DNA strand breaks in testicular cells from humans and rats following in vitro exposure to 1,2-dibromo-3-chloropropane (DBCP)

Assessing testicular germ cell DNA damage in the comet assay; introduction of a proof-of-concept

The in vivo comet assay is widely used to measure genotoxicity; however, the current OECD test guideline (TG 489) does not recommend using the assay to assess testicular germ cells, due to the presence of testicular somatic cells. An adapted approach to specifically assess testicular germ cells within the comet assay is certainly warranted, considering regulatory needs for germ cell-specific genotoxicity data in relation to the increasing global production of and exposure to potentially hazardous chemicals. Here, we provide a proof-of-concept to selectively analyze round spermatids and primary spermatocytes, distinguishing them from other cells of the testicle. Utilizing the comet assay recordings of DNA content (total fluorescence intensity) and DNA damage (% tail intensity) of individual comets, we developed a framework to distinguish testicular cell populations based on differences in DNA content/ploidy and appearance. Haploid round spermatid comets are identified through (1) visual inspection of DNA content distributions, (2) setting DNA content thresholds, and (3) modeling DNA content distributions using a normal mixture distribution function. We also describe an approach to distinguish primary spermatocytes during comet scoring, based on their high DNA content and large physical size. Our concept allows both somatic and germ cells to be analyzed in the same animal, adding a versatile, sensitive, rapid, and resource-efficient assay to the limited genotoxicity assessment toolbox for germ cells. An adaptation of TG 489 facilitates accumulation of valuable information regarding distribution of substances to germ cells and their potential for inducing germ cell gene mutations and structural chromosomal aberrations.

Reference cells and ploidy in the comet assay

In the comet assay single cells are analyzed with respect to their level of DNA damage. Discrimination of the individual cell or cell type based on DNA content, with concomitant scoring of the DNA damage, is useful since this may allow analysis of mixtures of cells. Different cells can then be characterized based on their ploidy, cell cycle stage, or genome size. We here describe two applications of such a cell type-specific comet assay: (i) Testicular cell suspensions, analyzed on the basis of their ploidy during spermatogenesis; and (ii) reference cells in the form of fish erythrocytes which can be included as internal standards to correct for inter-assay variations. With standard fluorochromes used in the comet assay, the total staining signal from each cell - whether damaged or undamaged - was found to be associated with the cell's DNA content. Analysis of the fluorescence intensity of single cells is straightforward since these data are available in scoring systems based on image analysis. The analysis of testicular cell suspensions provides information on cell type specific composition, susceptibility to genotoxicants, and DNA repair. Internal reference cells, either untreated or carrying defined numbers of lesions induced by ionizing radiation, are useful for investigation of experimental factors that can cause variation in comet assay results, and for routine inclusion in experiments to facilitate standardization of methods, and comparison of comet assay data obtained in different experiments or in different laboratories. They can also be used - in combination with a reference curve - to quantify the DNA lesions induced by a certain treatment. Fish cells of a range of genome sizes, both greater and smaller than human, are suitable for this purpose, and they are inexpensive.

Gonadotoxic Effects of DBCP: A Historical Review and Current Concepts

Dibromo-chloro-propane (DBCP), a persistent liphophilic brominated organochlorine, has been produced for agricultural purposes as a nematocide since the 1950s. Widespread use due to its effectiveness as a pesticide continued until the late 1970s when early reports of its toxicity emerged from the laboratories, particularly its impact on spermatogenesis and other adverse reproductive health effects. Since then innumerable cases and studies have surfaced with clear impact after exposure to DBCP, however, the sustained effect of this exposure has yet to be completely understood. As a result of these studies, environmental agencies banned almost all agricultural uses of DBCP in the United States in the late 1970s. This review will try to balance the known toxicity of DBCP with a scientific assessment of published data and a summary of the legal issues that have resulted.

