Characterization of mutagen-activated cellular oncogenes that confer anchorage independence to human fibroblasts and tumorigenicity to NIH 3T3 cells: sequence analysis of an enzymatically amplified mutant HRAS allele

Evolution of Molecular Targets in Melanoma Treatment

Melanoma is the deadliest type of skin cancers, accounting for more than 80% of skin cancer mortality. Although melanoma was known very early in the history of medicine, treatment for this disease had remained largely the same until very recently. Previous treatment options, including removal surgery and systemic chemotherapy, offered little benefit in extending the survival of melanoma patients. However, the last decade has seen breakthroughs in melanoma treatment, which all emerged following new insight into the oncogenic signaling of melanoma. This paper reviewed the evolution of drug targets for melanoma treatment based on the emergence of novel findings in the molecular signaling of melanoma. One of the findings that are most influential in melanoma treatment is that more than 50% of melanoma tumors contain BRAF mutations. This is fundamental for the development of BRAF inhibitors, which is the first group of drugs that significantly improves the overall survival of melanoma patients compared to the traditional chemotherapeutic dacarbazine. More recently, findings of the role of immune checkpoint molecules such as CTLA-4 and PD1/PD-L1 in melanoma biology have led to the development of a new therapeutic category: immune checkpoint inhibitors, which, for the first time in the history of cancer treatment, produced a durable response in a subset of melanoma patients. However, as this paper discussed next, there is still an unmet need for melanoma treatment. A significant population of patients did not respond to either BRAF inhibitors or immune checkpoint inhibitors. Of those patients who gained an initial response from those therapies, a remarkable percentage would develop drug resistance even when MEK inhibitors were added to the treatment. Finally, this paper discusses some possible targets for melanoma treatment.

Spectrum of K ras mutations in Pakistani colorectal cancer patients

The incidence of colorectal cancer (CRC) is increasing daily worldwide. Although different aspects of CRC have been studied in other parts of the world, relatively little or almost no information is available in Pakistan about different aspects of this disease at the molecular level. The present study was aimed at determining the frequency and prevalence of K ras gene mutations in Pakistani CRC patients. Tissue and blood samples of 150 CRC patients (64% male and 36% female) were used for PCR amplification of K ras and detection of mutations by denaturing gradient gel electrophoresis, restriction fragment length polymorphism analysis, and nucleotide sequencing. The K ras mutation frequency was found to be 13%, and the most prevalent mutations were found at codons 12 and 13. A novel mutation was also found at codon 31. The dominant mutation observed was a G to A transition. Female patients were more susceptible to K ras mutations, and these mutations were predominant in patients with a nonmetastatic stage of CRC. No significant differences in the prevalence of K ras mutations were observed for patient age, gender, or tumor type. It can be inferred from this study that Pakistani CRC patients have a lower frequency of K ras mutations compared to those observed in other parts of the world, and that K ras mutations seemed to be significantly associated with female patients.

From genes to drugs: targeted strategies for melanoma

The past decade has revealed that melanoma is comprised of multiple subclasses that can be categorized on the basis of key features, including the clinical stage of disease, the oncogenic molecular 'drivers', the anatomical location or the behaviour of the primary lesion and the expression of specific biomarkers. Although exercises in subclassification are not new in oncology, progress in this area has produced both conceptual and clinical breakthroughs, which, for melanoma, are unprecedented in the modern history of the disease. This Review focuses on these recent striking advances in the strategy of molecularly targeted approaches to the therapy of melanoma in humans.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll and Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in he environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll & Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in the environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll & Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in the environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll and Peto, 1981). An important and prevalent class of potent carcinogeniccompounds present in the environment is polycyclic aromatic hydrocarbons(PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

The role of polycyclic aromatic hydrocarbon-DNA adducts in inducing mutations in mouse skin

