Assessment of the three-test genetic toxicology battery for groundwater metabolites

The two-test in vitro battery for genotoxicity testing (Ames and micronucleus) has in the majority of cases replaced the three-test battery (as two-test plus mammalian cell gene mutation assay) for the routine testing of chemicals, pharmaceuticals, cosmetics, and agrochemical metabolites originating from food and feed as well as from water treatment. The guidance for testing agrochemical groundwater metabolites, however, still relies on the three-test battery. Data collated in this study from 18 plant protection and related materials highlights the disparity between the often negative Ames and in vitro chromosome aberration data and frequently positive in vitro mammalian cell gene mutation assays. Sixteen of the 18 collated materials with complete datasets were Ames negative, and overall had negative outcomes in in vitro chromosome damage tests (weight of evidence from multiple tests). Mammalian cell gene mutation assays (HPRT and/or mouse lymphoma assay (MLA)) were positive in at least one test for every material with this data. Where both MLA and HPRT tests were performed on the same material, the HPRT seemed to give fewer positive responses. In vivo follow-up tests included combinations of comet assays, unscheduled DNA synthesis, and transgenic rodent gene mutation assays, all gave negative outcomes. The inclusion of mammalian cell gene mutation assays in a three-test battery for groundwater metabolites is therefore not justified and leads to unnecessary in vivo follow-up testing.

Prognostic significance and immunological role of HPRT1 in human cancers

Hypoxanthine phosphoribosyl transferase 1 (HPRT1), once considered a housekeeping gene, has been identified as playing an important role in several tumors. Its role in pan-cancer, however, has not been systematically studied. This study evaluates the relationship between HPRT1 and clinical parameters, survival prognosis, and tumor immunity based on multi omics data from the Cancer Genome Atlas (TCGA) and the Gene Expression Omnibus (GEO) databases. Drug sensitivity analysis screened 16 effective drugs against HPRT1, exploring the interactions with chemicals and genes. The significance of HPRT1 in tumor immunotherapy has also been investigated. Immunohistochemistry confirmed significant differences in the expression of HPRT1 between five tumor types (colon adenocarcinoma [COAD], head-neck squamous cell carcinoma [HNSC], lung adenocarcinoma [LUAD], thyroid carcinoma [THCA], and uterine corpus endometrial carcinoma [UCEC]) and adjacent normal tissues (P < 0.05). HPRT1 competitive endogenous RNA network was constructed in HNSC. Through cytological experiments, it was verified that HPRT1 plays a carcinogenic role in HNSC and is associated with tumor cell proliferation, migration, invasion, and apoptosis. In addition, there was a significant positive correlation between HPRT1 and programmed cell death-1 (PD-1) expression in HNSC (P < 0.05). These findings suggest that HPRT1 may be a potential biomarker for predicting and treating cancer.

Reference Gene Validation in the Embryonic and Postnatal Brain in the Rat Hyperhomocysteinemia Model

Maternal hyperhomocysteinemia (HCY) induced by genetic defects in methionine cycle enzymes or vitamin imbalance is known to be a pathologic factor that can impair embryonal brain development and cause long-term consequences in the postnatal brain development as well as changes in the expression of neuronal genes. Studies of the gene expression on this model requires the selection of optimal housekeeping genes. This work aimed to analyze the expression stability of housekeeping genes in offspring brain. Pregnant female Wistar rats were treated daily with a 0.15% L-methionine solution in the period starting on the 4th day of pregnancy until delivery, to cause the increase in the homocysteine level in fetus blood and brain. Housekeeping gene expression was assessed by RT-qPCR on whole embryonic brain and selected rat brain areas at P20 and P90. The amplification curves were analyzed, and raw means Cq data were imported to the RefFinder online tool to assess the reference genes stability. Most of the analyzed genes showed high stability of mRNA expression in the fetal brain at both periods of analysis (E14 and E20). However, the most stably expressed genes at different age points differed. Actb, Ppia, Rpl13a are the most stably expressed on E14, Ywhaz, Pgk1, Hprt1 - on E20 and P20, Hprt1, Actb, and Pgk1 - on P90. Gapdh gene used as a reference in various studies demonstrates high stability only in the hippocampus and cannot be recommended as the optimal reference gene on HCY model. Hprt1 and Pgk1 genes were found to be the most stably expressed in the brain of rat subjected to HCY. These two genes showed high stability in the brain on E20 and in various areas of the brain on the P20 and P90. On E14, the preferred genes for normalization are Actb, Ppia, Rpl13a.

Selection of reference genes in liproxstatin-1-treated K562 Leukemia cells via RT-qPCR and RNA sequencing

BACKGROUND

Reverse transcription quantitative polymerase chain reaction (RT-qPCR) can accurately detect relative gene expression levels in biological samples. However, widely used reference genes exhibit unstable expression under certain conditions.

METHODS AND RESULTS

Here, we compared the expression stability of eight reference genes (RPLP0, RPS18, RPL13, EEF1A1, β-actin, GAPDH, HPRT1, and TUBB) commonly used in liproxstatin-1 (Lip-1)-treated K562 cells using RNA-sequencing and RT-qPCR. The expression of EEF1A1, ACTB, GAPDH, HPRT1, and TUBB was considerably lower in cells treated with 20 μM Lip-1 than in the control, and GAPDH also showed significant downregulation in the 10 μM Lip-1 group. Meanwhile, when we used geNorm, NormFinder, and BestKeeper to compare expression stability, we found that GAPDH and HPRT1 were the most unstable reference genes among all those tested. Stability analysis yielded very similar results when geNorm or BestKeeper was used but not when NormFinder was used. Specifically, geNorm and BestKeeper identified RPL13 and RPLP0 as the most stable genes under 20 μM Lip-1 treatment, whereas RPL13, EEF1A1, and TUBB were the most stable under 10 μM Lip-1 treatment. TUBB and EEF1A1 were the most stable genes in both treatment groups according to the results obtained using NormFinder. An assumed most stable gene was incorporated into each software to validate the accuracy. The results suggest that NormFinder is not an appropriate algorithm for this study.

CONCLUSIONS

Stable reference genes were recognized using geNorm and BestKeeper but not NormFinder. Overall, RPL13 and RPLP0 were the most stable reference genes under 20 μM Lip-1 treatment, whereas RPL13, EEF1A1, and TUBB were the most stable genes under 10 μM Lip-1 treatment.

What e-cigarette factors determine oral epithelial DNA damage in consumers?

DESIGN

Study participants were divided equally into three groups being exclusive vapers (never smokers), current exclusive smokers and non-users. Brush biopsy samples of oral epithelial cells were collected. DNA damage quantification was assessed using LA-QPCR, and analysis interrogated a 12.2 kb region of the DNA polymerase beta gene (POLB). An additional gene, hypoxanthine phosphoribosyltransferase 1 (HPRT), was also interrogated for validity. Enzyme-linked immunosorbent assay (ELISA) was used to measure plasma cotinine levels. Breath monitoring was measured using Bedfont Micro Smokerlyzer in order to quantify exhaled CO and %COHb levels in participants.

CASE SELECTION

72 subjects, consisting of both males and females of diverse ages, races and ethnicities, were recruited. Comprehensive interviews alongside biochemical studies were used to verify smoking and vaping status. Participants classified as vapers reported a minimum use of e-cigarettes three times weekly for 6 months, with no use of cigarettes or tobacco products in their lifetime. Smokers reported cigarette consumption for a minimum of three times weekly for at least 12 months, less than five vaping sessions ever and no use of other tobacco products in the previous 6 months. Participants reporting no or less than five uses of e-cigarettes or tobacco products were classified as non-users. Former smokers, vapers and those who were dual or poly users of e-cigarettes, cigarettes or tobacco products were excluded.

DATA ANALYSIS

R environment for statistical computing (RStudio), was used for data analysis. The Shapiro-Wilk test was used to evaluate the distribution of data. Student's t test allowed comparison of all variables between two groups (vapers and nonusers, smokers and nonusers, or vapers and smokers), specifically DNA damage levels. A one-way Analysis of Variance (ANOVA) followed by a post hoc Tukey HSD test allowed comparison of damage in three or more groups (heavy vapers, light vapers, and nonusers, as well as heavy smokers, light smokers, and nonusers). DNA damage was also analysed in this manner when assessing e-cigarette device type, liquid type or in non-users. Pearson correlation coefficient analysis allowed examination of relationships between different variables.

