Somatic recombination: a major genotoxic effect of two pyrimidine antimetabolitic chemotherapeutic drugs in Drosophila melanogaster

Chromatin Structure and Function in Mosquitoes

The principles and function of chromatin and nuclear architecture have been extensively studied in model organisms, such as Drosophila melanogaster. However, little is known about the role of these epigenetic processes in transcriptional regulation in other insects including mosquitoes, which are major disease vectors and a worldwide threat for human health. Some of these life-threatening diseases are malaria, which is caused by protozoan parasites of the genus Plasmodium and transmitted by Anopheles mosquitoes; dengue fever, which is caused by an arbovirus mainly transmitted by Aedes aegypti; and West Nile fever, which is caused by an arbovirus transmitted by Culex spp. In this contribution, we review what is known about chromatin-associated mechanisms and the 3D genome structure in various mosquito vectors, including Anopheles, Aedes, and Culex spp. We also discuss the similarities between epigenetic mechanisms in mosquitoes and the model organism Drosophila melanogaster, and advocate that the field could benefit from the cross-application of state-of-the-art functional genomic technologies that are well-developed in the fruit fly. Uncovering the mosquito regulatory genome can lead to the discovery of unique regulatory networks associated with the parasitic life-style of these insects. It is also critical to understand the molecular interactions between the vectors and the pathogens that they transmit, which could hold the key to major breakthroughs on the fight against mosquito-borne diseases. Finally, it is clear that epigenetic mechanisms controlling mosquito environmental plasticity and evolvability are also of utmost importance, particularly in the current context of globalization and climate change.

Validating the Demethylating Effects of 5-aza-2'-deoxycytidine in Insects Requires a Whole-Genome Approach

We previously demonstrated that treatment with the demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) alters the offspring sex ratios produced by females of the parasitoid wasp Nasonia vitripennis. Females allocate offspring sex ratio in line with local mate competition theory, producing more or less female-biased sex ratios as the number of other females laying eggs on a patch varies, thereby reducing competition among their sons for mates. Interestingly, treatment with 5-aza-dC did not ablate the facultative sex allocation response. Instead, sex ratios became less female biased, a shift in the direction of the optimum sex ratio for paternally inherited alleles according to genomic conflict theory. This was the first (albeit indirect) experimental evidence for genomic conflict over sex allocation. In their comment, Ellers and colleagues assayed the effects of 5-aza-dC on DNA methylation in 10 Nasonia genes, finding no evidence of demethylation in these 10 genes, from which they conclude that 5-aza-dC has no demethylating capability in N. vitripennis. Quantifying the efficacy of 5-aza-dC in terms of demethylation is indeed crucial to in-depth interpretation of studies using 5-aza-dC to link phenotypes to epigenetic regulation. Here we outline the mode of action of 5-aza-dC and demonstrate that determining the efficacy of 5-aza-dC in insect systems requires a whole-genome approach.

An insight into the genotoxicity assessment studies in dipterans

The dipterans have been widely utilized in genotoxicity assessment studies. Short life span, easy maintenance, production of large number of offspring in a single generation and the tissues with appropriate cell populations make these flies ideal for studies associated to developmental biology, diseases, genetics, genetic toxicology and stress biology in the group. Moreover, their cosmopolitan presence makes them suitable candidate for ecological bio-monitoring. An attempt has been made in the present review to reveal the significance of dipteran flies for assessing alterations in genetic content through various genotoxicity biomarkers and to summarize the gradual advancement in these studies. Recent studies on genotoxicity assays in dipterans have opened up a broader perspective for DNA repair related mechanistic studies, pre-screening of chemicals and environmental bio-monitoring. Studies in dipterans, other than Drosophila may be helpful in using them as an alternative model system for assessment of genotoxicity, especially at the gene level and further extension of these studies give a future insight to develop new strategies for maintaining environment friendly limits of the toxicants.

Methylation and worker reproduction in the bumble-bee (Bombus terrestris)

Insects are at the dawn of an epigenetics era. Numerous social insect species have been found to possess a functioning methylation system, previously not thought to exist in insects. Methylation, an epigenetic tag, may be vital for the sociality and division of labour for which social insects are renowned. In the bumble-bee Bombus terrestris, we found methylation differences between the genomes of queenless reproductive workers and queenless non-reproductive workers. In a follow up experiment, queenless workers whose genomes had experimentally altered methylation were more aggressive and more likely to develop ovaries compared with control queenless workers. This shows methylation is important in this highly plastic reproductive division of labour. Methylation is an epigenetic tag for genomic imprinting (GI). It is intriguing that the main theory to explain the evolution of GI predicts that GI should be important in this worker reproduction behaviour.

