Molecular analysis of lacI mutations in Rat2 cells exposed to 7,12-dimethylbenz[a]anthracene: evidence for DNA sequence and DNA strand biases for mutation

Valproic acid counteracts polycyclic aromatic hydrocarbons (PAHs)-induced tumorigenic effects by regulating the polarization of macrophages

Polycyclic aromatic hydrocarbons (PAHs) are common persistent organic pollutants that are carcinogenic, teratogenic and mutagenic, causing a variety of harm to human health. In this study, we investigated the mechanism of how valproic acid (VPA) interferes with the carcinogenesis of PAHs protect normal tissues via the regulation of macrophages' function. Using the established model of transformed malignant breast cancer by 7,12-dimethylbenz[a]anthracene (DMBA), a representative PAH carcinogen, we discovered VPA induces the polarization of macrophages toward the M1 phenotype in the tumor tissues, facilitates the expression of pro-inflammatory cytokines such as IFN-γ, IL-12 and TNF-α, activates CD8⁺ T cells to secret Granzyme B thus to promote the apoptosis of tumor cells and suppresses the viability of vascular endothelial cells in tissue stroma of tumor. Surprisingly, VPA selectively induces macrophages to polarize towards the M2 phenotype in normal tissues and promotes the expression of anti-inflammatory cytokines such as IL-10 to enhance cell proliferation. Additionally, at the cellular level, VPA can directly regulate the polarization of macrophages to affect the growth of vascular endothelial cells by simulating the living conditions of tumor and normal cells. Collectively, VPA exerts an interventional effect on tumor growth and a protective effect on normal tissues by regulation of selective macrophages' polarization in their microenvironment.

The intervention of valproic acid on the tumorigenesis induced by an environmental carcinogen of PAHs

This study investigated whether valproic acid (VPA, a histone deacetylase inhibitor) can interfere with the carcinogenicity of polycyclic aromatic hydrocarbons (PAHs). A typical representative compound of PAHs, 7,12-Dimethylbenz[a]anthracene (DMBA), was used to induce rat breast cancer. The results showed that therapeutic concentration of VPA (50 and 100 mg/kg) delayed the occurrence of tumors, reduced tumor formation rate and attenuated tumors growth, and have a protective effect on normal tissues. The macrophage-mediated inflammatory response was found to be associated with the observed effect of VPA. In addition, we screened and validated a possible gene, Sema3c, which was involved in DMBA-induced breast cancer development and can be inhibited by VPA.

Genetic screens to identify pathogenic gene variants in the common cancer predisposition Lynch syndrome

In many individuals suspected of the common cancer predisposition Lynch syndrome, variants of unclear significance (VUS), rather than an obviously pathogenic mutations, are identified in one of the DNA mismatch repair (MMR) genes. The uncertainty of whether such VUS inactivate MMR, and therefore are pathogenic, precludes targeted healthcare for both carriers and their relatives. To facilitate the identification of pathogenic VUS, we have developed an in cellulo genetic screen-based procedure for the large-scale mutagenization, identification, and cataloging of residues of MMR genes critical for MMR gene function. When a residue identified as mutated in an individual suspected of Lynch syndrome is listed as critical in such a reverse diagnosis catalog, there is a high probability that the corresponding human VUS is pathogenic. To investigate the applicability of this approach, we have generated and validated a prototypic reverse diagnosis catalog for the MMR gene MutS Homolog 2 (Msh2) by mutagenizing, identifying, and cataloging 26 deleterious mutations in 23 amino acids. Extensive in vivo and in vitro analysis of mutants listed in the catalog revealed both recessive and dominant-negative phenotypes. Nearly half of these critical residues match with VUS previously identified in individuals suspected of Lynch syndrome. This aids in the assignment of pathogenicity to these human VUS and validates the approach described here as a diagnostic tool. In a wider perspective, this work provides a model for the translation of personalized genomics into targeted healthcare.

