Integrating murine gene expression studies to understand obstructive lung disease due to chronic inhaled endotoxin.

RATIONALE: Endotoxin is a near ubiquitous environmental exposure that that has been associated with both asthma and chronic obstructive pulmonary disease (COPD). These obstructive lung diseases have a complex pathophysiology, making them difficult to study comprehensively in the context of endotoxin. Genome-wide gene expression studies have been used to identify a molecular snapshot of the response to environmental exposures. Identification of differentially expressed genes shared across all published murine models of chronic inhaled endotoxin will provide insight into the biology underlying endotoxin-associated lung disease. METHODS: We identified three published murine models with gene expression profiling after repeated low-dose inhaled endotoxin. All array data from these experiments were re-analyzed, annotated consistently, and tested for shared genes found to be differentially expressed. Additional functional comparison was conducted by testing for significant enrichment of differentially expressed genes in known pathways. The importance of this gene signature in smoking-related lung disease was assessed using hierarchical clustering in an independent experiment where mice were exposed to endotoxin, smoke, and endotoxin plus smoke. RESULTS: A 101-gene signature was detected in three murine models, more than expected by chance. The three model systems exhibit additional similarity beyond shared genes when compared at the pathway level, with increasing enrichment of inflammatory pathways associated with longer duration of endotoxin exposure. Genes and pathways important in both asthma and COPD were shared across all endotoxin models. Mice exposed to endotoxin, smoke, and smoke plus endotoxin were accurately classified with the endotoxin gene signature. CONCLUSIONS: Despite the differences in laboratory, duration of exposure, and strain of mouse used in three experimental models of chronic inhaled endotoxin, surprising similarities in gene expression were observed. The endotoxin component of tobacco smoke may play an important role in disease development.

Genotoxicity of 1,3-butadiene and its epoxy intermediates.

