Sabine Stolzenburg

Epigenetic reprogramming of cancer cells via targeted DNA methylation

An obstacle in the treatment of human diseases such as cancer is the inability to selectively and effectively target historically undruggable targets such as transcription factors. Here, we employ a novel technology using artificial transcription factors (ATFs) to epigenetically target gene expression in cancer cells. We show that site-specific DNA methylation and long-term stable repression of the tumor suppressor Maspin and the oncogene SOX2 can be achieved in breast cancer cells via zinc-finger ATFs targeting DNA methyltransferase 3a (DNMT3a) to the promoters of these genes. Using this approach, we show Maspin and SOX2 downregulation is more significant as compared with transient knockdown, which is also accompanied by stable phenotypic reprogramming of the cancer cell. These findings indicate that multimodular Zinc Finger Proteins linked to epigenetic editing domains can be used as novel cell resources to selectively and heritably alter gene expression patterns to stably reprogram cell fate.

Targeted silencing of the oncogenic transcription factor SOX2 in breast cancer

The transcription factor (TF) SOX2 is essential for the maintenance of pluripotency and self-renewal in embryonic stem cells. In addition to its normal stem cell function, SOX2 over-expression is associated with cancer development. The ability to selectively target this and other oncogenic TFs in cells, however, remains a significant challenge due to the ‘undruggable’ characteristics of these molecules. Here, we employ a zinc finger (ZF)-based artificial TF (ATF) approach to selectively suppress SOX2 gene expression in cancer cells. We engineered four different proteins each composed of 6ZF arrays designed to bind 18 bp sites in the SOX2 promoter and enhancer region, which controls SOX2 methylation. The 6ZF domains were linked to the Kruppel Associated Box (SKD) repressor domain. Three engineered proteins were able to bind their endogenous target sites and effectively suppress SOX2 expression (up to 95% repression efficiencies) in breast cancer cells. Targeted down-regulation of SOX2 expression resulted in decreased tumor cell proliferation and colony formation in these cells. Furthermore, induced expression of an ATF in a mouse model inhibited breast cancer cell growth. Collectively, these findings demonstrate the effectiveness and therapeutic potential of engineered ATFs to mediate potent and long-lasting down-regulation of oncogenic TF expression in cancer cells.

Stable oncogenic silencing in vivo by programmable and targeted de novo DNA methylation in breast cancer

With the recent comprehensive mapping of cancer genomes, there is now a need for functional approaches to edit the aberrant epigenetic state of key cancer drivers to reprogram the epi-pathology of the disease. In this study we utilized a programmable DNA-binding methyltransferase to induce targeted incorporation of DNA methylation (DNAme) in the SOX2 oncogene in breast cancer through a six zinc finger (ZF) protein linked to DNA methyltransferase 3A (ZF-DNMT3A). We demonstrated long-lasting oncogenic repression, which was maintained even after suppression of ZF-DNMT3A expression in tumor cells. The de novo DNAme was faithfully propagated and maintained through cell generations even after the suppression of the expression of the chimeric methyltransferase in the tumor cells. Xenograft studies in NUDE mice demonstrated stable SOX2 repression and long-term breast tumor growth inhibition, which lasted for >100 days post implantation of the tumor cells in mice. This was accompanied with a faithful maintenance of DNAme in the breast cancer implants. In contrast, downregulation of SOX2 by ZF domains engineered with the Krueppel-associated box repressor domain resulted in a transient and reversible suppression of oncogenic gene expression. Our results indicated that targeted de novo DNAme of the SOX2 oncogenic promoter was sufficient to induce long-lasting epigenetic silencing, which was not only maintained during cell division but also significantly delayed the tumorigenic phenotype of cancer cells in vivo, even in the absence of treatment. Here, we outline a genome-based targeting approach to long-lasting tumor growth inhibition with potential applicability to many other oncogenic drivers that are currently refractory to drug design.

