Distinct Levels of Radioresistance in Lgr5+ Colonic Epithelial Stem Cells versus Lgr5+ Small Intestinal Stem Cells

Abstract

While small and large intestines possess seemingly similar Wnt-driven Leucine-rich repeat-containing G protein coupled receptor 5 (Lgr5)+ adult epithelial stem cells, we report here that the two organs exhibit distinct mechanisms of tissue response to ionizing radiation. Employing Lgr5-lacZ transgenic mice and Lgr5 in situ hybridization, we found colonic epithelial stem cells (CESCs) markedly more radioresistant in vivo than small intestinal crypt base columnar stem cells (CBCs) (D0= 6.0±0.3 Gy vs. 1.3±0.1, respectively; p<0.01). Accordingly, CESCs survived 30 Gy exposure, while CBCs were completely depleted after 15 Gy. Edu incorporation studies indicated that after 19 Gy, CBCs exited growth arrest at 12 hours, resuming normal mitotic activity despite 60% of this population displaying residual γH2AX foci, indicative of persistent unrepaired DNA damage. Checkpoint recovery before complete DSB repair represents the sine qua non of a newly-defined potentially-lethal pathophysiology termed checkpoint adaptation. In the small intestinal mucosa, checkpoint adaptation resulted in CBCs succumbing to an eight-fold increase in incidence of highly-lethal chromosomal aberrations and mitotic catastrophe by 48 hours post-radiation. In contrast, Lgr5+ CESCs displayed delayed checkpoint recovery at 48 hours post-19 Gy, coordinated with complete DSB repair and regeneration of colonic mucosa originating, at least in part, from surviving CESCs. The discovery that small intestinal CBCs succumb to checkpoint adaptation is the first demonstration that this aberrant cell cycle response may drive mammalian tissue radiosensitivity.