Formaldehyde mutagenesis in Drosophila. Molecular analysis of ADH-negative mutants

The mutagenic properties of formaldehyde and acetaldehyde: Reflections on half a century of progress

Formaldehyde and acetaldehyde are reactive, small compounds that humans are exposed to routinely, variously from endogenous and exogenous sources. Both small aldehydes are classified as human carcinogens. Investigation of the DNA damaging properties of these two compounds began some 50 years ago. In this review, we summarize progress in this field since its inception over half a century ago, distilling insights gained by the collective efforts of many research groups while highlighting areas for future directions. Over the decades, general consensus about aspects of the mutagenicity of formaldehyde and acetaldehyde has been reached. But other characteristics of formaldehyde and acetaldehyde remain incompletely understood and require additional investigation. These include crucial details about the mutational signature(s) induced and possible mechanistic role(s) during carcinogenesis.

The implausibility of leukemia induction by formaldehyde: a critical review of the biological evidence on distant-site toxicity

Formaldehyde is a naturally occurring biological compound that is present in tissues, cells, and bodily fluids. It is also a potent nasal irritant, a cytotoxicant at high doses, and a nasal carcinogen in rats exposed to high airborne concentrations. The normal endogenous concentration of formaldehyde in the blood is approximately 0.1 mM in rats, monkeys, and humans, and it is 2- to 4-fold higher in the liver and nasal mucosa of the rat. Inhaled formaldehyde enters the one-carbon pool, and the carbon atom is rapidly incorporated into macromolecules throughout the body. Oxidation to formate catalyzed by glutathione-dependent and -independent dehydrogenases in nasal tissues is a major route of detoxication and generally precedes incorporation. The possibility that inhaled formaldehyde might induce various forms of distant-site toxicity has been proposed, but no convincing evidence for such toxicity has been obtained in experimental studies. This review summarizes the biological evidence that pertains to the issue of leukemia induction by formaldehyde, which includes: (1) the failure of inhaled formaldehyde to increase the formaldehyde concentration in the blood of rats, monkeys, or humans exposed to concentrations of 14.4, 6, or 1.9 ppm, respectively; (2) the lack of detectable protein adducts or DNA-protein cross-links (DPX) in the bone marrow of normal rats exposed to [3H]- and [14C]formaldehyde at concentrations as high as 15 ppm; (3) the lack of detectable protein adducts or DPX in the bone marrow of glutathione-depleted (metabolically inhibited) rats exposed to [3H]- and [14C]formaldehyde at concentrations as high as 10 ppm; (4) the lack of detectable DPX in the bone marrow of Rhesus monkeys exposed to [14C]formaldehyde at concentrations as high as 6 ppm; (5) the failure of formaldehyde to induce leukemia in any of seven long-term inhalation bioassays in rats, mice, or hamsters; and (6) the failure of formaldehyde to induce chromosomal aberrations in the bone marrow of rats exposed to airborne concentrations as high as 15 ppm or of mice injected intraperitoneally with formaldehyde at doses as high as 25 mg/kg. Biological evidence that might be regarded as supporting the possibility of leukemia induction by formaldehyde includes: (1) the detection of cytogenetic abnormalities in circulating lymphocytes in seven studies of human subjects exposed to ambient concentrations in the workplace (but not in seven other studies of human subjects or in rats exposed to 15 ppm); (2) the induction of leukemia in rats in a single questionable drinking water study with formaldehyde concentrations as high as 1.5 g/L (but not in three other drinking water studies with concentrations as high as 1.9 or 5 g/L); (3) the detection of chromosomal aberrations in the bone marrow of rats exposed to very low concentrations of formaldehyde (0.4 or 1.2 ppm) (but not in another study at concentrations as high as 15 ppm); and (4) an apparent increase in the fraction of protein-associated DNA (assumed to be due to DPX) in circulating lymphocytes of humans exposed to ambient concentrations in the workplace (1-3 ppm). This evidence is regarded as inconsequential for several reasons, including lack of reproducibility, inadequate reporting of experimental methods, inconsistency with other data, or insufficient analytical sensitivity, and therefore, it provides little justification for or against the possibility that inhaled formaldehyde may be a leukemogen. In contrast to these inconclusive findings, the abundance of negative evidence mentioned above is undisputed and strongly suggests that there is no delivery of inhaled formaldehyde to distant sites. Combined with the fact that formaldehyde naturally occurs throughout the body, and that multiple inhalation bioassays have not induced leukemia in animals, the negative findings provide convincing evidence that formaldehyde is not leukemogenic.

Smg1 nonsense mutations do not abolish nonsense-mediated mRNA decay in Drosophila melanogaster

Smg1 is a key component of nonsense-mediated decay (NMD) in Caenorhabditis elegans and mammals. Here we report that two nonsense alleles of the ortholog of Smg1 do not affect NMD in Drosophila melanogaster.

Compensatory evolution of a precursor messenger RNA secondary structure in the Drosophila melanogaster Adh gene

Evidence for the evolutionary maintenance of a hairpin structure possibly involved in intron processing had been found in intron 1 of the alcohol dehydrogenase gene (Adh) in diverse Drosophila species. In this study, the putative hairpin structure was evaluated systematically in Drosophila melanogaster by elimination of either side of the stem using site-directed mutagenesis. The effects of these mutations and the compensatory double mutant on intron splicing efficiency and ADH protein production were assayed in Drosophila melanogaster Schneider L2 cells and germ-line transformed adult flies. Mutations that disrupt the putative hairpin structure right upstream of the intron branch point were found to cause a significant reduction in both splicing efficiency and ADH protein production. In contrast, the compensatory double mutant that restores the putative hairpin structure was indistinguishable from the WT in both splicing efficiency and ADH level. It was also observed by mutational analysis that a more stable secondary structure (with a longer stem) in this intron decreases both splicing efficiency and ADH protein production. Implications for RNA secondary structure and intron evolution are discussed.