Analysis of an artificial zinc finger epigenetic modulator: widespread binding but limited regulation

Artificial transcription factors (ATFs) and genomic nucleases based on a DNA binding platform consisting of multiple zinc finger domains are currently being developed for clinical applications. However, no genome-wide investigations into their binding specificity have been performed. We have created six-finger ATFs to target two different 18 nt regions of the human SOX2 promoter; each ATF is constructed such that it contains or lacks a super KRAB domain (SKD) that interacts with a complex containing repressive histone methyltransferases. ChIP-seq analysis of the effector-free ATFs in MCF7 breast cancer cells identified thousands of binding sites, mostly in promoter regions; the addition of an SKD domain increased the number of binding sites ∼5-fold, with a majority of the new sites located outside of promoters. De novo motif analyses suggest that the lack of binding specificity is due to subsets of the finger domains being used for genomic interactions. Although the ATFs display widespread binding, few genes showed expression differences; genes repressed by the ATF-SKD have stronger binding sites and are more enriched for a 12 nt motif. Interestingly, epigenetic analyses indicate that the transcriptional repression caused by the ATF-SKD is not due to changes in active histone modifications.

Bidirectional modulation of endogenous EpCAM expression to unravel its function in ovarian cancer

Background:The epithelial cell adhesion molecule (EpCAM) is overexpressed on most carcinomas. Dependent on the tumour type, its overexpression is either associated with improved or worse patient survival. For ovarian cancer, however, the role of EpCAM remains unclear.Methods:Cell survival of ovarian cancer cell lines was studied after induction or repression of endogenous EpCAM expression using siRNA/cDNA or artificial transcription factors (ATF) consisting of engineered zinc-fingers fused to either a transcriptional activator or repressor domain.Results:Two ATFs were selected as the most potent down- and upregulator, showing at least a two-fold alteration of EpCAM protein expression compared with control. Downregulation of EpCAM expression resulted in growth inhibition in breast cancer, but showed no effect on cell growth in ovarian cancer. Induction or further upregulation of EpCAM expression decreased ovarian cancer cell survival.Conclusion:The bidirectional ATF-based approach is uniquely suited to study cell-type-specific biological effects of EpCAM expression. Using this approach, the oncogenic function of EpCAM in breast cancer was confirmed. Despite its value as a diagnostic marker and for immunotherapy, EpCAM does not seem to represent a therapeutic target for gene expression silencing in ovarian cancer.

Breaking through an epigenetic wall: Re-activation of Oct4 by KRAB-containing designer zinc finger transcription factors

The gene Oct4 encodes a transcription factor critical for the maintenance of pluripotency and self-renewal in embryonic stem cells. In addition, improper re-activation of Oct4 contributes to oncogenic processes. Herein, we describe a novel designer zinc finger protein (ZFP) capable of upregulating the endogenous Oct4 promoter in a panel of breast and ovarian cell lines carrying a silenced gene. In some ovarian tumor lines, the ZFP triggered a strong reactivation of Oct4, with levels of expression comparable with exogenous Oct4 cDNA delivery. Surprisingly, the reactivation of Oct4 required a KRAB domain for effective upregulation of the endogenous gene. While KRAB-containing ZFPs are traditionally described as transcriptional repressors, our results suggest that these proteins could, in certain genomic contexts, function as potent activators and, thus, outline an emerging novel function of KRAB-ZFPs. In addition, we document a novel ZFP that could be used for the epigenetic reprograming of cancer cells.

Modulation of Gene Expression Using Zinc Finger-Based Artificial Transcription Factors

Artificial transcription factors (ATFs) consist of a transcriptional effector domain fused to a DNA-binding domain such as an engineered zinc finger protein (ZFP). Depending on the effector domain, ATFs can up- or downregulate gene expression and thus represent powerful tools in biomedical research and allow novel approaches in clinical practice. Here, we describe the construction of ATFs directed against the promoter of the epithelial cell adhesion molecule and against the promoter of the RNA component of telomerase. Methods to assess DNA binding of the engineered ZFP as well as to determine and improve the cellular effect of ATFs on (endogenous) promoter activity are described.