The comet assay in male reproductive toxicology

Due to our lifestyle and the environment we live in, we are constantly confronted with genotoxic or potentially genotoxic compounds. These toxins can cause DNA damage to our cells, leading to an increase in mutations. Sometimes such mutations could give rise to cancer in somatic cells. However, when germ cells are affected, then the damage could also have an effect on the next and successive generations. A rapid, sensitive and reliable method to detect DNA damage and assess the integrity of the genome within single cells is that of the comet or single-cell gel electrophoresis assay. The present communication gives an overview of the use of the comet assay utilising sperm or testicular cells in reproductive toxicology. This includes consideration of damage assessed by protocol modification, cryopreservation vs the use of fresh sperm, viability and statistics. It further focuses on in vivo and in vitro comet assay studies with sperm and a comparison of this assay with other assays measuring germ cell genotoxicity. As most of the de novo structural aberrations occur in sperm and spermatogenesis is functional from puberty to old age, whereas female germ cells are more complicated to obtain, the examination of male germ cells seems to be an easier and logical choice for research and testing in reproductive toxicology. In addition, the importance of such an assay for the paternal impact of genetic damage in offspring is undisputed. As there is a growing interest in the evaluation of genotoxins in male germ cells, the comet assay allows in vitro and in vivo assessments of various environmental and lifestyle genotoxins to be reliably determined.

Critical evaluation of the cancer risk of dibromochloropropane (DBCP)

Dibromochloropropane (1,2-dibromo-3-chloropropane, DBCP), a pesticide used widely for over 20 years to control nematodes on crops, turf and in nurseries, was banned by the United States Environmental Protection Agency (US EPA) in 1977 because of evidence of infertility in men and induction of a variety of tumors in laboratory animals. Despite the ban on the use of DBCP, this pesticide remains persistent in soil and continues to be detected as a groundwater contaminant in areas of past high use, in particular California's Central Valley. In this review, we present a critical evaluation of the available scientific literature on the potential for DBCP to affect cancer risk, including the results of animal cancer bioassays, human epidemiological studies and in vitro and in vivo genotoxicity studies. In addition, we provide updated information on DBCP chemistry and metabolism, production and past use, current regulations, its environmental fate, potential for human exposure and current remediation efforts. Results from long-term cancer bioassays in rodents show a statistically significant increase in the incidence of malignant and benign mammary gland tumors in female rats treated orally with DBCP compared to controls and some evidence of increased incidence of mammary fibroadenomas in DBCP low-dose treated female rats exposed by inhalation. Significantly increased incidence of tumors of the forestomach occurred in both sexes of rats and mice treated orally. Rats exposed to DBCP by inhalation showed significant increases in tumors of the tunica vaginalis in males; tumors of the pharynx and adrenal gland in females; and tumors of the tongue, nasal turbinate and nasal cavity in both sexes compared to controls. Male and female mice exposed to DBCP by inhalation experienced increased tumor incidence in the lungs and nasal cavity compared to controls. Significant increases in tumors of the lung and forestomach have also been reported in female mice treated by a dermal route. Although high mortality rates in both rat and mouse bioassays limited the ability to detect tumors late in life, the induction of a variety of tumors by multiple routes of exposure in two rodent species provides clear evidence of a DBCP tumorigenic response. In vitro, in vivo and human genotoxicity studies indicate that DBCP is capable of acting as a mutagen and clastogen. Few studies have been conducted to assess whether DBCP workplace or drinking water exposures affect cancer risk in humans. While case-control, cohort and ecological epidemiology studies have not found significant, positive associations between DBCP exposure and cancer in exposed populations, these studies have numerous limitations including small numbers of participants, a lack of control for confounding factors, lack of exposure information on DBCP and other chemicals and short follow-up times. Given the persistent nature of DBCP contamination in areas of past use, efforts should be made to continue remediation efforts and follow previously exposed populations for development of certain human cancers, including breast, ovarian, stomach, respiratory, oral and nasal cancers, among others.

Limited repair of 8-hydroxy-7,8-dihydroguanine residues in human testicular cells

