Errors in DNA synthesis: a source of spontaneous mutations

Correction of monomeric enhanced green fluorescent protein (mEGFP) gene by short 5'-tailed duplexes

Mutations of important genes elicit various disorders, including cancer. Recently, a new version of a 5'-tailed duplex (short TD), consisting of a ∼100-base editor strand containing the wild-type sequence and a ∼35-base assistant strand, was shown to correct a base substitution mutation in a target gene in human cells. In that previous study, the target was the copepod green fluorescent protein (copGFP) gene. To examine the usefulness of the short TD, we performed gene correction experiments using a mutant form of the monomeric enhanced Aequorea victoria green fluorescent protein (mEGFP) gene containing a TAC to CAC mutation in codon 75 (corresponding to the tyrosine to histidine substitution in the chromophore). The short TDs with the wild-type sequence efficiently corrected the inactivated gene in human U2OS cells. These results indicated that the short TDs are effective for gene editing.

Mismatch repair system proteins in oral benign and malignant lesions

Different environmental agents may cause DNA mutations by disrupting its double-strand structure; however, even normal DNA polymerase function may synthesize mismatch nucleotide bases, occasionally demonstrating failure in its proofreading activity. To overcome this issue, mismatch repair (MMR) system, a group of proteins specialized in finding mispairing bases and small loops of insertion or deletion, works to avoid the occurrence of mutations that could ultimately lead to innumerous human diseases. In the last decades, the role of MMR proteins in oral carcinogenesis and in the development of other oral cavity neoplasms has grown, but their importance in the pathogenesis and their prognostic potential for patients affected by oral malignancies, especially oral squamous cell carcinoma (OSCC), remain unclear. Therefore, in this manuscript we aimed to review and critically discuss the currently available data on MMR proteins expression in oral potentially malignant lesions, in OSCC, and in other oral neoplasms to better understand their relevance in these lesions.

The involvement of a Nanog, Klf4 and c-Myc transcriptional circuitry in the intertwining between neoplastic progression and reprogramming

One undisputed milestone of traditional oncology is neoplastic progression, which consists of a progressive selection of dedifferentiated cells driven by a chance sequence of genetic mutations. Recently it has been demonstrated that the overexpression of well-defined transcription factors reprograms somatic cells to the pluripotent stem status. The demonstration raises crucial questions as to whether and to what extent this reprogramming contributes to tumorigenesis, and whether the epigenetic changes involved in it are reversible. Here, we show for the first time that a tumor produced in vivo by a chemical carcinogen is the product of the interaction between neoplastic progression and reprogramming. The experimental model employed the prototype of ascites tumors, the Yoshida AH130 hepatoma and other neoplasias, including human melanoma. AH130 hepatoma was started in the liver by the carcinogen o-aminoazotoluene. This compound binds to and abolishes the p53 protein, producing a genomic instability that promotes both the neoplastic progression and the hepatoma reprogramming. Eventually this tumor contained 100% CD133(+) elements and pO(2)-dependent percentages of the three embryonic transcription factors Nanog, Klf4 and c-Myc. Once transferred into aerobic cultures, the minor cellular fraction expressing this triad generates various types of adherent cells, which are progressively substituted by non-tumorigenic elements committed to fibromuscular, neuronal and glial differentiation. This reprogramming appears to be accomplished stepwise, with the assembly of the triad into a sophisticated transcriptional, oxygen-dependent circuitry, in which Nanog and Klf4 antagonistically regulate c-Myc, and hence, cell hypoxia survival and cell cycle activation.

Phenotypic plasticity and epithelial-mesenchymal transitions in cancer and normal stem cells?

Cancer stem cells (CSCs) are similar to normal stem cells in their ability to self-renew and to generate large populations of more differentiated descendants. In contrast to the hierarchical organization that is presumed to be the prevalent mode of normal tissue homeostasis, phenotypic plasticity allows cancer cells to dynamically enter into and exit from stem-cell states. The epithelial-mesenchymal transition (EMT) has been closely associated with the acquisition of both invasive and stem-cell properties in cancer cells. Thereby, EMT programs emerge as important regulators of phenotypic plasticity in cancer cells including their entrance into stem-cell states. Much is still to be learned about the regulation of EMTs through epigenetic mechanisms in cancer cells and the contributions that EMT programs make to normal tissue homeostasis.

