Sag/Rbx2 Partial Inactivation Sensitizes Mice to Radiation and Radiation-Induced Tumorigenesis1

SAG (sensitive to apoptosis gene)/RBX2 (RING box-2), is the second family member of RING component of cullin-RING ligase (CRL) complex required for its enzymatic activity. Using total or conditional Sag knockout mouse models, we previously showed that Sag plays an essential role in embryonic development, apoptosis, vasculogenesis, angiogenesis and tumorigenesis. We also found that Sag-null ES cells are more sensitive to radiation. In this study, we generated the SagΔ/flneo mice with partial Sag inactivation due to deletion in one allele (Δ allele), and disrupted expression in the another (by a neo cassette). Compared to wild-type, SagΔ/fl-neo mice are more sensitive to a lethal dose of radiation with significantly shortened life span, resulting from an increased tissue damage with reduced proliferation and increased apoptosis in the intestines. Similar observations were made when SagΔ/fl-neo mice received a high dose of radiation directly delivered to the abdomen with reduced proliferation and prolonged DNA damage repair. Mechanistically, we found accumulations of Sag substrates, p21 and p27, explaining the proliferation defect. Finally, we found that SagΔ/fl-neo mice are more prone to tumorigenesis induced by a low dose of radiation with shortened life-span and increased incidence of lymphoma. Collectively, our study demonstrates that Sag protects mice from radiation-induced tissue damages and tumorigenesis.