Oxidative damage in testicular DNA is associated with poor semen quality, reduced fertility and increased risk of stillbirths and birth defects. These DNA lesions are predominantly removed by base excision repair. Cellular extracts from human and rat testicular cells and three enriched populations of rat male germ cells (primary spermatocytes, round spermatids and elongating/elongated spermatids) all showed proficient excision/incision of 5-hydroxycytosine, thymine glycol and 2,6-diamino-4-hydroxy-5-formamidopyrimidine. DNA containing 8-oxo-7,8-dihydroguanine was excised poorly by human testicular cell extracts, although 8-oxoguanine-DNA glycosylase-1 (hOGG1) was present in human testicular cells, at levels that varied markedly between 13 individuals. This excision was as low as with human mononuclear blood cell extracts. The level of endonuclease III homologue-1 (NTH1), which excises oxidised pyrimidines, was higher in testicular than in somatic cells of both species. Cellular repair studies of lesions recognised by formamidopyrimidine-DNA glycosylase (Fpg) or endonuclease III (Nth) were assayed with alkaline elution and the Comet assay. Consistent with the enzymatic activities, human testicular cells showed poor removal of Fpg-sensitive lesions but efficient repair of Nth-sensitive lesions. Rat testicular cells efficiently repaired both Fpg- and Nth-sensitive lesions. In conclusion, human testicular cells have limited capacity to repair important oxidative DNA lesions, which could lead to impaired reproduction and de novo mutations.

Expression of Bcl-2 family proteins and spontaneous apoptosis in normal human testis

We investigated the frequency of spontaneous apoptosis and expression of the Bcl-2 family of proteins during normal spermatogenesis in man. Testicular tissue with both normal morphology and DNA content was obtained from necro-donors and fixed in Bouin's solution. A TdT-mediated dUTP end-labelling method (TUNEL) was used for the detection of apoptotic cells. Expression of apoptosis regulatory Bcl-2 family proteins and of p53 and p21(Waf1) was assessed by immunohistochemistry. Germ cell apoptosis was detected in all testes and was mainly seen in primary spermatocytes and spermatids and in a few spermatogonia. Bcl-2 and Bak were preferentially expressed in the compartments of spermatocytes and differentiating spermatids, while Bcl-x was preferentially expressed in spermatogonia. Bax showed a preferential expression in nuclei of round spermatids, whereas Bad was only seen in the acrosome region of various stages of spermatids. Mcl-1 staining was weak without a particular pattern, whereas expression of Bcl-w, p53 and p21(Waf1) proteins was not detected by immunohistochemistry. The results show that spontaneous apoptosis occurs in all male germ cell compartments in humans. Bcl-2 family proteins are distributed preferentially within distinct germ cell compartments suggesting a specific role for these proteins in the processes of differentiation and maturation during human spermatogenesis.

Highly efficient base excision repair (BER) in human and rat male germ cells

The quality of germ cell DNA is critical for the fate of the offspring, yet there is limited knowledge of the DNA repair capabilities of such cells. One of the main DNA repair pathways is base excision repair (BER) which is initiated by DNA glycosylases that excise damaged bases, followed by incision of the generated abasic (AP) sites. We have studied human and rat methylpurine-DNA glycosylase (MPG), uracil-DNA glycosylase (UNG), and the major AP endonuclease (HAP1/APEX) in male germ cells. Enzymatic activities and western analyses indicate that these enzymes are present in human and rat male germ cells in amounts that are at least as high as in somatic cells. Minor differences were observed between different cellular stages of rat spermatogenesis and spermiogenesis. Repair of methylated DNA was also studied at the cellular level using the Comet assay. The repair was highly efficient in both human and rat male germ cells, in primary spermatocytes as well as round spermatids, compared to rat mononuclear blood cells or hepatocytes. This efficient BER removes frequently occurring DNA lesions that arise spontaneously or via environmental agents, thereby minimising the number of potential mutations transferred to the next generation.

Nucleotide excision repair in rat male germ cells: low level of repair in intact cells contrasts with high dual incision activity in vitro

The acquisition of genotoxin-induced mutations in the mammalian germline is detrimental to the stable transfer of genomic information. In somatic cells, nucleotide excision repair (NER) is a major pathway to counteract the mutagenic effects of DNA damage. Two NER subpathways have been identified, global genome repair (GGR) and transcription-coupled repair (TCR). In contrast to somatic cells, little is known regarding the expression of these pathways in germ cells. To address this basic question, we have studied NER in rat spermatogenic cells in crude cell suspension, in enriched cell stages and within seminiferous tubules after exposure to UV or N-acetoxy-2-acetylaminofluorene. Surprisingly, repair in spermatogenic cells was inefficient in the genome overall and in transcriptionally active genes indicating non-functional GGR and TCR. In contrast, extracts from early/mid pachytene cells displayed dual incision activity in vitro as high as extracts from somatic cells, demonstrating that the proteins involved in incision are present and functional in premeiotic cells. However, incision activities of extracts from diplotene cells and round spermatids were low, indicating a stage-dependent expression of incision activity. We hypothesize that sequestering of NER proteins by mispaired regions in DNA involved in synapsis and recombination may underlie the lack of NER activity in premeiotic cells.

