What Can We Learn from a Pig Model of FAP?
A pig model of intestinal adenoma development, described in the November issue of Gastroenterology, will improve our understanding of colorectal cancer development and could be used to evaluate new therapeutics.
Familial adenomatous polyposis (FAP) is an inherited disease; patients develop dysplasias in the colon and rectum that develop to adenomatous polyps and adenocarcinoma. FAP is caused by germline mutations in adenomatous polyposis coli (APC), which encodes a tumor suppressor—somatic mutations in APC are found in early-stage sporadic colorectal tumors.
Although there are several mouse models of APC, they don’t fully resemble the human disease.
Tatiana Flisikowska et al. show that in pigs, unlike mice, a single heterozygous germline mutation in APC is sufficient to initiate the sequence that leads to adenomas in the large intestine, and replicate early-stage human FAP.
Using cloned pigs, Flisikowska et al. inserted translational stop signals in APC at codons 1061 and 1311—orthologous to common germline mutations (APC1061 and APC1309) that cause human FAP. The mutation at human codon 1309 is associated with a particularly severe phenotype, with early onset and prolific polyposis; a mutation at codon 1061 causes a less-severe form of polyposis.
After 1 year, wild-type pigs had normal gastrointestinal tracts. The intestines of the APC1061/+ pig showed no evidence of polyposis. However, colon and rectal tissues from each APC1311/+ pig had more than 100 macroscopically visible lesions, including more than 60 sessile polyps. Polyps ranged from barely visible mucosal nodules of 1–2 mm to flat polyps up to 1 cm. They were scattered along the entire large bowel, with most in the proximal colon (see below figure).
As in young patients with FAP, no gastric polyps or duodenal adenomas were observed in the pigs. Colonic adenomas develop during childhood, but gastric polyps and duodenal adenomas occur later, when patients reach adulthood.
Importantly for research purposes, the pigs with FAP could be evaluated using human-sized equipment, such as high-resolution magnification chromoendoscopic imaging of the small and large intestines.
Pathology evaluation of the pigs’ lesions showed serrated morphology without dysplasia, which are common in humans but not in mice with Apc mutations. These types of lesions increase risk of colorectal cancer in humans.
Some larger adenomas showed focal features of more advanced tumors, and were classified as adenomas with focal high-grade intraepithelial neoplasia. The pigs’ tumors appeared identical to human colonic adenomas with respect to surface involvement, with dysplastic cells on the superficial mucosal surface—this is in contrast to the adenomas that form in Apc mutant mice, which have a surface layer of nondysplastic epithelium.
In an editorial that accompanies the article, Joanna Groden and Randall Burt say that the new model created by Flisikowska et al. will permit the study of allelic variation at the APC locus and the polyposis phenotype.
Animal models of intestinal tumor formation have been important in identifying mechanisms of cancer pathogenesis, but mouse models are limited because most adenomas form in the small intestine, as well as their histopathology features and short lifespan, which prevents studies of cancer progression and therapies.
How do mutations in APC lead to adenoma and adenocarcinoma? APC is part of a complex that regulates the phosphorylation-dependent degradation of β-catenin and Wnt signaling. Cancer-associated mutations in APC most commonly insert stop codons onto the open reading frame, to truncate APC. It can therefore no longer bind or contribute to degradation of β-catenin, leading to constitutive stabilization of β-catenin and the expansion of the stem cell compartment in the intestinal crypts.
Flisikowska et al. assessed the wild-type APC allele in 5 porcine adenomas (0.4–1 cm; 3 with low-grade and 2 with low- and focal high-grade dysplasia). The wild-type APC allele was lost in each case. Aberrant crypt foci and colorectal tumors had high levels of nuclear and cytoplasmic β-catenin staining outside the proliferative zone, indicating that tumor initiation and progression occurs via Wnt pathway activation. Consistent with human FAP, there was no increase in epithelial proliferation of histologically normal intestinal mucosa.
Groden and Burt say that an animal model that more closely resembles human disease will improve our understanding of the relationships between APC genotype and phenotypes, and led to identification of other genes that affect disease development and therapeutic targets.
Read the article online.
Flisikowska T, Merkl C, Landmann M, et al. A porcine model of familial adenomatous polyposis.Gastroenterology 2012;143:1173−1175.e7.
Read the accompanying editorial.
Groden J, Burt R. Genotypes and phenotypes: Animal models of familial adenomatous polyposis coli. Gastroenterology 2012;143:1133−1135.