The importance of distinct metabolites of N-nitrosodiethylamine for its in vivo mutagenic specificity

Although N-nitrosodiethylamine (NDEA) is a potent carcinogen in rodents and a probable human carcinogen, little attempts were made to characterize its mutation spectrum in higher eukaryotes. We have compared forward mutation frequencies at multiple (700) loci with the mutational spectrum induced at the vermilion gene of Drosophila, after exposure of post- and pre-meiotic male germ cells to NDEA. Among 30 vermilion mutants collected from post-meiotic stages were 12 G:C-->A:T transitions (40%), 8 A:T-->T:A transversions (27%), and 4 structural rearrangements (13%). The remainder were three A:T-->G:C transitions, two G:C-->C:G transversions and one G:C-->T:A transversion. The results show that although NDEA induces predominantly transitions (40% G:C-->A:T and 10% A:T-->G:C), the frequencies of transversions (37%, of which 27% of A:T-->T:A transversions) and especially of rearrangements (13%) are remarkably high. This mutation spectrum differs significantly from that produced by the direct-ethylating agent N-ethylnitrosourea (ENU), although the relative distribution of ethylated DNA adducts is similar for both carcinogens. These differences, in particular the occurrence of rearrangements, are most likely the result of the requirement of NDEA for bioactivation. Since all four rearrangements were collected from non-metabolizing spermatozoa (or late spermatids), it is hypothesized that they derived from acetaldehyde, a stable metabolite of NDEA. Due to its cytotoxicity, attempts to isolate vermilion mutants from NDEA-exposed pre-meiotic cells were largely unsuccessful, because only two mutants (one A:T-->G:C transition and one 1bp insertion) were collected from those stages. Our results show that NDEA is capable of generating carcinogenic lesions other than base pair substitutions.

Pharmacodynamics of formaldehyde: applications of a model for the arrest of DNA replication by DNA-protein cross-links

A variety of evidence suggests that formaldehyde (HCHO)-induced DNA-protein cross-links (DPX) are genotoxic as a result of their ability to arrest DNA replication. Although DPX can be removed and the DNA can be repaired, failure to remove the blockage prior to cell division or excision followed by incomplete repair could cause cell death or a mutation. To characterize the concentration and time dependence of this mechanism, a biologically based model for DNA replication in the presence of DPX was developed based on the assumptions that (1) DPX are formed randomly in the DNA and (2) a replication fork can advance up to but not past a DPX. Using a combination of Poisson and binomial statistics, a quantitative relationship between the amount of newly synthesized DNA and the concentration of DPX was derived, which predicts that the rate of DNA replication should decrease nonlinearly with increasing concentrations of DPX. Because the latter is a nonlinear function of the airborne concentration of HCHO, an inverse sigmoidal relationship is predicted between the rate of DNA replication and the concentration of inhaled formaldehyde. The model was parameterized using data derived from a study of the incorporation of [methyl-(14)C]thymidine monophosphate into the DNA of the nasal respiratory mucosa of Fischer-344 rats exposed to (3)HCHO and H(14)CHO (6 ppm, 6 h). The model was then applied to measurements of DNA replication in the nasal mucosa of experimental animals exposed to wide ranges of H(14)CHO (rats: 0.7, 2, 6, or 15 ppm, 3 h; rhesus monkeys: 0.7, 2, or 6 ppm, 6 h). The results indicate that, at airborne concentrations above 6 ppm in rats, there is a marked decrease (ca. 62% at 15 ppm) in the amount of newly synthesized DNA due to DPX formation during a single 6-h exposure to HCHO. The arrest of DNA replication at high HCHO concentrations could result in cytolethality or genotoxicity, both of which are critical factors in the induction of rat nasal cancer by HCHO. However, at concentrations below 2 ppm in monkeys or 1 ppm in rats, the decrease in the rate of DNA replication is predicted to be <1% after a 6-h exposure. This small decrease is probably undetectable using currently available techniques. The parameterized model suggests that the arrest of DNA replication by DPX is mainly a high-dose phenomenon and that at ambient exposure concentrations it is unlikely to be a major risk factor.

DNA damage in the nasal passageway: a literature review

The purpose of this review is to provide a compilation of work examining DNA damage in the nasal cavity. There are numerous methods to identify and quantify damage to DNA and the diversity of methods and toxicologic endpoints is illustrated by the range of studies presented here. There are a large number of independent studies measuring endpoints in the upper respiratory tract; however, with regard to toxicant induced DNA damage in the nasal passageway, the effects of two compounds, 4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and formaldehyde (HCHO), appear to have been extensively characterized. The body of work on NNK and formaldehyde have provided insights into molecular mechanisms of DNA damage and repair and induced cell replication and its relationship to nasal cancer. With new technologies and molecular techniques, the sensitivity to enable evaluations of the minute quantities of nasal tissue available in test species and human biopsy impact the study of the nasal-toxicant interactions. As methods used to characterize DNA damage increase in sensitivity, the importance of both exogenous and endogenous sources of DNA damage, steady-state levels of cellular damage, repair, and resulting mutations, low-dose exposure assessments and inter-species extrapolation will become increasingly complex. Additional studies of DNA damage in the nasal passage will undoubtedly challenge future estimations of risk and impact what are perceived to be acceptable levels of exposure to known and predicted carcinogens. The aim of this paper is to provide to the interested scientist literature relevant to the effects of agents on nasal DNA, so that areas of insufficient information can be identified and used to further develop and expand the knowledge base for nasal DNA toxicant interactions.