Current risk assessments of 1,3-butadiene (BD*) are complicated by limited evidence of its carcinogenicity in humans. Hence, there is a critical need to identify early events and factors that account for the heightened sensitivity of mice to BD-induced carcinogenesis and to deter-mine which animal model, mouse or rat, is the more useful surrogate of potency for predicting health effects in BD-exposed humans. HEI sponsored an earlier investigation of mutagenic responses in mice and rats exposed to BD, or to the racemic mixture of 1,2-epoxy-3-butene (BDO) or of 1,2,3,4-diepoxybutane (BDO2; Walker and Meng 2000). In that study, our research team demonstrated (1) that the frequency of mutations in the hypoxanthine-guanine phosphoribosyl transferase (Hprt) gene of splenic T cells from BD-exposed mice and rats could be correlated with the species-related differences in cancer susceptibility; (2) that mutagenic-potency and mutagenic-specificity data from mice and rats exposed to BD or its individual epoxy intermediates could provide useful information about the BD metabolites responsible for mutations in each species; and (3) that our novel approach to measuring the mutagenic potency of a given chemical exposure as the change in Hprt mutant frequencies (Mfs) over time was valuable for estimating species-specific differences in mutagenic responses to BD exposure and for predicting the effect of BD metabolites in each species. To gain additional mode-of-action information that can be used to inform studies of human responses to BD exposure, experiments in the current investigation tested a new set of five hypotheses about species-specific patterns in the mutagenic effects in rodents of exposure to BD and BD metabolites: 1. Repeated BD exposures at low levels that approach the occupational exposure limit for BD workers (set by the U.S. Occupational Safety and Health Administration) are mutagenic in female mice. 2. The differences in mutagenic responses of the Hprt gene to BD in similarly exposed rodents of a given species (reported in various earlier studies) are primarily associated with age-related thymus activity and trafficking of T cells and with sex-related differences in BD metabolism. 3. The mutagenic potency of the stereochemical forms of BD's epoxy intermediates plays a significant role in the species-related mutagenicity of BD. 4. The hydrolysis-detoxification pathway of BD through 1,2-dihydroxy-3-butene (BD-diol) is a major contributor to mutagenicity at high-level BD exposures in mice and rats. 5. Significant and informative species-specific differences in mutation spectra can be identified by examining both large- and small-scale genetic alterations in the Hprt gene of BD-exposed mice and rats. The first four hypotheses were tested by exposing mice and rats to BD, meso-BDO2, or BD-diol and measuring Hprt Mfs as the primary biomarker. For this, we used the T-cell-cloning assay of lymphocytes isolated from the spleens of exposed and control (sham-exposed) mice and rats. The first hypothesis was tested by exposing female B6C3F1 mice (4 to 5 weeks of age) by inhalation for 2 weeks (6 hours/day, 5 days/week) to 0 or 3 ppm BD. Hprt Mfs were measured at the time of peak mutagenic response after exposure for this age of mice. We then compared the resulting data to those from mutagenicity studies with mice of the same age that had been exposed in a similar protocol to higher levels of BD (Walker and Meng 2000). In mice exposed to 3 ppm BD (n = 27), there was a significant 1.6-fold increase over the mean background Hprt Mf in control animals (n = 24, P = 0.004). Calculating the efficiency of Hprt mutant induction, by dividing induced Hprt Mfs by the respective BD exposure levels, demonstrated that the mutagenic potency of 3 ppm BD was twice that of 20 ppm BD and almost 20 times that of 625 or 1250 ppm BD in exposed female mice. Sample-size calculations based on the Hprt Mf data from this experiment demonstrated the feasibility of conducting a future experiment to find out whether induced Mfs at even lower exposure levels (between 0.1 and 1.0 ppm BD) fit the supralinear exposure-response curve found with exposures between 3.0 and 62.5 ppm BD, or whether they deviate from the curve as Mf values approach the background levels found in control animals. The second hypothesis was tested by estimating mutagenic potency for female mice exposed by inhalation for 2 weeks to 0 or 1250 ppm BD at 8 weeks of age and comparing this estimate to that reported for female mice exposed to BD in a similar protocol at 4 to 5 weeks of age (Walker and Meng 2000). For these two age groups, the shapes of the mutant splenic T-cell manifestation curves were different, but the mutagenic burden was statistically the same. These results support our contention that the disparity in responses reported in earlier Hprt-mutation studies of BD-exposed rodents is related more to age-related T-cell kinetics than to age-specific