Assessment of the mutagenic, recombinagenic and carcinogenic potential of orlistat in somatic cells of Drosophila melanogaster

In this study the mutagenic, recombinagenic, carcinogenic and anticarcinogenic potential of orlistat was assessed using the somatic mutation and recombination test (SMART) and the epithelial tumor detection test (wts). The experiments were conducted on Drosophila melanogaster. In the assessment using SMART, larvae, descendants from the standard (ST) cross and the high bioactivation (HB) cross, were treated chronically with three orlistat concentrations. The results revealed a recombinagenic effect, associated with orlistat, in the descendants of the HB cross, at all three levels of concentration. Homologous recombination can function as a determinant at different stages of carcinogenesis. For verification, larvae from the wts test, descendants of the wts/TM3 virgin female and mwh/mwh male cross, were treated with the same three orlistat concentrations separately and in association with mitomicin C (0.1mM). The results did not, however, provide evidence that orlistat has carcinogenic potential nor was it associated with the reduction of tumors induced by mitomicin C in D. melanogaster.

Comparative genotoxicity evaluation of imidazolinone herbicides in somatic cells of Drosophila melanogaster

In the present study, five analogous herbicides, namely Imazapyr (IMZR), Imazapic (IMZC), Imazethapyr (IMZT), Imazamox (IMZX) and Imazaquin (IMZQ), were evaluated for genotoxicity (mutagenic and recombinagenic activity) in the wing somatic mutation and recombination test (SMART) of Drosophila melanogaster. They are classified as imidazolinone (IMI) herbicides and their mode of action is to inhibit acetohydroxyacid synthase (AHAS), an enzyme involved in the biosynthesis of the amino acids leucine, isoleucine and valine. Two crosses were used: the standard (ST) cross and the high bioactivation (HB) cross. The latter is characterized by high levels of cytochrome P450 conferring increased sensitivity to promutagens and procarcinogens. Three-day-old larvae were exposed by chronic feeding (48 h) to four different concentrations of these herbicides (2.5, 5.0, 10.0 or 20.0 mM). For the evaluation of genotoxic effects, the frequencies of spots per individual in the treated series were compared to the concurrent negative control series (ultrapure water). Imazapyr, Imazapic and Imazethapyr gave negative results with both crosses of the wing spot test. In the ST cross, Imazamox showed positive results only for large single spots (20.0 mM IMZX) and weak positive results for total spots (10.0 and 20.0 mM IMZX), while Imazaquin showed positive results only for large single spots (5.0 and 20.0mM IMZQ) and a weak positive result for total spots (20.0 mM IMZQ). These positive results are mainly due to induced recombination and to a minor extent to mutations. In the HB cross, only Imazamox (5.0 mM IMZX) showed a weak positive result for small single spots. The positive control urethane, a promutagen, caused an increase in the number of all types of spots in both crosses. In conclusion, the results of chronic treatments performed at high doses (toxicity was observed at higher doses) shows the existence of a genotoxic risk for IMZX and IMZQ exposure under these experimental conditions, and indicate the need for further research to delineate the exact mechanisms involved.

Transbuccal delivery of 5-aza-2 -deoxycytidine: effects of drug concentration, buffer solution, and bile salts on permeation

Delivery of 5-aza-2 -deoxycytidine (decitabine) across porcine buccal mucosa was evaluated as an alternative to the complex intravenous infusion regimen currently used to administer the drug. A reproducible high-performance liquid chromatography method was developed and optimized for the quantitative determination of this drug. Decitabine showed a concentration-dependent passive diffusion process across porcine buccal mucosa. An increase in the ionic strength of the phosphate buffer from 100 to 400 mM decreased the flux from 3.57 +/- 0.65 to 1.89 +/- 0.61 microg/h/cm2. Trihydroxy bile salts significantly enhanced the flux of decitabine at a 100 mM concentration (P > .05). The steady-state flux of decitabine in the presence of 100 mM of sodium taurocholate and sodium glycocholate was 52.65 +/- 9.48 and 85.22 +/- 7.61 microg/cm2/h, respectively. Two dihydroxy bile salts, sodium deoxytaurocholate and sodium deoxyglycocholate, showed better enhancement effect than did trihydroxy bile salts. A 38-fold enhancement in flux was achieved with 10 mM of sodium deoxyglycocholate.

Genotoxicity (mitotic recombination) of the cancer chemotherapeutic agents cisplatin and cytosine arabinoside in Aspergillus nidulans

Cisplatin (cis-diamminedichloroplatinum, cis-DDP) and cytosine arabinoside (ara-C) are anticancer drugs used in the treatment of human cancer. The two chemotherapeutic drugs were tested in current research for their recombinogenic potential in diploid cells of Aspergillus nidulans. Non-cytotoxic concentrations of ara-C (0.4 and 0.8 microM) and cis-DDP (1.5, 3.0 and 6.0 microM) were strong recombinagens in A. nidulans UT448//A757 diploid strain, which induced homozygosis of recessive genetic markers, previously present in heterozygous condition. Drugs significantly increased homozygosity index (HI) values for five nutritional genetic markers when compared with those determined in the absence of anticancer drugs. Since mitotic recombination is a mechanism leading to malignant growth through loss of heterozygosity at tumor-suppressor loci, ara-C and cis-DDP may be characterized as secondary promoters of malignant neoplasia in diagnosed cancer patients, after chemotherapy treatment.

