Elucidating mechanisms regulating GC-C signalling in colorectal cancer

Abstract

Colorectal cancer is the third most frequently diagnosed cancer and the second major cause of cancer death worldwide. Classical therapies for treating colorectal cancer are mostly nonspecific and, therefore, are associated with lots of side effects. Thus, there is a pressing need to develop targeted therapies for treating colorectal cancer patients. This study utilised the TCGA (The Cancer Genome Atlas) and other datasets to identify differentially regulated genes in primary tumours of colon and rectum adenocarcinoma patients. Gene set enrichment analysis of the differentially regulated genes revealed that ‘guanylate cyclase activator activity’ was the most prominent molecular function associated with the under-expressed genes in primary tumours of both colon and rectum adenocarcinoma patients. The major components of the ‘guanylate cyclase activator activity’ include receptor guanylyl cyclase C (GC-C) and its endogenous ligands guanylin and uroguanylin. GC-C is a transmembrane protein predominantly expressed predominantly by the intestinal epithelial cells and converts GTP to cGMP, on activation with ligands namely guanylin and uroguanylin. Although, the primary function of GC-C was thought to be restricted to the maintenance of ion water homeostasis in the intestine, growing evidence has emerged over time indicating the involvement of GC-C signalling in intestinal tumourigeneis. An in-depth inspection of publicly available datasets demonstrated that GC-C and its ligands were downregulated in primary tumours of colon and rectum adenocarcinoma patients. Mechanistic analysis revealed that the downregulation of the GC-C ligands is due to suppression by the Wnt signalling pathway, as well as the loss of intestinal cell types that produce them, whereas downregulation of GC-C is due to the loss of CDX2 expression, a key transcription factor of GC-C, and the loss of major intestinal epithelial cell types that produce GC-C. The inverse correlation between the GC-C signalling pathway and colorectal cancer, coupled with its predominant expression exclusively in the intestinal epithelial cells, makes it an attractive target for developing anti-cancer therapies using GC-C agonists. However, a tumour microenvironment is highly vulnerable to the development of mutations, raising the possibility of having patients with loss of function mutations in GC-C. These mutations would impair the ability of GC-C to get activated on agonist treatment thereby making the patient refractive to the therapy. The study further extracted mutations reported in GC-C from tumour samples of colorectal cancer patients from multiple datasets, and tested their impact on the function of GC-C using bioinformatic and biochemical approaches. The study identified several loss of function mutations that disrupted the cGMP-producing ability of GC-C on ligand stimulation, and also uncovered the mechanisms responsible for their loss of activity. Overall, this study highlights the importance of screening colorectal cancer patients for mutations in GC-C prior to GC-C agonist treatment.