RESULTS

Mean levels of DNA damage in the POLB gene was 2.6-fold higher in vapers (p = 0.005) and 2.2-fold higher in smokers (p = 0.020), when compared to non-users. On comparing POLB gene DNA damage in vapers versus smokers, the results were not statistically significant (p = 0.522). Comparing DNA damage in the HPRT gene, levels were much higher in vapers (p = 0.029) and smokers (p = 0.033) versus non-users. Similarly to the POLB gene, DNA damage levels in the HPRT gene in vapers versus smokers were not statistically significant (p = 0.578). When assessing volume of e-cigarette liquid or smoking pack years, levels of DNA damage in increased in the POLB gene in a dose-dependent manner between 'light' and 'heavy' users versus non-users (F = 4.571, p = 0.0156 | Tukey's HSD p = 0.0195 in vapers, F = 4.368, p = 0.0185 | Tukey's HSD p = 0.0135 in smokers). Vaping device type was investigated showing mean level of DNA damage in the oral cells of pod device users was 3.3-fold higher compared to non-users (F = 3.886, p = 0.0152 | Tukey's HSD p = 0.0216). This was followed by a 2.6-fold increase in oral cell DNA damage in Mod device users, and a 1.6-fold increase in multiple device users. Levels of DNA damage was higher in those who consume sweet-flavoured e-liquid (F = 3.238, p = 0.0146 | Tukey's HSD p < 0.05), followed by vapers of multiple flavours, mint or menthol and tobacco, and fruit flavours. No correlation was found between DNA damage of oral cells and cumulative nicotine consumption in vapers (r = 0.3189, p = 0.1288). Plasma cotinine levels, a validated maker of tobacco in cigarettes and e-cigarettes, were not significantly different between vapers and smokers (p = 0.607), but were significantly higher compared to non-users (p < 0.0001). Whist compared to non-users, vapers had similar levels of CO and %COHb, smokers showed significantly increased levels (p = 0.0005 and p = 0.0002, respectively).

CONCLUSIONS

Based on the results of this study, there is evidence to support a dose-dependent formation of DNA damage in oral cells in those vapers who have never smoked cigarettes, and in those exclusive cigarette smokers. Additionally, e-cigarette device type and flavour, may also determine levels of DNA damage.

Toxicological investigation of lilial

Lilial (also called lysmeral) is a fragrance ingredient presented in many everyday cosmetics and household products. The concentrations of lilial in the final products is rather low. Its maximum concentration in cosmetics was limited and recently, its use in cosmetics products was prohibited in the EU due to the classification as reproductive toxicant. Additionally, according to the European Chemicals Agency, it was under assessment as one of the potential endocrine disruptors, i.e. a substance that may alter the function of the endocrine system and, as a result, cause health problems. Its ability to act as an androgen receptor agonist and the estrogenic and androgenic activity of its metabolites, to the best of our knowledge, have not yet been tested. The aim of this work was to determine the intestinal absorption, cytotoxicity, nephrotoxicity, mutagenicity, activation of cellular stress-related signal pathways and, most importantly, to test the ability to disrupt the endocrine system of lilial and its Phase I metabolites. This was tested using set of in vitro assays including resazurin assay, the CHO/HPRT mutation assay, γH2AX biomarker-based genotoxicity assay, qPCR and in vitro reporter assays based on luminescence of luciferase for estrogen, androgen, NF-κB and NRF2 signalling pathway. It was determined that neither lilial nor its metabolites have a negative effect on cell viability in the concentration range from 1 nM to 100 µM. Using human cell lines HeLa9903 and MDA-kb2, it was verified that this substance did not have agonistic activity towards estrogen or androgen receptor, respectively. Lilial metabolites, generated by incubation with the rat liver S9 fraction, did not show the ability to bind to estrogen or androgen receptors. Neither lilial nor its metabolites showed a nephrotoxic effect on human renal tubular cells (RPTEC/TERT1 line) and at the same time they were unable to activate the NF-κB and NRF2 signalling pathway at a concentration of 50 µM (HEK 293/pGL4.32 or pGL4.37). Neither lilial nor its metabolites showed mutagenic activity in the HPRT gene mutation test in CHO-K1 cells, nor were they able to cause double-strand breaks in DNA (γH2AX biomarker) in CHO-K1 and HeLa cells. In our study, no negative effects of lilial or its in vitro metabolites were observed up to 100 µM using different in vitro tests.

Identification of suitable house-keeping genes during chikungunya virus infection

PURPOSE

Quantitative PCR (qPCR) is a reliable and robust technique for gene expression analysis, but its efficacy is dependent on the normalization of qPCR data with the stably expressed reference gene. Selection of a suitable reference gene is mandatory for accurate gene expression analysis, till data the most appropriate reference gene during chikungunya virus infection has not been elucidated.

METHOD

In this study the expression of reference genes(GAPDH, GUSB, HPRT, Beta-actin, 18S rRNA) was analysed during chikungunya virus infection by quantitative PCR. The stability of the house-keeping genes was evaluated with three bioinformatics softwares: BestKeeper, NormFinder and GeNorm.

RESULT

The significant variation in the expression of house-keeping genes (GusB, Beta-actin, HPRT) was observed during chikungunya virus infection; whereas GAPDH and 18S rRNA was most stable. The stability of reference genes analysed by the bioinformatics software further corroborate the results of qPCR.

CONCLUSION

This is first study that identifies and validates the most suitable reference gene for normalization of qPCR data during chikungunya based gene expression analysis. This could serve as a reference study for the researchers working on different aspects of chikungunya virus infections.

Age, sex, and frailty modify the expression of common reference genes in skeletal muscle from ageing mice

Changes in gene expression with age are typically normalised to constitutively expressed reference genes (RGs). However, RG expression may be affected by age or overall health and most studies use only male animals. We investigated whether expression of common RGs (Gapdh, Gusb, Rplp0, B2m, Tubb5, Rpl7l1, Hprt, Rer1) was affected by age, sex and/or overall health (frailty index) in skeletal muscle from young (4-mos) and aged (25-26-mos) mice. Standard RG selection programs recommended Gapdh (RefFinder/Genorm/NormFinder) or Rpl7l1 (BestKeeper) without considering age and sex. Analysis of raw Cq values showed only Rplp0 was stable in both sexes at both ages. When qPCR data were normalised to Rplp0, age affected RG expression, especially in females. For example, Hprt expression declined with age (Hprt=9.8 ×10-2 ± 4.7 ×10-2 vs. 6.5 ×10-3 ± 8.8 ×10-4; mean±SEM), while Gusb expression increased (6.0 ×10-4 ± 5.5 ×10-5 vs. 1.7 ×10-3 ± 3.1 ×10-4; n = 5/group; p < 0.05). These effects were not seen in males. Tubb5 and Gapdh were not affected by age or sex when normalised to Rplp0. Similar results were seen with normalisation by Gapdh or the Rplp0/Gapdh pair. Interestingly, RG expression was graded not only by age but by frailty. These data demonstrate that age, sex, and frailty of animals must be carefully considered when selecting RGs to normalise mRNA abundance data.

Identification of suitable reference genes for normalization of reverse transcription quantitative real-time PCR (RT-qPCR) in the fibrotic phase of the bleomycin mouse model of pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a severe lung disease with a poor prognosis and few treatment options. In the most widely used experimental model for this disease, bleomycin is administered into the lungs of mice, causing a reaction of inflammation and consequent fibrosis that resembles the progression of human IPF. The inflammation and fibrosis together induce changes in gene expression that can be analyzed with reverse transcription quantitative real-time PCR (RT-qPCR), in which accurate normalization with a set of stably expressed reference genes is critical for obtaining reliable results. This work compares ten commonly used candidate reference genes in the late, fibrotic phase of bleomycin-induced pulmonary fibrosis and ranks them from the most to the least stable using NormFinder and geNorm. Sdha, Polr2a and Hprt were identified as the best performing and least variable reference genes when alternating between normal and fibrotic conditions. In order to validate the findings, we investigated the expression of Tnf and Col1a1, representing the hallmarks of inflammation and fibrotic changes, respectively. With the best three genes as references, both were found to be upregulated relative to untreated controls, unlike the situation when analyzed solely with Gapdh, a commonly used reference gene. We therefore recommend Sdha, Polr2a and Hprt as reference genes for RT-qPCR in the 4-week bleomycin challenge that represents the late fibrotic phase.