differences in the metabolism of BD. The third hypothesis was tested by estimating mutagenic potency for female mice and rats (4 to 5 weeks of age) exposed by inhalation to 2 or 4 ppm meso-BDO2 and comparing these estimates to those previously obtained for female mice and rats of the same age and exposed in a similar protocol to (+/-)-BDO2 (Meng et al. 1999b; Walker and Meng 2000). These exposures to stereospecific forms of BDO2 caused equivalent mutagenic effects in each species. This suggests that the small differences in the mutagenic potency of the individual stereoisomers of BDO2 appear to be of less consequence in characterizing the sources of BD-induced mutagenicity than the much larger differences between the mutagenic potencies of BDO2 and the other two BD epoxides (BDO and 1,2-dihydroxy-3,4-epoxybutane [BDO-diol]). The fourth hypothesis was tested in several experiments. First, female and male mice and rats (4 to 5 weeks of age) were exposed by nose only for 6 hours to 0, 62.5, 200, 625, or 1250 ppm BD or to 0, 6, 18, 24, or 36 ppm BD-diol primarily to establish BD and BD-diol exposure levels that would yield similar plasma concentrations of BD-diol. Second, animals were exposed in inhalation chambers for 4 weeks to 0, 6, 18, or 36 ppm BD-diol to determine the mutagenic potency estimates for these exposure levels and to compare these estimates with those reported for BD-exposed female mice and rats (Walker and Meng 2000) in which similar blood levels of BD-diol had been achieved. Measurements of plasma concentrations of BD-diol (via a gas chromatography and mass spectrometry [GC/MS] method developed for this purpose) showed these results: First, BD-diol accumulated in a sublinear manner during a single 6-hour exposure to more than 200 ppm BD. Second, BD-diol accumulated in a linear manner during single (6-hour) or repeated (4-week) exposure to 6 or 18 ppm BD and in a sublinear manner with increasing levels of BD-diol exposure. Third, exposure of female mice and rats to 18 ppm BD-diol produced plasma concentrations equivalent to those produced by exposure to 200 ppm BD (exposure to 36 ppm BD-diol produced plasma concentrations of about 25% of those produced by exposure to 625 ppm BD). In general, 4-week exposure to 18 or 36 ppm BD-diol was significantly mutagenic in female and male mice and rats. The differences in mutagenic responses between the species and sexes were not remarkable, except that the mutagenic effects were greatest in female mice. The substantial differences in the exposure-related accumulation of BD-diol in plasma after rodents were exposed to more than 200 ppm BD compared with the relatively small differences in the mutagenic responses to direct exposures to 6, 18, or 36 ppm BD-diol in female mice provided evidence that the contribution of BD-diol-derived metabolites to the overall mutagenicity of BD has a narrow range of effect that is confined to relatively high-level BD exposures in mice and rats. This conclusion was supported by the results of parallel analyses of adducts in mice and rats concurrently exposed to BD-diol (Powley et al. 2005b), which showed that the exposure-response curves for the formation of N-(2,3,4-trihydroxybutyl)valine (THB-Val) in hemoglobin, formation of N7-(2,3,4-trihydroxybutyl)guanine (THB-Gua) in DNA, and induction of Hprt mutations in exposed rodents were remarkably similar in shape (i.e., supralinear). Combined, these data suggest that trihydroxybutyl (THB) adducts are good quantitative indicators of BD-induced mutagenicity and that BD-diol-derived BDO-diol (the major source of the adducts) might be largely responsible for mutagenicity in rodents exposed to BD-diol or to hight levels of BD. The mutagenic-potency studies of meso-BDO2 and BD-diol reported here, combined with our earlier studies of BD, (+/-) BDO, and(+/-)-BDO2 (Walker and Meng 2000), revealed important trends in species-specific mutagenic responses that distinguish the relative degree to which the epoxy intermediates contribute to mutation induction in rodents at selected levels of BD exposures. These data as a whole suggest that , in mice, BDO2 largely causes mutations at exposures less than 62.5 ppm BD and that BD-diol-derived metabolites add to these mutagenic effects at higher BD exposures. In rats, it appears that the BD-diol pathway might account for nearly all the mutagenicity at the hight-level BD exposures where significant increases in Hprt Mfs are found and cancers are induced. Additional exposure-response studies of hemoglobin and DNA adducts specifics to BDO2, BDO-diol, and other reactive intermediates are needed to determine more definitively the relative contribution of each metabolite to the DNA alkylation and mutation patterns induced by BD exposure in mice and rats. For the fifth hypothesis, a multiplex polymerase chain reaction (PCR) procedure for the analysis of genomic DNA mutations in the Hprt gene of mice was developed. (ABSTRACT TRUNCATED)

