BMP-SMAD Signaling Regulates Lineage Priming, but Is Dispensable for Self-Renewal in Mouse Embryonic Stem Cells

Naive mouse embryonic stem cells (mESCs) are in a metastable state and fluctuate between inner cell mass- and epiblast-like phenotypes. Here, we show transient activation of the BMP-SMAD signaling pathway in mESCs containing a BMP-SMAD responsive reporter transgene. Activation of the BMP-SMAD reporter transgene in naive mESCs correlated with lower levels of genomic DNA methylation, high expression of 5-methylcytosine hydroxylases Tet1/2 and low levels of DNA methyltransferases Dnmt3a/b. Moreover, naive mESCs, in which the BMP-SMAD reporter transgene was activated, showed higher resistance to differentiation. Using double Smad1;Smad5 knockout mESCs, we showed that BMP-SMAD signaling is dispensable for self-renewal in both naive and ground state. These mutant mESCs were still pluripotent, but they exhibited higher levels of DNA methylation than their wild-type counterparts and had a higher propensity to differentiate. We showed that BMP-SMAD signaling modulates lineage priming in mESCs, by transiently regulating the enzymatic machinery responsible for DNA methylation

A new conditional Apc-mutant mouse model for colorectal cancer

Mutations of the adenomatous polyposis coli (APC) gene predispose individuals to familial adenomatous polyposis (FAP), characterized by multiple tumours in the large intestine. Most mouse models heterozygous for truncating mutant Apc alleles mimic FAP, however, the intestinal tumours occur mainly in the small intestine. To model large intestinal tumours, we generated a new conditional Apc-mutant allele, Apc15lox, with exon 15 flanked by loxP sites. Similar survival of Apc1638N/15loxand Apc1638N/1mice indicated that the normal function of Apc was not impaired by the loxP sites. Deletion of exon 15, encoding nearly all functional Apc domains and containing the polyadenylation signal, resulted in a mutant allele expressing low levels of a 74 kDa truncated Apc protein. Germ line Cre-mediated deletion of exon 15 resulted in ApcD15/1mice, showing a severe ApcMin/1-like phenotype characterized by multiple tumours in the small intestine and early lethality. In contrast, conditional Cre-mediated deletion of exon 15 specifically directed to the epithelia of distal small and large intestine of FabplCre;Apc15lox/1mice led to longer survival and to tumours that developed predominantly in the large intestine, mimicking human FAP-associated colorectal cancer and sporadic colorectal cancer. We conclude that the FabplCre;Apc15lox/1mouse should be an attractive model for studies on prevention and treatment of colorectal cancer