Validation of droplet digital PCR for cytokeratin 19 mRNA detection in canine peripheral blood and mammary gland

In humans, peripheral blood cytokeratin 19 (CK19) mRNA-positive circulating tumor cells (CTCs) was utilized to identify early-stage breast cancer patients with micrometastatic disease who are at risk for disease progression and monitor treatment response in patients with advanced disease. To our knowledge, there has been little research regarding CK19 in canine mammary tumors (CMTs) using molecular methods. A droplet digital PCR (ddPCR) is proposed as a precise and sensitive quantification of nucleic acid targets. Hence, this study aimed to validate a newly designed assay for CK19 detection in canine blood and mammary tissue, along with the reference gene HPRT, by ddPCR. All primers and probes showed a precise match with the exon region of target genes. The assay exhibited PCR efficacy of 90.4% and 91.0% for CK19 and HPRT amplifications with linearity, respectively. The annealing temperature (Ta) for duplex ddPCR was 55 °C, providing the highest concentrations of both genes tested by the synthetic plasmid DNA. The limit of detection (LOD) of CK19 and HPRT were 2.16 ± 1.27 and 2.44 ± 1.31 copies/µL, respectively. Finally, the ddPCR assay was validated with canine peripheral blood, non-neoplastic mammary tissues and spiked samples. Our findings provide a new platform for CK19 studies in CMT diagnosis through blood and mammary tissues.

Pretreatment serum uric acid level is not a surrogate marker for the outcome of favipiravir treatment in COVID-19 patients

----.

RNA-guided retargeting of Sleeping Beauty transposition in human cells

An ideal tool for gene therapy would enable efficient gene integration at predetermined sites in the human genome. Here we demonstrate biased genome-wide integration of the Sleeping Beauty (SB) transposon by combining it with components of the CRISPR/Cas9 system. We provide proof-of-concept that it is possible to influence the target site selection of SB by fusing it to a catalytically inactive Cas9 (dCas9) and by providing a single guide RNA (sgRNA) against the human Alu retrotransposon. Enrichment of transposon integrations was dependent on the sgRNA, and occurred in an asymmetric pattern with a bias towards sites in a relatively narrow, 300 bp window downstream of the sgRNA targets. Our data indicate that the targeting mechanism specified by CRISPR/Cas9 forces integration into genomic regions that are otherwise poor targets for SB transposition. Future modifications of this technology may allow the development of methods for specific gene insertion for precision genetic engineering.

Mutational signatures of non-homologous and polymerase theta-mediated end-joining in embryonic stem cells

Cells employ potentially mutagenic DNA repair mechanisms to avoid the detrimental effects of chromosome breaks on cell survival. While classical non-homologous end-joining (cNHEJ) is largely error-free, alternative end-joining pathways have been described that are intrinsically mutagenic. Which end-joining mechanisms operate in germ and embryonic cells and thus contribute to heritable mutations found in congenital diseases is, however, still largely elusive. Here, we determined the genetic requirements for the repair of CRISPR/Cas9-induced chromosomal breaks of different configurations, and establish the mutational consequences. We find that cNHEJ and polymerase theta-mediated end-joining (TMEJ) act both parallel and redundant in mouse embryonic stem cells and account for virtually all end-joining activity. Surprisingly, mutagenic repair by polymerase theta (Pol θ, encoded by the Polq gene) is most prevalent for blunt double-strand breaks (DSBs), while cNHEJ dictates mutagenic repair of DSBs with protruding ends, in which the cNHEJ polymerases lambda and mu play minor roles. We conclude that cNHEJ-dependent repair of DSBs with protruding ends can explain de novo formation of tandem duplications in mammalian genomes.

Expression of tlr4, md2 and cd14 in equine blood leukocytes during endotoxin infusion and in intestinal tissues from healthy horses

The expression of tlr4, md2 and cd14 was studied in equine blood leukocytes and in intestinal samples using real time PCR. The stability of three commonly used reference genes, glyceraldehyde-3P-dehydrogenase (GAPDH), hypoxantine ribosyltransferase (HPRT) and succinate dehydrogenase complex subunit A (SDHA), was evaluated using qbase(PLUS). The equine peripheral blood mononuclear cells (eqPBMC) examined were either stimulated in vitro with Phorbol 12-myristate 13-acetate (PMA) and ionomycin or with the CpG oligodeoxynuclotide 2216 (CpG-ODN 2216) or obtained from horses before, during and after infusion of endotoxin. Intestinal tissue from healthy horses was sampled at ileum, right dorsal colon and rectum. Ranking of the three reference genes used for normalisation identified the combination HPRT/SDHA as most suitable both when determined ex vivo in leukocytes obtained from experimentally induced endotoxaemia and in eqPBMC activated in vitro while HPRT/GAPDH were most appropriate for the intestinal samples. The relative amounts of mRNA for TLR4 and MD-2 increased threefold during in vitro activation of the cells with CpG-ODN 2216 but was decreased in cultures stimulated with PMA/ionomycin. A transient elevation in the transcription of tlr4 and md2 was also evident for equine blood leukocytes following endotoxaemia. The levels of mRNA for CD14 on the other hand remained unaffected both during the induction of endotoxaemia and in the in vitro stimulated PBMCs. A low steady expression of TLR4, MD-2 and CD14 mRNA was demonstrated for the intestinal samples with no variation between the intestinal segments analysed. Thus, the foundation for real time PCR based levels of analysis of mRNA for all three components in the equine LPS receptor complex in different intestinal segments was set, making it possible to carry out future expression studies on clinical material.

Extensive sugar modification improves triple helix forming oligonucleotide activity in vitro but reduces activity in vivo

We are developing triple helix forming oligonucleotides (TFOs) for gene targeting. Previously, we synthesized bioactive TFOs containing 2'-O-methylribose (2'-OMe) and 2'-O-aminoethylribose (2'-AE) residues. Active TFOs contained four contiguous 2'-AE residues and formed triplexes with high thermal stability and rapid association kinetics. In an effort to further improve bioactivity, we synthesized three series of TFOs containing the 2'-AE patch and additional ribose modifications distributed throughout the remainder of the oligonucleotide. These were either additional 2'-AE residues, the conformationally locked BNA/LNA ribose with a 2'-O,4'-C-methylene bridge, or the 2'-O,4'-C-ethylene analogue (ENA). The additionally modified TFOs formed triplexes with greater thermal stability than the reference TFO, and some had improved association kinetics. However, the most active TFOs in the biochemical and biophysical assays were the least active in the bioassay. We measured the thermal stability of triplexes formed by the TFOs in each series on duplex targets containing a change in sequence at a single position. The Tm value of the variant sequence triplexes increased as the number of all additional modifications increased. A simple explanation for the failure of the improved TFOs in the bioassay was that the increased affinity for nonspecific targets lowered the effective nuclear concentration. Enhancement of TFO bioactivity will require chemical modifications that improve interaction with the specific targets while retaining selectivity against mismatched sequences.

Children's exposure to environmental pollutants and biomarkers of genetic damage. II. Results of a comprehensive literature search and meta-analysis

The present review is based on findings from 178 publications retrieved through an extensive search of the MedLine/PubMed database for a 25 years time period (1980-2004) and 10 manually identified papers. Among the cytogenetic biomarkers that are frequently used in field studies, chromosome aberrations (CA) and micronuclei (MN) but not sister chromatid exchanges (SCE) were found consistently increased in children exposed to environmental pollutants. Meta-analysis of the studies reporting SCE in cord blood showed similar levels of SCE in exposed and in non-exposed newborns. Exposure to airborne pollutants, soil and drinking water contaminants, mostly increased CA and, to a lesser extent, MN levels in children. The effect of exposure to airborne urban pollutants was consistently reported by field studies measuring DNA, albumin and hemoglobin adducts. Prenatal (in utero) and postnatal exposure (environmental tobacco smoke, ETS) to tobacco smoke compounds were associated with increased frequencies of DNA and hemoglobin adducts and CA. The limited number of field studies measuring DNA fragmentation (Comet assay), hypoxanthine-guanine phosphoribosyltransferase (HPRT) and the glycophorinA (GPA) mutation frequency in environmentally exposed children precluded a meaningful evaluation of the usefulness of these assays. Meta-analyses performed in children exposed to ETS and in newborns exposed in utero to their mothers' smoke showed 1.3 and 7 times higher levels of hemoglobin adducts compared to referent subjects, respectively. These increases are consistent with the epidemiological evidence of higher lung cancer risks reported in adults who had never smoked and were exposed to ETS during childhood and with 7-15 times higher lung cancer risks reported in smokers than in non-smokers. Higher levels of PAH-DNA adducts were found in fetal than in maternal tissue, suggesting a specific susceptibility of the fetus to this class of ubiquitous environmental pollutants. According to these findings, future research and biomonitoring programs on children would greatly benefit from the inclusion of selected biomarkers that could provide biologically based evidence for the identification of intervention priorities in environmental health.

