Lack of DNA Damage Response at Low Radiation Doses in Adult Stem Cells Contributes to Organ Dysfunction

Abstract

Radiotherapy for head and neck cancer may result in serious side effects, such as hyposalivation, impairing the patient's quality of life. Modern radiotherapy techniques attempt to reduce the dose to salivary glands, which, however, results in low-dose irradiation of the tissue stem cells. Here we assess the low-dose sensitivity of tissue stem cells and the consequences for tissue function. Postirradiation rat salivary gland secretory function was determined after pilocarpine induction. Murine and patient-derived salivary gland and thyroid gland organoids were irradiated and clonogenic survival was assessed. The DNA damage response (DDR) was analyzed in organoids and modulated using different radiation modalities, chemical inhibition, and genetic modification. Relative low-dose irradiation to the high-density stem cell region of rat salivary gland disproportionally impaired function. Hyper-radiosensitivity at doses <1 Gy, followed by relative radioresistance at doses ≥1 Gy, was observed in salivary gland and thyroid gland organoid cultures. DDR modulation resulted in diminished, or even abrogated, relative radioresistance. Furthermore, inhibition of the DDR protein ATM impaired DNA repair after 1 Gy, but not 0.25 Gy. Irradiation of patient-derived salivary gland organoid cells showed similar responses, whereas a single 1 Gy dose to salivary gland-derived stem cells resulted in greater survival than clinically relevant fractionated doses of 4 × 0.25 Gy. We show that murine and human glandular tissue stem cells exhibit a dose threshold in DDR activation, resulting in low-dose hyper-radiosensitivity, with clinical implications in radiotherapy treatment planning. Furthermore, our results from patient-derived organoids highlight the potential of organoids to study normal tissue responses to radiation.

Cellular senescence contributes to radiation-induced hyposalivation by affecting the stem/progenitor cell niche

“…The observation that after aging 37 or upon an ablative radiation dose 38 the remaining SGSCs, when taken out of their niche and cultured as organoids, have the same regenerative potential as young/untreated SGSCs agrees with this hypothesis. Indeed, evidence of DNA damage response activation in organoids presented here and in a previously published study 32 and in irradiated salivary gland tissue 19 and the high level of senescence and SASP gene expression after irradiation as shown here in SGSC-derived organoids may contribute to this phenomenon.…”

Prevention and treatment of radiotherapy‐induced side effects

“…A deeper knowledge of the mechanisms that underlie normal tissue damage might also help to develop better preventive and therapeutic strategies. We need to progress from understanding local molecular/cellular events toward having a better understanding of tissue and organ interactions; this progress does not occur automatically and needs to be supported by subsequent translational research using animal models or tissue resembling models [63,127]. Typically, studies of the importance of different structures, in particular for the adult and pediatric brain, are needed to define (functional) structures that need to be spared or that can tolerate a somehow larger dose.…”

“…However, the complete sparing of the area that contains the highest proportion of SSPCs proved to be difficult due to the close vicinity of the tumor. This and the finding that these stem/progenitor cells might be very sensitive to low doses of irradiation [63] warrant the use of very accurate radiotherapy technologies, such as proton therapy (Box 1). The above-described approaches have all been developed after obtaining a deeper knowledge of the response of the salivary gland to irradiation.…”

Preclinical models of head and neck squamous cell carcinoma for a basic understanding of cancer biology and its translation into efficient therapies

“…Importantly, not only treatment-related effects in tumors but also unwanted therapy side effects in normal tissue can be studied in organoid models. For example, patient-derived salivary gland organoids have been used for dissecting the molecular basis of hyposalivation, a frequent severe side effect of radiation [30].…”