The first CTCF patent issued in 1999 described full length cDNAs coding for exceptionally conserved gene ubiquitusly expressed in somatic cells from a wide range of the evolutionary diverged species, including Drosophila, birds, mice, and humans. CTCF is unique multifunctional nuclear factor with multiple sequence-specificity which controls global genome architecture and many aspects of allele-specific epigenetic regulation including direct repression and/or activation of transcription, pausing-associated splicing, replication timing, long-range enhancer-promoter communications and hormone-induced silencing. Initially demonstrated by us (MCB 2004) in vitro dimerization between distinct CTCF/DNA complexes was recently shown to mediate universal site-specific intergenic, intra-chromosomal, and inter-chromosomal linking-looping in vivo through interactions between distinct classes of regulatory elements containing varying DNA sequences of CTCF-target-sites (CTSes). As we discovered CTCF may act through two major mechanisms: either direct regulation of a gene downstream of CTSes or indirect regulation via the formation of chromatin loops stabilized by CTCF dimerization that affects allele-specific relationships between the promoter, enhancer, and/or an imprinted control region (ICR) on either same or different chromosome. Dimerization of DNA-bound CTCF may potentially be at the core of its activity as a versatile chromatin-bridging and chromatin-looping agent, underlying its fundamental biological functions. The loop-forming activity of CTCF can be naturally extended to include formation of localized somatic inter-chromosome pairing sites that may acquire potential for epigenetic co-regulation through allele-specific transcription factories, DNA replication factories, 5-mC/5-hmC excisions, and DNA repair foci. Many other chromatin-anchored functions, such as the establishment of imprinting marks and their reading, X-chromosome inactivation, and apoptosis, are also regulated by CTCF. CTCF has emerged as a key facilitator of 3D organization of interphase chromatin, as well as a major player in cell proliferation control. In some cases, the loop-forming activity of CTCF was found to be accompanied/complemented by the more direct regulation of a particular gene. This mixed mode regulation is likely the most appropriate representation of a native gene regulation framework. In addition to phosphorylation and poly(ADP)rybozylation of CTCF, we also identified first CTCF-interacting partner proteins, Sin3A and YB-1, which have recently gained our increased attention in course of ongoing interpretation of the NGS data generated by MPS (in collaboration with Dr. B. Ren) with regard to the genomewide CTCF and CTCFL occupancy, Pol 2 initiation, transcriptional pausing coupled with splicing, and rewiring regulatory elements of CTCFL positive cancer cells by mobile retrotransposones with variable length tandem repeats affected often by a gain of CTCF site specific SNPs. Moreover, we expanded our previous studies of important CTCF activity that directly links CTCF to transcriptional machinery: the binding of CTCF to the Pol II. This novel pathway is sensitive to external signals that affect post-translational modifications of critical CTCF amino acids and provides either a mechanism for opening loop-independent transcription start sites downstream of the promoter-determined +1 site (at intron/exon sequences)and it may have specifically evolved to induce non-coding transcripts throughout the genome depending on the presence of BORIS in a particular cell type under study. Mechanistically, the regulated recruitment and the subsequent release of Pol II from a DNA-bound CTCF complex indicates that the CTCF site itself could act as an attenuator and/or promoter in some locations in the genome. At imprinted genes, CTCF likely works together with BORIS (Brother of the Regulator of Immmprinted Sites), the testis- and cancer-specific CTCF paralog, which we discovered and characterized. We continued previously initiated (Cell 2007) genomewide mapping of CTCF targets by using a mix of our 9 CTCF Mabs for ChIP Seq (Nature, 2011;2012) to better our understanding of the fundamental CTCF roles in spatiotemporal coordination of major cellular functions. By virtue of having so many vital functions CTCF became an essential gene in vertebrates, as CTCF KO mice are non-viable, and early lethality occurs at the very early embryonic stages (PloS One, 2011). With respect to human disease, CTCF is a candidate tumor suppressor gene (TSG);several functional point mutations in the 11ZF DBD of CTCF have been characterized in primary cancers, in combination with the LOH of the CTCF locus. In the past year, we studied several genes and their associated regulatory sequences in an effort to elucidate the contributions of CTCF and CTCF binding sites to the regulation of gene expression. These studies included genes important for immune responses, mono-allelic multigene families of sensory receptors, as well as genes with a potential for a breakthrough in the development of new approaches for cancer treatment. Unlike somatic cells, testicular germ cells undergo meiosis rather that mitosis, - whereby CTCF is normally likely to work together with BORIS. Our recent results have identified BORIS as an anti-silencing component of undifferentiated ES and cancer cells that directly interacts with CTCF. We found earlier that functionally important dimerization between two different CTCF/DNA complexes is based on the capability of CTCF to interact with itself that requires Zn-fingers (MCB, 2004), and showed that literally same fingers are uniquely duplicated and preserved in evolution of mouse and human BORIS proteins (PNAS, 2002). Since formation of homodimeric CTCF/CTCF complexes on DNA underlie site-specific long-range interactions that serve for chromatin linking-folding in normal somatic cells (which do not express BORIS), we expect that formation of heterodimeric CTCF/BORIS complexes observed in chromatin of germ/stem and in cancer cells is likely to have many important structure-functional implications for understanding functional consequences of abnormal CTCF/BORIS dimerization, including an altered chromatin packaging mode normally driven by CTCF alone. While CTCF is mostly known as a regulator of gene expression, our data point to its potential functions in nuclear and nucleolar compartmentalization and heterochromatinization, as well as in in formation of centrosomes. We have also characterized an unusual form of CTCF protein in condensed mitotic chromosomes pointing to its roles in mitosis and meiosis, and thereby suggesting a significant housekeeping role of CTCF in spatiotemporal coordination of genome organization and chromosome segregation in dividing diploid and haploid cells. CTCF was previously shown to undergo a variety of post-translational modifications and we expanded these studies to characterize novel modifications. Another pathological aspect of the misregulated CTCF occupancy on promoter target sites is aberrant alteration of DNA methylation pattern at CTCF sites that can loose protection by DNA-bound CTCF in cancer. Both of these novel biological roles of CTCF are subjects of ongoing studies in the MPS. We also have obtained direct evidence that at least one role of BORIS is to facilitate DNA re-methylation and Pol 2 recruitment at certain intergenic CTS-sequences found in intrones (MCB 20011) and in many promoters of cancer-testis genes (JBC 2011).
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