PORCINE MODELS FOR HUMAN CANCER

Abstract

Cancers are a leading cause of death worldwide and a major priority for biomedical research. Most animal models of solid cancers are in rodents, particularly genetically engineered mice. However, mice differ significantly from humans in size, lifespan, physiology, anatomy, and diet, limiting their usefulness for some studies. Pigs are increasingly recognised as a valuable adjunct to pre-clinical research. Our aim is to provide a series of genetically defined pigs that model serious and common human cancers. These will allow new diagnostic and therapeutic strategies to be investigated at human scale, and longitudinal studies under conditions that mimic the human patient. We are thus engaged in a program of gene targeting to replicate in pigs a series of genetic lesions known to underlie human cancers. Here, we describe results from two key tumour suppressor genes: adenomatous polyposis coli (APC) and p53 (TP53). Somatic mutations resulting in inactivation or altered p53 function are present in most human cancers, and germline TP53 mutations are responsible for Li-Fraumeni multiple cancer syndrome. TP53^R175H is the most frequent missense mutation in many sporadic human cancers. We have created gene-targeted knockout pigs and pigs carrying a latent TP53^R167H mutant allele orthologous to human mutant TP53^R175H that can be activated by Cre recombination to model the occurrence of oncogenic mutant p53 in chosen tissues (Leuchs et al. 2012 PLoS One, in press). In vitro studies indicate that porcine TP53^R167H resembles human TP53^R175H in altered function, and homozygous knockout of porcine TP53 results in transformation of porcine MSCs. APC plays a vital initiating role in both sporadic colorectal cancer (CRC) and the inherited predisposition to colorectal cancer, familial adenomatous polyposis (FAP). We generated gene-targeted cloned pigs carrying two different nonsense mutations in APC (APC¹⁰⁶¹ and APC¹³¹¹) at sites orthologous to human germline mutations responsible for FAP. At 1 year of age, the APC¹³¹¹ mutation resulted in >100 lesions, including ~60 polyps, exclusively in the large intestine. Importantly, this accords with the location and onset of human FAP in early adulthood, and contrasts with equivalent mutations in mice where polyps develop in the small intestine. Histological and molecular analysis showed that the porcine model recapitulates all major features of early stage human FAP (Flisikowska et al. 2012 Gastroenterology, in press). Tumorigenesis involves multiple genetic alterations over time. It will now be possible to mimic this progression in pigs by combining these and other mutations. We are confident that pig models will make a significant contribution to human oncology.