Zidovudine-didanosine coexposure potentiates DNA incorporation of zidovudine and mutagenesis in human cells

Drug combinations that include nucleoside reverse transcriptase inhibitors (NRTIs) are remarkably effective in preventing maternal-viral transmission of HIV during pregnancy. However, there may be potential long-term risks for children exposed in utero. Examination of the genotoxic and mutagenic effects of two NRTIs, zidovudine [AZT (3'-azido-3'-deoxythymidine)] and didanosine [ddI (2',3'-dideoxyinosine)], in cultured human lymphoblastoid cells revealed multiplicative synergistic enhancement of AZT-DNA incorporation and mutant frequency induction in response to the combined drug exposure, as compared with single-drug exposures. Dose-related increases in DNA incorporation of AZT (as measured by a competitive RIA) and mutagenicity at the HPRT and TK loci (as assessed by cell-cloning assays) were observed in cells exposed in culture to AZT, or equimolar combinations of AZT + ddI, at exposure concentrations ranging from 3 to 30 times the maximum plasma levels found in humans. Because mutagenesis is strongly associated with tumor induction in experimental models, children exposed transplacentally to combinations of NRTIs may be at risk for cancer development later in life.

Somatic mutation of hPMS2 as a possible cause of sporadic human colon cancer with microsatellite instability

Inactivation of DNA-mismatch repair underlies the genesis of microsatellite unstable (MSI) colon cancers. hPMS2 is one of several genes encoding components of the DNA-mismatch repair complex, and germline hPMS2 mutations have been found in a few kindreds with hereditary nonpolyposis colorectal carcinoma (HNPCC), in whom hereditary MSI colon cancers develop. However, mice bearing null hPMS2 genes do not develop colon cancers and hPMS2 mutations in sporadic human colon cancers have not been described. Here we report that in Vaco481 colon cancer the hPMS2 gene is inactivated by somatic mutations of both hPMS2 alleles. The cell line derived from this tumor is functionally deficient in DNA mismatch repair. This deficiency can be biochemically complemented by addition of a purified hMLH1-hPMS2 (hMutLalpha) complex. The hPMS2 deficient Vaco481 cancer cell line demonstrates microsatellite instability, an elevated HPRT gene mutation rate, and resistance to the cytotoxicity of the alkylator MNNG. We conclude that somatic inactivation of hPMS2 can play a role in development of sporadic MSI colon cancer expressing the full range of cancer phenotypes associated with inactivation of the mismatch repair system.

Chemopreventive doses of amifostine confer no cytoprotection to tumor nodules growing in the lungs of mice treated with cyclophosphamide

In addition to the cytoprotective benefits of amifostine (Ethyol; Alza Pharmaceuticals, Palo Alto, CA/US Bioscience, West Conshohocken, PA) to normal cells, it also prevents the induction of somatic mutations that can lead to therapy-induced second cancers. The mutagenic effects of cyclophosphamide, an agent that is known to be mutagenic to normal cells, were determined in mouse splenocytes using a mutational assay system. Cyclophosphamide 100 mg/kg increased mutant frequencies 10-fold. In contrast, amifostine 100 mg/kg, whether administered 30 minutes before or 2 hours after cyclophosphamide administration, resulted in eightfold lowered mutant frequencies. To address potential cytoprotective effects on tumors exposed to this dose, amifostine was administered to tumor-bearing mice either 30 minutes before or 2 hours after the administration of cyclophosphamide. Cyclophosphamide (range, 10 to 100 mg/kg) was administered intraperitoneally into mice 4 days following the injection of 3.5 x 10(5) viable fibrosarcoma (FSa) cells. At this time, microcolonies of FSa tumors containing 50 to 200 cells were present in the lung. The number of FSa lung nodules formed at the end of 14 days in control animals was compared with that of animals treated with cyclophosphamide +/- amifostine. No cytoprotection of murine FSa tumors by amifostine was observed across the entire cyclophosphamide dose range tested, regardless of time of administration, demonstrating the utility of amifostine as a chemopreventive drug under conditions that do not allow cytoprotection for tumor cells.

Large domains of apparent delayed replication timing associated with triplet repeat expansion at FRAXA and FRAXE

Trinucleotide repeat expansions have been implicated in the causation of a number of neurodegenerative disorders. In the case of fragile X syndrome, full expansion of the FMR1 repeat element (CGG)n has also been correlated with replication timing delay of the locus and proximal flanking sequences in male lymphoblasts. To define more extensively this altered region of DNA replication, as well as to extend these studies to female cells containing premutant and mutant alleles, study of the replication timing properties of a >2-Mb zone in the FRAXA region (Xq27.3-q28) was undertaken by using a FISH technique. In this assay, relative times of replication of specific loci are inferred from the ratios of singlet and doublet hybridization signals in interphase nuclei. In all individuals with a full expansion of the trinucleotide repeat, a large (1-1.2-Mb) region of delayed timing was observed; the apparent timing of the earlier-replicating allele in female cells in this region was intermediate between normal and affected alleles in males, which is in accordance with expectations of a mixed population of cells resulting from random X inactivation. In addition, expansion of the nearby FRAXE locus also was found to correlate with replication timing delay, although the extent of the altered region was somewhat less. Trinucleotide repeat expansion thus may be acting in the Xq27.3-q28 region to alter long-range chromatin structure that could influence transcription of gene sequences within the affected domain.

Clonal analysis of human adrenocortical carcinomas and secreting adenomas

OBJECTIVES

Adrenocortical tumours in man are characterized mainly on biochemical, anatomical and histological grounds which establish their secretory pattern and, with some uncertainty, their benign or malignant nature. To study further these tumours and eventually to shed some light on their pathogenesis, we determined their clonal composition.

METHODS

Clonal composition was determined by X-chromosome inactivation analysis on tumour and leucocyte DNA using three markers: M27 beta, phospho-glycero-kinase (PGK) and hypoxanthine-phosphoribosyl transferase (HPRT) with 88, 33 and 27% heterozygosity rates respectively.

PATIENTS

Clonal analysis was performed on 25 tumours from 19 heterozygous female patients: four had a carcinoma, 14 had a single secreting adenoma, and one had autonomous bilateral macronodular hyperplasia with Cushing's syndrome (seven adenomas examined).

RESULTS

The malignant tumours had patterns indicative of monoclonality. The single adenomas displayed contrasting results with patterns indicative of monoclonality in eight cases, and patterns indicative of polyclonality in six cases; monoclonal adenomas were larger and had a higher prevalence of nuclear pleomorphism than the apparently polyclonal adenomas. In the patient with bilateral macronodular hyperplasia, different clonal patterns were present in different adenomas: whereas a clear monoclonal pattern was observed in the three adenomas of the right gland, in which the active X-allele was not always the same, in two interpretable adenomas of the left gland, a moderately skewed pattern suggested a partial monoclonal component.

CONCLUSIONS

These data show that adrenocortical carcinomas are monoclonal and suggest that adenomas may arise from a single cell or from more than one cell under the putative action of local growth factors. In adenomas, which until now had appeared homogeneous, this genetic heterogeneity may reflect different pathophysiological mechanisms or it may represent different stages of a common multistep process exceptionally occurring in a single patient with bilateral macronodular hyperplasia.

Effects of low-doses of X-rays on the expression of human cell surface CD2 antigen in CD2+ CHO cells

Thioguanine resistant CHO cells (HPRT-) were stably cotransfected with pSV2-gpt and pi H3-CD2 vectors using the calcium phosphate coprecipitation technique. The effects of single low doses of ionizing radiation were studied in a CD2+ CHO clone. The CD2+ phenotype responsible for binding sheep erythrocytes and rosette formation, was not affected by X-rays doses in the range 2-6 cGy. However, after 10 cGy of X-irradiation, 50% of the cells lost the CD2+ phenotype. These results suggest that this CD2+ clone might be a very sensitive indicator of very low X-ray doses. The implications of the phenotypic changes, observed after very low doses of irradiation, are discussed.