Transgenic lambda medaka as a new model for germ cell mutagenesis

To address the need for improved approaches to study mutations transmitted to progeny from mutagen-exposed parents, we evaluated lambda transgenic medaka, a small fish that carries the cII mutation target gene, as a new model for germ cell mutagenesis. Mutations in the cII gene in progeny derived from ethyl-nitrosourea (ENU)-exposed males were readily detected. Frequencies of mutant offspring, proportions of mosaic or whole body mutant offspring, and mutational spectra differed according to germ cell stage exposed to ENU. Postmeiotic germ cells (spermatozoa/late spermatids) generated a higher frequency of mutant offspring (11%) compared to premeiotic germ cells (3.5%). Individuals with cII mutant frequencies (MF) elevated more than threefold above the spontaneous MF (3 x 10(-5)) in the range of 10(-4) to 10(-3) were mosaic mutant offspring, whereas those with MFs approaching 1 x 10(-2) were whole body mutant offspring. Mosaic mutant offspring comprised the majority of mutant offspring derived from postmeiotic germ cells, and unexpectedly, from spermatogonial stem cells. Mutational spectra comprised of two different mutations, but at identical sites were unusual and characteristic of delayed mutations, in which fixation of a second mutation was delayed following fertilization. Delayed mutations and prevalence of mosaic mutant offspring add to growing evidence that implicates germ cells in mediating processes postfertilization that contribute to genomic instability in progeny. This model provides an efficient and sensitive approach to assess germ cell mutations, expands opportunities to increase understanding of fundamental mechanisms of mutagenesis, and provides a means for improved assessment of potential genetic health risks.

Enhanced susceptibility to chemical induction of ovarian tumors in mice with a germ line p53 mutation

Mice with a germ line p53 mutation (p53(Ala135Val/wt)) display increased susceptibility to lung, skin, and colon carcinogenesis. Here, we show that p53(Ala135Val/wt) mice developed ovarian tumors significantly more rapidly than their wild-type littermates after 7,12-dimethylbenz(a)anthracene (DMBA) treatment. Approximately 50% of the ovarian tumors in p53(wt/wt) mice and 23% in p53(Ala135Val/wt) mice are adenocarcinomas and the remaining tumors were adenocarcinoma mixed with sarcoma or ovarian sarcomas. All of the p53(Ala135Val/wt) mice had died of ovarian tumors 25 weeks after the initial DMBA treatment, whereas >50% of p53(wt/wt) mice were still alive. These mice not only have a shortened tumor latency but also closely resemble a subset of human ovarian tumors containing the p53 mutation. Microarray and GenMAPP analyses revealed that the mutant p53 (Ala135Val) affected several cellular processes, including the cell cycle, apoptosis, and Wnt pathways. These findings indicate that a germ line p53 mutation significantly enhanced DMBA-induced ovarian tumor development and progression.

Impact of transforming viruses on cellular mutagenesis, genome stability, and cellular transformation

It is estimated that 15% of all cancers are etiologically linked to viral infection. Specific cancers including adult T-cell leukemia, hepatocellular carcinoma, and uterine cervical cancer are associated with infection by human T-cell leukemia virus type I, hepatitis B virus, and high-risk human papilloma virus, respectively. In these cancers, genomic instability, a hallmark of multistep cancers, has been explicitly linked to the expression of oncoproteins encoded by these viruses. This review discusses mechanisms utilized by these viral oncoproteins, Tax, HBx, and E6/E7, to mediate genomic instability and cellular transformation.