Lack of evidence for the involvement of formaldehyde in the hepatocarcinogenicity of methyl tertiary-butyl ether in CD-1 mice

The oxygenated fuel additive methyl tertiary-butyl ether (MTBE) induced hepatocellular adenomas in female but not male CD-1 mice exposed to 8000 ppm; liver cancer was not induced in female or male mice exposed to 3000 or 400 ppm. Since MTBE is metabolized by cytochrome P450 to formaldehyde (HCHO), a potentially mutagenic intermediate capable of forming DNA-protein cross-links (DPX), the formation of DPX and of another HCHO derivative, RNA-formaldehyde adducts (RFA), from MTBE was investigated using freshly isolated hepatocytes from female CD-1 mice incubated with MTBE-(O-methyl-14C). DPX and RFA were detected, but the adduct yields were very small and were independent of the concentration of MTBE in the hepatocyte suspension over a wide concentration range (0.33-6.75 mM). Similar results were obtained using hepatocytes from male B6C3F1 mice and male F344 rats. Induction of cytochrome P450 by pretreatment of mice with MTBE prior to isolation of hepatocytes did not result in a measurable increase in the yields of either DPX or RFA. In contrast to the absence of concentration-dependent DPX and RFA formation from MTBE, there was a marked, concentration-dependent increase in the yields of both DPX and RFA when [14C]formaldehyde was added directly to the medium. These results suggest that the metabolism of MTBE to HCHO approaches saturation at concentrations below 0.33 mM, and that the rate of HCHO production from metabolism of MTBE is slow relative to the rate of HCHO metabolism. The lack of concentration dependence and the absence of species or sex differences in the formation of DPX and RFA from MTBE indicate that metabolism of MTBE to HCHO is not a critical component of its carcinogenic mechanism in mice.

DNA-protein cross-links produced by various chemicals in cultured human lymphoma cells

Chemicals such as cis-platinum, formaldehyde, chromate, copper, and certain arsenic compounds have been shown to produce DNA-protein cross-links in human in vitro cell systems at high doses, such as those in the cytotoxic range. Thus far there have only been a limited number of other chemicals evaluated for their ability to produce cross-links. The purpose of the work described here was to evaluate whether select industrial chemicals can form DNA-protein cross-links in human cells in vitro. We evaluated acetaldehyde, acrolein, diepoxybutane, paraformaldehyde, 2-furaldehyde, propionaldehyde, chloroacetaldehyde, sodium arsenite, and a deodorant tablet [Mega Blue; hazardous component listed as tris(hydroxymethyl)nitromethane]. Short- and long-term cytotoxicity was evaluated and used to select appropriate doses for in vitro testing. DNA-protein cross-linking was evaluated at no fewer than three doses and two cell lysate washing temperatures (45 and 65 degrees C) in Epstein-Barr virus (EBV) human Burkitt's lymphoma cells. The two washing temperatures were used to assess the heat stability of the DNA-protein cross-link, 2-Furaldehyde, acetaldehyde, and propionaldehyde produced statistically significant increases in DNA-protein cross-links at washing temperatures of 45 degrees C, but not 65 degrees C, and at or above concentrations of 5, 17.5, and 75 mM, respectively. Acrolein, diepoxybutane, paraformaldehyde, and Mega Blue produced statistically significant increases in DNA-protein cross-links washed at 45 and 65 degrees C at or above concentrations of 0.15 mM, 12.5 mM, 0.003%, and 0.1%, respectively. Sodium arsenite and chloroacetaldehyde did not produce significantly increased DNA-protein cross-links at either temperature nor at any dose tested. Excluding paraformaldehyde and 2-furaldehyde treatments, significant increases in DNA-protein cross-links were observed only at doses that resulted in complete cell death within 4 d following dosing. This work demonstrates that DNA-protein cross-links can be formed in vitro following exposure to a variety of industrial compounds and that most cross-links are formed at cytotoxic concentrations.

Interactions between the regulatory regions of two Adh alleles

A region (NS1) that acts like an enhancer is located approximately 300 bp upstream of the larval cap site in the Adh gene of D. melanogaster. When this sequence is deleted (delta NS1), the gene fails to express ADH protein. Gene expression can be restored by placing a second Adh gene with an intact enhancer elsewhere on the same plasmid. In these circumstances, both genes are expressed equally regardless of their orientation on the plasmid. In this report we further characterize the interactions that occur when a single enhancer activates expression from a proximal and distant promoter. We have made the following observations: (1) While the two genes are expressed equivalently, their expression relative to a plasmid carrying two intact genes is reduced by a factor of 2 to 6 depending on the orientation of the two genes. (2) The single enhancer drives expression of both genes on any given plasmid molecule. (3) The enhancer does not interact with the Adh gene from which the NS7 region (which spans the larval TATA box) is removed. (4) Expression of the delta NS1 gene can be restored by an intact gene when both are inserted together into the Drosophila genome via P element-mediated transformation. (5) Increasing the separation between the two genes on a plasmid by up to 15 kbp does not prevent the restoration of expression of the delta NS1 gene. We propose a model that explains how a single enhancer can stimulate equal expression from two genes.

Dichloromethane (methylene chloride): metabolism to formaldehyde and formation of DNA-protein cross-links in B6C3F1 mice and Syrian golden hamsters

Dichloromethane (DCM) is metabolized via a glutathione transferase (GST)-dependent pathway to formaldehyde (HCHO), a mutagenic compound that could play an important role in the carcinogenic effects of DCM observed in the liver and lungs of B6C3F1 mice at 2000 and 4000 ppm. Syrian hamsters metabolize DCM more slowly than mice via this pathway, and hamsters exposed to 3500 ppm showed no apparent carcinogenic response. The possible formation of DNA-protein cross-links (DPX) from DCM in both species was examined. Male mice and hamsters were pre-exposed for 2 days (6 hr/day) to 4000 ppm of DCM and on the third day were exposed (6 hr) to a decaying concentration (4500 to 2500 ppm) of [14C]DCM. DPX were detected in mouse liver, but not in mouse lung, hamster liver, or hamster lung. The failure to detect DPX in mouse lung does not exclude their possible formation in a subpopulation of lung cells. Metabolic incorporation of 14C derived from [14C]DCM into DNA suggested a higher rate of turnover of some mouse lung cells than of hamster lung cells, but no large difference in the turnover rates of liver cells in the two species under these conditions. These results demonstrate that HCHO derived from DCM can form DNA-protein cross-links in the liver of the B6C3F1 mouse. The formation of DPX is dependent on the activity of the GST pathway, and species such as hamsters and humans having much lower rates of DCM metabolism via this pathway may not generate toxicologically significant concentrations of HCHO and DPX.