Retinopathy of prematurity and maternal age

PURPOSE

To determine the risk factors, especially maternal risk factors, associated with the development of retinopathy of prematurity (ROP) in premature babies.

METHODS

A matched case-control study involving premature patients was undertaken retrospectively. The case group consisted of premature babies with the subsequent development of ROP. The control group consisted of gestational age-matched and sex-matched premature babies that did not develop ROP during the follow-up period. Risk factors involving patient demographics and maternal characteristics were compared between the case and control groups.

RESULTS

A total of 144 patients were included in this study (72 patients in the case group and 72 patients in the control group). Among the 66 possible risk factors compared, only birth weight and maternal age were found to be significant risk factors. Birth weight was significantly lower in the case group (1,248.7 +/- 257.8 g vs. 1,335.5 +/- 297.2 g, P = 0.01), and maternal age was significantly older in the case group compared with that in the control group (31.2 +/- 5.1 years vs. 28.2 +/- 5.3 years, P < 0.001). The odds ratio of having babies with ROP was 2.9 when the maternal age was >30 years.

CONCLUSION

Older maternal age is a newly identified risk factor for the development of ROP in premature babies.

Nitrosative cytosine deamination. An exploration of the chemistry emanating from deamination with pyrimidine ring-opening

A discussion of nitrosative deamination of cytosine 1 is presented that argues for the formation of 6 by diazotization of 1 to cytosinediazonium ion 2 and its electrostatic complex 3, dediazoniation to 4 <--> 5, and amide-bond cleavage to 6. The reaction channels available to 6 include hydrolytic deglycation to 3-isocyanatoacrylonitrile 7, water addition to carbamic acid 9 with the possibility for re-closure to uracil 13, water addition to carbamic acid 9, and decarboxylation to 3-aminoacrylonitrile 10. With a view to the instability of the carbamic acid 9, the carbamate models ethyl (Z)-2-cyanovinylcarbamate 14 and (Z)-2-cyano-1-tert-butylvinylcarbamate 20 were studied. Acid-catalyzed hydrolysis of 14 leads to 2-amino-carbonylphenylcarbamate 15, and its cyclization yields the benzo-fused uracil quinazoline-2,4-dione 16. In contrast to the aromatic system 14, acid-catalyzed cyclization cannot compete with oligomerization in the case of 20, and 5-tert-butyluracil 22 is accessible only with base-catalysis. It is shown that 23, the parent of 10, also easily polymerizes. The experimental results provide a rationale as to why 9, 10, and 12 would have escaped detection in in vitro studies: they would have oligomerized. In contrast to the in vitro experiments, the oligomerizations of 9, 10, or 12 clearly are not relevant in vivo because of low monomer concentrations. With the exclusion of recyclization and of oligomerization in vivo, attention thus needs to focus on (Z)-3-aminoacrylonitrile 10 as the most likely deamination product of cytosine aside from uracil.

tRNAPhe cleavage by aminoglycosides is triggered off by formation of an abasic site

This communication reports the characteristics of the mechanism of highly specific tRNA(Phe) cleavage, which occurs in the anticodon loop in the presence of aminoglycoside antibiotic-neomycin B. The data prove that the cleavage requires previous depurination of the polynucleotide chain at position 37, which is occupied by a hypermodified guanine base-wybutine. The results suggest that the phenomenon, previously considered as selective with respect to the presence of tRNA hypermodification, may concern far more RNA molecules, namely the ones carrying abasic sites.

Effects of low-dose ionizing radiation on gene expression in human skin biopsies

PURPOSE

Several investigations have demonstrated that significant biologic effects can occur in animals, animal cells, immortalized human cell lines, and primary human cells after exposure to doses of ionizing radiation in the low-dose, < or =1-10 cGy region (LDIR). However, little information is available as to how these and other observations pertain to human responses to LDIR, though such knowledge is required for reducing the uncertainty of assessing human risks due to these exposures. We therefore undertook these translational studies to begin the development of a unique data set of human cellular responses to LDIR as measured by gene expression changes when exposure occurs to a normal tissue with its complex cellular mixture and three-dimensional architecture.