17 Beta-estradiol and not genistein modulates lacI mutant frequency and types of mutation induced in the heart of ovariectomized big blue rats treated with 7, 12-dimethylbenz[a]anthracene

In industrialized countries, heart disease rates are higher among women after menopause. Recent studies indicate that consumption of phytoestorogens, e.g., isoflavones such as genistein (GE), may have potential cardiovascular health benefits; however, no studies have evaluated the effect of these agents on toxicant-induced damage in the heart. Since estrogen receptors are found in the heart, and GE mimics estrogenic effects, we have examined whether or not dietary GE or 17 beta-estradiol (E2) modulates the lacI mutant frequency (MF) in the heart of ovariectomized (OVX) Big Blue rats exposed to the model carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). Groups of female rats were administered 80 mg/kg DMBA or vehicle by gavage and were chronically fed with diets containing 0, 250, or 1,000 microg/g GE or 5 microg/g E2. Sixteen weeks after carcinogen treatment, the animals were sacrificed and the hearts were removed and processed for determining the frequency and types of mutations in the heart tissue. GE and E2 supplementation alone resulted in nonsignificant increases in MF. The DMBA-induced lacI MF in the heart was sevenfold higher than the control (119.8 +/- 18.7 x 10(-6) vs. 17.4 +/- 3.2 x 10(-6); P < 0.001). GE in the diet had no significant effect on DMBA mutagenicity, while feeding E2 to DMBA-treated rats caused a significant reduction in the MF (119.8+/- 18.7 x 10(-6) vs. 61.4 +/- 13.5 x 10(-6); P < 0.017). DNA sequence analysis revealed that the majority of DMBA-induced mutations in rats fed control diet were A:T-->T:A (42%) and G:C-->T:A (19%) transversions, followed by G:C-->A:T (13%) and A:T-->G:C (8%) transitions. Feeding E2 altered the DMBA-induced mutational spectra by decreasing A:T-->T:A (23%) and G:C-->T:A (13%) transversions and increasing G:C-->A:T (24%) and A:T-->G:C (21%) transitions. Taken together, the results suggest that DMBA can induce gene mutations in heart tissue of OVX rats, and while dietary GE had little or no effect on DMBA-induced mutation, dietary E2 reduced the mutagenicity of DMBA.

Inducible mutagenesis in TEPC 2372, a mouse plasmacytoma cell line that harbors the transgenic shuttle vector lambdaLIZ

The plasmacytoma cell line, TEPC 2372, was derived from a malignant plasma cell tumor that developed in the peritoneal cavity of a BALB/c mouse that harbored the transgenic shuttle vector for the assessment of mutagenesis in vivo, lambdaLIZ. TEPC 2372 was found to display the typical features of a BALB/c plasmacytoma. It consisted of pleomorphic plasma cells that secreted a monoclonal immunoglobulin (IgG2b/lambda), was initially dependent on the presence of IL-6 to grow in cell culture, contained a hyperdiploid chromosome complement with a tendency to undergo tetraploidization, and harbored a constitutively active c-myc gene by virtue of a T(6;15) chromosomal translocation. TEPC 2372 was further characterized by the ability to respond to in vitro exposure with 4-NQO (4-nitroquinoline-1-oxide), an oxidative model mutagen, with a vigorous dose-dependent increase in mutagenesis that peaked at a 7.85-fold elevation of mutant rates in lambdaLIZ when compared to background mutant rates in untreated controls. Cotreatment with 4-NQO and BSO (buthionine sulfoximine), a glutathione-depleting compound that causes endogenous oxidative stress, resulted in a 9.03-fold increase in the mutant frequency in lambdaLIZ. These results demonstrated that TEPC 2372, the malignant plasma cell counterpart of the lambdaLIZ-based in vivo mutagenesis assay, may be useful as an in vitro reference point for the further elucidation of oxidative mutagenesis in lymphoid tissues.