Tissue-specific position effects on alcohol dehydrogenase expression in Drosophila melanogaster

Twenty transformed lines have been isolated as a result of the germ line insertion of a 3.2 kb alcohol dehydrogenase (Adh) gene fragment into an Adh negative strain of Drosophila melanogaster by P element-mediated transformation. More than half of these lines exhibited abnormal ADH expression. The level of ADH expression ranges from zero in some lines to near normal levels in others, and the pattern of ADH expression in the larval gut is also abnormal in many of these lines. Each of the abnormal tissue-specific patterns is stable and characterized by the absence or reduction of ADH expression in certain tissues. High levels of ectopic expression were not observed. In two of these lines, the pattern of ADH staining is highly restricted: it is limited to the medial midgut in line MM-50, and to the gastric caecae and the proventriculus in line GC-1. In heterozygotes between these two lines ADH is expressed in both of these tissues. To test the hypothesis that this abnormal expression is due to position effects, inserts were mobilized to new locations. The mobilized inserts exhibited new patterns of tissue-specific expression associated with new cytological insert locations, showing that the abnormal expression in lines MM-50 and GC-1 is due to tissue-specific position effects and not to mutations. The results are discussed in the context of chromatin structure as a possible cause of these position effects.

A Drosophila Adh gene can be activated in trans by an enhancer

The ability of a segment of the Drosophila Adh gene to produce ADH activity in larvae is dependent upon the presence of a 53 bp sequence (called NS1) located between 289 and 341 bp upstream of the larval transcription start site. This sequence behaves like an enhancer in that it can stimulate gene activity when it is placed at various distances from, or on either side of, an Adh gene. Like a typical enhancer, NS1 does not ordinarily function in trans. However, when an Adh gene lacking NS1 is placed on one plasmid, and a second gene carrying NS1 is placed on another, and the two plasmids are interlocked in a catenane, both genes are active. This finding supports the mechanism of loop-mediated enhancer action.

Introduction of single-stranded ADH genes into Drosophila results in tissue-specific expression

We injected single-stranded circular DNA containing a Drosophila Adh gene into ADH-negative embryos of Drosophila melanogaster and performed ADH histochemical staining on third instar larvae of the injected generation. Introduction of either the coding or non-coding strand resulted in correct tissue-specific expression of the Adh gene in larvae. Southern blotting revealed that the bulk of the injected DNA became double-stranded shortly after injection and was not integrated into the genome.

Analysis of formaldehyde-induced Adh mutations in Drosophila by RNA structure mapping and direct sequencing of PCR-amplified genomic DNA

Two formaldehyde-induced mutations at the Drosophila Adh locus (Adhfn45 and Adhfn46) were analyzed by determining RNA structures at different developmental stages, polymerase chain reaction (PCR) amplification of the affected genomic regions, and direct sequencing of the resulting double-stranded DNA fragments. Adhfn46 adults and larvae accumulate abundant ADH-like distal (adult) and proximal (larval) transcripts that are shorter than transcripts in wild-type flies by a lesion located in the second ADH protein-coding exon. Direct sequencing of the amplified DNA region showed that Adhfn46 contains a 69-bp in-frame deletion that removes 23 amino acids near one border of the second exon. Consistent with these findings, we observed a shorter ADHfn46 protein present at only 3% of wild-type levels. In contrast, Adhfn45 adults and larvae accumulate much smaller amounts of ADH-like distal and proximal transcripts. Both RNAs have an identical aberration in RNA splicing of the 65-base intron sequence. Direct sequencing of the amplified mutated DNA region showed that Adhfn45 contains a 21-bp deletion that removed and rearranged DNA at the 5' splice junction of the 65-bp intron. No ADH cross-reacting material is detected in Adhfn45 flies. Direct-repeat sequences (3-11 bp) are present flanking and within the mutated DNA regions. The patterns of DNA deletion and deletion accompanied by sequence addition at the mutant sites suggest a slipped mispairing mechanism during DNA replication or repair that involves local DNA homology.

Mutations induced at the white and vermilion loci in Drosophila melanogaster

The white and vermilion loci in D. melanogaster were selected as target genes for the study of the mutational specificity of ionizing radiation and N-ethyl-N-nitrosourea (ENU) in a whole organism. Analysis of X-ray- and neutron-induced white mutants by a combination of genetic and molecular techniques showed that ionizing radiation induces primarily break-type mutations against a repair-proficient background, the majority of these alterations being deletions. Both very large multi-locus deficiencies and deletions of only a few base pairs were observed. These small deletions are flanked by repeats of 2-3 nucleotides, one copy of which is retained at the new junction. Presumably these small repeats are involved in the generation of the X-ray-induced deletions. In excision-repair-deficient mus201D1 flies, the frequency of whole-body white mutants recovered after X-ray irradiation is the same as in the wild-type strain. The percentage of mosaic mutations, however, is enhanced by a factor 3-4. Analysis by blot hybridization of ENU-induced white mutants strongly indicates that most mutations are due to base-pair changes. This was confirmed by sequence analysis of 25 ENU-induced vermilion mutants. In all mutants the alterations are due to base-pair changes, the majority being GC to AT transitions (61%).