Frequent lack of repressive capacity of promoter DNA methylation identified through genome-wide epigenomic manipulation

It is widely assumed that the addition of DNA methylation at CpG rich gene promoters silences gene transcription. However, this conclusion is largely drawn from the observation that promoter DNA methylation inversely correlates with gene expression in natural conditions. The effect of induced DNA methylation on endogenous promoters has yet to be comprehensively assessed. Here, we induced the simultaneous methylation of thousands of promoters in the genome of human cells using an engineered zinc finger-DNMT3A fusion protein, enabling assessment of the effect of forced DNA methylation upon transcription, histone modifications, and DNA methylation persistence after the removal of the fusion protein. We find that DNA methylation is frequently insufficient to transcriptionally repress promoters. Furthermore, DNA methylation deposited at promoter regions associated with H3K4me3 is rapidly erased after removal of the zinc finger-DNMT3A fusion protein. Finally, we demonstrate that induced DNA methylation can exist simultaneously on promoter nucleosomes that possess the active histone modification H3K4me3, or DNA bound by the initiated form of RNA polymerase II. These findings suggest that promoter DNA methylation is not generally sufficient for transcriptional inactivation, with implications for the emerging field of epigenome engineering. One Sentence Summary Genome-wide epigenomic manipulation of thousands of human promoters reveals that induced promoter DNA methylation is unstable and frequently does not function as a primary instructive biochemical signal for gene silencing and chromatin reconfiguration.

Rewriting DNA Methylation Signatures at Will: The Curable Genome Within Reach?

Epigenetic regulation of gene expression is vital for the maintenance of genome integrity and cell phenotype. In addition, many different diseases have underlying epigenetic mutations, and understanding their role and function may unravel new insights for diagnosis, treatment, and even prevention of diseases. It was an important breakthrough when epigenetic alterations could be gene-specifically manipulated using epigenetic regulatory proteins in an approach termed epigenetic editing. Epigenetic editors can be designed for virtually any gene by targeting effector domains to a preferred sequence, where they write or erase the desired epigenetic modification. This chapter describes the tools for editing DNA methylation signatures and their applications. In addition, we explain how to achieve targeted DNA (de)methylation and discuss the advantages and disadvantages of this approach. Silencing genes directly at the DNA methylation level instead of targeting the protein and/or RNA is a major improvement, as repression is achieved at the source of expression, potentially eliminating the need for continuous administration. Re-expression of silenced genes by targeted demethylation might closely represent the natural situation, in which all transcript variants might be expressed in a sustainable manner. Altogether epigenetic editing, for example, by rewriting DNA methylation, will assist in realizing the curable genome concept.

Engineering Transcription Factors in Breast Cancer Stem Cells

Breast cancers are classified in at least six different subtypes (normal-like, luminal A, luminal B, Her2, basal-like, claudin-low), which are characterized by distinct genome-wide transcriptional profiles and response to therapy [1]. Recently, it has been shown that these intrinsic types of breast cancers are associated with unique DNA-methylation patterns [2,3,4]. In 2009, a large-scale genomic analysis of breast cancer cohorts has identified a novel subtype of breast cancer enriched in putative cancer stem-cell (CSC) markers, named claudin-low [5]. In addition to cancer or stem cell signatures, claudin-low tumors are enriched in Epithelial-to-Mesenchymal transition (EMT) markers, such as high expression of the Transcription Factors (TFs) Twist and Snail, and loss of epithelial junction proteins, such as cadherins, claudins and ocludins. Together with basal-like breast cancers, claudin-low carcinomas are mostly triple negative, hence their lack of expression of the Estrogen Receptor (ER), Progesterone Receptor (PR) and Her2. Consequently, these carcinomas are refractory to regimens to treat breast cancers, such as anti-estrogens and conventional chemotherapy. Similarly to these breast cancers, a subtype of serious epithelial ovarian cancers also appear to be poorly differentiated, high grade, and associated with poor clinical outcome. These serous epithelial ovarian tumors, named type II, are often associated with p53 and BRCA mutations [6]. Thus, there is a need to develop novel and more effective strategies to target poorly differentiated carcinomas. This will begin with a better understanding of molecular pathways that are activated in these tumors, which maintain aberrant proliferation and potentially, tumor initiation. Little is known regarding the molecular determinants of tumor initiation and progression in poorly differentiated cancers. it has been proposed that claudin-low and basal-like breast tumors are originated by oncogenic transformation of bipotent stem and progenitor cells, respectively. Consistent with this idea, we found that many Transcription Factors (TFs) normally expressed in both, adult and embryonic stem cells (hESCs), are also over-expressed in poorly differentiated breast and ovarian carcinomas. In the first part of this chapter we will overview oncogenic TFs and TF networks that could play a role in maintaining aberrant selfrenewal, with special focus on the OCT4-SOX2-NANOG embryonic TF network. In addition to abnormal reactivation of oncogenic TFs, tumor suppressor genes undergo epigenetic silencing