Pathway Analysis of Transcriptional Alterations in the Salivary Gland Following Radiation Exposure

Abstract

Radiation-induced xerostomia (RIX) is the subjective condition of dry mouth caused by radiation therapy to the head and neck and a result of hyposalivation and altered sialochemistry. Our long-term goal is to develop novel approaches to improve salivary function after radiation therapy. To better understand the molecular alterations induced by radiation in the salivary glands, we performed transcriptional profiling aimed at describing acute and long-term alterations in the murine salivary glands induced by radiation to better design effective regenerative therapies. The submandibular gland (SMG) of male C57Bl/6 mice received 15 Gy in a single fraction using a small animal image-guided irradiator. The mice were followed for 90 days. Saliva was collected at baseline and 7, 60 and 90 days after radiation. Salivary glands were collected for histology 3, 7, 60, and 90 days after radiation. The tissues were analyzed using a multiplex analysis platform processed with the murine whole transcriptome panel to investigate changes in gene expression over time. Using the multiplex analysis platform software, 96 regions of interest corresponding to acini, ducts, and granular convoluted tubules (GCTs) were delineated and gene expression was assessed separately in each area. To confirm the identified alterations, SMG tissue was histologically evaluated for fibrosis (masons tri-chrome), mucin (alcian blue), and amylase (α-amylase). Alterations in the immune microenvironment (CD19, CD3, and F4/80) and salivary gland stem cells (MIST1, SCA, Sox2, and c-Kit) were assessed by immunohistochemistry. Radiation resulted in a significant decrease in salivary production compared to nonirradiated controls (p-value = 0.03). The analysis of the transcriptional pathway identified early alterations in cell communication, DNA damage, and the immune response. Protein metabolism and extracellular matrix remodeling were up-regulated at later time points. Histological evaluation of tissues showed an increase in glandular structure disorganization over time, with a decrease in the size and distribution of the acinar compartments throughout the gland (mean area stained: No RT = 25.81% vs 90d RT = 19.98%, p-value = 0.005). There was an increase in periductal fibrosis within the tissue after 60 days compared to controls. Acutely, there was an increase in the expression of MIST1 and macrophages (mean area stained: No RT = 14.8% vs 7d RT = 18.76%, p-value = 0.0004) within the SMG that had resolved by the 60- and 90-day time points. The results implicate macrophage alterations in radiation-induced salivary gland damage and provide valuable mechanistic insight guiding potential approaches to intervene in RIX. We are pursuing cell therapy-based approaches to prevent and repair damage caused by radiation in order to improve the lives of our patients.