See figure 1.

76776 bp gene (Ensembl).

Ubiquitously highly expressed gene (GeneCards), 12 exons, 11 introns with at least 5 differentially spliced transcripts (Ensembl).

No.

CTCF was originally described as a c-myc activator (Klenova et al., 1993). It is a 727 aa protein with a MW of 82.8 kD, a charge of 8.5 and an iso electric point of 6.95 (Ensembl). The central domain with 11 zinc fingers of the C2H2 type is highly conserved.

CTCF is an abundant and ubiquitously expressed protein, yet absent in primary spermatocytes (Loukinov et al., 2002). It is downregulated during differentiation of human myeloid leukemia cells (Delgado et al., 1999; Torrano et al., 2005). Post-traductional modifications include acetylation (Choudhary et al., 2009), sumoylation (Kitchen and Schoenherr, 2010; MacPherson et al., 2009), which is regulated by hypoxic stress (Wang et al., 2012a), phosphorylation, in particular ser604-612 by CKII (El-Kady and Klenova, 2005; Klenova et al., 2001), and poly(ADPribo)sylation (see figure 2). The latter modification is lost or decreased in proliferating cells and in BT (Docquier et al., 2009) (for sites and role see Farrar et al., 2010; Yu et al., 2004; Guastafierro et al., 2013). CTCF is a downstream target protein of growth factor-induced pathways and is regulated by EGF and insulin through activation of ERK and AKT signaling cascades (Gao et al., 2007). It was recently shown to be regulated by NF-kB (Lu et al., 2010).

CTCF is localized in the nucleoplasm of proliferating cells with exclusion from the nucleolus. It was detected at the centrosomes and midbody during mitosis (Zhang et al., 2004). It is associated with the nuclear matrix (Dunn et al., 2003; Yusufzai and Felsenfeld, 2004) and the Lamina (Guelen et al., 2008; Ottaviani et al., 2009). Nucleolar translocation after growth arrest is accompanied by inhibition of nucleolar transcription (Torrano et al., 2006). Cytoplasmic expression was described in sporadic breast tumors (Rakha et al., 2004).

CTCF is an essential protein, since KO mice die before ED 9.5 (Heath et al., 2008) (reviewed in Filippova, 2008; Phillips and Corces, 2009). It interacts with numerous ubiquitous and cell-type specific genomic sites (Chen et al., 2008; Bao et al., 2008; Barski et al., 2007; Kim et al., 2007; Chen et al., 2012). The 11 Zn fingers would provide flexibility in DNA recognition (Filippova et al., 1996), the central 4 bind to a consensus DNA sequence (Filippova et al., 1998; Renda et al., 2007). Multiple interacting proteins were described including RNA polymerase II (Chernukhin et al., 2007), cohesin (Parelho et al., 2008; Rubio et al., 2008; Wendt et al., 2008; Xiao et al., 2011), Suz12 (Li et al., 2008), CHD8 (Ishihara et al., 2006), YY1 (Donohoe et al., 2007), nucleophosmin (Yusufzai et al., 2004), Kaiso (Defossez et al., 2005) and Sin3A (Lutz et al., 2000). XPG endonuclease promotes DNA breaks and DNA demethylation at promoters allowing the recruitment of CTCF and gene looping, which is further stabilized by XPF (Le May et al., 2012). By mediating intra and interchromosomal contacts through its interaction with cohesin, CTCF plays a central role in organization of topological domains inside the nucleus. Cell-type specific binding sites lead to specific interactomes and transcriptional programs (Hou et al., 2010; Handoko et al., 2011; Dixon et al., 2012; Botta et al., 2010; reviewed by Merkenschlager and Odom, 2013). The plasticity in binding sites occupancy is linked to DNA methylation (Wang et al., 2012b) and could depend also on CTCF interaction with other factors (see concept of modular insulators in Weth et al., 2010). One thoroughly studied factor is the thyroid receptor (Awad et al., 1999; Lutz et al., 2003). Its peculiar chromosomal environment could explain the multiple (not necessarily exclusive) functions that were described for CTCF, including chromatin barrier (Cuddapah et al., 2009; Witcher and Emerson, 2009), promoter insulation from enhancer (Bell et al., 1999) or silencer (Hou et al., 2008), transcriptional activation (Gombert and Krumm, 2009) (for instance of the tumour suppressor genes INK4A/ARF (Rodriguez et al., 2010) and p53 (Soto-Reyes and Recillas-Targa, 2010)), repression (for instance hTERT (Renaud et al., 2005)), nucleosome positioning (Fu et al., 2008b), protection from DNA methylation (Mukhopadhyay et al., 2004; Schoenherr et al., 2003; Guastafierro et al., 2008), preservation of triplet-repeat stability (Cho et al., 2005; Filippova et al., 2001; Libby et al., 2008), imprinting (Fedoriw et al., 2004; Fitzpatrick et al., 2007), X chromosome inactivation (Chao et al., 2002), chromosome "kissing" (Ling et al., 2006), transvection (Liu et al., 2008), death signaling (Docquier et al., 2005; Gomes and Espinosa, 2010; Li and Lu, 2007), replication timing (Bergstrom et al., 2007), mitotic bookmarking (Burke et al., 2005), MHC class II gene expression (Majumder et al., 2008), V(D)J recombination (Guo et al., 2011; reviewed by Chaumeil and Skok, 2012), miRNA expression (Saito and Saito, 2012), telomere end protection (Deng et al., 2012), neuronal diversity (Monahan et al., 2012), myogenesis (Delgado-Olguin et al., 2011), splicing (Shukla et al., 2011) and angiogenesis (Tang et al., 2011). Considering the central role of CTCF in transcriptional regulation, it is likely to play a role in adaptive evolution in Drosophila (Ni et al., 2012), and in the evolutionary succes of bilateria (Heger et al., 2012). Remodeling of CTCF binding sites and the accompanying interactome during evolution could be driven by retrotransposon expansion (Schmidt et al., 2012).

49 orthologues were described including D. melanogaster (Smith et al., 2009) and C. elegans proteins (Moon et al., 2005), 3 paralogues: CTCFL or BORIS, originating from a gene duplication in reptiles (Hore et al., 2008; Loukinov et al., 2002), and possibly ZFP64 (Mack et al., 1997) and the Histone H4 transcription factor HINF-P (van Wijnen et al., 1991).

SNP at AA 630 /K /E 90 /D /G 447 fR (NCBI).

Non-coding mutations only.

Mutations are rare and include point mutations of Zn-fingers in breast (BT) (Aulmann et al., 2003), prostate (PT) and Wilms tumor (WT) (Filippova et al., 2002; Tiffen et al., 2013), insertion in BT (Aulmann et al., 2003) (see figure 2), and indels in AML (Dolnik et al., 2012).