Expression of the CTCFL Gene during Mouse Embryogenesis Causes Growth Retardation, Postnatal Lethality, and Dysregulation of the Transforming Growth Factor β Pathway

BORIS/CTCFL epigenetically reprograms clustered CTCF binding sites into alternative transcriptional start sites

BACKGROUND

Pervasive usage of alternative promoters leads to the deregulation of gene expression in carcinogenesis and may drive the emergence of new genes in spermatogenesis. However, little is known regarding the mechanisms underpinning the activation of alternative promoters.

RESULTS

Here we describe how alternative cancer-testis-specific transcription is activated. We show that intergenic and intronic CTCF binding sites, which are transcriptionally inert in normal somatic cells, could be epigenetically reprogrammed into active de novo promoters in germ and cancer cells. BORIS/CTCFL, the testis-specific paralog of the ubiquitously expressed CTCF, triggers the epigenetic reprogramming of CTCF sites into units of active transcription. BORIS binding initiates the recruitment of the chromatin remodeling factor, SRCAP, followed by the replacement of H2A histone with H2A.Z, resulting in a more relaxed chromatin state in the nucleosomes flanking the CTCF binding sites. The relaxation of chromatin around CTCF binding sites facilitates the recruitment of multiple additional transcription factors, thereby activating transcription from a given binding site. We demonstrate that the epigenetically reprogrammed CTCF binding sites can drive the expression of cancer-testis genes, long noncoding RNAs, retro-pseudogenes, and dormant transposable elements.

CONCLUSIONS

Thus, BORIS functions as a transcription factor that epigenetically reprograms clustered CTCF binding sites into transcriptional start sites, promoting transcription from alternative promoters in both germ cells and cancer cells.

The function of brother of the regulator of imprinted sites in cancer development

As Douglas Hanahan and Robert Weinberg compiled, there are nine hallmarks of cancer that are conducive to cancer cell development and survival. Previous studies showed that brother of the regulator of imprinted sites (BORIS) might promote cancer progression through these aspects. The competition between BORIS and CCCTC-binding factor (CTCF), which is crucial in the formation of chromatin loops, affects the normal function of CTCF and leads to neoplasia and deformity. In addition, BORIS belongs to the cancer-testis antigen families, which are potential targets in cancer diagnosis and treatment. Herein, we discuss the function and mechanisms of BORIS, especially in cancer development.

CTCFL expression is associated with cerebral vascular abnormalities

CTCFL is expressed in testis, oocytes and embryonic stem cells, and is aberrantly expressed in malignant cells, and is classified as a cancer-testis gene. We have previously shown by using a tetracycline-inducible Ctcfl transgene that inappropriate expression of Ctcfl negatively impacts fetal development and causes early postnatal lethality in the mouse. The affected pups displayed severe vascular abnormalities and localized hemorrhages in the brain evocative of cerebral cavernous malformations (CCM) and arteriovenous malformations (AVM) in humans. Thus, we aim to analyze; a) the presence of CCM-related proteins CCM1/KRIT1, CCM2/malcavernin and CCM3/PDCD10 in Ctcfl transgenic animals and, b) whether there is CTCFL expression in human CCM and AVM tissues. Ctcfl transgenic animals exhibited increased CD31 expression in vascular areas of the dermis and periadnexal regions but no difference was observed for vWF and α-SMA expressions. CCM-related proteins CCM1/KRIT1, CCM2/malcavernin and CCM3/PDCD10 were aberrantly expressed in coronal sections of the head in transgenic animals. We also observed CTCFL expression in human CCMs and AVMs. The induced expression of CTCFL resulting in vascular brain malformations in mice combined with the presence of CTCFL in human vascular malformations provide new insights into the role of this gene in vascular development in humans.

BMP6/TAZ-Hippo signaling modulates angiogenesis and endothelial cell response to VEGF

The BMP/TGFβ-Smad, Notch and VEGF signaling guides formation of endothelial tip and stalk cells. However, the crosstalk of bone morphogenetic proteins (BMPs) and vascular endothelial growth factor receptor 2 (VEGFR2) signaling has remained largely unknown. We demonstrate that BMP family members regulate VEGFR2 and Notch signaling, and act via TAZ-Hippo signaling pathway. BMPs were found to be regulated after VEGF gene transfer in C57/Bl6 mice and in a porcine myocardial ischemia model. BMPs 2/4/6 were identified as endothelium-specific targets of VEGF. BMP2 modulated VEGF-mediated endothelial sprouting via Delta like Canonical Notch Ligand 4 (DLL4). BMP6 modulated VEGF signaling by regulating VEGFR2 expression and acted via Hippo signaling effector TAZ, known to regulate cell survival/proliferation, and to be dysregulated in cancer. In a matrigel plug assay in nude mice BMP6 was further demonstrated to induce angiogenesis. BMP6 is the first member of BMP family found to directly regulate both Hippo signaling and neovessel formation. It may thus serve as a target in pro/anti-angiogenic therapies.

Defining the relative and combined contribution of CTCF and CTCFL to genomic regulation

BACKGROUND

Ubiquitously expressed CTCF is involved in numerous cellular functions, such as organizing chromatin into TAD structures. In contrast, its paralog, CTCFL, is normally only present in the testis. However, it is also aberrantly expressed in many cancers. While it is known that shared and unique zinc finger sequences in CTCF and CTCFL enable CTCFL to bind competitively to a subset of CTCF binding sites as well as its own unique locations, the impact of CTCFL on chromosome organization and gene expression has not been comprehensively analyzed in the context of CTCF function. Using an inducible complementation system, we analyze the impact of expressing CTCFL and CTCF-CTCFL chimeric proteins in the presence or absence of endogenous CTCF to clarify the relative and combined contribution of CTCF and CTCFL to chromosome organization and transcription.

