. Finally, knowledge of the development of increased drug catabolism was used to increase the therapeutic effectiveness of FUra by its incorporation into an increasing-dose regimen. Mice bearing 24-h transplants of the murine breast tumor were treated with a constant dose of FUra for 12 weeks or with a dose that was increased, after 7 weeks, to a dose normally causing a high degree of drug-related mortality. The group receiving the incremented FUra dose had a significantly slower tumor growth rate without an increase in drug-related toxicity. These results are discussed in light of their obvious clinical implications.

Isolation and characterization of a Chinese hamster ovary cell mutant which accumulates UDP glucuronic acid and requires uridine for growth

The isolation and characterization of a new pyrimidine requiring mutant of Chinese hamster ovary cells (CHO-Kl, pro-) is described. This mutant (P192) shows an absolute requirement for uridine for growth but is fully capable of pyrimidine nucleotide synthesis. P192 cells accumulate exceedingly large amounts of a compound identified by a variety of chromatographic, chemical, and labelling criteria as UDP glucuronic acid (UDPglcUA). This compound comprises approximately 60% of the acid-soluble nucleotides of P192 cells. Possible genetic mechanisms leading to the phenotype of P192 are examined, and the role of UDPglcUA in cellular metabolism is discussed.

Structural gene coding for multifunctional protein carrying orotate phosphoribosyltransferase and OMP decarboxylase activity is located on long arm of human chromosome 3

In humans, deficiency in the last two enzymes of UMP biosynthesis, orotate phosphoribosyltransferase (OPRT) and OMP decarboxylase results in the inborn error of metabolism hereditary orotic aciduria, type 1. In this manuscript, we present immunologic, molecular, biochemical, and genetic evidence that the gene coding for this set of enzymatic activities is located on the long arm of human chromosome 3. The evidence presented here is consistent with both these activities being carried on the same multifunctional protein in mammalian cells. These studies allow further genetic analysis of human chromosome 3, confirming that human markers ACY-1, previously assigned to 3p21, and beta-gal, previously assigned by others to the region 3(p21-q21), must be in the region 3 (cen-p21) and confirming the regional assignment of a human DNA segment, D3S1, to 3q12. The significance of these studies to genetic analysis of genes on human chromosome 3, some of which appear to play a role in some forms of malignancy, is discussed.

A selection system specific for the Thy mutator phenotype

Thy- mutants, in addition to being resistant to arabinosyl cytosine (arcC), show cross-resistance to 5-fluorouracil (5FU). When Chinese hamster ovary (CHO) cells were exposed to a selection system using both araC and 5FU, the resistant clones isolated were identical to thy- mutants by the following criteria: (1) all were auxotrophic for thymidine with a high reversion frequency to thymidine prototrophy; (2) those tested had a high level of dCTP relative to wild-type cells, while dTTP and dATP levels were unaffected, and (3) all tested had a 7- to 50-fold higher rate of spontaneous mutation than the wild-type strain for at least one independent genetic marker. Although spontaneous thy- mutants were rare, the frequencies of thy- mutants in untreated and mutagenized cultures are consistent with the conclusion that the thy- phenotype is the consequence of a single mutation in CHO cells.

Synergistic effect of purine derivatives on the toxicity of pyrazofurin and 6-azauridine towards cultured mammalian cells

A novel synergistic effect of several purine derivatives such as adenine, adenosine, hypoxanthine, and guanine on the toxicity of nucleoside analogs pyrazofurin and 6-azauridine towards cultured Chinese hamster ovary (CHO) cells has been observed. The presence of the above purine derivatives enhanced the toxicity of pyrazofurin and 6-azauridine, in a dose dependent manner. The growth inhibitory effects of these nucleoside analogs either alone or in combination with the purine derivatives were reversed by uridine and cytidine, providing evidence that the synergistic effect of the purine derivatives was exerted at the level of pyrimidine nucleotide biosynthesis. Studies with mutant cells lacking various purine phosphorylating enzymes show that phosphorylation of purine derivatives through reactions utilizing phosphoribosylpyrophosate (PRPP) is essential for observing the synergistic response. It is suggested that the above purine derivatives (including adenosine, via conversion to hypoxanthine) exert their synergistic effects by depleting the cellular pool of PRPP by two separate mechanisms (direct utilization and feedback inhibition of its synthesis), which as a result becomes rate limiting in the synthesis of orotidine monophosphate (OMP). The reduced levels of OMP, which is a competing substrate with pyrazofurin- and 6-azauridine-5'-monophosphates for binding to the target enzyme OMP decarboxylase, could then account for the inhibition of the enzyme at lower concentrations of these analogs.

Demonstration, by somatic cell genetics, of coordinate regulation of genes for two enzymes of purine synthesis assigned to human chromosome 21

A method for determining coordinate genetic regulation is proposed for mammalian cells. The method involves (i) isolation of a set of mutants defective in the relevant pathway; (ii) complementation analysis of these mutants to determine dominance and to categorize the mutants into various different complementation groups; (iii) determination of the biochemical blocks in the mutants; (iv) identification of individual mutants that fail to complement the members of at least two distinct complementation groups that complement each other, such mutants being said to show coordinate regulation of the affected functions; (v) biochemical and reversion analysis of the relevant cell types to confirm the basis for the observed coordinate regulation; (vi) assignment of the individual genes to particular human chromosomes; (vii) mapping of the genes to determine contiguity on the genome; and (viii) examination of the structure of the relevant gene products. This method has allowed the demonstration of coordinate regulation between the gene coding for phosphoribosylglycineamide synthetase [5-phosphoribosylamine:glycine ligase (ADP-forming), EC 6.3.4.13], defective in our Ade-C mutants, and the gene coding for phoshoribosylaminoimidazole synthetase [5'-phosphoribosylformylglycinamidine cyclo-ligase (ADP-forming), EC 6.3.3.1], defective in our Ade-G mutants. Moreover, both genes can be assigned to human chromosome 21. Because at least two genes for purine biosynthesis have now been assigned to chromosome 21, and because patients with trisomy 21 (Down syndrome) show increased levels of serum purines, it may be that cells of these patients overproduce purines and that this overproduction may be relevant to the pathology of the syndrome.