Dose-rate effects of neutrons and gamma-rays on the induction of mutation and oncogenic transformation in plateau-phase mouse m5S cells

The dose-rate effect of 252-californium neutrons was investigated using confluent cultures of mouse m5S cells. The relative biological effectiveness (RBE) of neutrons for oncogenic transformation was increased from 3.3 to 5.1 when the dose-rate was reduced from 1.8 to 0.12 cGy/min. Similarly, neutron RBE values for HPRT- mutation were 4.9 and 7.4 at dose-rates of 1.8 and 0.12 cGy/min, respectively. The increases in RBE as dose-rate was reduced were due mainly to diminished transformation- and mutation-induction by gamma-rays (the standard radiation). The yields of neutron-induced oncogenic transformation as well as neutron-induced mutation were constant for both dose rates. Our observation contrasts with reports by others using proliferating cells where both oncogenic transformation and mutation were enhanced with neutron exposure at a reduced dose-rate, the so-called inverse dose-rate effect. Since m5S cells are sensitive to postconfluent inhibition of cell division, this observation could be ascribed to cell growth conditions used in these experiments. The mechanism of the inverse dose-rate effect of neutrons suggests that the enhancement of neutron-induced mutation and oncogenic transformation at a reduced dose-rate is strongly associated with cell proliferation during exposure.

Hypoxanthine phosphoribosyl transferase assay of lead mutagenicity on keratinocytes

An improved hypoxanthine phosphoribosyl transferase (HPRT) assay system was used to investigate the genotoxicity in human and rat keratinocytes exposed to Pb2+ 0.1-100 mumol.ml-1 in vitro. Autoradiography was applied to determine the number of labeled cells/cm2 of culture with [3H]TdR and liquid scintillation spectrometry was used to determine the incorporation of [3H]TdR into DNA counting of 6-thioguanine (TG)-resistant cells. The ratio between the number of labeled cells in the Pb2+ treated group (T) and in the control group (C) was calculated. When the cells exposed to Pb2+ 6 mumol.L-1 for 4 h, the T/C ratios reached 1.75 (scintillation, S), and 2.07 (autoradiography, A) in human and 1.37 (S), and 1.77 (A) in rat cells. A positive relation existed between the concentration of Pb2+ and mutagenicity. Lead should be considered as a weak mutagen in human and rat keratinocytes.

[Comparison of point mutation induced by ethylnitrosourea in human fetus and Syrian hamster pulmonary epithelial cells]

Cytotoxicity and mutation at the HGPRT (hypoxanthineguanine phosphoribosyl transferase) locus induced by ethylnitrosourea (ENU) in human fetus and Syrian hamster pulmonary epithelial cells were studied. A dose-dependent relationship between ENU and both parameters was observed. Cell killing of ENU to both cell lines was similar when determined as a reduction in colony-forming efficiency. However, after treatment with 0.01 mg/ml of ENU, the frequency of point mutation was zero in the human cells, but 4.1/10(5) cells in the hamster cells. ENU at 0.8 mg/ml induced 248.4 mutants per 10(5) cells in the hamster cells, but only 9.4 mutants per 10(5) cells in the human cells. The average frequency of mutation from all doses in the hamster cells was 10.5-fold higher than in human cells. These results imply that hamster cells are more sensitive to mutation induction than human cells and the cytotoxic effect of ENU may not be responsible for point mutation in both cell lines. These results support our previous finding that human chromosomes are more stable than those of rodents.

Epstein-Barr virus (EBV) recombinants: use of positive selection markers to rescue mutants in EBV-negative B-lymphoma cells

The objective of these experiments was to develop strategies for creation and identification of recombinant mutant Epstein-Barr viruses (EBV). EBV recombinant molecular genetics has been limited to mutations within a short DNA segment deleted from a nontransforming EBV and an underlying strategy which relies on growth transformation of primary B lymphocytes for identification of recombinants. Thus, mutations outside the deletion or mutations which affect transformation cannot be easily recovered. In these experiments we investigated whether a toxic drug resistance gene, guanine phosphoribosyltransferase or hygromycin phosphotransferase, driven by the simian virus 40 promoter can be recombined into the EBV genome and can function to identify B-lymphoma cells infected with recombinant virus. Two different strategies were used to recombine the drug resistance marker into the EBV genome. Both utilized transfection of partially permissive, EBV-infected B95-8 cells and positive selection for cells which had incorporated a functional drug resistance gene. In the first series of experiments, B95-8 clones were screened for transfected DNA that had recombined into the EBV genome. In the second series of experiments, the transfected drug resistance marker was linked to the plasmid and lytic EBV origins so that it was maintained as an episome and could recombine with the B95-8 EBV genome during virus replication. The recombinant EBV from either experiment could be recovered by infection and toxic drug selection of EBV-negative B-lymphoma cells. The EBV genome in these B-lymphoma cells is frequently an episome. Virus genes associated with latent infection of primary B lymphocytes are expressed. Expression of Epstein-Barr virus nuclear antigen 2 (EBNA-2) and the EBNA-3 genes is variable relative to that of EBNA-1, as is characteristic of some naturally infected Burkitt tumor cells. Moreover, the EBV-infected B-lymphoma cells are often partially permissive for early replicative cycle gene expression and virus replication can be induced, in contrast to previously reported in vitro infected B-lymphoma cells. These studies demonstrate that dominant selectable markers can be inserted into the EBV genome, are active in the context of the EBV genome, and can be used to recover recombinant EBV in B-lymphoma cells. This system should be particularly useful for recovering EBV genomes with mutations in essential transforming genes.

Genetic toxicology studies comparing the activity of sidestream smoke from cigarettes which burn or only heat tobacco

The results of in vitro genetic toxicology studies of sidestream cigarette smoke (SSCS) from cigarettes which heat but do not burn tobacco were compared to those of sidestream smoke from cigarettes which burn tobacco. SSCSs from 5 cigarettes were compared. Three of the cigarettes, the Kentucky reference research cigarette (1R4F), a commercially available ultra-low-tar brand (ULT) and a commercially available ultra-low-tar menthol brand (ULT-menthol) burn tobacco while two of the cigarettes, a regular (TEST) and a menthol (TEST-menthol) heat tobacco. SSCSs from all cigarettes were prepared by identical techniques, which involved collecting sidestream smoke particulate matter on Cambridge filter pads and combining the particulate matter with the vapor-phase materials collected by bubbling the smoke exiting the Cambridge pad through DMSO. The SSCSs obtained (equivalent to 0.4 cigarettes/ml DMSO) were evaluated at identical concentrations in an in vitro genetic toxicology test battery. SSCS from 1R4F, ULT and ULT-menthol cigarettes produced positive results in Ames bacterial strains TA98, TA100, TA1537 and TA1538 in the presence of metabolic activation (S9 from Aroclor-induced rat liver) but negative results in strain TA1535. In the absence of metabolic activation, 1R4F, ULT and ULT-menthol SSCSs were not significantly mutagenic. TEST and TEST-menthol SSCSs produced negative results in all 5 bacterial strains, both with and without metabolic activation. SSCS from 1R4F, ULT and ULT-menthol cigarettes produced positive results in the CHO chromosomal aberration assay and in the CHO sister-chromatid exchange assay both with and without metabolic activation while TEST and TEST-menthol SSCSs produced negative results in both assays, either with or without metabolic activation. The SSCSs from 1R4F, ULT and ULT-menthol cigarettes were weakly positive in inducing DNA repair in cultured rat hepatocytes while TEST and TEST-menthol SSCSs were negative in this assay. All 5 SSCSs were nonmutagenic in the CHO-HGPRT assay both with and without metabolic activation. SSCSs from the 1R4F, ULT and ULT-menthol cigarettes were cytotoxic in the CHO-HGPRT assay, both with and without metabolic activation, while TEST and TEST-menthol SSCSs were not cytotoxic under either condition. These results demonstrate that sidestream smoke from cigarettes which heat but do not burn tobacco (TEST and TEST-menthol) was neither genotoxic nor cytotoxic under conditions where sidestream smoke from cigarettes which burn tobacco (1R4F, ULT and ULT-menthol) was genotoxic and/or cytotoxic in a concentration-dependent manner.

Use of a selectable marker regulated by alpha interferon to obtain mutations in the signaling pathway

We have selected mutations in genes encoding components of the signaling pathway for alpha interferon (IFN-alpha) by using a specially constructed cell line. The upstream region of the IFN-regulated human gene 6-16 was fused to the Escherichia coli guanine phosphoribosyltransferase (gpt) gene and transfected into hypoxanthine-guanine phosphoribosyltransferase-negative human cells. These cells express gpt only in the presence of IFN-alpha. They grow in medium containing hypoxanthine, aminopterin, and thymidine plus IFN and are killed by 6-thioguanine plus IFN. Two different types of mutants were obtained after treating the cells with mutagens. A recessive mutant, selected in 6-thioguanine plus IFN, was completely resistant to IFN-alpha but responded normally to IFN-gamma and, unexpectedly, partially to IFN-beta. A constitutive mutant, selected in hypoxanthine-aminopterin-thymidine alone, was abnormal in expressing endogenous genes in the absence of IFN. Both types revert infrequently, allowing selection for complementation of the defects by transfection.

