Trp53 null and R270H mutant alleles have comparable effects in regulating invasion, metastasis, and gene expression in mouse colon tumorigenesis.

Somatic APC (adenomatous polyposis coli), TP53, KRAS mutations are present in roughly 80%, 60%, and 40%, respectively, of human colorectal cancers (CRCs). Most TP53 mutant alleles in CRCs encode missense mutant proteins with loss-of-function (LOF) of p53's transcriptional activity and dominant negative (DN) effects on wild-type p53 function. Missense mutant p53 proteins have been reported to exert gain-of-function (GOF) effects in cancer. We compared the phenotypic effects of the common human cancer-associated TP53 R273H missense mutation to p53 null status in a genetically engineered mouse CRC model. Inactivation of one allele of Apc together with activation of a Kras mutant allele in mouse colon epithelium instigated development of serrated and hyperplastic epithelium and adenomas (AK mice). Addition of a Trp53R270H or Trp53null mutant allele to the model (AKP mice) led to markedly shortened survival and increased tumor burden relative to that of AK mice, including adenocarcinomas in AKP mice. Comparable life span and tumor burden were seen in AKP mice carrying Trp53R270H or Trp53null alleles, along with similar frequencies of spontaneous metastasis to lymph nodes, lung, and liver. The fraction of adenocarcinomas with submucosa or deeper invasion was higher in AKP270/fl mice than in AKPfl/fl mice, but the incidence of adenocarcinomas per mouse did not differ significantly between AKPfl/fl and AKP270/fl mice. In line with their comparable biological behaviors, mouse primary tumors and tumor-derived organoids with the Trp53R270H or Trp53null alleles had highly similar gene expression profiles. Human CRCs with TP53 R273 missense mutant or null alleles also had essentially homogeneous gene expression patterns. Our findings indicate the R270H/R273H p53 mutant protein does not manifest definite GOF biological effects in mouse and human CRCs, suggesting possible GOF effects of mutant p53 in cancer phenotypes are likely allele-specific and/or context-dependent.

BRAFV600E cooperates with CDX2 inactivation to promote serrated colorectal tumorigenesis.

While 20-30% of colorectal cancers (CRCs) may arise from precursors with serrated glands, only 8-10% of CRCs manifest serrated morphology at diagnosis. Markers for distinguishing CRCs arising from 'serrated' versus 'conventional adenoma' precursors are lacking. We studied 36 human serrated CRCs and found CDX2 loss or BRAF mutations in ~60% of cases and often together (p=0.04). CDX2Null/BRAFV600E expression in adult mouse intestinal epithelium led to serrated morphology tumors (including carcinomas) and BRAFV600E potently interacted with CDX2 silencing to alter gene expression. Like human serrated lesions, CDX2Null/BRAFV600E-mutant epithelium expressed gastric markers. Organoids from CDX2Null/BRAFV600E-mutant colon epithelium showed serrated features, and partially recapitulated the gene expression pattern in mouse colon tissues. We present a novel mouse tumor model based on signature defects seen in many human serrated CRCs - CDX2 loss and BRAFV600E. The mouse intestinal tumors show significant phenotypic similarities to human serrated CRCs and inform about serrated CRC pathogenesis.