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29 CXXC-Finger Protein 1 Regulates DNA Methyltransferase 1 Protein Expression and Activity.

Epigenetic regulation refers to changes in gene expression that occur without changes in the nucleotide sequence. Cytosine methylation and histone tail modifications are two epigenetic modifications that influence gene expression. Elucidating epigenetic mechanisms of gene regulation is becoming increasingly important as deregulation of epigenetic processes is observed in many diseases, including cancer. The CXXC1 gene encodes CXXC finger protein 1 (CFP1), a transcriptional activator that specifically binds unmethylated CpG dinucleotides. The specific binding activity of CFP1 makes it unique in that most CpG binding proteins bind methylated CpG dinucleotides and facilitate heterochromatin formation. CFP1 has recently been identified as a member of the mammalian SET1 histone H3 lysine 4 methyltransferase complex. Disruption of CXXC1 in mice results in an early embryonic lethal phenotype, therefore CXXC1−/− embryonic stem (ES) cells were isolated from CXXC1−/− blastocysts to further characterize the function of CFP1. Embryonic stem cells lacking expression of CFP1 exhibit multiple epigenetic modification defects including altered histone modifications and reduced global cytosine methylation. DNA methyltransferase 1 (Dnmt1) is the major source of DNA methyltransferase activity in mammalian cells and is responsible for copying methylation patterns during DNA replication. Dnmt1 protein level and methyltransferase activity are decreased by ∼50% in CXXC1−/− ES cells and are rescued by stable expression of murine CFP1. Dnmt1 transcript level is not reduced in CXXC1 −/− ES cells, however. Immunoprecipitation experiments revealed an interaction between CFP1 and Dnmt1 in vivo. Regulation of Dnmt1 protein level and activity by CFP1 is an exciting discovery in that it is the first example of reduced Dnmt1 protein without direct disruption of Dnmt1 gene function. The functional significance of this novel intersection of epigenetic regulatory proteins is the principle focus of ongoing experiments.