METHODS AND MATERIALS

Using full-thickness human skin resected during esthetic surgery, we obtained biopsy cores and exposed the tissue to LDIR ex vivo. Gene expression changes in five core regulatory genes were assessed by real-time RT-PCR.

RESULTS

Results indicate that skin is a good biologic model for assessing LDIR in humans, though meticulous attention to sample processing is necessary. LDIR does produce changes in gene expression, though time- and dose-response relationships may be complex.

CONCLUSION

These proof-of-principle studies have provided a crucial initial step toward validation of LDIR risk assessment models in humans. We have demonstrated the feasibility of this approach and provide initial evidence that ionizing radiation exposures as low as 1 cGy are biologically active in human skin.

Different subtypes of human lung adenocarcinoma caused by different etiological factors. Evidence from p53 mutational spectra

Human lung adenocarcinomas are only relatively weakly associated with tobacco smoke, and other etiological factors need to be clarified. These may also vary with the histopathology. Because the p53 mutation status (frequency and spectrum) of a carcinoma can provide clues to causative agents, we subclassified 113 adenocarcinomas into five cell types: hobnail, columnar/cuboidal, mixed, polygonal, and goblet (54, 23, 18, 13, and 5, respectively) and investigated relationships with p53 mutations and smoking history. In the hobnail cell type, a low mutational frequency (37%) and a high proportion of transitions (65%), especially G:C to A:T transitions at CpG dinucleotides (45%) associated with spontaneous mutations, were found with a weak relation to tobacco smoke. In contrast, a high mutation frequency (70%) with a higher proportion of transversions (50%), especially G:C to T:A (44%) on the nontranscribed DNA strand, caused by exogenous carcinogenic agents like tobacco smoke, were observed for the columnar cell type, as with squamous cell carcinomas. These results indicate that two major subtypes of lung adenocarcinoma exist, one probably caused by tobacco smoke, and the other possibly due to spontaneous mutations. For the prevention of lung adenocarcinomas, in addition to stopping tobacco smoking, the elucidation of endogenous mechanisms is important.

Formation of abasic sites in DNA by t-butyl peroxyl radicals: implication for potent genotoxicity of lipid peroxyl radicals

We investigated abasic site formation in calf thymus DNA after exposure to a model compound of lipid-derived peroxyl radical that was generated by the reaction of tert-butyl hydroperoxide (t-BuOOH) with hemoglobin. Abasic site density in DNA was quantified by use of an enzyme-linked immunosorvent assay-like assay. In the presence of 10 mM t-BuOOH and 12.5 or 25 microM hemoglobin, 0.6-1.0 abasic sites/10(4) nucleotides were formed. However, abasic sites were not detected after replacing hemoglobin with nonheme iron, e.g. EDTA/Fe(2+), which initiates the production of alkyl and alkoxyl radicals. Therefore, the present results suggest that lipid peroxyl radicals may have a genotoxic potential through unique reactions, including depurination and depyrimidination, which lead to DNA strand breakage.

Substrate specificity of the p53-associated 3'-5' exonuclease

p53 exhibits 3'-5' exonuclease activity and the significance of this biochemical function is currently not defined. In order to gain information about the potential role(s) of this exonuclease activity, recombinant and wild-type human p53 was examined for excision of nucleotides from defined synthetic DNA substrates. p53 removes nucleotides threefold faster from single-strand DNA than from DNA duplexes, exhibits a 1.5-fold preference for 3'-terminals of DNA that contain a single nucleotide mispair (mismatch) as compared to correctly paired DNA and efficiently excises nucleotides from 3'-ends of blunt and cohesive (staggered) DNA double-strand breaks. The p53 exonuclease is predominantly non-processive on DNA which is 17 nucleotides long (or shorter) and processive on the longer 30-mers. The processivity of nucleotide excision is decreased in the presence of 50 mM potassium phosphate and eliminated when full-length p53 is replaced with the core domain, comprised of amino acids 82-292. Photoaffinity labeling indicates that (1) p53 monomers, rather than dimers, bind to single-strand forms of these oligomers; (2) complexes between p53 and 30-mers are more stable than those formed with 17-mers. The stability of these complexes determines processivity during nucleotide removal and modulates the 3'-5' exonuclease activity of p53. The relevance of substrate specificity of the p53 exonuclease to DNA repair is discussed.

