Silencing the intestinal GUCY2C tumor suppressor axis requires APC loss of heterozygosity

Abstract

Most sporadic colorectal cancer reflects acquired mutations in the adenomatous polyposis coli (APC) tumor suppressor gene, while germline heterozygosity for mutant APC produces the autosomal dominant disorder Familial Adenomatous Polyposis (FAP) with a predisposition to colorectal cancer. In these syndromes, loss of heterozygosity (LOH) silences the remaining normal allele of APC, through an unknown mechanism, as the initiating step in transformation. Guanylyl cyclase C receptor (GUCY2C) and its hormones, uroguanylin and guanylin, have emerged as a key signaling axis opposing mutations driving intestinal tumorigenesis. Indeed, uroguanylin and guanylin are among the most commonly repressed genes in colorectal cancer. Here, we explored the role of APC heterozygosity in mechanisms repressing hormone expression which could contribute to LOH. In genetic mouse models of APC loss, uroguanylin and guanylin expression were quantified following monoallelic or biallelic deletion of the Apc gene. Induced biallelic loss of APC repressed uroguanylin and guanylin expression. However, monoallelic APC loss in Apc min/+ mice did not alter hormone expression. Similarly, in FAP patients, normal colonic mucosa (monoallelic APC loss) expressed guanylin while adenomas and an invasive carcinoma (biallelic APC loss) were devoid of hormone expression. Thus, uroguanylin and guanylin expression by normal intestinal epithelial cells persists in the context of APC heterozygosity and is lost only after tumor initiation by APC LOH. These observations reveal a role for loss of the hormones silencing the GUCY2C axis in tumor progression following biallelic APC loss, but not in mechanisms creating the genetic vulnerability in epithelial cells underlying APC LOH initiating tumorigenesis.

A β-Catenin-TCF-Sensitive Locus Control Region Mediates GUCY2C Ligand Loss in Colorectal Cancer

“…[22][23][24] However, transformation universally orphans the receptor through loss of hormones. 7,16,[21][22][23]25,26 Hormone loss silencing GUCY2C occurs early in dysplastic crypts and is conserved across species and tumor subtypes. 16,22,23,25,26 GUCA2A is lost following APC inactivation in mouse models of conditional biallelic Apc deletion (Apc CKO/CKO ) and Apc loss of heterozygosity (Apc min/þ ).…”

“…7,16,[21][22][23]25,26 Hormone loss silencing GUCY2C occurs early in dysplastic crypts and is conserved across species and tumor subtypes. 16,22,23,25,26 GUCA2A is lost following APC inactivation in mouse models of conditional biallelic Apc deletion (Apc CKO/CKO ) and Apc loss of heterozygosity (Apc min/þ ). Further, silencing mutant b-catenin-TCF signaling restores GUCA2A levels.…”

The Intestinal Tract Brush Border in Young Children Uniformly Expresses Guanylate Cyclase C

“…As the receptor for the heat-stable enterotoxin, and for endogenous paracrine ligands uroguanylin and guanylin, GCC has a clear role in intestinal fluid homeostasis. GCC also has a role as a tumor suppressor and biomarker [12][13][14] with GCC targeting as therapy for oncologic and gastrointestinal disorders. [15][16][17][18][19][20] GCC is also a focus of satiety and obesity studies because of the observation that it is a satiety modulator with activation relative to serum pro-uroguanylin levels.…”

An update on guanylyl cyclase C in the diagnosis, chemoprevention, and treatment of colorectal cancer

“…However, an alternative hypothesis suggests that APC loss of heterozygosity precedes guanylin loss, necessarily lifting a barrier to tumorigenesis imposed by GUCY2C. A recent pair of studies demonstrated that guanylin is comparably expressed in wild type and APC min/+ mice, and is only lost following biallelic APC deletion, suggesting that guanylin expression is unable to prevent sporadic APC loss of heterozygosity [16,17]. Reconstitution of wild type APC in colorectal cancer cells was sufficient to restore guanylin expression in vitro, supporting the hypothesis that APC loss drives guanylin loss [16].…”