Analysis of ENU-induced mutations at the Adh locus in Drosophila melanogaster

N-Ethyl-N-nitrosourea (ENU) was used to induce mutations in the Drosophila melanogaster, alcohol dehydrogenase (Adh) gene. Flies were treated with ENU and mated to homozygous intragenic Adh null mutants; Adh null mutations were selected by exposure of the F1 generation to 1-penten-3-ol. Fourteen Adh null mutations were recovered which included 11 from spermatozoa, 2 from oocytes and 1 from a premeiotic spermatocyte. 2 mutations from spermatozoa and 1 of the mutations from oocytes were multilocus deficiencies which included the Adh locus as determined by complementation tests. The remaining 11 intragenic Adh null mutations were sequenced using the Sanger dideoxy method. One Adh null mutation induced in an oocyte was an AT to TA transversion and the mutation induced in a premeiotic spermatocyte was a GC to AT transition, both of which resulted in a single amino acid substitution. The 11 null mutations induced in spermatozoa were a data set in which both the dose of ENU and the treated germ-cell stage were held constant; therefore, only these 11 mutations were used to calculate the mutation frequency and compare the mutations at the Adh locus with those recovered in other studies. The dose of ENU induced a sex-linked recessive lethal frequency approximately 300 times that of the spontaneous frequency; therefore, these mutations were assumed to have been induced by ENU. 2 of the 11 mutations induced in spermatozoa were multilocus deficiencies and 9 were intragenic mutations. 7 of the 9 intragenic mutations were GC to AT transitions which resulted in 5 single amino acid substitutions, 1 premature translation termination codon, and 1 splice site mutation.(ABSTRACT TRUNCATED AT 400 WORDS)

Conversion and reciprocal exchange between tandem repeats in Drosophila melanogaster

We have developed an experimental system to assay conversion and reciprocal exchange between tandem repeats in Drosophila melanogaster. In this system, the recombining markers map 0.76 kb apart within the Adh gene, and the length of the repeated unit is 4.75 kb. Our results provide a preliminary record of germline frequencies of gene conversion and unequal exchange between these markers. Conversions involving dispersed repeats were not observed, and may be less frequent. This work demonstrates that conversion takes place at an appreciable frequency between tandem repeats in metazoan germline. It confirms that gene conversion can mediate homogenization of reiterated sequences in higher eukaryotes.

Ecdysteroid regulation of glucose dehydrogenase and alcohol dehydrogenase gene expression in Drosophila melanogaster

The temporal patterns of glucose dehydrogenase (Gld) and alcohol dehydrogenase (Adh) expression in Drosophila are correlated positively and negatively, respectively, with ecdysterone titers during the late third instar. In mutant l(3)ecdysone-1ts (ecd-1) larvae, the normal peak of Gld mRNA late in the third instar is not expressed. Conversely, the normal decrease in Adh mRNA at this stage fails to occur in ecd-1. These two abnormal patterns can be reversed by treatment with exogenous ecdysterone. Premature exposure of wild type mid-third instar larvae to ecdysterone also results in the rapid accumulation of Gld mRNA and signals the repression of Adh mRNA. The observed decrease in Adh mRNA expression is accompanied by a transient switch in promoter usage from proximal to distal transcription start sites, which normally occurs later in the third instar.

Use of promoter fusions in Drosophila genetics: characterization of a YP1-ADH fusion gene

In Drosophila melanogaster the yolk protein (YP) genes are normally expressed only in the fat body and follicular epithelium of adult females--never in males or in larvae. We describe here a first step toward a genetic examination of the developmental controls that restrict the activity of the YP genes to adult female tissues. A YP1 promoter that contains the tissue-, temporal-, and sex-specific controlling elements for expression was fused to the reporter gene, alcohol dehydrogenase (Adh). The gene fusion was transformed into an Adh-deficient genotype. As assayed by a number of criteria, that the fusion gene is expressed in the same physiological manner as the endogenous yolk protein genes. The fusion gene's activity is modulated in trans by a temperature-sensitive allele of the sex determination gene, tra-2. The Adh enzyme serves as a selectable marker and therefore these flies are suitable for use in genetic screens for trans-acting mutations that affect the expression of the yolk protein genes.

Quantitative analysis of RNA produced by slow and fast alleles of Adh in Drosophila melanogaster

The alcohol dehydrogenase (ADH) locus (Adh) of Drosophila melanogaster in polymorphic on a world-wide basis for two allozymes, Fast and Slow. This study was undertaken to determine whether the well-established difference in ADH protein concentration between the allozymes is due to a difference in mRNA levels. RNA gel blot hybridization and an RNase protection assay were used to quantify ADH mRNA levels. Each method used an Adh null mutant as an internal standard. Several Slow and Fast allele pairs of different geographic origins were analyzed. The results provide strong evidence that the ADH protein concentration difference is not accounted for by RNA level.

Spectra of molecular changes induced in DNA of Drosophila spermatozoa by 1-ethyl-1-nitrosourea and X-rays

Mutations induced in Drosophila spermatozoa at the alcohol dehydrogenase Adh locus by 1-ethyl-1-nitrosourea (ENU) were compared to X-ray-induced mutations using genetic tests for complementation, southern blotting, western blotting and northern blotting. 8 of 10 ENU-induced mutations complemented all known adjacent loci and were presumed to be intragenic. In contrast, 8 of 30 X-ray-induced mutations were intragenic. Southern blot analysis showed that 2 of 7 intragenic mutations induced by X-rays were altered at the Adh locus, whereas all 8 intragenic ENU mutants appeared normal. Western blot analysis showed 4 of 7 intragenic mutants induced by X-rays produced a detectable polypeptide; 1 of the 4 had normal molecular weight and charge. In contrast, 7 of the 8 intragenic mutants induced by ENU produced a polypeptide of normal molecular weight and charge. One ENU and two X-ray-induced mutants, which had normal southern blots and no detectable polypeptide, produced normal molecular weight mRNA by northern blots. The interpretation of these results is that in spermatozoa X-rays induce primarily deletions that either produce deficiencies of the Adh locus or nonsense mutations within the locus, whereas ENU induces primarily missense mutations. This forward mutation assay based on loss of enzymatic activity efficiently recovered a broad spectrum of mutations ranging from missense to intragenic deletions and multi-locus deficiencies. Only 3 of these 40 mutations produced a polypeptide detectable as an electrophoretic variant.