Gene- and tissue-specificity of mutation in Big Blue rats treated with the hepatocarcinogen N-hydroxy-2-acetylaminofluorene

In a previous study, we found that treating transgenic Big Blue rats with the hepatocarcinogen N-hydroxy-2-acetylaminofluorene (N-OH-AAF) produced the same major DNA adduct in the target liver and the nontarget spleen lymphocytes and bone marrow cells, induced lacI mutants in the liver, and induced much lower frequencies of lacI and hprt mutants in spleen lymphocytes. In the present study, sequence analysis was conducted on lacI DNA and hprt cDNA from the mutants, to determine the mutational specificity of N-OH-AAF in the rat. All the mutation spectra from N-OH-AAF-treated rats differed significantly from corresponding mutation profiles from untreated animals (P = 0.02 to P < 0.0001). Although there were similarities among the mutational patterns derived from N-OH-AAF-treated rats (e.g., G:C --> T:A transversion was the most common mutation in all mutation sets), there were significant differences in the patterns of basepair substitution and frameshift mutation between the liver and spleen lymphocyte lacI mutants (P = 0.02) and between the spleen lymphocyte lacI and hprt mutants (P = 0.04). Also, multiplex PCR analysis of genomic DNA from the hprt mutants indicated that 12% of mutants from treated rats had major deletions in the hprt gene; no corresponding incidence of large deletions was evident among lacI mutations. All the mutation profiles reflect the general mutational specificity of the major DNA adduct formed by N-OH-AAF. The differences between N-OH-AAF mutation in the endogenous gene and transgene can be partially explained by the structures of the two genes. The tissue-specificity of the mutation spectra may contribute to targeting tumor formation to the liver. Environ. Mol. Mutagen. 37:203-214, 2001. Published 2001 Wiley-Liss, Inc.

7,12-dimethylbenz[a]anthracene-induced mutation in the Tk gene of Tk(+/-) mice: automated scoring of lymphocyte clones using a fluorescent viability indicator

7,12-Dimethylbenz[a]anthracene (DMBA) is a rodent carcinogen and a potent in vivo mutagen for the X-linked hypoxanthine guanine phosphoribosyl transferase (hprt) gene of rats and for the lacI transgene of Big Blue mice and rats. Although DMBA is also a powerful clastogen, molecular analysis of these DMBA-induced hprt and lacI mutations indicates that most are single base-pair (bp) substitutions and 1- to 3-bp frameshifts. In the present study, we evaluated the types of mutations induced by DMBA in the autosomal thymidine kinase (Tk) gene of Tk(+/-) mice. Male and female 5- to 6-week-old animals were injected i.p. with DMBA at a dose of 30 mg/kg. Five weeks after the treatment, hprt and Tk mutant frequencies were determined using a limiting dilution clonal assay in 96-well plates. We established conditions for the automated identification of wells containing expanded lymphocyte clones using the fluorescent indicator alamarBlue. This procedure allowed the unbiased identification of viable clones and calculation of mutant frequencies. In male mice, DMBA treatment increased the frequency of hprt mutants from 1.8 +/- 1.1 to 34 +/- 9 x 10(-6), and Tk mutants from 33 +/- 12 to 78 +/- 26 x 10(-6); treated female mice had a significant but lower increase in hprt mutant frequency than did males. Molecular analysis of DMBA-induced Tk mutants revealed that at least 75% had the entire wild-type Tk allele missing. The results indicate that the predominant types of DMBA-induced mutation detected by the autosomal Tk gene are different from those detected by the X-linked hprt gene. The Tk gene mainly detects loss of heterozygosity mutation, whereas the majority of mutations previously found in the hprt gene were point mutations.

Mutant frequency and molecular analysis of in vivo lacI mutations in the bone marrow of Big Blue rats treated with 7, 12-dimethylbenz[a]anthracene

