A Chinese study has shown that the recently discovered transcription factor Bach2, which is important in B-cell development and progression of inflammatory disease, also regulates gastrointestinal (GI) regeneration by facilitating DNA repair after ionizing radiation (IR) exposure.
This study finding, made in Bach2-knockout (KO) mice and in an intestinal organoid-based model, has significant implications for the development of new drugs for the treatment of IR-induced GI syndrome, the authors reported in the December 30, 2020, online edition of Stem Cell Reports.
"High-dose radiation exposure induces GI stem cell death, resulting in denudation of the intestinal mucosa and mortality from GI syndrome," said study leader Guoqiang Hua, a professor in the Fudan University Shanghai Cancer Center in Shanghai.
Acute high-dose IR can damage the GI mucosa and cause GI tract inflammation that can result in adverse reactions including nausea, vomiting and appetite loss in cancer patients undergoing radiotherapy. Those side effects can adversely impact compliance.
Moreover, "mice exposed to higher IR doses have been shown to die within 10 days, owing to disruption in the small intestinal epithelium," Hua told BioWorld Science.
"Due to the profound need for efficient intestine function, pathways or genes through which intestinal regeneration can be expedited could dramatically help overall survival."
The pathophysiology underlying such damage involves depletion of stem cells in the crypts of Lieberkuhn, which impairs regeneration of villus/crypt units and compromises mucosal integrity and function.
However, as yet there are no effective FDA-approved treatments for radiation-induced GI syndrome, although intestinal stem cells (ISCs) have been reported to promote regeneration.
GI epithelial cells undergo continuous turnover via stem cells, which persist indefinitely, and 'transit amplifying' (TA) cells, which arise from stem cells and finitely divide until they differentiate.
Evidence increasingly indicates that stem cells can prevent the accumulation of genetic lesions and promote their repair.
For example, hair-follicle-bulge stem cells are relatively radio-resistant due to higher expression of an antiapoptotic gene, while ISCs can repair DNA double-strand breaks (DSBs) better than differentiated GI cells.
Moreover, hematopoietic stem cells (HSCs) are known to use multiple mechanisms to prevent mutations or their transmission to offspring, while baseline DNA repair is also very efficient in HSCs.
Genetic lesions are particularly harmful to stem cell maintenance and tissue regeneration, providing the rationale for developing specific therapeutic strategies, such as those targeting Bach2.
Recent research has suggested that Bach2 is important in maintaining tissue immune homeostasis and regulating the cell cycle and DNA repair.
An important transcription factor, Bach2 has primarily been reported to induce apoptosis via inhibition of the NRF2-mediated antioxidant defense pathway.
Genetic silencing of Bach2 stimulates proliferation and increases cell survival in response to acute oxidative stress in fibroblasts.
In addition, Bach2 is known to be required for the efficient formation of regulatory T cells and suppressing inflammation.
Importantly, young Bach2-deficient mice show a hypertrophic crypt phenotype in the small intestinal epithelium and develop progressive wasting, resulting in reduced survival versus wild-type (WT) controls.
These studies suggest Bach2 may play a key role in determining cell cycle, but its detailed functional role in GI regeneration and IR-induced DNA damage remains unclear.
In their new Stem Cell Reports study, Hua and colleagues showed that Bach2 deficiency promoted intestinal epithelial cell proliferation during homeostasis.
In particular, they demonstrated that genetic inactivation of Bach2 in the mouse GI epithelium facilitated post-IR crypt regeneration, resulting in reduced mortality.
"We found that genetic ablation of Bach2 in mice resulted in hypertrophic crypts in [their] small intestines, which is consistent with a previous report showing that Bach2-KO mice developed a progressive wasting disease, resulting in diminished survival compared with controls," said Hua.
"Bach2-deficient mice also exhibited significantly faster crypt regeneration after IR, and [their] regenerated crypts were much larger in size and in quantity, indicating that surviving TA cells might coordinate with ISCs in reconstructing the GI epithelium."
The Fudan University researchers then tested mouse survival following a lethal dose of radiation.
"Our data demonstrated that Bach2 ablation rescued mice from radiation-induced GI syndrome, [showing that] Bach2 is an important regulator of cell proliferation and survival in ISCs and TA cells after IR damage," said Hua.
Moreover, RNA-sequencing analysis of isolated crypts showed that Bach2 deficiency altered the expression of numerous genes, including those regulating DSB repair.
"Our data suggest that Bach2 deficiency stimulates intestinal regeneration by upregulating genes that are critical for DNA DSBs," said Hua.
Moreover, mechanistic studies demonstrated that "both ISCs and TA cells from Bach2-KO mice repaired DSBs more efficiently than controls."
Collectively, these findings illustrate that Bach2 deficiency promotes intestinal regeneration by accelerating DNA repair in intestinal stem cells after radiation damage.
This has significance in terms of new drug development, noted Hua, since "small-molecule inhibitors of Bach2 expression or promoting Bach2 protein degradation could be a useful strategy for treating IR-induced GI injury associated with delayed GI regeneration and repair."