Molecular analysis of Drosophila melanogaster AdhnLA405 confirms reliability of DNA-sequencing methodology

An alcohol dehydrogenase (ADH) null mutant of Drosophila melanogaster (AdhnLA405) originally recovered following X-ray irradiation of mature sperm (Aaron, 979) is analyzed by Southern blotting, Western blotting, and DNA sequencing. The genetic, immunologic, and nucleic acid sequence data are consistent with the hypothesis that a cross-over event, independent of X-irradiation, between parental chromosomes is responsible for the ADH null phenotype of AdhnLA405. By DNA-sequence analysis we show that molecular cloning of this locus (i.e., propagation in prokaryotic hosts) apparently does not introduce any spurious changes (substitutions, additions, deletions, or rearrangements) within the DNA.

A mutagenicity assessment of acetaldehyde

Acetaldehyde has been shown in studies by several different laboratories to be a clastogen (chromosome-breaking) and inducer of sister-chromatid exchanges in cultured mammalian cells (Chinese hamster cells and human lymphocytes). Although there have been very few studies in intact mammals, the available evidence suggests that acetaldehyde produces similar cytogenetic effects in vivo. The production of cytogenetic abnormalities may be related to the ability of acetaldehyde to form DNA-DNA and/or DNA-protein cross-links. Acetaldehyde apparently has not been evaluated for its ability to cause gene mutations in cultured mammalian cells, but it has been shown to produce sex-linked recessive lethals in Drosophila. In general, bacteria tests have been negative. Although acetaldehyde is a genotoxic cross-linking agent, it does not appear to cause DNA strand breaks. There were no studies available regarding the potential of acetaldehyde to produce genetic damage in mammalian germ cells in vivo. Most mutagenicity testing on acetaldehyde has been motivated by attempts to define the proximate mutagen in ethanol metabolism.

Molecular analysis of formaldehyde-induced mutations in human lymphoblasts and E. coli

The molecular nature of formaldehyde (HCHO)-induced mutations was studied in both human lymphoblasts and E. coli. Thirty HPRT- human lymphoblast colonies induced by eight repetitive 150 microM HCHO treatments were characterized by Southern blot analysis. Fourteen of these mutants (47%) had visible deletions of some or all of the X-linked HPRT bands, indicating that HCHO can induce large losses of DNA in human lymphoblasts. In E. coli, DNA alterations induced by HCHO were characterized with use of the xanthine guanine phosphoribosyl transferase (gpt) gene as the genetic target. Exposure of E. coli to 4 mM HCHO for 1 hr induced large insertions (41%), large deletions (18%), and point mutations (41%). Dideoxy DNA sequencing revealed that most of the point mutations were transversions at GC base pairs. In contrast, exposure of E. coli to 40 mM HCHO for 1 hr produced 92% point mutations, 62% of which were transitions at a single AT base pair in the gene. Therefore, HCHO is capable of producing different genetic alterations in E. coli at different concentrations, suggesting fundamental differences in the mutagenic mechanisms operating at the two concentrations used. Naked pSV2gpt plasmid DNA was exposed to 3.3 or 10 mM HCHO and transformed into E. coli. Most of the resulting mutations were frameshifts, again suggesting a different mutagenic mechanism.

DNA sequence analysis of X-ray-induced deletions at the white locus of Drosophila melanogaster

We have determined the nucleotide sequence of 5 X-ray-induced white mutants containing small rearrangements. Comparison with wild-type sequences showed deletions in the coding region ranging in size between 6 bp and 29 bp. These small deletions are distributed non-randomly over the white locus. Two mutants contain the same 29-bp deletion, while the other 3 deletions are clustered. All 5 deletions have occurred between 2 and 3 bp repeats. One of the repeats is preserved in the novel junction formed by the deletion. Our results suggest that recombinational processes may be involved in the generation of X-ray-induced deletions.

Molecular consequences of two formaldehyde-induced mutations in the alcohol dehydrogenase gene of Drosophila melanogaster

Adhfn23 and Adhfn24 are two formaldehyde-induced, homozygous-viable, alcohol dehydrogenase-null mutants that bear lesions in the gene that codes for the alcohol dehydrogenase (ADH; EC 1.1.1.1) of Drosophila melanogaster. Adhfn23 contains a 34-base pair deletion in the C-terminal coding region of the alcohol dehydrogenase structural gene. By immunological and molecular analysis, we show that the deletion shifts the translation reading frame and results in a prematurely truncated polypeptide product (10 amino acids shorter than wild type) that cross-reacts with antibody raised against ADH. The steady-state level of alcohol dehydrogenase mRNA present in this mutant is close (97%) to that in the wild type, but the steady-state level of alcohol dehydrogenase-like protein is 50% lower. Moreover, the rate of alcohol dehydrogenase synthesis in Adhfn23 flies is reduced to 60% of that found in the wild type. Hence both the rate of synthesis and the rate of degradation of alcohol dehydrogenase are affected. In contrast, Adhfn24 which contains an 11-base pair deletion in the N-terminal coding region of the ADH gene, synthesizes no immunodetectable protein, and the amount of alcohol dehydrogenase mRNA is less than half that of wild-type flies. As with Adhfn23, the deletion in Adhfn24 results in a change in the reading frame. Unlike Adhfn23, however, nucleic acid sequence data indicate that polypeptide chain elongation can proceed for a considerable distance (over 130 amino acids) beyond the deletion. Based upon antigenic binding-site predictions, the resultant aberrant protein (projected 195 amino acids in length) would share few antigenic sites with the alcohol dehydrogenase from the wild type, which may account for the lack of immunoprecipitable material in this mutant. The contrasting effects these two deletions have on the Drosophila ADH mRNA levels and ADH protein levels are discussed.