Recently, we evaluated lacI mutations in lymphocytes and mammary tissue of Big Blue (BB) rats exposed to 7, 12-dimethylbenz[a]anthracene (DMBA). The results on the time course of mutant induction suggested that the lacI gene may manifest a tissue-specific increase in mutant frequency (MF). To test whether a tissue-specific increase in lacI MF is dependent on the cell proliferation rate of a tissue, we examined rapidly proliferating bone marrow cells for DMBA-induced lacI mutations. Seven-week-old female BB rats were given single doses of 0, 20, and 130 mg/kg DMBA by gavage and the lacI MFs in the bone marrow were measured over a period of 14 weeks following treatment. Bone marrow cells had a remarkably low average background MF (3.1 +/- 1.6 x 10(-6) plaque-forming units) and the DMBA-induced lacI MFs were significantly higher than control MFs for both doses and at all time points (P < 0.01). The lacI MF in the bone marrow increased for 2 weeks and then remained relatively constant; 20 and 130 mg/kg DMBA produced 34- and 106-fold increases in MF over control MF. DNA sequencing revealed that the majority of DMBA-induced lacI mutations were base-pair substitutions and that A:T --> T:A (48%) and G:C --> T:A (24%) transversions were the predominant types. Thus, the different lacI mutation fixation times observed for bone marrow (2 weeks), mammary (10 weeks), and lymphocytes (6 weeks) suggest that the lacI gene manifests a tissue-specific mutation fixation time, which may depend on the cell proliferation rate of a tissue. In addition, the relatively low spontaneous MF in bone marrow compared with that in other tissues may be useful for increasing the sensitivity of the assay for detecting induced MFs in BB rats.

A functional and quantitative mutational analysis of p53 mutations in yeast indicates strand biases and different roles of mutations in DMBA- and BBN-induced tumors in rats

In order to analyze the mutational events and to understand the biological significance of the p53 gene in chemical carcinogenesis, we applied a new yeast-based p53 functional assay to ovarian tumors induced by 7, 12-dimethylbenz[a]anthracene (DMBA), as well as to transitional cell carcinomas of the urinary bladder induced by N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN) in rats. The assay demonstrated that 15 of 19 DMBA induced tumors harbored clonal p53 mutations, which is consistent with the expectations of the "clonal expansion" hypothesis. The majority of the mutations were purine (AG) to pyrimidine (CT) transversions (12/19) on the non-transcribed (sense) strand (NTS), which is likely to be due to depurination created by DMBA adduct formation on the NTS. In contrast, we found no pyrimidine to purine [corrected] transversion on the NTS. After cessation of BBN treatment, BBN-induced multifocal lesions in the bladder contained heterogeneous p53 mutations at an early stage. In the later stage, however, clonal p53 mutations were identified in 4 out of 7 bladders analyzed, conforming with the concept of "field cancerization". The observed base substitutions were G-->A (1/6) or C -->T transitions (2/6), and mutations at T (3/6) on the NTS in clonal mutations, together with non-clonal mutations, showing a preference of C-->T to G-->A (17 vs. 0). Thus, preferential repair was found in the transcribed strand of the p53 gene, whether modified by DMBA or by BBN carcinogens. Very similar mutation patterns were observed between clonal and non-clonal mutations in the DMBA- and BBN-induced tumors, indicating that the rat yeast p53 functional assay can be a potential tool for the characterization of in vivo mutation patterns of p53, when modified by chemical carcinogens.

Mutation spectrum of 4-nitroquinoline N-oxide in the lacI transgenic Big Blue Rat2 cell line

This paper describes the spectrum of mutations induced by 4-nitroquinoline N-oxide (4-NQO) in the lacI target gene of the transgenic Big Blue Rat2 cell line. There are only a few report for the mutational spectrum of 4-NQO in a mammalian system although its biological and genetic effects have been well studied. Big Blue Rat2 cells were treated with 0.03125, 0.0625 or 0.125 microg/ml of 4-NQO, the highest concentration giving 85% survival. Our results indicated that the mutant frequency (MF) induced by 4-NQO was dose-dependent with increases from three- to seven-fold. The DNA sequence analysis of lacI mutants from the control and 4-NQO treatment groups revealed an obvious difference in the spectra of mutations. In spontaneous mutants, transition (60%) mutations, especially G:C-->A:T transition (45%), were most frequent. However, the major type of base substitution after treatment of 4-NQO was transversions (68.8%), especially G:C-->T:A (43.8%), while only 25% of mutants were transitions. These results are consistent with those produced by 4-NQO in other systems and the transgenic assay system will be a powerful tool to postulate more accurately the mechanism of chemical carcinogenesis involved.