Cytotoxicity and gene expression profiles in cell cultures exposed to whole smoke from three types of cigarettes.

The purpose of this study was to evaluate and compare the cytotoxicity and gene expression profiles in cell cultures exposed to whole smoke generated from a full flavor cigarette (Test 1), a low tar cigarette (Test 2), and an ultra-low tar cigarette (Test 3). In addition, a reference cigarette 2R4F was evaluated for cytotoxicity. Neutral red (NR) cytotoxicity assay was performed to determine relative cell death at each exposure concentration (n = 6). LC(50) was generated using wet total particular matter (WTPM), cigarette number, or nicotine concentrations. The overall order of cytotoxicity was Test 1 >> 2R4F approximately Test 2 > Test 3. Cell culture samples were collected for RNA extraction at WTPM concentrations of each cigarette that gave similar nicotine concentrations. Affymetrix mouse whole genome 430 2.0 array was used to characterize the gene expression profiles for each cigarette. A total of 598 genes in Test 1, 176 genes in Test 2, and 234 genes in Test 3 samples were differentially expressed compared to the concurrent sham controls. The major biological processes associated with the changed genes in Test 1 samples were down-regulated DNA replication and cell proliferation; the same biological processes were much less affected in Test 2 and Test 3 samples. The common findings in all three cigarettes types were increased glutathione biosynthesis/consumption and inflammatory response, which are known biological effects caused by smoke exposure. The most significantly up-regulated genes were CYP1A1, GSTs, Hmox1, and Procr in smoke-exposed samples, which are either related to well-studied mechanisms of smoke exposure-related diseases or potential new biomarkers for assessing and monitoring biological effects of cigarette smoke exposure in vivo and in smokers. In summary, both the NR cytotoxicity assay and gene expression profiling were able to differentiate the three types of test cigarettes, and the results demonstrated reduced biological effects for the Test 2 and Test 3 cigarettes compared to the Test 1 cigarette in BALB/c-3T3 Cells.

Plasma and cellular markers of 3'-azido-3'-dideoxythymidine (AZT) metabolism as indicators of DNA damage in cord blood mononuclear cells from infants receiving prepartum NRTIs.

Several systemic and cellular markers of 3'-azido-3'-dideoxythymidine (AZT) metabolism and AZT incorporation into nuclear DNA were measured in cord blood from uninfected infants born to HIV-1-infected mothers receiving prepartum therapies based on AZT or AZT in combination with 2',3'-dideoxy-3'-thiacytidine (3TC). In addition, the relationships among these pharmacological end points, levels of AZT-DNA incorporation, and the previously reported mutagenic responses in these infants were evaluated. AZT- and 3TC-specific radioimmunoassays (RIAs), or HPLC coupled with AZT-RIA, were used to measure plasma levels of AZT and the AZT-glucuronide, and cellular levels of AZT, phosphorylated AZT, and DNA incorporation of AZT or 3TC in cord blood mononuclear cells from treated infants compared with unexposed controls born to HIV-uninfected mothers. Fewer infants had detectable AZT-DNA incorporation levels in the group exposed to AZT (71%; n = 7) compared with those receiving AZT-3TC (100%; n = 21), and the mean AZT-DNA incorporation for AZT-exposed infants (14.6 +/- 6.3 AZT/10(6) nucleotides) was significantly lower than that in AZT-3TC exposed infants (51.6 +/- 10.2 AZT/10(6) nucleotides; P = 0.028). Low levels of 3TC-DNA incorporation found in a few AZT-3TC-exposed newborns correlated with AZT-DNA incorporation values in the same samples. Among the metabolites studied, there were positive correlations between levels of AZT-diphosphate and AZT-triphosphate, and AZT-triphosphate and AZT-DNA incorporation, in nucleoside analog-exposed infants. Levels of AZT-DNA incorporation, however, did not correlate well with the reported frequencies of somatic mutations in the same population of nucleoside analog-treated children. While these data support the continued use of AZT-based therapies during pregnancy, infants receiving prepartum AZT should be monitored long-term for adverse health effects.

Genotoxicity assessed by the comet and GPA assays following in vitro exposure of human lymphoblastoid cells (H9) or perinatal exposure of mother-child pairs to AZT or AZT-3TC.