Pooled analysis of p53 mutations in hematological malignancies

A computerized database is described that contains information about 507 mutations in the p53 gene of hematologic tumors and corresponding cell lines. Analysis of these mutations indicated the following findings: First, mutational spectrum analysis in these tumors was found to be similar to the pattern found for other solid tumors. However, when the patterns of base substitutions were examined separately according to the types of hematologic malignancies, followed by subgroup analysis, notable differences (in some cases of statistical significance) emerged. Second, mutational pattern analysis indicates that about 48% of base substitutions in hematologic tumors are suspected to be associated with carcinogen exposure. Third, deletions and insertions are localized mainly to exons 5-8 and repeated DNA sequences. However, the unusual profile of variations in frequency within each type of tumor suggests that, in addition to endogenous damage to template DNA, there is the factor of exposure to environmental physical and chemical carcinogens/mutagens. Fourth, p53 protein alterations analysis indicate that most of the changes in the amino acids are "semiconservative," presumably in order to avoid disrupting the structure of the p53 monomer. Consistent with this notion, structural mutations are more conservative than the binding mutations. Finally, molecular mechanisms that lead to p53 mutations, etiological factors that play a role in their formation, and the pathophysiological significance of consequent p53 protein alterations are discussed.

Significance of multiple mutations in cancer

There is increasing evidence that in eukaryotic cells, DNA undergoes continuous damage, repair and resynthesis. A homeostatic equilibrium exists in which extensive DNA damage is counterbalanced by multiple pathways for DNA repair. In normal cells, most DNA damage is repaired without error. However, in tumor cells this equilibrium may be skewed, resulting in the accumulation of multiple mutations. Among genes mutated are those that function in guaranteeing the stability of the genome. Loss of this stability results in a mutator phenotype. Evidence for a mutator phenotype in human cancers includes the frequent occurrence of gene amplification, microsatellite instability, chromosomal aberrations and aneuploidy. Current experiments have centered on two mechanisms for the generation of genomic instability, one focused on mutations in mismatch repair genes resulting in microsatellite instability, and one focused on mutations in genes that are required for chromosomal segregation resulting in chromosomal aberrations. This dichotomy may reflect only the ease by which these manifestations can be identified. Underlying both pathways may be a more general phenomenon involving the selection for mutator genes during tumor progression. During carcinogenesis there is selection for cells harboring mutations that can overcome adverse conditions that limit tumor growth. These mutations are produced by direct DNA damage as well as secondarily as a result of mutations in genes that cause a mutator phenotype. Thus, as tumor progression selects for cells with specific mutations, it also selects for cancer cells harboring mutations in genes that normally function in maintaining genetic instability.

Spectra of spontaneous frameshift mutations at the hisD3052 allele of Salmonella typhimurium in four DNA repair backgrounds

To characterize the hisD3052 -1 frameshift allele of Salmonella typhimurium, we analyzed approximately 6000 spontaneous revertants (rev) for a 2-base deletion hotspot within the sequence (CG)4, and we sequenced approximately 500 nonhotspot rev. The reversion target is a minimum of 76 bases (nucleotides 843-918) that code for amino acids within a nonconserved region of the histidinol dehydrogenase protein. Only 0.4-3.9% were true rev. Of the following classes, 182 unique second-site mutations were identified: hotspot, complex frameshifts requiring DeltauvrB + pKM101 (TA98-specific) or not (concerted), 1-base insertions, duplications, and nonhotspot deletions. The percentages of hotspot mutations were 13.8% in TA1978 (wild type), 24.5% in UTH8413 (pKM101), 31.6% in TA1538 (DeltauvrB), and 41.0% in TA98 (DeltauvrB, pKM101). The DeltauvrB allele decreased by three times the mutant frequency (MF, rev/10(8) survivors) of duplications and increased by about two times the MF of deletions. Separately, the DeltauvrB allele or pKM101 plasmid increased by two to three times the MF of hotspot mutations; combined, they increased this MF by five times. The percentage of 1-base insertions was not influenced by either DeltauvrB or pKM101. Hotspot deletions and TA98-specific complex frameshifts are inducible by some mutagens; concerted complex frameshifts and 1-base insertions are not; and there is little evidence for mutagen-induced duplications and nonhotspot deletions. Except for the base substitutions in TA98-specific complex frameshifts, all spontaneous mutations of the hisD3052 allele are likely templated. The mechanisms may involve (1) the potential of direct and inverted repeats to undergo slippage and misalignment and to form quasi-palindromes and (2) the interaction of these sequences with DNA replication and repair proteins.