Comparison of selectable and plaque assay systems to detect menadione- and UV-induced lacI mutations in mammalian cells

We have compared the spontaneous mutation frequency and spectrum of lacI genes recovered from a rat embryonic fibroblast line transfected with a lambda-phage shuttle vector (Rat2lambdalacI) using both the traditional plaque assay as well as a positive selection assay. In addition, mutation frequencies and spectrum were determined after treatment of the cells with either the intracellular superoxide-generating compound, menadione, or UVC light. The differences in mutation frequency between the two systems suggested that the selectable assay was better at discerning relatively small mutation frequency increases, more rapidly and at lower cost, than the plaque assay method. Some novel lacI mutations were observed in mutants derived from the selectable assay. This indicates that the selectable assay system may be a useful tool for assessing the mutagenic potential of different agents.

Spontaneous mutation spectrum at the lambda cII locus in liver, lung, and spleen tissue of Big Blue transgenic mice

Big Blue mice harbor a recoverable transgene in a lambda/LIZ shuttle vector. In the standard assay, in vivo mutations are measured in the bacterial lacI gene using a labor-intensive color plaque assay. Applying a simpler assay [Jakubczak et al. (1996): Proc Natl Acad Sci USA 93:9073-9078], we measured mutations in the lambda cII gene portion of the transgene. Spontaneous clear plaque mutants were analyzed from liver, lung, and spleen of five untreated mice. Of 314 mutants, 182 (58%) had independent mutations, 74 (23.5%) appeared clonal, and 58 (18.5%) showed no cII mutations. Of 182 independent cII mutations, 156 (85.7%) were base substitutions, 20 (10.9%) were frameshifts, and 6 (3.2%) were multiple substitutions and one deletion. G:C --> A:T transitions were the predominant base substitution (78% of these at CpG sites). The major mutation hotspot, a six G run and its 3' flanking T at bases 179 to 185, comprised 18.7% of the independent mutations. Other hotspots were positions 103, 196, and 212. The in vivo cII spectrum had a significantly higher proportion of G --> A and G --> T mutations and fewer frameshifts than reported in vitro. The cII and published lacI spectra are similar, though G --> A transitions and deletions were fewer in the cII gene. The cI gene was sequenced in 48 mutants with no cII mutations and most had cI mutations: 81.3% base substitutions and 18.7% frameshifts. We conclude that the cII/cI system is insensitive to deletion events, but is useful for detecting point mutations.

Comparison of mutant frequencies and types of mutations induced by thiotepa in the endogenous Hprt gene and transgenic lacI gene of Big Blue rats