The molecular nature of mutants induced by X rays is altered by the presence of the radioprotector cysteamine

The molecular structure of mutants induced in human lymphoblast cells by 500 cGy X rays in the presence of the radioprotector cysteamine (25 mM) has been compared with that induced by an equally mutagenic treatment of 150 cGy X rays alone. Sets of mutants at the hypoxanthine-guanine phosphoribosyl transferase locus were analyzed by Southern blot. Of 24 mutants induced by X rays in the presence of cysteamine, 67% exhibited no change in the restriction fragment pattern and thus were defined as point mutations; 8% appeared to be total gene deletions and 25% were partial deletions or rearrangements. In contrast, among 28 mutants induced by X rays alone (Liber et al., Mutat. Res. 178, 143-153 (1987)), 46% were point mutations, while 50% were total gene deletions and only 1 mutant (4%) was a partial deletion or rearrangement. Thus mutants isolated in the presence of cysteamine consisted of more point mutations and partial deletions/rearrangements, and considerably fewer total gene deletions. These results suggest that cysteamine may protect selectively against processes which lead to large-scale molecular changes.

Molecular analysis of spontaneous mutations at the gpt locus in Chinese hamster ovary (AS52) cells

AS52 cells are Chinese hamster ovary (CHO) cells that carry a single functional copy of the bacterial gpt gene and allow the isolation of 6-thioguanine-resistant (6TGr)mutants arising from mutation at the chromosally integrated gpt locus. The gpt locus in AS52 cells is extremely stable, giving rise to 6TGr mutants at frequencies comparable to the endogenous CHO hprt locus. In this study, we describe the spectrum of spontaneous mutations observed in AS52 cells by Southern blot and DNA sequence analyses. Using the polymerase chain reaction (PCR) and the Thermus aquaticus (Taq) polymerase, we have enzymatically amplified 6TGr mutant gpt sequences in vitro. The PCR product was then sequenced without further cloning manipulations to directly identify gpt structural gene mutations. Deletions predominant among the 62 spontaneous 6TGr-AS52 mutant clones analyzed in this study. Of these, 79% (49/62) of the mutations were identified as deletions either by Southern blotting, PCR amplification or DNA sequence analysis. Among these deletions is a predominant 3-base deletion that was observed in 31% (19/62) of the mutants. These data provide a basis for future comparisons of induced point mutational spectra derived in the AS52 cell line, and demonstrate the utility of PCR in the generation of DNA sequence spectra derived from chromosomally integrated mammalian loci.

Mutagenic treatments result in inactivation of expression of a transfected bacterial gene integrated into a human cell line

The cell line E2 is a SV40-transformed human fibroblast cell line containing a single integrated copy of the bacterial guanine phosphoribosyl transferase (gpt) gene. Treatment of E2 with ultraviolet light (UV) or ethyl methanesulphonate (EMS) induced the formation of Gpt- derivatives. Several induced derivatives have been isolated, and the structure, expression and revertibility of the gpt gene have been analysed. In the majority of cases the Gpt- phenotype resulted from switching off the gpt gene, in most instances by methylation, but in a few cases by phenotypic switching. Thus mutagenic treatment can result in the inactivation of gene expression in human cells. In a small proportion of Gpt- derivatives the gpt sequences were deleted.

A review and analysis of the Chinese hamster ovary/hypoxanthine guanine phosphoribosyl transferase assay to determine the mutagenicity of chemical agents. A report of phase III of the U.S. Environmental Protection Agency Gene-Tox Program

Published literature on the Chinese hamster ovary cell/hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) assay from mid-1979 through June 1986 was reviewed and evaluated. Data from the papers considered acceptable include test results on 121 chemicals belonging to 25 chemical classes. A total of 87 chemicals were evaluated positive, 3 negative, and 31 inconclusive. Mutagenicity data on 49 of the 121 chemicals evaluated could also be compared with in vivo animal carcinogenicity data. 40 of the 43 reported animal carcinogens were considered mutagenic. Caprolactam, the only definitive noncarcinogen in the group of 49, was not mutagenic. The CHO/HGPRT assay was concluded to be an appropriate assay system for use in the screening of chemicals for genotoxicity.

The lack of genotoxicity of sodium fluoride in a battery of cellular tests

In a comprehensive assessment of genotoxicity, sodium fluoride was evaluated in a battery of cellular tests providing different genetic end points and biotransformation capabilities. The tests included the following: rat hepatocyte primary culture/DNA repair assay, Salmonella typhimurium histidine locus reversion assay, adult rat liver epithelial cell/hypoxanthine guanine phosphoribosyl transferase mutation assay, and sister chromatid exchange in two target cell types, human peripheral blood lymphocytes and Chinese hamster ovary cells. Negative findings were made in all assays, indicating that sodium fluoride is not genotoxic in these assays.

High-frequency deletional rearrangement of immunoglobulin kappa gene segments introduced into a pre-B-cell line

We describe an immunoglobulin gene recombination indicator in which a specific rearrangement via deletion results in the acquisition of a dominant phenotype. The indicator consists of the Escherichia coli xanthine/guanine phosphoribosyltransferase (gpt) gene, whose translation is prevented by the presence of an upstream initiation codon out of frame with respect to the gpt coding sequence. Flanking this barrier initiation codon are the heptamer-spacer-nonamer recognition sequences from a kappa chain variable region (V kappa) and from a kappa chain joining region (J kappa). A proper V-J joint results in the deletion of the translational barrier and allows expression of the selectable marker. When tested by transfection into fibroblasts, no rearrangements were detected and the presence of the barrier initiation codon was sufficient to completely abolish gpt expression in these cells. Similarly, no rearrangements were detected after transfer of the test gene into myeloma cells. However, when the construct was introduced into the pre-B-cell line 38B9, greater than 80% of the transfected cells showed evidence of a specific rearrangement. These rearrangements were associated with the translation of gpt, although no selection for its expression was needed. DNA sequence analysis of six different V-J joints revealed that the rearrangement proceeded with a high degree of accuracy. These results indicate that only very minimal DNA sequences (21 base pairs 5' of the V heptamer and 4 base pairs 3' of its nonamer; less than 45 base pairs 5' of the J nonamer and 3' of its heptamer) are required for efficient rearrangement and provide formal proof that kappa gene segments can rearrange by a deletional mechanism.

Inactivation of a transfected gene in human fibroblasts can occur by deletion, amplification, phenotypic switching, or methylation

Plasmids containing the bacterial gpt gene under control of the simian virus 40 promoter were transfected into a simian virus 40-transformed human fibroblast line. Two transfectants, E2 and C10, which contain stably integrated single copies of the gpt gene, were isolated. These two lines produce Gpt- variants spontaneously with a frequency of about 10(-4). We carried out a detailed molecular analysis of the spectrum of alterations which gave rise to the Gpt- phenotype in these variants. DNA from 14 of 19 Gpt- derivatives of one of the cell lines (E2) contains deletions or rearrangements of gpt-containing sequences. In four of the remaining five lines, the Gpt- phenotype was correlated with reduced levels of expression rather than with changes in the gross structure of the gpt gene, and it was possible to reactivate the gpt gene. In one Gpt- line, gpt mRNA was present at normal levels, but no active enzyme was produced. Spontaneous Gpt- derivatives of the other cell line (C10) produced a completely different spectrum of alterations. Very few deletions were found, but several derivatives contained additional extrachromosomal gpt sequences, and, remarkably, in two other Gpt- lines, gpt-containing sequences were amplified more than 100-fold. The phenotypes of the majority of the Gpt- derivatives of C10 could be attributed to alterations in gene expression caused by methylation.

Hypoxanthine:guanine phosphoribosyltransferase activity in primary human osteosarcomas. A rationale for therapy with methotrexate-thymidine rescue?

Hypoxanthine:guanine phosphoribosyltransferase (HPRT) activity was measured in 14 human osteosarcomas to test whether a subset of these tumors was deficient in the purine salvage pathway enzyme and thus provide a rationale for therapy with methotrexate-thymidine rescue. All tumors contained HPRT activity within the range previously reported for xenografts of human osteosarcoma. Three patients received methotrexate (3.375 g/m2/24 hours) as a 72-hour continuous infusion with thymidine rescue (2.0 g/m2/24 hours) beginning 24 hours after the start of the methotrexate infusion. The methotrexate-thymidine infusion was well tolerated by all patients with no significant toxicity; however, there were no responses. We conclude that osteosarcomas are not deficient in HPRT activity. Therefore, the previously reported rationale for therapy of osteosarcoma with methotrexate-thymidine based on lack of activity of this enzyme is not valid. This combination, although well tolerated, is inconvenient and requires prolonged hospitalization. Therefore, without a valid rationale it cannot be recommended for therapy of patients with osteosarcoma.