DNA sequence analysis of spontaneous mutagenesis in Saccharomyces cerevisiae

To help elucidate the mechanisms involved in spontaneous mutagenesis, DNA sequencing has been applied to characterize the types of mutation whose rates are increased or decreased in mutator or antimutator strains, respectively. Increased spontaneous mutation rates point to malfunctions in genes that normally act to reduce spontaneous mutation, whereas decreased rates are associated with defects in genes whose products are necessary for spontaneous mutagenesis. In this article, we survey and discuss the mutational specificities conferred by mutator and antimutator genes in the budding yeast Saccharomyces cerevisiae. The implications of selected aspects of the data are considered with respect to the mechanisms of spontaneous mutagenesis.

Cancer cells exhibit a mutator phenotype

This review analyzes the concept and evidence in support of a mutator phenotype in human cancer. The large number of mutations reported in tumor cells cannot be accounted for by the low mutation rates observed in normal somatic cells; rather, it must be a manifestation of a mutator phenotype present early during the tumorigenic process. The interaction between increased mutagenesis and clonal selection provides a mechanism for the selection of cells with increased proliferative advantage. The concept of a mutator phenotype in cancer has gained considerable support from the findings of enormous numbers of somatic mutations in repetitive sequences in human tumors. Moreover, cell lines exhibiting microsatellite instability demonstrate an increased mutation frequency in expressed genes. A knowledge of mechanisms that generate multiple mutations in cancer cells has important implications for prevention. For many tumors, a delay in the rate of accumulation of mutations by a factor of two could drastically reduce the death rates from these tumors.

The mutation rate and cancer

The selection of advantageous mutations underlies tumorigenesis. The growth of a tumor is therefore a form of evolution at the somatic level, in which the population is comprised of individual cells within the tumor. Models of tumorigenesis have considered the relative importance of mutation and selection. We show that selection is more important than an increased mutation rate in the growth of a tumor. Some cancers may acquire a "mutator phenotype," probably leading to faster growth, but mutator phenotypes are not necessary for carcinogenesis.

Tissue regeneration as the basic oncogenic factor

A new hypothesis of oncogenesis has been suggested. The errors of deoxyribonucleic acid (DNA) synthesis are supposed to be the main cause of oncogenic mutations. Since cell division and consequently DNA synthesis are continually needed for the renewal of some tissues in adult organisms, the physiological and reparative regeneration of these tissues (epithelium, blood) is the basic factor of oncogenesis. The exogenous and endogenous agents increase the probability of tumor frequency via increasing the cell renovation rate in tissues and decreasing the DNA synthesis fidelity. This hypothesis is in accord with the experimental and epidemiological data including the results which are not explained reasonably until now, namely, the association of the carcinogenic activity of drugs with their toxicity, the organ and tissue specificity of tumor frequency, etc.

Effects of hyperoxia and caffeine on the expression of fragile site at Xq27.3

To enhance the cytogenetic expression of the fragile X chromosome, we studied the effects of hyperoxia and caffeine on the induction of fragile Xq27.3. A lymphoblastoid cell line (GM 06912) derived from a fragile X male proband was cultured in RPMI 1640 containing 16% dialyzed fetal calf serum. The cells were synchronously subjected to one of 3 different atmospheric oxygen tensions (21%, 21.3 kPa, normoxic; 40%, 40.5 kPa, hyperoxic; or 60%, 60.8 kPa, hyperoxic) during the last 24 hours of the 72 hour culture, immediately after the addition of 2'-deoxy-5-fluorouridine (FUdR) at 25 ng/ml. To study the enhancing effect of caffeine, with or without hyperoxia, a second set of cultures was additionally subjected to caffeine (2.5 mM) during the last 6 hours of the culture. When the fragility of hyperoxic cells (38.1 kPa dissolved oxygen) was compared to that of normoxic control cells (13.3 kPa dissolved oxygen), the difference was significant (P < 0.05). These data suggest that there is a mean increase in the fragile Xq27.3 expressivity as the dissolved oxygen tension increases. Additionally, we observed that caffeine, with or without hyperoxia, significantly (P < 0.05) suppressed the expression of the fragile X site in this lymphoblastoid cell line.