Polycyclic aromatic hydrocarbons (PAH) form stable and depurinating DNA adducts in mouse skin to induce preneoplastic mutations. Some mutations transform cells, which then clonally expand to establish tumors. Strong clues about the mutagenic mechanism can be obtained if the PAH-DNA adducts can be correlated with both preneoplastic and tumor mutations. To this end, we studied mutagenesis in PAH-treated early preneoplastic skin (1 day after exposure) and in the induced papillomas in SENCAR mice. Papillomas were studied by PCR amplification of the H-ras gene and sequencing. For benzo[a]pyrene (BP), BP-7,8-dihydrodiol (BPDHD), 7,12-dimethylbenz[a]anthracene (DMBA) and dibenzo[a,l]pyrene (DB[a,l]P), the codon 13 (GGC to GTC) and codon 61 (CAA to CTA) mutations in papillomas corresponded to the relative levels of Gua and Ade-depurinating adducts, despite BP and BPDHD forming significant amounts of stable DNA adducts. Such a relationship was expected for DMBA and DB[a,l]P, as they formed primarily depurinating adducts. These results suggest that depurinating adducts play a major role in forming the tumorigenic mutations. To validate this correlation, preneoplastic skin mutations were studied by cloning H-ras PCR products and sequencing individual clones. DMBA- and DB[a,l]P-treated skin showed primarily A.T to G.C mutations, which correlated with the high ratio of the Ade/Gua-depurinating adducts. Incubation of skin DNA with T.G-DNA glycosylase eliminated most of these A.T to G.C mutations, indicating that they existed as G.T heteroduplexes, as would be expected if they were formed by errors in the repair of abasic sites generated by the depurinating adducts. BP and its metabolites induced mainly G.C to T.A mutations in preneoplastic skin. However, PCR over unrepaired anti-BPDE-N(2)dG adducts can generate similar mutations as artifacts of the study protocol, making it difficult to establish an adduct-mutation correlation for determining which BP-DNA adducts induce the early preneoplastic mutations. In conclusion, this study suggests that depurinating adducts play a major role in PAH mutagenesis.

Frequency and spectrum of K-RAS codons 12 and 13 mutations in colorectal adenocarcinomas from Taiwan

Mutations in codons 12 and 13 of the K-RAS oncogene are detected at a remarkably high frequency in colorectal adenocarcinoma and are believed to be a critical event in oncogenesis. In the present study, we evaluated colorectal tumor specimens from Taiwan for mutations in K-RAS codons 12 and 13 using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis and direct DNA sequencing. Mutations were found in 48 of 181 (26.5%) tumors, 30 mutations were G-->A transitions (62.5% of all mutations), 14 were G-->T transversions (29.2%), and only 4 were G-->C transversions (8.3%). Similar relative mutation frequencies and spectra were found regardless of the sex of the patient, the tumor grade, or the tumor stage. The high frequency of transitions among K-RAS mutation suggests that G/T mismatches play an important role in the oncogenesis of colorectal adenocarcinoma, implying that alkylating carcinogens may be involved in the colorectal carcinogenesis. Although the frequency of mutation (26.5%) appears to be lower than those reported in the United States (40%), France (49%), and the Netherlands (34%), the spectrum of point mutations in codons 12 and 13 of the K-RAS gene in the Taiwan Chinese population appears to be similar. The reason for these results may be that diet and ethnicity are not rate limit factors in controlling the spectra of mutations but influence on the frequency of K-RAS mutations in human colorectal adenocarcinomas.

Enzymatic deglycation of proteins

Reducing sugars such as glucose react with amino groups in proteins to form the Amadori product, which can undergo a wide range of chemical modifications and form cross-links in tissue proteins. There is growing evidence to suggest that accumulation of glycation products is associated with aging and disease progression, as in diabetes. Thus, the design and discovery of inhibitors for the glycation cascade would potentially offer a promising therapeutic approach for the prevention of glycation related diseases, especially diabetes. Two types of enzymes, fructosyl lysine oxidase and fructose lysine 3-phosphokinase, catalyze the deglycation reaction and generate free amine groups. This paper reviews the biochemical properties of these "amadoriase" enzymes, such as structural-function relationship, kinetic mechanism, and substrate specificity, as well as their biological roles and applications in the protein deglycation.

DNA damaging agents improve stable gene transfer efficiency in mammalian cells

Gene therapy is an evolving discipline which today relies primarily on viral systems for gene transfer. The primary reason that plasmid vectors have not been widely used for gene therapy trials is their relatively low rate of stable gene transfer. We show here that both ionizing irradiation and hydrogen peroxide can each increase the gene transfer efficiency of plasmids. Hydrogen peroxide improves gene transfer in a linear dose-dependent manner. At equitoxic doses, hydrogen peroxide improves gene transfer by 20-fold over untreated cells and approximately 5 times above that seen for radiation, and this improvement correlates with both the total amount of DNA damage induced and the amount of residual damage after 4 hr of repair. These data suggest that DNA damaging agents may be useful to improve human gene therapy.