Evidence for tetrameric structure of mammalian hypoxanthine phosphoribosyltransferase

A fast electrophoretic variant of hypoxanthine phosphoribosyltransferase (HPRT) has been detected in Mus musculus bactrianus, a mouse subspecies from Middle Asia (USSR). The electrophoretic HPRT pattern yielded by hybrids between the somatic cell of LMTK- (deficient in thymidine kinase) and the splenocytes of a male of M. m. bactrianus was five-banded. The pattern obtained from the germ cells of the ovaries from 14.5-day-old embryos from laboratory CBA mice X M. m. bactrianus crosses was also composed of five bands. The hybrids between the somatic cells of human fibroblasts X LMTK- cells gave a three-banded electrophoretic HPRT pattern because the asymmetrical heteropolymeric isozymes are probably unstable. Taken together, all the evidence is in favor of a tetrameric structure of mammalian HPRT.

Purification and characterization of guanine phosphoribosyltransferase from Giardia lamblia

Giardia lamblia, a flagellated parasitic protozoan and the causative agent of giardiasis, lacks de novo purine biosynthesis and exists on salvage of adenine and guanine by adenine phosphoribosyltransferase and guanine phosphoribosyltransferase. Guanine phosphoribosyltransferase from G. lamblia crude extracts has been purified to apparent homogeneity by Sephacryl S-200 gel filtration followed by C-8-GMP-agarose and 2',3'-GMP-agarose affinity chromatography, resulting in an overall recovery of 77% and a purification of 83,000-fold. The molecular weight of the native enzyme as estimated by gel filtration and isokinetic sucrose gradients was found to be 58,000-63,000, with a subunit molecular weight of approximately 29,000, as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mono P chromatofocusing chromatography gives rise to a major activity peak eluting from the column at a pH of 6.75 and two minor activity peaks at pH of 5.3 and 5.2. Hypoxanthine and xanthine can be recognized by the enzyme as substrates but at Km values 20 times higher than that observed with guanine. G. lamblia guanine phosphoribosyltransferase is immunologically distinct from human hypoxanthine-guanine phosphoribosyltransferase and Escherichia coli xanthine-guanine phosphoribosyltransferase, and G. lamblia DNA fragments are incapable of hybridizing with mouse neuroblastoma hypoxanthine-guanine phosphoribosyltransferase DNA or E. coli xanthine phosphoribosyltransferase DNA under relatively relaxed conditions. All evidence presented suggests that G. lamblia guanine phosphoribosyltransferase may be qualified as a potential target for antigiardiasis chemotherapy.

Trans-acting nuclear protein responsible for induction of rearranged human immunoglobulin heavy chain gene

A complete human gamma 1 heavy chain gene (HIG1) was transferred into mouse cells by protoplast fusion. The HIG1 gene was strongly expressed in mouse myeloma cells but not in mouse fibroblasts (L cells). Nuclear extracts from myeloma cells were injected into L cell transformants containing one copy of the HIG1 gene; this triggered accurate transcription of the HIG1 gene in the transformants. The induction of HIG1 gene expression by a myeloma nuclear factor (or factors) appeared to depend on the enhancers in the heavy chain gene. Nuclear proteins prepared from cells of the B lineage could induce the transcription of HIG1 gene in the L cell transformants, while those from the cells of non-B-lineage could not. The present study shows that positive regulatory trans-acting factors are involved in the activation of the immunoglobulin heavy chain gene through its enhancers and are contained only in cells of the B lineage.

One pyrimidine dimer inactivates expression of a transfected gene in xeroderma pigmentosum cells

We have developed a host cell reactivation assay of DNA repair utilizing UV-treated plasmid vectors. The assay primarily reflects cellular repair of transcriptional activity of damaged DNA measured indirectly as enzyme activity of the transfected genes. We studied three plasmids (pSV2cat, 5020 base pairs; pSV2catSVgpt, 7268 base pairs; and pRSVcat, 5027 base pairs) with different sizes and promoters carrying the bacterial cat gene (CAT, chloramphenicol acetyltransferase) in a construction that permits cat expression in human cells. All human simian virus 40-transformed cells studied expressed high levels of the transfected cat gene. UV treatment of the plasmids prior to transfection resulted in differential decrease in CAT activity in different cell lines. With pSV2catSVgpt, UV inactivation of CAT expression was greater in the xeroderma pigmentosum group A and D lines (D0 = 56 J X m-2) than in the other human cell lines tested (normal, ataxia-telangiectasia, Lesch-Nyhan, retinoblastoma)(D0 = 680 J X m-2)(D0 is the dose that reduces the percentage of CAT activity by 63% along the exponential portion of the dose-response curve). The D0 of the CAT inactivation curve was 50 J X m-2 for pSV2cat and for pRSVcat in the xeroderma pigmentosum group A cells. The similarity of the D0 data in the xeroderma pigmentosum group A cells for three plasmids of different size and promoters implies they all have similar UV-inactivation target size. UV-induced pyrimidine dimer formation in the plasmids was quantified by assay of the number of UV-induced T4 endonuclease V-sensitive sites. In the most sensitive xeroderma pigmentosum cells, with all three plasmids, one UV-induced pyrimidine dimer inactivates a target of about 2 kilobases, close to the size of the putative CAT mRNA.

Genetic linkage but independent expression of functional HSV-1 tk and mammalian aprt genes after cotransfer to L cells

DNA-mediated gene transformation of mouse Ltk-aprt-hprt-cells was used to obtain stable, doubly selected transformants simultaneously expressing herpes virus thymidine kinase (TK) and mammalian adenine phosphoribosyltransferase (APRT). Cotransformants occurred at a frequency of 5 X 10(-6), a similar frequency for the transfer of the aprt marker has been previously observed. Isozyme and Southern blot analysis show that the TK and APRT expressed in these transformants resulted from gene transfer. For one stable cotransformant, [3H]thymidine [( 3H]TdR) selection against TK activity resulted in the loss of APRT activity as well, suggesting that these genes had become genetically linked together. Similarly selection against APRT expression resulted in the loss of a subset of the transferred herpes simplex virus tk genes. 5-Bromodeoxyuridine (BUdR) selected TK- variants differed from [3H]TdR selected TK- variants, in that they retained tk genes. However, BUdR-selected variants expressed full levels of APRT. Therefore, even though the transferred tk and aprt genes had become genetically linked together, they were, in this case, independently expressed since these cells were phenotypically TK- and APRT+.

The genotoxicity of alpha particles in human embryonic skin fibroblasts

Cell inactivation and induced mutation frequencies at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus have been measured in cultured human fibroblasts (GM10) exposed to alpha particles from 238Pu (LET at the cell surface was 100 keV/microns) and 250 kVp X rays. The survival curves resulting from exposure to alpha particles are exponential. The mean lethal dose, D0, is approximately 1.3 Gy for X rays and 0.25 Gy for alpha particles. As a function of radiation dose, mutation induction at the HGPRT locus was linear for alpha particles whereas the X-ray-induced mutation data were better fitted by a quadratic function. When mutation frequencies were plotted against the log of survival, mutation frequency at a given survival level was greater in cells exposed to alpha particles than to X rays.

A simplified method for estimating 5-phosphoribosyl 1-pyrophosphate in mouse liver and spleen

A simplified method for the estimation of 5-phosphoribosyl 1-pyrophosphate (PRPP) in mouse liver and spleen is described. The method uses the enzymic conversion of [8-14C] hypoxanthine to [8-14C] inosine 5'-monophosphate in the presence of PRPP and has the advantage over previously published methods in that the enzyme used in this assay, hypoxanthine-guanine phosphoribosyltransferase (HG-PRTase), is present in the tissue being analyzed and does not require preparation. In addition, each assay uses as its internal standard PRPP, thus overcoming the problem of the chemical instability inherent in this molecule. The two-minute heat treatment used in the method was found to destroy most interfering enzymes and resulted in average recovery of PRPP of 56% and 85% for the liver and spleen, respectively. The counts obtained in the assay were about ten-fold above background (80 c.p.m) and allowed accurate measurement of the phosphoribose sugar. The levels of PRPP (+/- standard deviation) found for liver and spleen in twelve (12) separate Balb/C male mice were 6.3 +/- 1.8 and 10.8 +/- 5.0 nmoles/g wet weight tissue, respectively, and agree with the tissue concentrations already reported for this metabolite. A further advantage of this assay is that it gives an index of the HG-PRTase activity in the tissues.

