Distinct response of adult neural stem cells to low versus high dose ionising radiation.

Radiosusceptibility is the sensitivity of a biological organism to ionising radiation (IR)-induced carcinogenesis, an outcome of IR exposure relevant following low doses. The tissue response is strongly influenced by the DNA damage response (DDR) activated in stem and progenitor cells. We previously reported that in vivo exposure to 2 Gy X-rays activates apoptosis, proliferation arrest and premature differentiation in neural progenitor cells (transit amplifying cells and neuroblasts) but not in neural stem cells (NSCs) of the largest neurogenic region of the adult brain, the subventricular zone (SVZ). These responses promote adult quiescent NSC (qNSC) activation after 2 Gy. In contrast, neonatal (P5) SVZ neural progenitors continue proliferating and do not activate qNSCs. Significantly, the human and mouse neonatal brain is radiosusceptible. Here, we examine the response of stem and progenitor cells in the SVZ to low IR doses (50-500 mGy). We observe a linear dose-response for apoptosis but, in contrast, proliferation arrest and neuroblast differentiation require a threshold dose of 200 or 500 mGy, respectively. Importantly, qNSCs were not activated at doses below 500 mGy. Thus, full DDR activation in the neural stem cell compartment in vivo necessitates a threshold dose, which can be considered of significance when evaluating IR-induced cancer risk and dose extrapolation.