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Several types of cancer are characterized by global hypomethylation accompanied by regional hypermethylation and overexpression of DNA methyltransferase (cytosine-5) 1 (Dnmt1). In addition to the established role of Dnmt1 as maintenance methyltransferase, it has been suggested that Dnmt1 might also methylate certain target sites de novo. We created a transgenic mouse model to investigate whether the overexpression of the somatic form of Dnmt1, Dnmt1s, is sufficient to cause erroneous methylation and disease. Because ubiquitous Dnmt1 overexpression has been reported to be embryonic lethal, we designed a CAG promoter-driven Cre-loxP conditional transgene containing a floxed EGFP sequence followed by the Dnmt1s coding sequence. The EGFP sequence is excised and transgenic Dnmt1s expression is activated at specific time points or in specific tissues depending on the Cre deleter strain used for cross-ins. Pronucleus injections with the Dnmt1s transgene construct resulted in six founder lines as verified by PCR, Southern blot and EGFP fluorescence. We performed cross-ins with a CMV-Cre deleter strain to clarify if ubiquitous overexpression of Dnmt1s alone causes the previously observed embryonic lethality. Unexpectedly, these cross-ins yielded viable recombined offspring that ubiquitously overexpressed Dnmt1 mRNA at tissue-dependent levels of up to 229-fold. However, the recombined offspring did not significantly overexpress Dnmt1 protein and showed no apparent signs of disease or pathological phenotype. Here, we describe the establishment of our Dnmt1s-transgenic mouse model and propose possibilities for the absence of transgenic protein.
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