Purine metabolism in high- and low-uric acid lines of chickens: hypoxanthine/guanine phosphoribosyltransferase activities

The activity of hypoxanthine/guanine phosphoribosyltransferase (HGPRT) was examined in the livers and kidneys of two genetic lines of chickens selected for different plasma uric acid levels. Previous work demonstrated that the high-uric acid line (HUA) had significantly greater de novo uric acid synthesis rates in kidney tissue compared to the low-uric acid line (LUA). In addition, phosphoribosylpyrophosphate (PRPP) synthetase and xanthine dehydrogenase activities in livers and kidneys were significantly higher in the HUA compared to the LUA line. PRPP pool sizes were also significantly higher in both livers and kidneys of HUA birds. HGPRT activities in livers of HUA birds were significantly (P less than 0.05) greater than in LUA birds. The mean value of liver HGPRT was 7.36 +/- 0.25 pmole inosine-5'-monophosphate (IMP) and 6.05 +/- 0.27 pmole IMP produced/micrograms protein/hr, respectively, for the HUA and LUA lines. There were no significant differences (P greater than 0.05) in kidney HGPRT activities between the two groups. The mean value of kidney HGPRT was 52.87 +/- 1.62 pmole IMP and 50.72 +/- 1.62 pmole IMP produced/micrograms protein/hr, respectively, for the HUA and LUA line. Elevated liver HGPRT may serve to enhance the regeneration of PRPP in the HUA liver. Elevated liver PRPP synthetase and PRPP pool size suggest an increased flux through the de novo purine biosynthetic pathway in HUA birds. The resulting additional pyrophosphate from the glutamine PRPP amidotransferase reaction would stimulate recovery of PRPP and spare the system from a substantial loss of energy.

Androgen-regulated expression of a cloned rat prostatic c3 gene transfected into mouse mammary tumor cells

The expression of the rat prostatic steroid binding protein gene C3 is androgen-responsive in vivo in the rat prostate. Recombinant transfection vectors were constructed containing a cloned C3 rat gene ligated directly into the dominant acting selection vector pSV2-gpt, and were used to transfect androgen-responsive mouse mammary tumor cells (S115 cells). Transformants containing intact copies of the C3 gene were isolated and shown by primer extension analysis to produce C3 transcripts that had authentic 5' termini. The steady state level of the transcripts was under androgen control in three cell lines tested which had low copy numbers of the intact C3 gene, but was not regulated in a cell line which had a very high C3 gene copy number. Despite this difference in the androgenic regulation of integrated C3 rat genes, both type of cell lines still demonstrated androgen-regulated growth characteristics.

Salvage capacity of hepatoma 3924A and action of dipyridamole

The role and behavior of the salvage enzymes in the biosynthesis of purines (adenine and hypoxanthine-guanine phosphoribosyltransferases) and pyrimidines (uridine-cytidine, deoxycytidine and thymidine kinases) were elucidated. In liver purine metabolism the transferase activities were orders of magnitude higher than the activities of the enzymes of de novo biosynthesis. In both purine and pyrimidine biosynthesis the activities of the enzymes of the de novo pathways were low (23 pmol to 70 nmol/hr/mg protein), whereas those of salvage synthetic pathways ranged from 0.8 to 1,470 nmol/hr/mg protein. In purine metabolism the salvage enzymes had markedly higher affinity to the shared substrate PRPP (4 to 40 microM) than the rate-limiting enzyme of de novo synthesis, amidophosphoribosyltransferase (900 microM). In rapidly growing hepatoma 3924A the activities of the enzymes of de novo purine biosynthesis increased, whereas those of the salvage pathway changed little. However, the activities of the enzymes of the salvage pathways remained much higher than those of the enzymes of de novo purine production. In pyrimidine production in the hepatomas the activities of both de novo and salvage enzymes markedly increased. However, the activities of the salvage enzymes far outstripped those of the enzymes of the de novo pathways. To inhibit the operation of the salvage pathways, the action of the transport inhibitor, dipyridamole, was examined. In tissue culture, dipyridamole inhibited the transport of purine and pyrimidine nucleosides with an IC50 of 10(-6) or 10(-7) M. As measured by colony-forming assay, dipyridamole killed hepatoma cells with an IC50 of 20 microM. Dipyridamole markedly depressed the pools of ATP, GTP, CTP and UTP; in combination chemotherapy with acivicin, an anti-glutamine agent, synergistic action was observed on the pools of nucleotides in hepatoma 3924A in vivo. These investigations emphasize the importance of the capacity to utilize precursors by the salvage enzymes and may explain, in part at least, the failure of inhibitors of the de novo pathways to yield lasting chemotherapeutic results. Combination chemotherapy of inhibitors of the de novo pathways with an inhibitor of the salvage pathways (dipyridamole) should impact on our understanding of the contribution of salvage pathways and provide a rational basis for successful combination chemotherapy of neoplastic diseases.

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.

[Eremothecium ashbyii mutants resistant to 8-azaguanine. III. Nucleotide pyrophosphorylase activity of mutants with a varying degree of 8-azaguanine resistance]

Eremothecium ashbyii mutants resistant to low and high concentrations of 8-azaguanine (AZG), have been obtained. In low resistant mutants (10(-4) M AZG) isolated by one step selection, the activity of GMP- and AMP-pyrophosphorylases was decreased, as compared with the initial sensitive strain. The stepwise increase of the mutants resistance to AZG resulted in increasing of resistance to 8-azaadenine and decreasing of the activity of GMP-pyrophosphorylase, while this did not affect the level of AMP-pyrophosphorylase. Characteristics of the cross-resistance of mutant to purine analogues and the level of nucleotide-pyrophosphorylases activity were discussed in the light of their possible influence on riboflavin biosynthesis.

Guanine uptake and metabolism in Neurospora crassa

Guanine is transported into germinated conidia of Neurospora crassa by the general purine base transport system. Guanine uptake is inhibited by adenine and hypoxanthine but not xanthine. Guanine phosphoribosyltransferase (GPRTase) activity was demonstrated in cell extracts of wild-type germinated conidia. The Km for guanine ranged from 29 to 69 micro M in GPRTase assays; the Ki for hypoxanthine was between 50 and 75 micro M. The kinetics of guanine transport differ considerably from the kinetics of GPRTase, strongly suggesting that the rate-limiting step in guanine accumulation in conidia is not that catalyzed by GPRTase. Efflux of guanine or its metabolites appears to have little importance in the regulation of pools of guanine or guanine nucleotides since very small amounts of 14C label were excreted from wild-type conidia preloaded with [8-14C]guanine. In contrast, excretion of purine bases, hypoxanthine, xanthine, and uric acid appears to be a mechanism for regulation of adenine nucleotide pools (Sabina et al., Mol. Gen. Genet. 173:31-38, 1979). No label from exogenous [8-14C]guanine was ever found in any adenine nucleotides, nucleosides, or the base, adenine, upon high-performance liquid chromatography analysis of acid extracts from germinated conidia of wild-type of xdh-l strains. The 14C label from exogenous [8-14C]guanine was found in GMP, GDP, GTP, and the GDP sugars as well as in XMP. Xanthine and uric acid were also labeled in wild-type extracts. Similar results were obtained with xdh-l extracts except that uric acid was not present. The labeled xanthine and XMP strongly suggest the presence of guanase and xanthine phosphoribosyltransferase in germinated conidia.

Red cell proteins. I. Two-dimensional mapping of human erythrocyte lysate proteins

Human erythrocyte lysate proteins were resolved into over 250 discrete spots by two-dimensional electrophoresis using isoelectric focusing in the first dimension and electrophoresis in the presence of sodium dodecyl sulfate, (SDS) in the second. The overwhelming excess of hemoglobin has made such analyses difficult in the past. However, with the ISO-DALT two-dimensional electrophoresis system, large numbers of red cell proteins can be mapped in the presence of hemoglobin. When hemoglobin and several other major proteins are removed by adsorption to DEAE-cellulose, additional minor components are seen, giving a total of over 275. With the use of purified preparations, the map positions of five cell enzymes or their subunits were determined: pyruvate kinase, catalase, glucose-6-phosphate dehydrogenase, hypoxanthine phosphoribosyltransferase, and carbonic anhydrase. The mapping techniques described complement and extend those traditionally used to find human red cell protein variants.