DNA damage and repair in somatic and germ cells in vivo

Alkylation-induced germ cell mutagenesis in the mouse versus Drosophila is compared based on data from forward mutation assays (specific-locus tests in the mouse and in Drosophila and multiple-locus assays in the latter species) but not including assays for structural chromosome aberrations. To facilitate comparisons between mouse and Drosophila, forward mutation test results have been grouped into three categories. Representatives of the first category are MMS (methyl methanesulfonate) and EO (ethylene oxide), alkylating agents with a high s value which predominantly react with ring nitrogens in DNA. ENU (N-ethyl-N-nitrosourea), MNU (N-methyl-N-nitrosourea), PRC (procarbazine), DEN (N-nitrosodiethylamine), and DMN (N-nitrosodimethylamine) belong to the second category. These agents have in common a considerable ability for modification at oxygens in DNA. Cross-linking agents (melphalan, chlorambucil, hexamethylphosphoramide) form the third category. The most unexpected, but encouraging outcome of this study is the identification of common features for three vastly different experimental indicators of genotoxicity: hereditary damage in Drosophila males, genetic damage in male mice, and tumors (TD50 estimates) in rodents. Based on the above three category classification scheme the following tentative conclusions are drawn. Monofunctional agents belonging to category 1, typified by MMS and EO, display genotoxic effects in male germ cell stages that have passed meiotic division. This phenomenon seems to be the consequence of a repair deficiency during spermiogenesis for a period of 3-4 days in Drosophila and 14 days in the mouse. We suggest that the reason for the high resistance of premeiotic stages, and the generally high TD50 estimates observed for this class in rodents, is the efficient error-free repair of N-alkylation damage. If we accept this hypothesis, then the increased carcinogenic potential in rodents, seen when comparing category 2 (ENU-type mutagens) to category 1 (MMS-type mutagens), along with the ability of category 2 genotoxins to induce genetic damage in premeiotic stages, must presumably be due to their enhanced ability for alkylations at oxygens in DNA; it is this property that actually distinguishes the two groups from each other. In contrast to category 1, examination of class 2 genotoxins (ENU and DEN) in premeiotic cells of Drosophila gave no indication for a significant role of germinal selection, and also removal by DNA repair was less dramatic compared to MMS. Thus category 2 mutagens are expected to display activity in a wide range of both post- and premeiotic germ cell stages. A number of these agents have been demonstrated to be among the most potent carcinogens in rodents.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectrum of spontaneous missense mutations causing cyclic AMP-resistance phenotypes in cultured S49 mouse lymphoma cells differs markedly from those of mutations induced by alkylating mutagens

Mutants of S49 mouse lymphoma cells resistant to cytolysis by analogs of cyclic AMP (cAMP) generally have missense mutations in the gene encoding the regulatory subunit of cAMP-dependent protein kinase. We have compared the mutations in 95 spontaneous isolates with those in 60 mutagen-induced isolates by sequence analysis of amplified cDNAs. Twenty-nine single basepair substitutions in 19 codons produced selectable phenotypes. The spontaneous mutant spectrum was dominated by a CpG transition hotspot in the codon for Arg334. This and other nearby CpG sites were found to be methylated in genomic S49 cell DNA by restriction enzyme analyses. Most of the remaining spontaneous mutants had either G-C-->C-G or T-A-->G-C transversions, which have been associated with damage caused by oxygen radicals. In contrast, the majority of mutants induced with the alkylating mutagens ethyl methanesulfonate (EMS) and N-methyl-N'-nitro-N-nitrosoguanidine had G-C-->A-T mutations at non-CpG sites; in addition, EMS induced several A-T-->G-C, A-T-->T-A, and G-C-->T-A substitutions. A single ICR191-induced mutant analyzed had a unique A-T-->G-C lesion. A number of spontaneous and mutagen-induced isolates had closely linked double or triple substitutions, and two isolates had tandem triple substitutions.