Further studies of the metabolic incorporation and covalent binding of inhaled [3H]- and [14C]formaldehyde in Fischer-344 rats: effects of glutathione depletion

Glutathione (GSH) is required for the oxidation of formaldehyde (HCHO) to formate catalyzed by formaldehyde dehydrogenase (FDH). The effects of GSH depletion on the mechanisms of labeling of macromolecules in the rat nasal mucosa and bone marrow by 3HCHO and H14CHO were investigated. Male rats were exposed for 3 hr to atmospheres containing 3HCHO and H14CHO at concentrations of 0.9, 2, 4, 6, or 10 ppm, 1 day after a single 3-hr preexposure to the same concentration of unlabeled HCHO. Two hours prior to the second exposure, the animals were injected either with phorone (300 mg/kg, ip) or with corn oil. The concentration of nonprotein sulfhydryls in the nasal respiratory mucosa of phorone-injected rats was decreased to 10% of that of corn oil-injected rats. The metabolic incorporation of 3HCHO and H14CHO into DNA, RNA, and proteins in the respiratory and olfactory mucosa and bone marrow (femur) was significantly decreased, and DNA-protein crosslinking was significantly increased in the respiratory mucosa of phorone-injected relative to corn oil-injected rats at all HCHO concentrations. DNA-protein crosslinks were not detected in the respiratory mucosa of corn oil-injected rats at 0.9 ppm. Evidence was obtained for the formation of adducts of HCHO with the RNA from the nasal respiratory mucosa of phorone-injected rats at concentrations above 0.9 ppm. Covalent binding of HCHO to macromolecules in the bone marrow was not detected. These results indicate that the GSH-dependent oxidation of HCHO catalyzed by FDH is an important defense mechanism against the covalent reactions of HCHO with nucleic acids in the respiratory mucosa. Experiments using phorone-injected rats exposed to 10 ppm of [3H]- and [14C]formaldehyde showed that the DNA from the respiratory mucosa was enriched in 3H relative to 14C in comparison to the inhaled vapor. The enrichment is explained by an isotope effect in the oxidation of 3HCHO and H14CHO (H. d'A, Heck and M. Casanova (1987). Toxicol. Appl. Pharmacol. 89, 122-134), which results in 3H enrichment of the residual (unoxidized) HCHO that binds to DNA. A non-linear pharmacokinetic model is proposed that depicts the potential effects of FDH saturation on the relative concentrations of intracellular to extracellular HCHO.

Formaldehyde mutagenesis and formation of DNA-protein crosslinks in human lymphoblasts in vitro

Human lymphoblasts were exposed in vitro to various concentrations of formaldehyde (HCHO) in single and multiple treatment regimens to determine relative mutagenic efficiency. Single treatments of HCHO (0-150 microM X 2 h) resulted in a nonlinear increase in induced mutant fraction at the thymidine kinase locus with increasing slope at concentrations above 125 microM. Only HCHO exposures of 125 microM X 2 h or greater produced significant effects on the growth rate of the lymphoblasts. Cultures were also exposed to either three treatments of 50 microM X 2 h, five treatments of 30 microM X 2 h, or ten treatments of 15 microM X 2 h; multiple treatments were administered on different days. These multiple treatments resulted in increases in mutant fraction, although their combined effect was less than a single treatment of equivalent concentration X time (150 microM X 2 h). Exposure of lymphoblasts to four treatments of 150 microM X 2 h HCHO failed to induce mutations at the ouabain resistance locus. Cultures of lymphoblasts receiving a single treatment of HCHO (0-600 microM X 2 h) were analyzed by the alkaline elution technique to detect the presence of DNA-protein crosslinks. HCHO treatment resulted in a significant nonlinear increase in DNA-protein crosslinks at concentrations greater than 50 microM X 2 h, which correlated with the onset of significant toxicity in this cell line. Holding the culture for 24 h resulted in complete removal of the crosslinks. These data indicate that both the induction of mutations and the formation of DNA-protein crosslinks by HCHO are nonlinear functions in human lymphoblasts and occur at overlapping concentration ranges.

Drosophila glue gene Sgs-3: sequences required for puffing and transcriptional regulation

The 68C intermolt puff of Drosophila melanogaster contains a cluster of three glue protein genes, Sgs-3, Sgs-7, and Sgs-8. By analysis of chromosomal rearrangements which break near the glue gene cluster, we have established that a region of no more than 20 kb is required for normal expression of the glue genes and for formation of the 68C puff. Using P element-mediated transformation, we have introduced defined segments of the 68C region into the fly genome and assayed the expression of the Sgs-3 gene. Based on the criteria of correct tissue- and stage-specific expression, transcription of an RNA of appropriate size and abundance, and production of an sgs-3 protein, the correctly regulated expression of the Sgs-3 gene requires less than 3.4 kb of total flanking sequences, approximately 2.3 kb 5' and 1.1 kb 3'. Formation of a new intermolt puff at the site of insertion is not observed for all transformants which produce high levels of Sgs-3 RNA. Only transformants in which the introduced DNA from 68C also contains the Sgs-7 and Sgs-8 genes cause a new intermolt puff at the chromosomal location of the insert.

The partial characterization of alcohol dehydrogenase null alleles from natural populations of Drosophila melanogaster

Twenty-three alcohol dehydrogenase (ADH) putative null alleles extracted from four Tasmanian (Australia) populations of Drosophila melanogaster produce no ADH activity and are unable to form active heterodimers with either AdhF or AdhS. Twelve of these nulls were tested by enzyme-linked immunosorbent assay (ELISA) and did not produce any ADH cross-reacting material (CRM). The null homozygotes had similar, but slightly lower, mortalities on ethanol-supplemented media compared to an artificially induced null allele. Heterozygotes between the null alleles and standard AdhF and AdhS alleles had intermediate ADH activity and CRM levels.