The genotoxicity of zidovudine (AZT) based treatments was investigated in human H9 lymphoblastoid cells in an in vitro study and in red blood cells (RBCs) from perinatally exposed HIV-1-infected mothers and their infants in an observational cohort study. Exposure of H9 cells for 24 hr to AZT produced dose-dependent increases in Comet assay tail moment (TM) when electrophoresed at pH 13.0, but not at pH 12.1 or pH 8.0, suggesting that DNA damage was via alkali-labile lesions and not double-stranded DNA strand breaks. The TM dose response at pH 13.0 correlated directly with AZT-DNA incorporation determined by AZT-radioimmunoassay. Levels of DNA damage in utero, measured by Comet assay TM, were similar in cord blood mononuclear cells of nucleoside analog-exposed newborns (n = 43) and unexposed controls (n = 40). In contrast, the glycophorin A (GPA) somatic cell mutation assay (which screens for large-scale DNA damage in RBCs) showed clear evidence that GPA N/N variants, arising from chromosome loss and duplication, somatic recombination, and gene conversion, were significantly elevated in mother-child pairs receiving prepartum AZT plus lamivudine (3TC). Cord blood from newborns exposed to AZT-3TC had GPA N/N variant frequencies of 4.7 +/- 0.7 (mean +/- SE) x 10(-6) RBCs (n = 26 infants) compared with 2.2 +/- 0.3 x 10(-6) RBCs for unexposed controls (n = 30 infants; P < 0.001). Elevations in GPA N/N variants generally persisted through 1 year of age in nucleoside analog-exposed children. Overall, the mutagenic effects found in mother-child pairs receiving AZT-based treatments justify their surveillance for long-term genotoxic consequences.

Measurement of plasma or urinary metabolites and Hprt mutant frequencies following inhalation exposure of mice and rats to 3-butene-1,2-diol.

Studies were performed to determine if the detoxification pathway of 1,3-butadiene (BD) through 3-butene-1,2-diol (BD-diol) is a major contributor to mutagenicity in BD-exposed mice and rats. First, female and male mice and rats (4-5 weeks old) were exposed by nose-only for 6h to 0, 62.5, 200, 625, or 1250 ppm BD or to 0, 6, 18, 24, or 36 ppm BD-diol primarily to establish BD and BD-diol exposure concentrations that yielded similar plasma levels of BD-diol, and then animals were exposed in inhalation chambers for 4 weeks to BD-diol to determine the mutagenic potency estimates for the same exposure levels and to compare these estimates to those reported for BD-exposed female mice and rats where comparable blood levels of BD-diol were achieved. Measurements of plasma levels of BD-diol (via GC/MS methodology) showed that (i) BD-diol accumulated in a sub-linear fashion during single 6-h exposures to >200 ppm BD; (ii) BD-diol accumulated in a linear fashion during single or repeated exposures to 6-18 ppm BD and then in a sub-linear fashion with increasing levels of BD-diol exposure; and (iii) exposures of mice and rats to 18 ppm BD-diol were equivalent to those produced by 200 ppm BD exposures (with exposures to 36 ppm BD-diol yielding plasma levels approximately 25% of those produced by 625 ppm BD exposures). Measurements of Hprt mutant frequencies (via the T cell cloning assay) showed that repeated exposures to 18 and 36 ppm BD-diol were significantly mutagenic in mice and rats. The resulting data indicated that BD-diol derived metabolites (especially, 1,2-dihydroxy-3,4-epoxybutane) have a narrow range of mutagenic effects confined to high-level BD (>or=200 ppm) exposures, and are responsible for nearly all of the mutagenic response in the rat and for a substantial portion of the mutagenic response in the mouse following high-level BD exposures.

Mutagenicity of stereochemical configurations of 1,2-epoxybutene and 1,2:3,4-diepoxybutane in human lymphblastoid cells.