Spectrum of spontaneous HPRT- mutations in TK6 human lymphoblasts

The occurrence of deletions, coding sequence alterations, and intronic changes leading to aberrant splicing has been characterized among 33 spontaneous HPRT- mutants in TK6 human lymphoblasts. Deletions detectable by multiplex PCR amplification accounted for 45% (15/33) of the mutant collection. Base substitutions represented 30% (10/33) of the total, and were predominated by changes at G:C base pairs. The remaining mutants were distributed among frameshifts (9%, 3/33), small deletions (6%, 2/33), and compound alterations (9%, 3/33). Five mutants (15%) demonstrated aberrant splicing of the hprt transcript. A cluster of 4 deletion/insertion events was identified in hprt exon 6. A nearly perfect 13 bp duplication differed from the original sequence only by an A:T to G:C transition, which was observed as a unique alteration in another HPRT- mutant. A model involving correction of a mismatch in a secondary structure formed by the duplicated sequence may account for these results.

Multistep carcinogenesis: a 1992 perspective

Cancer is the leading cause of death in Japan. Recent changes in cancer incidence patterns may reflect the trend toward a more Western diet and life-style. Among the dietary factors that contribute to carcinogenesis are the heterocyclic amines, a group of mutagenic compounds present in cooked meat and fish. Carcinogenesis is a multistep process in which cells accumulate multiple genetic alterations as they progress to a more malignant phenotype. Recognition of the growing number of interacting factors that contribute to carcinogenesis may force reconsideration of current methods of risk assessment.

Spontaneous mutagenesis: experimental, genetic and other factors

Spontaneous mutations are "the net result of all that can go wrong with DNA during the life cycle of an organism" (Glickman et al., 1986). Thus, the types and amounts of spontaneous mutations produced are the resultant of all the cellular processes that are mutagenic and those that are antimutagenic. It is not widely appreciated that the types and frequencies of spontaneous mutations change markedly with subtle changes in experimental conditions. All types of mutations are produced spontaneously, i.e., base substitutions, frameshifts, insertions and deletions. However, very few papers have appeared that are devoted exclusively to the study of the mechanisms of spontaneous mutagenesis, and of the subtle experimental factors that affect the types and frequencies of spontaneous mutations. This is unfortunate because spontaneous mutagenesis appears to play a major role in evolution, aging, and carcinogenesis. This review emphasizes subtle experimental variables that markedly affect the results of a spontaneous mutation experiment. A thorough understanding of these variables eliminates the need for a theory of "directed" mutagenesis. The intrinsic instability of DNA, and the types of normal metabolic lesions that are produced in DNA that lead to mutations via errors made in replication, repair, and recombination are reviewed, as is the genetic control of spontaneous mutagenesis. As with spontaneous mutagenesis, spontaneous carcinogenesis can also be considered to be the net result of all that can go wrong with DNA during the life of an organism.

Molecular mechanisms of the formation of DNA double-strand breaks and induction of genomic rearrangements

The probability that damage occurs in closely opposed sites on complementary DNA strands increases when DNA is heavily modified with mutagenic agents. Enzymatic excision of the opposite lesions produces DNA double-strand breaks which give rise to genomic rearrangements (deletions, insertions, etc.). Plasmid systems were developed for studying chemical lesions leading to double-strand breaks and the fate of broken plasmid molecules within bacterial cells. Deletions result from the base-pairing of fortuitously located direct repeats flanking the DNA broken ends; as a consequence, the latter are joined, while the DNA fragment between the direct repeats is deleted. Genomic rearrangements arise during the repair of the DNA double-strand breaks, and both events are due to similar repair enzymes which maintain the integrity of the DNA primary structure when conditions are not stressful. A number of genomic rearrangements and point mutations seem to be predetermined by the DNA primary structure.