High-efficiency stable gene transfer of adenovirus into mammalian cells using ionizing radiation

We report a novel method for targeting adenovirus-mediated gene delivery. By irradiating mammalian cells prior to adenoviral transduction, adenoviral gene transfer is greatly improved and the adenoviral genome integrates into cellular DNA. In this work, human and rodent cell lines were irradiated and subsequently transduced with the adenovirus vector Ad5CMVlacZ. Initial levels of transduction were as much as 40-fold higher in irradiated cells, and this improvement in transduction was radiation dose dependent. The duration of lacZ expression in irradiated cells was also much longer than in nonirradiated cells and reached a plateau after 21 days. At doses of 7 Gy, long-term (< 50 day) expression of lacZ could be detected in 15% of cells by flow cytometry. This long-lasting expression of lacZ was due to viral DNA integration into the host genome. Thus, pretreatment of cells with ionizing radiation improves both immediate transduction efficiency and duration of transgene expression. This may lead to the development of new protocols combining radiation and gene therapy in treating human malignancy.

Isolation, purification, and characterization of amadoriase isoenzymes (fructosyl amine-oxygen oxidoreductase EC 1.5.3) from Aspergillus sp

Four "amadoriase" enzyme fractions, which oxidatively degrade glycated low molecular weight amines and amino acids under formation of hydrogen peroxide and glucosone, were isolated from an Aspergillus sp. soil strain selected on fructosyl adamantanamine as sole carbon source. The enzymes were purified to homogeneity using a combination of ion exchange, hydroxyapatite, gel filtration, and Mono Q column chromatography. Molecular masses of amadoriase enzymes Ia, Ib, and Ic were 51 kDa, and 49 kDa for amadoriase II. Apparent kinetic constants for Nepsilon-fructosyl Nalpha-t-butoxycarbonyl lysine and fructosyl adamantanamine were almost identical for enzymes Ia, Ib, and Ic, but corresponding values for enzyme II were significantly different. FAD was identified in all enzymes based on its typical absorption spectrum. N-terminal sequence was identical for enzymes Ia and Ib (Ala-Pro-Ser-Ile-Leu-Ser-Thr-Glu-Ser-Ser-Ile-Ile-Val-Ile-Gly-Ala-Gly- Thr-Trp-Gly-) and Ic except that the first 5 amino acids were truncated. The sequence of enzyme II was different (Ala-Val-Thr-Lys-Ser-Ser-Ser-Leu-Leu-Ile-Val-Gly-Ala-Gly-Thr-Trp-Gly- Thr-Ser-Thr-). All enzymes had the FAD cofactor-binding consensus sequence Gly-X-Gly-X-X-Gly within the N-terminal sequence. In summary, these data show the presence of two distinct amadoriase enzymes in the Aspergillus sp. soil strain selected on fructosyl adamantanamine and induced by fructosyl propylamine. In contrast to previous described enzymes, these novel amadoriase enzymes can deglycate both glycated amines and amino acids.

Covalent binding of benzo[a]pyrene 7,8-dihydrodiol 9,10-epoxides to DNA: molecular structures, induced mutations and biological consequences

Optical spectroscopic techniques have been used to characterize adducts formed upon reaction of the (+)- and (-)-enantiomers of 7R,8S-dihydroxy 9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) to DNA or synthetic oligonucleotides. The reaction yields preferentially adducts in which the exocyclic aminogroup of deoxyguanosine is bound to the C10 position of the diol epoxide either cis (BPDEc-N2-G adduct) or trans (BPDEt-N2-G adduct) relative to the hydroxyl group at the C9 position. The BPDEc-N2-G and BPDEt-N2-G adducts fall into the categories of type I and type II complexes, respectively. Two-dimensional NMR in conjunction with energy minimization computation have provided detailed information on the solution structure of single adducts localized in oligonucleotides. The results demonstrate that the pyrenyl chromophores of both the (+)- and (-)-BPDEt-N2-G adduct are located in a widened minor groove and directed towards the 5'-end [(+)-BPDEt-N2-G] or the 3'-end [(-)-BPDEt-N2-G] of the modified strand. The chromophore of the (+)-BPDEc-N2-G adduct is quasi-intercalated into the oligonucleotide and associated with a displacement of the deoxyguanosine ring into the minor groove. Replication of racemic or (+)-anti-BPDE modified DNA in mammalian cells leads predominantly to single point mutations of transversion type (GC-->TA). The mutagenic specificity however, appears to be determined by the base sequence context and local conformation at the adduct site. Cooperative adduct formation at certain base sequences is suggested by excimer fluorescence, most probably derived from two closely located (+)-BPDEt-N2-G adducts in adjacent base pairs on opposite DNA-strands.