Oral sildenafil citrate lacks genotoxicity and cytotoxicity in a primate model: Callithrix jacchus

OBJECTIVE

To determine sildenafil citrate (SC) genotoxicity and cytotoxicity in the Callithrix jacchus.

STUDY DESIGN

Fifteen organisms were assigned to one of three groups as follows: experimental (25 mg/kg of SC); negative control (glucose solution 5%); and positive control (3 mg/kg of cytocine arabinoside). Systemic hemodynamic changes were monitored in each animal before and after each treatment. A drop of blood was obtained before and after the treatment at 24-120 h. Smears were made and the frequency of micronucleated erythrocytes (MNE), micronucleated polychromatic erythrocytes (MNPCE) and polychromatic erythrocytes (PCE) was counted.

RESULTS

No significant differences in MNE, MNPCE and PCE were found in the group that received sildenafil and negative control. A significant increase in genotoxicity and cytotoxicity was observed in the positive control group. No changes were observed in systemic hemodynamic changes.

CONCLUSION

The macro-dose of SC lacks genotoxic, cytotoxic or systemic hemodynamic changes effects in this species.

Induction of A:T to G:C transition mutations by 5-(2-chloroethyl)-2'-deoxyuridine (CEDU), an antiviral pyrimidine nucleoside analogue, in the bone marrow of Muta Mouse

5-(2-chloroethyl)-2'-deoxyuridine (CEDU) is a pyrimidine nucleoside analogue formerly in development for the treatment of herpes simplex virus infections. The compound proved clearly mutagenic in the mouse spot test and exhibited weak activity in the Salmonella reverse mutation test, which led to the termination of the compound's development. In another study, CEDU, administered orally to beta-galactosidase (lacZ) transgenic mice (Muta Mouse) for five days, induced a clear increase in lacZ mutant frequencies in spleen, lung, and bone marrow. In the present follow-up study, we analyzed 32 of those lacZ mutants isolated from the bone marrow of the Muta Mouse animals of the experiments mentioned above, in order to obtain further information on the type of mutations induced by CEDU. CEDU induced a pronounced increase in A:T to G:C transitions. The distribution of A:T to G:C transitions was clearly non-random, showing a bias towards T to C substitutions in the coding DNA strand and a preference to occur in the sequence motif 5'-(G or C)-T-G-3'. Our data support the hypothesis that CEDU, after being phosphorylated, is incorporated into cellular DNA in place of thymidine, which leads to mispairing with guanosine during subsequent DNA replication. As a result, the compound is thought to exert its mutagenicity by inducing mismatches leading to T to C transitions. Our findings point towards a mode of mutagenic action of CEDU that differs fundamentally from that of other antiviral antinucleosides whose clastogenic and recombinogenic activities prevail.

Drosophila wing-spot test for genotoxic assessment of pollutants in water samples from urban and industrial origin

The Caí River (Rio Grande do Sul, Brazil) is an important watercourse that receives large amounts of industrial and untreated municipal discharges in its lower course. We employed the SMART in Drosophila melanogaster to evaluate the genotoxicity of surface waters collected from Caí sites receiving direct sewage discharge: from Montenegro (Km 52) and from São Sebastião do Caí (Km 78 and 80), and from two sites under the industrial influence (Km 13.6 and 18.6). The genotoxic analysis included three collections: March, June and September 1999, which were tested at crude sample and at 50 and 25% concentrations. Considering the industrial samples from Km 18.6 and 13.6, collected in March, June and September 1999, they were characterized as not having genetic toxicity. The urban samples collected in March--Km 52, 78 and 80--showed a significant increment in the frequencies of total spots. In Km 52 and 78 the genotoxic effect was associated to both mutational and recombinational events, although for Km 80 the increases observed were mainly related to the occurrence of homologous recombination. Moreover, the Km 80 crude sample from June and all the concentrations analyzed for Km 52 in September were also able to induce mitotic recombination. These effects were only observed in the ST cross, demonstrating the genotoxins present in the urban discharges act by direct interaction with the DNA of the somatic cells. The SMART in D. melanogaster was shown to be highly sensitive to detect genotoxic agents present in the aquatic environment, and must be better exploited for monitoring areas under anthropogenic discharges.

Silencing of genes by promoter hypermethylation: key event in rodent and human lung cancer

Transcriptional silencing by CpG island hypermethylation has become a critical component in the initiation and progression of lung cancer. The ability of pharmacologic agents to reverse promoter hypermethylation also makes it an attractive target to pursue for prevention of lung cancer. Animal models, together with studies in humans have fostered significant advances in elucidating the role of gene-specific methylation in cancer initiation and progression, the modulation of promoter methylation by carcinogen exposure and the ability to block tumor development by preventing epigenetically mediated gene silencing. These advances represent the beginning of efforts to develop a progression model for lung cancer that should aid efforts for early detection and gene targeting for therapy, and the development of preventive interventions that reverse epigenetic-mediated gene silencing.