Evaluation of DNA damage and DNA repair by the comet assay in workers exposed to organic solvents
Swanepoel, Andrew Johnstone
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Rapid development of the chemical industry over the past two decades makes the study of the effects of chemical substances (such as benzene) on the human body essential, Benzene toxicity involves both bone marrow depression and Ieukemogenesis caused by damage to various classes of hematopoietic cells and a variety of hematopoietic cell functions. Studies of the relationship between the metabolism and toxicity of benzene indicate that several metabolites of benzene play significant roles in generating benzene toxicity. Benzene is metabolized, primarily in the liver, to a variety of hydroxylated and ringopened products that are transported to the bone marrow where subsequent secondary metabolisms occur. Maintenance of the genomic integrity is of crucial importance for all organisms. Damage to the DNA that makes up human genes is constantly inflicted by a large number of agents and can have severe effects if it persists. Modification of DNA can lead to mutations, which alter the coding sequence of DNA and can lead to cancer in mammals. Other DNA lesions interfere with normal cellular transactions such as DNA replication or transcription, and are deleterious to the cell. Thus it is obvious that the stability of the genome must be under continuous surveillance. This is accomplished by DNA repair mechanisms, which remove DNA lesions in an error-free, or in some cases, error-prone way. Defects in DNA repair give rise to hypersensitivity to DNA damaging agents, accumulation of mutations in the genome, and finally to the development of cancer and various metabolic disorders. The alkaline single cell gel electrophoresis assay (comet assay) was applied to study the occurrence of DNA damage and repair in peripheral lymphocytes of 27 experimental human subjects with occupational exposure to benzene and 9 control human subjects with no occupational exposure to benzene. Two blood samples (pre-shift and after-shift respectively) were obtained from each of the control as well as the experimental subjects. After electrophoresis, scoring was done by using the CASP software program. Two of several variables were of primary interest, i.e., Tail DNA% and Tail Moment. Statistical analysis clearly indicated that significant differences exist between pre-shift and after-shift data for both variables. Results revealed that there are also different behaviour patterns for the experimental and control groups, pre-shift and after-shift, for both variables during all stages of the comet assay. It became evident for both Tail DNA%-data and Tail Momentdata that DNA damage was indeed present at the exposed (experimental) group of workers. For the experimental group, after-shift data exceeded pre-shift data in general. Different behaviour was noticed between the experimental and control-data during the repair process. Also, the initial pre-shift data-values of the experimental group, is much higher (about 9%) than that of the control group, which may be an indication of a carried-over effect which may exist from the previous shift, due to the exposure to benzene. After-shift urine samples were also obtained from the experimental and control subjects and analysed for urinary trans-trans-muconic acid (ttMA), a metabolite of benzene, with the aid of a high-performance liquid chromatography (HPLCIUV) detector. The exposed workers showed a markedly higher (1,8 times) ttMA mean value than the control persons, and the two-sample t-test confirmed this significant difference.
- Health Sciences