Prevalence of G-to-T transversions among K-ras oncogene mutations in human colorectal tumors in Yugoslavia

Human colorectal carcinoma tissue sampled from 37 patients, routinely graded into Dukes' stages A, B and C and histologically examined for the level of differentiation, were analyzed for the presence of point mutations in the K-ras oncogene. Seventeen cases out of the 37 analyzed were found to have a mutation in either the 12th or the 13th codon of the K-ras gene, giving an overall frequency of mutation of 46%. The incidence of mutations in Dukes' stages A, B and C was 33, 46 and 58% respectively. Although the frequency of mutation appears to be similar to that reported for the USA population, the spectrum of point mutations in codons 12 and 13 of the K-ras gene in the Yugoslav population appears to differ significantly. G-to-T transversions make up 77% of all mutations present, with the distribution as follows: 18% at the first base and 59% at the second base of codons 12 and 13. G-to-A transitions at the second base is the only other mutation identified, occurring mainly in codon 13 in colorectal tumors of all 3 stages.

Malignant transformation of human fibroblast cell strain MSU-1.1 by (+-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene

Treatment of MSU-1.1 cells, a near-diploid, karyotypically stable, infinite life-span human fibroblast strain, with (+-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene induced focus formation. Eight independent foci were isolated and the cell strains developed from them were examined for characteristics of malignant cells. Each grew to a higher density in medium containing 1% serum than did the MSU-1.1 cells. Three of the eight grew rapidly in serum-free medium without added growth factors, formed colonies in agarose with diameters of greater than or equal to 120 microns at a frequency of 5-19%, exhibited loss of genetic material, and, when injected into athymic mice, formed sarcomas that reached 6 mm in diameter within 2-3 wk. One produced high-grade sarcomas (progressively growing, invasive tumors exhibiting high mitotic activity); the other two produced low-grade sarcomas (tumors with a lower degree of mitotic activity) that developed focal areas of high-grade malignant cells if left in the animals for greater than 4 wk. A fourth cell strain formed high-grade sarcomas only after 2.5-3 mo, but the tumor-derived cells analyzed showed the same growth properties as the three malignant cell strains described above, exhibited loss of genetic material, and, when reinjected into athymic mice, produced high-grade sarcomas with a short latency period.

The endo/lysosomal protease cathepsin B is able to process conalbumin fragments for presentation to T cells

The protein antigens conalbumin (CA) and ovalbumin (OVA) are known to require uptake into antigen-presenting cells (APC) for their presentation to major histocompatibility complex (MHC) class II-restricted T cells. In both cases proteolytic cleavage is thought to be a necessary step for the generation of the respective antigenic peptides. A specific inhibitor of the endosomal protease cathepsin B, Cbz-Phe-Ala-CHN2, blocks the presentation of both CA and OVA, whereas this inhibitor has no effect on the presentation of a processing-independent OVA peptide. Furthermore, the presentation of insulin, an antigen that needs processing but no proteolytic cleavage, is enhanced when cathepsin B is inhibited during antigen pulsing. When the APC were treated with an inhibitor of acid proteases, the CA response was not affected, while the presentation of OVA was diminished under these conditions. To estimate the relevance of these findings for the generation of the antigenic CA peptide, extracellular digestions of CA by cathepsin B were carried out. The fragment(s) present in these digests was recognized by T cells without further processing. Furthermore, the time-course of intra- and extracellular CA processing with respect to the capacity to stimulate T cells was similar. Taken together these data suggest that degradation by cathepsin B may be sufficient in vivo to generate the antigenic CA fragment. On the other hand, the blocking of cathepsin B does not appear to have an adverse effect on the general mechanisms of antigen presentation.