The utility of the lacI transgene of Big Blue rats as a reporter of in vivo mutation was evaluated by comparing the frequency and types of mutations induced by thiotepa in the transgene and the endogenous Hprt gene. Transgenic rats were given i.p. injections of 1.4 mg/kg of thiotepa three times per week over a period of 4 weeks (a total dose of 16.8 mg/kg); 1 week after the last injection, mutation assays were performed on spleen lymphocytes isolated from the animals. Thiotepa treatment increased the lacI mutant frequency from 34.8 +/- 4.1 x 10(-6) in control animals to 140.9 +/- 64.8 x 10(-6) (p = 0.0020) and the Hprt mutant frequency from 3.5 +/- 1.5 x 10(-6) to 41.1 +/- 23.2 x 10(-6) (p = 0.0028). Sequence analysis of lacI mutant DNA and Hprt mutant cDNA produced similar overall mutation patterns: G:C-->T:A transversion was the most common base pair substitution (32% of independent mutations in the lacI gene and 28% of Hprt mutations), and deletions and insertions accounted for 34% of mutations in the lacI gene and 28% in the Hprt gene. The majority of thiotepa-induced base pair substitutions in the Hprt gene occurred with the mutated purine on the non-transcribed DNA strand, while no strand-related bias was found for mutations in the lacI gene. Substitutions at G:C base pairs in the lacI gene, but not in the Hprt gene, were found disproportionately in CpG sites. In addition, multiplex polymerase chain reaction analysis of genomic DNA from the Hprt mutants indicated that 34% had relatively large deletions; none of these deletions was detected by the cDNA analysis. The results indicate that the frequency of thiotepa-induced mutants in Big Blue rats was 2.8-fold greater in the lacI gene than in the Hprt gene. Although the Hprt gene recovered a fraction of large deletions not found among the lacI mutants, the effects of transcription-coupled DNA repair in the Hprt gene and the targeting of base pair substitutions to G:C base pairs in CpG sites may have contributed to the higher mutant frequencies induced by thiotepa in the lacI transgene compared with the Hprt gene.

Comparison of in vivo mutagenesis in the endogenous Hprt gene and the lacI transgene of Big Blue(R) rats treated with 7, 12-dimethylbenz[a]anthracene

The lacI transgene of Big Blue(R) (BB) rats was evaluated as a reporter of in vivo mutation by comparing mutant frequencies (MFs) in it and in the endogenous Hprt gene. Seven-week old female BB rats were given single doses of 0, 20, 75 and 130 mg/kg of 7, 12-dimethylbenz(a)anthracene (DMBA) by gavage, and Hprt and lacI MFs in splenic lymphocytes were measured over a period of 18 weeks. The Hprt MFs in treated rats increased for 10 weeks and then declined; 130 mg/kg of DMBA produced a maximum Hprt MF of 168+/-11.4x10-6 clonable lymphocytes, while the MF in control rats was 7.4+/-1. 5x10-6. DMBA exposure of generic F344 rats resulted in a similar time-course of mutant induction but produced about 50% higher Hprt MFs with the 75 and 130 mg/kg doses. In contrast, the lacI MFs increased for 6 weeks and then remained relatively constant; 130 mg/kg of DMBA produced a maximum increase in lacI MF of 341+/-83x10-6 PFU compared with 25+/-5x10-6 PFU in control rats. The Hprt mutant frequencies in DMBA-treated BB and F344 rats were significantly increased over control values for every dose-time combination examined, while only the 130 mg/kg dose consistently produced lacI MFs that were significantly above the controls. In addition, the fold-increase in MF for treated vs. control rats was two times higher for the Hprt gene than the lacI gene due to the higher MFs in the lacI gene of control rats. Differences between the lacI and Hprt genes in the kinetics of mutant induction, in the frequency of induced mutants, and in the sensitivity of mutant detection could be explained at least partially by the properties of these two genes.

Comparison of the types of mutations induced by 7,12-dimethylbenz[a]anthracene in the lacI and hprt genes of Big Blue rats

An important question regarding the use of transgenic reporter genes to detect mutation in rodents is how the types of mutations recovered in transgenes compare with the types of mutations found in endogenous genes. In this study, we examined mutations induced by 7,12-dimethylbenz[a]anthracene in the lacI transgene and the endogenous hprt gene of lymphocytes from Big Blue rats and in the hprt gene of lymphocytes from nontransgenic Fischer 344 rats. The overall mutation profiles found in these genes were remarkably similar: the majority of mutations were base pair substitutions, with the most common mutation being A:T-->T:A transversion. Differences were found for the mutational profiles in the endogenous gene and transgene with respect to the location of the mutations and the orientation of basepair substitutions in the DNA strands. In most cases, these differences could be explained by the nature of the target genes. The results support the use of the lacI transgene for detecting in vivo mutation.