Reduction of gene repair by selenomethionine with the use of single-stranded oligonucleotides

Protective effects of selenomethionine against ionizing radiation under the modulation of p53 tumor suppressor

Ionizing radiation (IR) therapy has been widely employed in the treatment of cancer. However, certain issues, including toxicity, have been raised in conjunction with IR therapy for cancer. Recently, selenomethionine (SeMet) as an antioxidant has been the subject of a great deal of attention for its chemopreventive effects. In this study, we found that DNA repair activity has been enhanced in response to SeMet against IR. In addition, our data showed that p53 functional activity was significantly reduced against IR in the cells expressing a mutant form of redox factor 1 (Ref-1) contrast with Ref-1 wild-type cells treated with SeMet, suggesting that p53 activation under the modulation of Ref-1 might play an important role in IR-treated cells in the presence of SeMet. Furthermore, IR-induced micronuclei numbers were also reduced after treatment with SeMet, strongly implicating protection by SeMet in genomic stability against IR-induced genotoxicity. From this study, we suggest that the p53-mediated protective mechanism of SeMet might provide clues for reducing side effects of IR therapy.

The effect of selenium, as selenomethionine, on genome stability and cytotoxicity in human lymphocytes measured using the cytokinesis-block micronucleus cytome assay

A supranutritional intake of selenium (Se) may be required for cancer prevention, but an excessively high dose could be toxic. Therefore, the effect on genome stability of seleno-L-methionine (Se-met), the most important dietary form of Se, was measured to determine its bioefficacy and safety limit. Peripheral blood lymphocytes were isolated from six volunteers and cultured with medium supplemented with Se-met in a series of Se concentrations (3, 31, 125, 430, 1880 and 3850 microg Se/litre) while keeping the total methionine (i.e. Se-met + L-methionine) concentration constant at 50 microM. Baseline genome stability of lymphocytes and the extent of DNA damage induced by 1.5-Gy gamma-ray were investigated using the cytokinesis-block micronucleus cytome assay after 9 days of culture in 96-microwell plates. High Se concentrations (>or=1880 microg Se/litre) caused strong inhibition of cell division and increased cell death (P < 0.0001). Baseline frequency of nucleoplasmic bridges and nuclear buds, however, declined significantly (P trend < 0.05) as Se concentration increased from 3 to 430 microg Se/litre. Se concentration (<or=430 microg Se/litre) had no significant effect on baseline frequency of micronuclei and had no protective effect against genome damage induced by exposure to 1.5-Gy gamma-ray irradiation. In conclusion, Se, as Se-met, may improve genome stability at concentrations up to 430 microg Se/litre, but higher doses may be cytotoxic. Therefore, a cautious approach to supplementation with Se-met is required to ensure that optimal genome health is achieved without cytotoxic effects.

Simultaneous targeted exchange of two nucleotides by single-stranded oligonucleotides clusters within a region of about fourteen nucleotides

Background

Transfection of cells with gene-specific, single-stranded oligonucleotides can induce the targeted exchange of one or two nucleotides in the targeted gene. To characterize the features of the DNA-repair mechanisms involved, we examined the maximal distance for the simultaneous exchange of two nucleotides by a single-stranded oligonucleotide. The chosen experimental system was the correction of a hprt-point mutation in a hamster cell line, the generation of an additional nucleotide exchange at a variable distance from the first exchange position and the investigation of the rate of simultaneous nucleotide exchanges.

Results

The smaller the distance between the two exchange positions, the higher was the probability of a simultaneous exchange. The detected simultaneous nucleotide exchanges were found to cluster in a region of about fourteen nucleotides upstream and downstream from the first exchange position.

Conclusion

We suggest that the mechanism involved in the repair of the targeted DNA strand utilizes only a short sequence of the single-stranded oligonucleotide, which may be physically incorporated into the DNA or be used as a matrix for a repair process.