Severe congenital diarrhea occurs in about half of patients with ARX null mutations. The cause of this diarrhea is unknown. In a mouse model of intestinal Arx deficiency, the prevalence of a subset of enteroendocrine cells is altered, leading to diarrhea. Since polyalanine expansions within the ARX protein are the most common mutations found in ARX-related disorders, we sought to characterize the enteroendocrine population in human tissue of an ARX(GGC)7 mutation and in a mouse model of the corresponding polyalanine expansion (Arx(GCG)7).
Immunohistochemistry and quantitative RT-PCR were the primary modalities utilized to characterize the enteroendocrine populations. Daily weights were determined for the growth curves, and Oil-Red-O staining on stool and tissue identified neutral fats.
An expansion of seven alanines in the first polyalanine tract of both human ARX and mouse Arx altered enteroendocrine differentiation. In human tissue, cholecystokinin (CCK), glucagon-like peptide 1 (GLP-1), and somatostatin (SST) populations were reduced, while the Chromogranin A population was unchanged. In the mouse model, CCK and GLP-1 populations were also lost, though the SST-expressing population was increased. The ARX(GGC)7 protein was present in human tissue, while the Arx(GCG)7 protein was degraded in the mouse intestine.
ARX/Arx is required for the specification of a subset of enteroendocrine cells in both humans and mice. Due to protein degradation, the Arx(GCG)7 mouse recapitulates findings of the intestinal Arx null model, but is not able to further the study of the differential effects of the ARX(GCG)7 protein on its transcriptional targets in the intestine.