Radiation-induced gastrointestinal syndrome (RIGS) is a limiting factor when delivering radiotherapy to the abdomen and pelvis. It is also predicted to be a major source of morbidity in the event of a nuclear accident or radiological terrorism. Given the organ complexities it is unlikely that cell culture systems alone can fully predict responses. Well-characterised animal models that mimic the human response to both the high, but focal, radiation doses used in the clinic and the potentially lethal more widespread doses of radiation experienced after a nuclear accident are required to assess the efficacy of both prophylactic and mitigating therapies. Development of a model for RIGS requires knowledge of the radiation dose-response relationship and time course of mortality and morbidity across the acute and prolonged gastrointestinal (GI) radiation syndrome, with severity correlated to the histopathology.
Whole-body irradiation (WBI) doses necessary to elicit RIGS in mice invariably result in concomitant haematopoietic syndrome, which precludes analysis of RIGS beyond the first week after irradiation. While hematopoietic syndrome can be rescued by supportive care (i.e., hydration and/or antibiotics) or haematopoietic stem cell transplantation, there are currently no effective therapies to mitigate intestinal damage after radiation injury.
Intestinal epithelial regeneration after radiation injury is dependent on intestinal stem cell (ISC) repopulation and regeneration of the differentiated cells that cover the functional villus. Lgr5 is a Wnt target gene that marks a population of self-renewing and multipotent ISCs. Apc is a tumour suppressor protein that acts as an antagonist of the Wnt signaling pathway. Depletion of Lgr5+ ISCs impairs regeneration post-irradiation. Indeed, Wnt agonists, such as R-spondin, can enhance intestinal cell proliferation in clonogenic survival assays and reduce GI injury in mice. The mechanism of protection remains elusive, as systemic Wnt potentiation can result in a wide range of effects in various organ systems.
In a recent study by Romesser et al, a mouse model was developed that permitted inducible and reversible gene suppression following abdomen only irradiation, thereby excluding the confounding effects of concomitant hematopoietic syndrome that occur following WBI. They demonstrated that transient intestinal Apc suppression stimulated intestinal regeneration and mitigated lethality post-irradiation, thus validating pulsed Wnt pathway agonism as a therapeutic strategy. They believe this platform can be readily adopted to study any gene associated with the biology and treatment of intestinal radiation injury.
A small-animal linear accelerator was used to develop a whole-abdomen irradiation (WAI) system using contrast markers to delineate normal organs by micro-computerised topography (CT), permitting real-time animal dosimetry.
As expected, and in contrast to WBI, WAI allowed radiation dose escalation to the small intestine while minimising the dose delivered to the bone marrow, thoracic and abdominopelvic organs.
Serial monitoring of complete blood counts confirmed reduced bone marrow toxicity compared to WBI.
By using an inducible short-hairpin RNA transgenic platform they showed that transient Apc suppression (and hence Wnt activation) accelerated intestinal regeneration and mitigated radiation-induced gastrointestinal lethality following WAI by accelerating intestinal stem cell regeneration.
While sustained Apc suppression can promote carcinogenesis, the long-term side effects of transient Apc suppression after lethal WAI were minimal after 1 year (normal intestinal proliferation and no abdominopelvic tumours). Thus, transient Wnt activation can ameliorate RIGS without producing unacceptable toxicities.
Although this is a valid mouse model for assessing the radiation response during oncology therapy (with the caveat that most radiotherapies are delivered in a fractionated schedule) it is not suitable for the evaluation of radiation mitigators that may be used in the event of a radiation attack or accident. Such an event is unlikely to cause a focal exposure with differential responses to other exposed organs impacting the severity of RIGS. It is therefore implicit that, for this application, the characterisation of RIGS links to the other sub-syndromes, particularly that of the haematopoietic system in which toxicity is slower to manifest than RIGS but is induced by much lower radiation doses. For evaluation of such radiation mitigators a converse model, which spares only a minimal level of bone marrow protection but exposes all other organs (as may be expected in a real-life situation), is preferable.
Romesser et al., Preclinical murine platform to evaluate therapeutic countermeasures against radiation-induced gastrointestinal syndrome. Proc Natl Acad Sci USA. 2019, Oct 8; 116(41):20672-20678.