Alternative titles; symbols
HGNC Approved Gene Symbol: CXXC1
Cytogenetic location: 18q21.1 Genomic coordinates (GRCh38) : 18:50,282,347-50,287,692 (from NCBI)
Proteins that contain a CXXC motif within their DNA-binding domain, such as CXXC1, recognize CpG sequences and regulate gene expression (Carlone and Skalnik, 2001).
By screening a myeloid cell cDNA library with a CpG motif probe, followed by EST database analysis, Voo et al. (2000) cloned full-length CXXC1, which they called CGBP. The deduced 656-amino acid protein has a calculated molecular mass of 76 kD. CGBP contains an N-terminal PHD domain, followed by a CXXC domain, a central basic region, a coiled-coil domain, and an imperfect PHD domain near its C terminus. Northern blot analysis detected a 2.6-kb CGBP transcript in all tissues examined. Western blot analysis of nuclear extracts of a myeloid cell line detected endogenous CGBP at an apparent molecular mass of 88 kD.
Using tandem repeats of the upstream element of the collagenase gene (120353) in a 1-hybrid screen of a fibroblast cDNA library, Fujino et al. (2000) cloned CXXC1, which they called PCCX1. They identified an acidic region just prior to the basic region in CXXC1, as well as 2 potential PEST sequences and several sequences similar to the nuclear localization signal.
Using immunofluorescence staining, Tian et al. (2018) found that Cxxc1 was present in sperm cell nuclei during all meiosis stages until decreasing to undetectable levels in spermatids.
Voo et al. (2000) demonstrated that CGBP bound a probe containing a CpG motif. CGBP failed to bind oligonucleotides in which the CpG motif was mutated or methylated, and it did not bind single-stranded DNA or RNA probes. Introduction of a CpG dinucleotide into an unrelated oligonucleotide sequence was sufficient to produce a binding site for CGBP. Electrophoretic mobility shift assays detected DNA-binding activity by endogenous CGBP. CGBP showed transactivation activity in a reporter gene assay using promoters containing CpG motifs.
Fujino et al. (2000) found that the acidic region of CXXC1 showed high transactivation ability, but the full-length protein was inactive due to regions that inhibited the acidic region, including the C-terminal region. They found that CXXC1 was expressed constitutively through all stages of cellular aging and immortalization, but at the protein level, a shorter form lacking the C-terminal region appeared as the cells approached crisis. Fujino et al. (2000) concluded that CXXC1 is activated by proteolytic cleavage that removes the C-terminal inhibitory region.
Lee and Skalnik (2002) found that endogenous CGBP localized to nuclear speckles in human and mouse cell lines. The speckles were concentrated in areas of DAPI light staining, consistent with localization of CGBP to euchromatic regions of the nucleus. CGBP was almost exclusively associated with the nuclear matrix fraction of fractionated cells. Mutation analysis indicated that multiple regions within the acidic, basic, and coiled-coil domains of the protein contribute cooperative signals that lead to a speckled nuclear distribution.
By immunoprecipitation and mass spectrometry, Lee and Skalnik (2005) determined that CXXC1, which they called CFP1, is a component of the mammalian Set1 histone H3-Lys4 methyltransferase complex (see 611052). Confocal microscopy revealed that CXXC1 and Set1 colocalize to nuclear speckles associated with euchromatin. Mouse embryonic stem cells lacking CXXC1 exhibited a 4-fold excess of histone H3-K4 methylation following induction of differentiation, indicating that CXXC1 restricts the activity of the Set1 histone methyltransferase. Lee and Skalnik (2005) suggested that CXXC1 is a critical epigenetic regulator of histone modification in addition to cytosine methylation.
By searching for proteins that are common to all CpG islands, Thomson et al. (2010) found high enrichment for Cfp1, which selectively binds to nonmethylated CpGs in vitro. Chromatin immunoprecipitation of a monoallelically methylated CpG island confirmed that Cfp1 specifically associates with nonmethylated CpG sites in vivo. High-throughput sequencing of Cfp1-bound chromatin identified a notable concordance with nonmethylated CpG islands and sites of H3K4me3 in mouse brain. Levels of H3K4me3 at CpG islands were markedly reduced in Cfp1-depleted cells, consistent with the finding that Cfp1 associates with the H3K4 methyltransferase Setd1 (see 611052). To test whether nonmethylated CpG-dense sequences are sufficient to establish domains of H3K4me3, Thomson et al. (2010) analyzed artificial CpG clusters that were integrated into the mouse genome. Despite the absence of promoters, the insertions recruited Cfp1 and created new peaks of H3K4me3. The data of Thomson et al. (2010) indicated that a primary function of nonmethylated CpG islands is to genetically influence the local chromatin modification state by interaction with Cfp1 and perhaps other CpG-finding proteins.