The carcinogenicity of 1,3-butadiene (BD) is related to its bioactivation to several DNA-reactive metabolites; accumulating evidence suggests that the stereochemistry of these BD intermediates may play a significant role in the mutagenic and carcinogenic actions of the parent compound. The objective of this study was to evaluate the cytotoxicity and mutagenicity of stereochemical forms of 1,2-epoxybutene (EB) and 1,2:3,4-diepoxybutane (DEB), two genotoxic BD metabolites, in a human lymphoblastoid cell line, TK6. Cytotoxicity was measured by comparing cloning efficiencies in chemical-exposed cells versus those in control cells. The hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK) mutant frequencies (MFs) were measured using a cell cloning assay. HPRT mutants collected from cells exposed to the three forms of DEB were analyzed by PCR to characterize large genetic alterations. All the three stereoisomers of DEB caused increased HPRT and TK MFs compared to the concurrent control samples. There were no significant differences in cytotoxicity or mutagenicity among the three isomers of DEB in TK6 cells. Molecular analysis of HPRT mutants revealed similar distributions of types of mutations among the three isomers of DEB. There were also no statistically significant differences in mutagenic efficiencies between the two isomers of EB in TK6 cells. These results were consistent with the in vivo findings that there was little difference in the mutagenic efficiencies of racemic-DEB versus meso-DEB in rodents. Thus, in terms of mutagenic efficiency, stereochemical configurations of EB and DEB are not likely to play a significant role in the mutagenicity and carcinogenicity of BD.

Age-, gender-, and species-dependent mutagenicity in T cells of mice and rats exposed by inhalation to 1,3-butadiene.

Experiments were performed: (i) to investigate potential age- and gender-dependent differences in mutagenic responses in T cells following exposures of B6C3F1 mice and F344 rats by inhalation for 2 weeks to 0 or 1250 ppm butadiene (BD), and (ii) to determine if exposures for 2 weeks to 62.5 ppm BD produce a mutagenic effect in female rats. To evaluate the effect of age on mutagenic response, mutant manifestation curves for splenic T cells of female mice exposed at 8-9 weeks of age were defined by measuring Hprt mutant frequencies (MFs) at multiple time points after BD exposure using a T cell cloning assay and comparing the resulting mutagenic potency estimate (calculated as the difference of areas under the mutant manifestation curves of treated versus control animals) to that reported for female mice exposed to BD in the same fashion beginning at 4-5 weeks of age. The shapes of the mutant T cell manifestation curves for spleens were different [e.g., the maximum BD-induced MFs in older mice (8.0+/-1.0 [S.D.]x10(-6)) and younger mice (17.8+/-6.1 x 10(-6)) were observed at 8 and 5 weeks post-exposure, respectively], but the mutagenic burden was the same for both age groups. To assess the effect of gender on mutagenic response, female and male rodents were exposed to BD at 4-5 weeks of age and Hprt MFs were measured when maximum MFs are expected to occur post-exposure. The resulting data demonstrated that the pattern for mutagenic susceptibility from high-level BD exposure is female mice>male mice>female rats>male rats. Exposures of female rats to 62.5 ppm BD caused a minor but significant mutagenic response compared with controls (n=16/group; P=0.03). These results help explain part of the differing outcomes/interpretations of data in earlier Hprt mutation studies in BD-exposed rodents.

Characterization of Hprt mutations in cDNA and genomic DNA of T-cell mutants from control and 1,3-butadiene-exposed male B6C3F1 mice and F344 rats.

A multiplex PCR procedure for analysis of genomic DNA mutations in the mouse hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene was developed and then used with other established methods for the coincident identification of large- and small-scale genetic alterations in the Hprt gene of mutant T-cell isolates propagated from sham- and 1,3-butadiene (BD)-exposed mice and rats. The spectra data for RT-PCR/cDNA analysis and multiplex PCR of genomic DNA from Hprt mutants were combined, and statistical analyses of the mutant fractions for the classes of mutations identified in control versus exposed animals were conducted. Under the assumption that the mutant fractions are distributed as Poisson variates, BD exposure of mice significantly increased the frequencies of (1) nearly all types of base substitutions; (2) single-base deletions and insertions; and (3) all subcategories of deletions. Significantly elevated fractions of G:C-->C:G and A:T-->T:A transversions in the Hprt gene of BD-exposed mice were consistent with the occurrence of these substitutions as the predominant ras gene mutations in multiple tumor types increased in incidence in carcinogenicity studies of BD in mice. BD exposure of rats produced significant increases in (1) base substitutions only at A:T base pairs; (2) single-base insertions; (3) complex mutations; and (4) deletions (mainly 5' partial and complete gene deletions). Future coincident analyses of large- and small-scale mutations in rodents exposed to specific BD metabolites should help identify species differences in the sources of deletion mutations and other types of mutations induced by BD exposures in mice versus rats.