The toxicology of 1,2-dibromo-3-chloropropane (DBCP): a brief review

The accumulating data demonstrating the reproductive toxicity of 1, 2-dibromo-3-chloropropane (DBCP) are reviewed. The sentinel event was the discovery of infertility in male pesticide manufacturing workers. In spite of early evidence of testicular damage, first in laboratory animals and later in humans, DBCP has been widely used as a nematocide in the United States and is still used in other countries. The spermatogenic effects of DBCP are usually irreversible, and there is also evidence of toxicity to the female reproductive system. DBCP is also a CNS depressant, a liver and kidney toxin, and a skin, eye, and respiratory irritant, and is probably carcinogenic. Environmental contamination of air and water may be an additional source of exposure. Materials often used to protect workers from toxic chemicals are not being deployed for use by exposed workers, and in any case are relatively ineffective.

Single cell gel electrophoresis assay: methodology and applications

The single cell gel electrophoresis or Comet assay is a sensitive, reliable, and rapid method for DNA double- and single-strand breaks, alkali-labile sites and delayed repair site detection, in eukaryotic individual cells. Given its overall characteristics, this method has been widely used over the past few years in several different areas. In this paper we review the studies published to date about the principles, the basic methodology with currently used variations. We also explore the applications of this assay in: genotoxicology, clinical area, DNA repair studies, environmental biomonitoring and human monitoring.

A comparative study of chemically induced DNA damage in isolated human and rat testicular cells

Testicular cells prepared from human organ transplant donors or from Wistar rats were used to compare 15 known reproductive toxicants with respect to their ability to induce DNA damage, measured as single-strand DNA breaks and alkali labile sites (ssDNA breaks) with alkaline filter elution. The compounds tested included various categories of chemicals (i.e., pesticides, industrial chemicals, cytostatics, and mycotoxins) most of which are directly acting genotoxicants (i.e., reacting with DNA either spontaneously or via metabolic activation). In addition, a few indirect genotoxic and nongenotoxic reproductive toxicants were included. Six of the chemicals induced no significant levels of ssDNA breaks in human and rat testicular cells; methoxychlor (10 to 100 microM, human and rat), benomyl (10 to 100 microM, human and rat), thiotepa (10 to 1000 microM, human and rat), cisplatin (30 to 1000 microM, human; 100 to 1000 microM, rat), Cd2+ (30 to 1000 microM, human; 100 to 1000 microM, rat), and acrylonitrile (30 to 1000 microM, human; 30 to 300 microM, rat). Four chemicals induced significant levels of ssDNA breaks in testicular cells from both species: styrene oxide (> or = 100 microM, rat and human), 1,2-dibromoethane (EDB) (> or = 100 microM, rat; 1000 microM human), thiram (> or = 30 microM, rat; > or = 100 microM, human), and chlordecone (300 microM, rat; > or = 300 microM, human). Finally, five chemicals induced ssDNA breaks in one of the two species. Four chemicals induced significant ssDNA breaks in rat testicular cells only: 1,2-dibromo-3-chloropropane (DBCP) (> or = 10 microM), 1,3-dinitrobenzene (1,3-DNB) (> or = 300 microM), Cr6+ (1000 microM), and aflatoxin B1 (> or = 100 microM), the last two of these produced only a minor positive response. One chemical, acrylamide, induced a marginal increase in ssDNA breaks in human at 1000 microM, but not in rat testicular cells. Although based on a limited number of donors, the data indicate a close correlation between the induction of DNA damage in human and rat testicular cells in vitro. For some chemicals, however, there appears to be differences in the susceptibility to chemically induced ssDNA breaks of isolated testicular cells from the two species. The data indicate that the parallel use of human and rat testicular cells provides a valuable tool in the assessment of human testicular toxicity.