Spontaneous and induced micronuclei and UDS in peripheral lymphocytes

A second life in science—working after the age of 65

I was born in January, 1921 and was fortunate in working for a research organization that had no fixed retirement age. I was permitted to continue Science as long as there were some resources to support research that had some relevance to the organization's goals. A number of projects on which I worked were continuations of ones begun before the age of 65 (1986) and several new ones were based on both previous interests and ideas and some on new ideas. A number of the ideas arose from participation on Committees of the US National Research Council. I was able to extend my earlier interests in DNA repair to include experiments on the variations in DNA repair among apparently normal humans. In collaborations with other researchers we showed that the repair abilities following exposures to chemicals or to ionizing or ultraviolet (UV) radiation did not follow Poisson distributions. I participated in experiments, using a fish model to estimate the wavelength ranges in sunlight responsible for inducing melanoma and another fish model to estimate the germ cell mutations that might arise from exposures to the heavily ionizing particles in cosmic rays beyond low Earth orbit. A transgenic fish model was used to investigate the possibilities of using the fish to assay for mutagens in sediments in Long Island Sound. These Reflections summarize the atmosphere necessary for a second life and the scientific results of this life.

Mutagenic and cytotoxic effectiveness of diisopropyl xanthogen polysulphide in human lymphocyte cultures

The mutagenic and cytotoxic effectiveness of the new rubber vulcanisation accelerator diisopropyl xanthogen polysulphide (Robac AS 100) was tested in human lymphocyte cultures of four healthy probands. The concentrations of Robac AS 100 were 0.57, 5.7 and 57.0 microg/ml. Higher concentrations showed too high cytotoxicity to be evaluable. Without external activation, incubation time with Robac AS 100 was 21 h. In the presence of rat liver microsomes from aroclor-induced rats (2mg microsomal protein/ml), incubation of the test compound was 2h. Mutagenicity testing was performed by analysis of micronuclei (MN), structural chromosome aberrations (CAs) and sister chromatid exchanges (SCEs). The MN-rate was determined using the cytochalasin B (cyt B) block method. For evaluation of cytotoxicity, mitotic index (MI) and nuclear division index (NDI) were determined. The validity of the test methods was ascertained by positive controls: mitomycin C (MMC) and bleomycin (BLM) were used in experiments without exogenous activation and cyclophosphamide (CP) in experiments with exogenous activation. The presence of rat liver microsomes increased the mutagenic effect of Robac AS 100 in the SCE- and MN-test. But only the highest Robac AS 100-concentration (57.0 microg/ml) showed significantly increased mutagenic activity in all tests. However, cytotoxicity at this concentration was already substantial. Therefore, we consider the evidence for mutagenicity of Robac AS 100 as limited.

Does reduced DNA repair capacity play a role in HIV infection and progression in the lymphocytes of opiate addicts?

Opiate use in vivo and in vitro reduces the ability of human peripheral lymphocytes to repair DNA damage caused by both the physical and chemical mutagens that produce single-strand adducts. This decrease in repair leads to increased genetic damage to the individual cell as measured by cytogenetic damage, including sister chromatid exchanges and formation of micronuclei. The expected consequences of this increase in damage can also be established by increases in host cell mutation rate and rate of apoptosis. The effect of this increase in genetic damage can be expected to have significant consequences for HIV-1 or simian immunodeficiency virus infecting those lymphocytes. For example, DNA damaging agents have long been known to induce lentiviral growth and propagation, and this has been found to be true for HIV-1 following ultraviolet light treatment of lymphocytes. However, to date, no one has fully explored the consequences of increased host mutation rate on HIV growth and maintenance. Recent reports have demonstrated the role of viral mutation in such key physiologic processes as resistance to highly active antiretroviral therapy (HAART). Beyond the effects of random mutations in the viral genome, specific mutations in the HIV-1 transcriptase and protease lead to increased accumulation of mutant viruses and the gradual failure of HAART. It therefore remains to be tested whether changes in host cell mutation rate will also predict changes in susceptibility to drug therapy. This also leads to questions about whether the higher rate of viral mutation in HIV-infected drug addicts might be the basis for higher rates of neuroAIDS in this population. It would be attractive to speculate that the increase in the heterogeneity of the virus in addicts produces mutants with a greater capacity to attack neuronal tissue and a high affinity to replicate there.

Ultraviolet light sensitivity, unscheduled DNA synthesis and DNA repair in C7-10 Aedes albopictus mosquito cells

We have examined the relative sensitivity of Aedes albopictus C7-10 mosquito cells to irradiation with ultraviolet light from a germicidal lamp. On the basis of plating efficiency, C7-10 cells were approximately two times more resistant to UV light than human 293 leukemia cells. Recovery after UV irradiation was accompanied by an increase in unscheduled DNA synthesis (UDS), which was measured by incorporation of (3)H-thymidine into acid-precipitable DNA in the presence of hydroxyurea. Under standardized conditions, UDS was maximal after a 10 min exposure (120 J/m(2)), and declined after longer exposures. In addition, UV treatment is associated with a small but reproducible increase in repair of plasmid DNA in transiently transfected cells. We anticipate that analysis of DNA repair activities in mosquito cells will identify molecular targets that might control longevity in transgenic mosquitoes.

Mutagenic and cytotoxic effectiveness of zinc dimethyl and zinc diisononyldithiocarbamate in human lymphocyte cultures

The mutagenic and cytotoxic effectiveness of the vulcanisation accelerators zinc dimethyldithiocarbamate (ZDMC; ziram) and zinc diisononyldithiocarbamate (ZDINDC; arbestab Z) was tested in lymphocyte cultures of five healthy probands. ZDMC and ZDINDC (c=0.1, 1.0 and 10.0microg/ml) were studied in lymphocyte cultures without external metabolic activation. Additionally, incubation of the compounds (c=10.0microg/ml) was performed in the presence of liver microsomes from aroclor-induced rats (1 and 2h, 1 and 2mg microsomal protein). Genotoxicity testing was performed by analysis of chromosomal aberrations (CA), sister chromatid exchanges (SCEs) and micronuclei (MN). For evaluation of antiproliferative effects, mitotic index (MI) and cell cycle kinetics (CCK) were determined. In contrast to earlier investigations we found no significantly increased mutagenic or cytotoxic activity of ZDMC; ZDINDC also was inactive under these conditions.

Sources of variability in the determination of micronuclei in irradiated peripheral blood lymphocytes

Following a dose of 4 Gy 250 kVp X-rays to the blood lymphocytes from three healthy donors the frequency of micronuclei was enumerated by three scorers. Eight slides were prepared from each blood sample with 1000 cytokinesis-blocked binucleate cells being scored for micronuclei and a recount undertaken 4 weeks later. Seven possible sources of variation were investigated: differences between donors, scorers, the four stages of sample preparation and time. A complete factorial analysis indicated that the major sources of variation were differences between donors and differences between scorers. One scorer identified fewer micronuclei than did the other two scorers in all slides of the irradiated blood samples and gave wide and inconsistent results for baseline counts. These results suggest that the performance of scorers should be screened before undertaking analysis of micronuclei. The most striking result, however, was the consistent performance of the scorers on the repeat counts compared with the very large differences between the scorers.