Increased repair and cell survival in cells treated with DIR1 antisense oligonucleotides: implications for induced radioresistance.

PURPOSE: To determine whether repression of a recently isolated, X-ray-responsive gene, DIR1, using antisense oligonucleotides could affect clonogenic cell survival and repair of DNA strand breaks and have a possible role in the mechanism underlying the phenomenon of 'induced radioresistance' (IRR). MATERIALS AND METHODS: Three cell lines, V79, RT112 and UM-UC-3, which are known to exhibit low-dose hypersensitivity (HRS) and induced radioresistance (IRR), and the radiosensitive cell line ATBIVA, were transfected with antisense oligonucleotides directed towards the DIR1 gene. Scrambled oligonucleotides were used as controls. DNA single-strand break (ssb) repair, using the alkaline comet assay, and cell survival using a standard clonogenic assay was measured after exposure to X-rays. RESULTS: Following treatment with 4Gy X-rays, the V79, RT112 and UM-UC-3 cell lines all exhibited significantly increased rates of ssb repair after transfection with DIR1 antisense oligonucleotides compared with cells transfected with scrambled oligonucleotides. They also demonstrated significantly enhanced survival after exposure to 2 Gy X-rays; the radiosensitive ATBIVA cells did not show these effects. CONCLUSIONS: Repression of the DIR1 gene product leads to an increase in the rate of repair and cell survival in three radioresistant cells lines but not in the radiosensitive ATBIVA cell line. Because DIR1 is repressed by X-rays in the dose range where IRR is observed, it may represent a candidate gene involved in the IRR phenomenon.