Telomere repeat binding factor TRF2 is a member of shelterin complex with an important role in protecting and stabilizing chromosomal ends. In the present study, we investigated the effect of partial knockdown of TRF2 on radiosensitivity of telomerase immortalized human mesenchymal stem cells (hMSC-telo1), which have a higher radioresistance compared to non telomerized counterpart. Partial knockdown of the protein achieved 15-20% reduction in TRF2 protein levels. The study compared the effect of 2.5 Gy radiation in two four days after irradiation for hMSC-telol cells and the cells transfected with siTRF2 and null control vector. Radio-response of the cells were examined using senescence associated beta-Gal assay (beta-Gal), colony forming assay (CFU) and gamma-H2AX phosphorylation. TRF2 deficiency substantially increased radiosensitivity of cells compared to controls in both proliferation and senescence assay (2.4 fold increase in beta-Gal, 1.6 fold decrease in CFU). In addition, it increased the gamma-H2AX foci as revealed by both immunfluorescence and Western blot analysis. Our data suggests that partial knockdown of TRF2 in hMSC-telol cells cause increased gamma-H2AX foci which led to fail TRF2 to protect telomeres from radiation thus TRF2 deficiency led to a 1,5-2 fold increase in the radiosensitivity of hMSC-telol cells through telomere destabilization. (C) 2015 Elsevier B.V. All rights reserved.