Alcohol dehydrogenase of Drosophila melanogaster: metabolic differences mediated through cryptic allozymes

Acetone formation from propan-2-ol, a saturated secondary alcohol, has been analysed in flies of three different Adh-genotypes of D. melanogaster. The in vivo oxidation of propan-2-ol was mainly mediated through ADH activity. It could be demonstrated that flies homozygous for the Adh71k allele produced more acetone than flies homozygous for AdhF. This difference in metabolic flux mediated through the cryptic allozymes under non-saturated ADH-substrate conditions seems to be based on their different kinetic properties in vivo. Product inhibition of ADH monitored by means of ADH-isozymes conversion as observed after electrophoresis was similar for both cryptic allozymes. ADH-71k and ADH-F showed immunological identity, and the in vivo protein levels of ADH-71k were 25-30 per cent higher than ADH-F. The population-genetic implications of our findings have been evaluated.

Molecular analysis of chemically-induced mutations at the RpII215 locus of Drosophila melanogaster

A substantial fraction, perhaps 50% or more, of spontaneous mutations in Drosophila melanogaster have been shown by molecular analyses to be associated with the presence of a transposable element (TE) inserted into the affected gene. We are interested in the molecular structure of induced mutations in Drosophila, in particular whether TEs are also responsible for a significant proportion of chemically-induced mutations. We report here the molecular analysis of 58 mutations at the RpII215 locus induced with EMS or ENU. While we find evidence for moderately sized deletions at this locus (in 3/58, or 5% of the examined mutants), we failed to detect any mutations which were associated with an insertion event. It may be the case that induced mutations are qualitatively different from spontaneous mutations.

Sister-chromatid exchanges in lymphocytes of anatomy students exposed to formaldehyde-embalming solution

Sister-chromatid exchanges measured in the peripheral lymphocytes of 8 non-smoking persons after exposure to formaldehyde-embalming solution during a 10-week anatomy class showed a small (P = 0.02) average increase when compared with samples obtained from the same individuals immediately before exposure began. Breathing-zone air samples collected during dissection procedures showed a mean concentration of 1.2 ppm (1.5 mg/m3) formaldehyde.

Molecular analysis of the Adh region of the genome of Drosophila melanogaster

A small region of the genome of Drosophila melanogaster has been cloned in a series of overlapping phage. A length of 165 X 10(3) base-pairs of contiguous DNA that spans polytene chromosome region 35A4 to 35B1 and includes the structural gene for alcohol dehydrogenase (Adh) as well as at least two other genes, outspread (osp) and no-ocelli (noc), has been characterized by mapping chromosome aberrations to the DNA. The relationship between osp and Adh is surprising: of nine osp alleles associated with chromosome breakpoints, five map distal (i.e. 5') to Adh and four map proximal (i.e. 3') to this gene. None affects the expression of Adh. As defined by these and other breakpoints, the osp gene spans at least 52 X 10(3) base-pairs and overlaps the Adh gene. The noc gene, as defined by the mapping of nearly 30 breakpoints, is at least 50 X 10(3) base-pairs in size. Alleles of noc and noc- deletions show either of two kinds of interaction with the recessive lethality of l(2)br29ScoR+1, a lethal that maps immediately distal to noc. One class of noc allele is viable when heterozygous with ScoR+1, while the other class is lethal or semi-lethal. Both classes, however, are homozygous or hemizygous viable. The locations of these two classes of noc allele on the DNA fall into two clusters, with those that are viable with ScoR+1 located proximal to those that are not. The physical boundary between these classes lies at a site just distal to that of the breakpoint of the inversion associated with ScoR+1 itself.

Mutation of the Adh gene of Drosophila melanogaster containing an internal tandem duplication

AdhnLA248 is an X-ray-induced mutation of the alcohol dehydrogenase gene of Drosophila melanogaster that lacks detectable ADH protein but is transcribed. The transcript of this mutant allele is longer than that of the wild type. This is because the mutation is a duplication of parts of the second and third exons of Adh and of the intron that normally separates them. The primary transcript of the mutant allele is processed by the removal of both of the identical copies of intron 3. This mutation presumably originated, in the haploid sperm, as two staggered single-stranded breaks that gave rise to the duplication as a consequence of replication after fertilization.

P-element transformation with period locus DNA restores rhythmicity to mutant, arrhythmic Drosophila melanogaster

Mutations at the period (per) locus of Drosophila melanogaster disrupt several biological rhythms. Molecular cloning of DNA sequences encompassing the per+ locus has allowed germ-line transformation experiments to be carried out. Certain subsegments of the per region, transduced into the genome of arrhythmic pero flies, restore rhythmicity in circadian locomotor behavior and the male's courtship song.

Inducible transcription and puffing in Drosophila melanogaster transformed with hsp70-phage lambda hybrid heat shock genes

A series of hsp70-phage lambda hybrid genes having various amounts of 5' flanking DNA was introduced into the germ line of Drosophila melanogaster by P-element-mediated transformation. Heat-induced transcription was normal in lines transformed with hsp70-lambda genes having 194 and 146 base pairs of DNA upstream from the mRNA initiation site. Lines transformed with genes having 70 base pairs of upstream DNA accumulated correctly initiated transcripts in response to heat shock, but the amount was somewhat reduced and minor amounts of incorrectly initiated transcripts were observed. No transcription, with or without heat shock, was seen in lines transformed with hsp70-lambda genes having only 52, 44, or 25 base pairs of upstream DNA. Heat shock induced polytene chromosome puffing at the site of integration in a line transformed with a gene having 194 base pairs of upstream DNA. The hsp70-lambda gene and a cotransformed Adh gene closely linked on the same integrating fragment were expressed independently, each apparently responding only to its own normal control signals.