RESULTS

We demonstrate that the N terminus of CTCF interacts with cohesin which explains the requirement for convergent CTCF binding sites in loop formation. By analyzing CTCF and CTCFL binding in tandem, we identify phenotypically distinct sites with respect to motifs, targeting to promoter/intronic intergenic regions and chromatin folding. Finally, we reveal that the N, C, and zinc finger terminal domains play unique roles in targeting each paralog to distinct binding sites to regulate transcription, chromatin looping, and insulation.

CONCLUSION

This study clarifies the unique and combined contribution of CTCF and CTCFL to chromosome organization and transcription, with direct implications for understanding how their co-expression deregulates transcription in cancer.

BORIS/CTCFL promotes a switch from a proliferative towards an invasive phenotype in melanoma cells

Melanoma is among the most aggressive cancers due to its tendency to metastasize early. Phenotype switching between a proliferative and an invasive state has been suggested as a critical process for metastasis, though the mechanisms that regulate state transitions are complex and remain poorly understood. Brother of Regulator of Imprinted Sites (BORIS), also known as CCCTC binding factor-Like (CTCFL), is a transcriptional modulator that becomes aberrantly expressed in melanoma. Yet, the role of BORIS in melanoma remains elusive. Here, we show that BORIS is involved in melanoma phenotype switching. Genetic modification of BORIS expression in melanoma cells combined with whole-transcriptome analysis indicated that BORIS expression contributes to an invasion-associated transcriptome. In line with these findings, inducible BORIS overexpression in melanoma cells reduced proliferation and increased migration and invasion, demonstrating that the transcriptional switch is accompanied by a phenotypic switch. Mechanistically, we reveal that BORIS binds near the promoter of transforming growth factor-beta 1 (TFGB1), a well-recognized factor involved in the transition towards an invasive state, which coincided with increased expression of TGFB1. Overall, our study indicates a pro-invasive role for BORIS in melanoma via transcriptional reprogramming.

Targeting CTCFL/BORIS for the immunotherapy of cancer

Cancer vaccines have great potential in the fight against metastatic malignancies. Current anti-tumor immunotherapy is hindered by existing tolerance to tumor-associated antigens (TAA) and tumor escape using various mechanisms, highlighting the need for improved targets for immunotherapy. The cancer-testis antigen CTCFL/BORIS was discovered 16 years ago and possesses all features necessary for an ideal TAA. Recently CTCFL/BORIS has received additional attention as a target expressed in cancer stem cells (CSC). These cells drive tumor growth recurrence, metastasis, and treatment resistance. CTCFL/BORIS silencing leads to senescence and death of CSC. Therefore, an immunotherapeutic strategy that targets CTCFL/BORIS may lead to the selective destruction of CSC and potential eradication of metastatic disease. The high immunotherapeutic potential of CTCFL/BORIS antigen was shown in a stringent 4T1 mouse model of breast cancer. Using these highly metastatic, poorly immunogenic carcinoma cells inoculated into T-helper2 prone mice, we showed that DC fed with recombinant CTCFL/BORIS as an immunogen inhibited tumor growth and reduced the number of metastases in distant organs. About 20% of CTCFL/BORIS immunized animals were tumor free. 50% of animals remained metastasis free. Those having metastasis showed at least tenfold fewer metastases compared to controls. In a rat model of breast cancer, we showed that alphavirus-based CTCFL/BORIS immunotherapy was capable of cancer elimination as we were able to cure 50% of animals. Based on the above data, we believe that translation of CTCFL/BORIS-targeting immunotherapeutic strategies to the clinic will provide new avenues for improving survival of breast cancer patients with advanced metastatic disease.

Molecular mechanisms of intrauterine growth restriction

Intrauterine growth restriction (IUGR) is a pregnancy specific disease characterized by decreased growth rate of fetus than the normal growth potential at particular gestational age. In the current scenario it is a leading cause of fetal and neonatal morbidity and mortality. In the last decade exhilarating experimental studies from several laboratories have provided fascinating proof for comprehension of molecular basis of IUGR. Atypical expression of enzymes governed by TGFβ causes the placental apoptosis and altered expression of TGFβ due to hyper alimentation causes impairment of lung function. Crosstalk of cAMP with protein kinases plays a prominent role in the regulation of cortisol levels. Increasing levels of NOD1 proteins leads to development of IUGR by increasing the levels of inflammatory mediators. Increase in leptin synthesis in placental trophoblast cells is associated with IUGR. In this review, we emphasize on the regulatory mechanisms of IUGR and its associated diseases. They may help improve the in-utero fetal growth and provide a better therapeutic intervention for prevention and treatment of IUGR.

Cancer/testis antigens as molecular drug targets using network pharmacology

Present chemotherapeutic drugs have limited efficacy and severe side effects. Considering the complexity of cancer, an effective strategy is necessary to discover multiple new drug targets. Cancer/testis antigens are vital for cancer cell progression. We have performed a computational network analysis of cancer/testis antigens and assessed these antigens as drug targets. During this analysis, protein interaction network of 700 human CT antigens was investigated. CT antigen network consisted of eight independent components. Four major hubs and two minor hubs were identified that play nodal role in the flow of information across the largest network. We have predicted 30 potential drug targets by analysing several topological parameters such as betweenness centrality, cluster coefficient and probable protein complexes. Structural and functional roles of potential drug targets have also been anatomized. Analysis of the CT antigen network enables us to pinpoint a set of candidate proteins that if targeted could be detrimental for cancerous cell without affecting any normal cell. The list of putative proteins is a starting point for experimental validation and may help further in the discovery of new anticancer drug targets.