Using coimmunoprecipitation, Tian et al. (2018) showed that Cxxc1 interacted with Prdm9 (609760) and bound to trimethylated histone-3 (see 602810) lysine-4 (H3K4me3) in mouse spermatocytes.
By genomic sequence analysis, Fujino et al. (2000) mapped the CXXC1 gene to chromosome 18q21.1. Using Southern blot analysis, Voo et al. (2000) determined that CXXC1 is a single-copy gene.
By homologous recombination, Carlone and Skalnik (2001) created mice lacking Cgbp. No viable Cgbp null animals were obtained, and no Cgbp null embryos were obtained between 6.5 and 12.5 days postcoitum (dpc). At 6.5 dpc, about one-fourth of the implantation sites appeared empty with no intact embryos. Cgbp null blastocysts were viable and capable of hatching and forming both an inner cell mass and a trophectoderm. Carlone and Skalnik (2001) concluded that CGBP is crucial for periimplantation development.
Tian et al. (2018) found that male mice with germ cell-specific Cxxc1 knockout were fertile with normal testis size, no detectable spermatogenesis defects, and no increased apoptosis in germ cells. In contrast, Cxxc1 germ cell-specific knockout female mice were sterile despite normal ovary morphology and follicle formation. Further investigation revealed that even though Cxxc1 interacted with Prdm9 and H3K4me3 in spermatocytes, it was not essential for meiotic recombination in mice. Cxxc1 was not required for Prdm9 binding at hotspots, their subsequent activation by Prdm9-dependent H3K4 trimethylation, or DNA double-strand break formation, repair, or crossover formation.
Carlone, D. L., Skalnik, D. G. CpG binding protein is crucial for early embryonic development. Molec. Cell. Biol. 21: 7601-7606, 2001. [PubMed: 11604496] [Full Text: https://doi.org/10.1128/MCB.21.22.7601-7606.2001]
Fujino, T., Hasegawa, M., Shibata, S., Kishimoto, T., Imai, S., Takano, T. PCCX1, a novel DNA-binding protein with PHD finger and CXXC domain, is regulated by proteolysis. Biochem. Biophys. Res. Commun. 271: 305-310, 2000. [PubMed: 10799292] [Full Text: https://doi.org/10.1006/bbrc.2000.2614]
Lee, J.-H., Skalnik, D. G. CpG-binding protein is a nuclear matrix- and euchromatin-associated protein localized to nuclear speckles containing human trithorax: identification of nuclear matrix targeting signals. J. Biol. Chem. 277: 42259-42267, 2002. [PubMed: 12200428] [Full Text: https://doi.org/10.1074/jbc.M205054200]
Lee, J.-H., Skalnik, D. G. CpG-binding protein (CXXC finger protein 1) is a component of the mammalian Set1 histone H3-Lys4 methyltransferase complex, the analogue of the yeast Set1/COMPASS complex. J. Biol. Chem. 280: 41725-41731, 2005. [PubMed: 16253997] [Full Text: https://doi.org/10.1074/jbc.M508312200]
Thomson, J. P., Skene, P. J., Selfridge, J., Clouaire, T., Guy, J., Webb, S., Kerr, A. R. W., Deaton, A., Andrews, R., James, K. D., Turner, D. J., Illingworth, R., Bird, A. CpG islands influence chromatin structure via the CpG-binding protein Cfp1. Nature 464: 1082-1086, 2010. [PubMed: 20393567] [Full Text: https://doi.org/10.1038/nature08924]
Tian, H., Billings, T., Petkov, P. M. CXXC1 is not essential for normal DNA double-strand break formation and meiotic recombination in mouse. PLoS Genet. 14: e1007657, 2018. Note: Electronic Article. [PubMed: 30365547] [Full Text: https://doi.org/10.1371/journal.pgen.1007657]
Voo, K. S., Carlone, D. L., Jacobsen, B. M., Flodin, A., Skalnik, D. G. Cloning of a mammalian transcriptional activator that binds unmethylated CpG motifs and shares a CXXC domain with DNA methyltransferase, human trithorax, and methyl-CpG binding domain protein 1. Molec. Cell. Biol. 20: 2108-2121, 2000. [PubMed: 10688657] [Full Text: https://doi.org/10.1128/MCB.20.6.2108-2121.2000]