DNA deaminating ability and genotoxicity of nitric oxide and its progenitors

Nitric oxide (NO), a multifaceted bioregulatory agent and an environmental pollutant, can also cause genomic alterations. In vitro, NO deaminated deoxynucleosides, deoxynucleotides, and intact DNA at physiological pH. That similar DNA damage can also occur in vivo was tested by treating Salmonella typhimurium strain TA1535 with three NO-releasing compounds, including nitroglycerin. All proved mutagenic. Observed DNA sequence changes were greater than 99% C----T transitions in the hisG46 (CCC) target codon, consistent with a cytosine-deamination mechanism. Because exposure to endogenously and exogenously produced NO is extensive, this mechanism may contribute to the incidence of deamination-related genetic disease and cancer.

Kinds and locations of mutations arising spontaneously in the coding region of the HPRT gene of finite-life-span diploid human fibroblasts

Spontaneous thioguanine-resistant mutants were derived from populations of finite-life-span, diploid human fibroblasts by means of a fluctuation analysis. cDNA was prepared from mutant HPRT mRNA and amplified by the polymerase chain reaction, and the sequence of the product was analyzed. Exon deletions, which very likely arose from mutations in the intron splice site consensus sequences, were found in 10 of the 37 mutants examined (27% of the total). Among the 28 mutations in the coding sequence, base pair substitutions predominated (89%). With the exception of one base pair involved in a tandem mutation, all base pair substitutions resulted in alterations in the predicted amino acid sequence of the protein. In addition there were three frameshift mutations, consisting of the deletion of one or two base pairs. Although mutations occurred throughout the coding sequence, 50% (14/28) were found in the 5' portion of exon 3.

Formation of the tetraploid intermediate is associated with the development of cells with more than four centrioles in the elastase-simian virus 40 tumor antigen transgenic mouse model of pancreatic cancer

The development of pancreatic cancer in transgenic mice expressing the simian virus 40 tumor antigen placed under controlling regions of the elastase I gene is characterized by the sequential appearance of tetraploid and then multiple aneuploid cell populations. Pancreatic tissues from such transgenic mice were studied between 8 and 32 days of age. Virtually 100% of acinar cell nuclei had immunohistochemically detectable tumor antigen by 18 days. Tetraploid cells were demonstrated by DNA content flow cytometry by 20 days and were associated with the appearance of interphase cells that had 5-11 centrioles per cell in single thin sections of pancreatic tissue examined by electron microscopy. Mitotic cells also were observed that had 5 or more centrioles per cell that were incorporated into the poles of bipolar or at least tripolar spindle apparatuses. These observations indicate that formation of the tetraploid intermediate in the diploid----tetraploid----aneuploid sequence of pancreatic tumor formation in elastase-simian virus 40 tumor antigen transgenic mice is accompanied by the development of cells with 5 or more centrioles that can be incorporated into the poles of abnormal mitotic spindles. We speculate that cells with more than 4 centrioles are predisposed to the formation of multipolar mitoses that may yield daughter cells with chromosomal gains and losses, resulting in the subsequent development of aneuploid tumors.

p53 mutations in human cancers

Mutations in the evolutionarily conserved codons of the p53 tumor suppressor gene are common in diverse types of human cancer. The p53 mutational spectrum differs among cancers of the colon, lung, esophagus, breast, liver, brain, reticuloendothelial tissues, and hemopoietic tissues. Analysis of these mutations can provide clues to the etiology of these diverse tumors and to the function of specific regions of p53. Transitions predominate in colon, brain, and lymphoid malignancies, whereas G:C to T:A transversions are the most frequent substitutions observed in cancers of the lung and liver. Mutations at A:T base pairs are seen more frequently in esophageal carcinomas than in other solid tumors. Most transitions in colorectal carcinomas, brain tumors, leukemias, and lymphomas are at CpG dinucleotide mutational hot spots. G to T transversions in lung, breast, and esophageal carcinomas are dispersed among numerous codons. In liver tumors in persons from geographic areas in which both aflatoxin B1 and hepatitis B virus are cancer risk factors, most mutations are at one nucleotide pair of codon 249. These differences may reflect the etiological contributions of both exogenous and endogenous factors to human carcinogenesis.