Molecular analysis of mutations at the HPRT and TK loci of human lymphoblastoid cells after combined treatments with 3'-azido-3'-deoxythymidine and 2',3'-dideoxyinosinedagger.

Combinations of antiretroviral drugs that include nucleoside reverse transcriptase inhibitors (NRTIs) are superior to single-agent regimens in treating or preventing HIV infection, but the potential long-term health hazards of these treatments in humans are uncertain. In earlier studies, our group found that coexposure of TK6 human lymphoblastoid cells to 3'-azido-2',3'-dideoxythymidine (AZT) and 2',3'-dideoxyinosine (ddI), the first two NRTIs approved by the FDA as antiretroviral drugs, produced multiplicative synergistic enhancement of DNA incorporation of AZT and mutagenic responses in both the HPRT and TK reporter genes, as compared with single-drug exposures (Meng Q et al. [2000a]: Proc Natl Acad Sci USA 97:12667-12671). The purpose of the current study was to characterize the mutational specificity of equimolar mixtures of 100 microM or 300 microM AZT + ddI at the HPRT and TK loci of exposed cells vs. unexposed control cells, and to compare the resulting mutational spectra data to those previously found in cells exposed to AZT alone (Sussman H et al. [1999]: Mutat Res 429:249-259; Meng Q et al. [2000b]: Toxicol Sci 54:322-329). Molecular analyses of HPRT mutant clones were performed by reverse transcription-mediated production of cDNA, PCR amplification, and cDNA sequencing to define small DNA alterations, followed by multiplex PCR amplification of genomic DNA to define the fractions of deletion events. TK mutants with complete gene deletions were distinguished by Southern blot analysis. The observed HPRT mutational categories included point mutations, microinsertions/microdeletions, splicing-error mutations, and macrodeletions including partial and complete gene deletions. The only significant difference or shift in the mutational spectra for NRTI-treated cells vs. control cells was the increase in the frequency of complete TK gene deletions following exposures (for 3 days) to 300 microM AZT-ddI (P = 0.034, chi-square test of homogeneity); however, statistical analyses comparing the observed mutant fraction values (measured mutant frequency x percent of a class of mutation) between control and NRTI-treated cells for each class of mutation showed that the occurrences of complete gene deletions of both HPRT and TK were significantly elevated over background values (0.34 x 10(-6) in HPRT and 6.0 x 10(-6) in TK) at exposure levels of 100 microM AZT-ddI (i.e., 1.94 x 10(-6) in HPRT and 18.6 x 10(-6) in TK) and 300 microM AZT-ddI (i.e., 5.6 x 10(-6) in HPRT and 34.6 x 10(-6) in TK) (P < 0.05, Mann-Whitney U-statistic). These treatment-related increases in complete gene deletions were consistent with the spectra data for AZT alone (ibid.) and with the known mode of action of AZT and ddI as DNA chain terminators. In addition, cotreatments of ddI with AZT led to substantial absolute increases in the mutant fraction of other classes of mutations, unlike cells exposed solely to AZT [e.g., the frequency of point mutations among HPRT mutants was significantly increased by 130 and 323% over the background value (4.25 x 10(-6)) in cells exposed to 100 and 300 microM AZT-ddI, respectively]. These results indicate that, at the same time that AZT-ddI potentiates therapeutic or prophylactic efficacy, the use of a second NRTI with AZT may confer a greater cancer risk, characterized by a spectrum of mutations that deviates from that produced solely by AZT.