Analysis of ras gene mutations in rainbow trout liver tumors initiated by aflatoxin B1

The suspect human hepatocarcinogen aflatoxin B1 (AFB1) is a well-known potent initiator of hepatic tumors in rainbow trout (Oncorhynchus mykiss). Both hepatocellular carcinomas and mixed hepatocellular/cholangiocellular carcinomas are induced by AFB1 in trout, with the mixed form predominating. We previously isolated two c-ras genes from trout liver cDNA, and in the present study we analyzed DNA from 14 AFB1-induced trout liver tumors for point mutations in exon 1 of both genes. Using the polymerase chain reaction (PCR) and oligonucleotide hybridization methods, a high proportion (10/14) of the AFB1-initiated tumor DNAs showed evidence of activating point mutations in the trout c-Ki-ras gene. Of the 10 mutant ras genotypes, seven were codon 12 GGA----GTA transversions, two were codon 13 GGT----GTT transversions, and one was codon 12 GGA----AGA transition. Nucleotide sequence analysis of cloned PCR products from four of these tumor DNAs provided definitive evidence for two codon 12 GGA----GTA mutations, one codon 12 GGA----AGA mutation, and one codon 13 GGT----GTT mutation, in complete agreement with the oligonucleotide hybridization results. No mutations were detected in exon 1 of a second trout ras gene also expressed in liver, nor in DNA from control livers. This is the first report of experimentally induced ras gene point mutations in a lower vertebrate fish model. The results indicate that the hepatocarcinogen AFB1 induces c-Ki-ras gene mutations in trout similar to those in rat liver tumors.

ras mutations in human leukemia and related disorders

The clinical association of an increased incidence of acute myelogenous leukemia (AML) with previous chemoradiotherapy, the detection of specific karyotypic changes in these secondary (therapy-induced) cases of AML and the discovery of increasing levels of oncogene-specific RNA in leukemia cells suggest that one potential site of action of environmental agents might be the proto-oncogenes in human hematopoietic stem cells. The location of human proto-oncogenes at the sites of chromosome breaks and/or translocations in cells from some patients with leukemia or lymphoma is a striking observation. These data stimulated research into the mechanism of activation of specific oncogenes that change the biology of human hematopoietic cells. Recent investigations have focused upon several areas that might alter cell biology including: 1) translocation and/or inversion of chromosome fragments containing a proto-oncogene to a location where other gene sequences can stimulate oncogene activation, 2) replication of copy number of proto-oncogenes or increased transcriptional activity and 3) point mutation in proto-oncogenes leading to a structurally altered protein. The third area of research has recently received significant attention with respect to the potential role of three ras genes (c-Harvey-ras, c-Kirsten-ras and N-ras) in human leukemias and myelodysplastic syndromes. Recent studies have proposed a model for leukemogenic transformation of human hematopoietic cells by the product of a mutated ras oncogene. Mutations at codons 12, 13 or 61 of the first exon of its 4.7 Kb of DNA (for c-Ha-ras) have been described. Other data revealing an absence of such mutations in the ras genes of many human leukemias and the absence of detectable transcription of ras genes in many alkylating agent-associated cases of AML, suggest that while ras mutations may be involved in some settings, there are probably multiple genetic pathways to leukemogenic transformation of human hematopoietic cells.

Activation of the Ki-ras protooncogene in spontaneously occurring and chemically induced lung tumors of the strain A mouse

The strain A mouse has a high incidence of spontaneous lung tumors and is susceptible to lung tumor induction by chemical carcinogens. By utilizing transfection assay, Southern blot analysis, and DNA amplification techniques, we have detected an activated Ki-ras gene in the DNAs of both spontaneously occurring and chemically induced lung tumors of strain A mice. The point mutations in the spontaneous lung tumors were in both codon 12 (60%) and codon 61 (30%). In contrast, 100% of the mutations in the Ki-ras gene detected in methylnitrosourea-induced lung tumors and 93% of the mutations in the Ki-ras genes detected in benzo[a]pyrene-induced lung tumors were in codon 12, whereas 90% of the mutations in the Ki-ras genes detected in ethyl carbamate-induced lung tumors were in codon 61. The selectivity of mutations in the Ki-ras oncogene observed in chemically induced tumors, as compared to spontaneous tumors, suggests that these chemicals directly induce point mutations in the Ki-ras protooncogene. These data indicate that the strain A mouse lung tumor model is a very sensitive system to detect the ability of chemicals to activate the Ki-ras protooncogene in lung tissue.