DNA methylation is an essential covalent modification that is required for growth and development. Once considered to be a relatively stable epigenetic mark, many studies have established that DNA ...methylation is dynamic. The 5-methylcytosine (5-mC) mark can be removed through active DNA demethylation in which 5-mC is converted to an unmodified cytosine through an oxidative pathway coupled to base excision repair (BER). The BER enzyme Thymine DNA Glycosylase (TDG) plays a key role in active DNA demethylation by excising intermediates of 5-mC generated by this process. TDG acts as a key player in transcriptional regulation through its interactions with various nuclear receptors and transcription factors, in addition to its involvement in classical BER and active DNA demethylation, which serve to protect the stability of the genome and epigenome, respectively. Recent animal studies have identified a connection between the loss of
and the onset of tumorigenesis. In this review, we summarize the recent findings on TDG's function as a transcriptional regulator as well as the physiological relevance of TDG and active DNA demethylation in cancer.
Retinoic acid (RA) plays important roles in development, growth, and homeostasis through regulation of the nuclear receptors for RA (RARs). Herein, we identify Hypermethylated in Cancer 1 (Hic1) as ...an RA-inducible gene. HIC1 encodes a tumor suppressor, which is often silenced by promoter hypermethylation in cancer. Treatment of cells with an RAR agonist causes a rapid recruitment of an RAR/RXR complex consisting of TDG, the lysine acetyltransferase CBP, and TET 1/2 to the Hic1 promoter. Complex binding coincides with a transient accumulation of 5fC/5caC and concomitant upregulation of Hic1 expression, both of which are TDG dependent. Furthermore, conditional deletion of Tdg in vivo is associated with Hic1 silencing and DNA hypermethylation of the Hic1 promoter. These findings suggest that the catalytic and scaffolding activities of TDG are essential for RA-dependent gene expression and provide important insights into the mechanisms underlying targeting of TET-TDG complexes.
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•The gene Hypermethylated in Cancer 1 (Hic1) is a direct retinoic acid receptor (RAR) target•RA-dependent induction of HIC1 requires a TDG-containing demethylation complex•Deletion of TDG in vivo leads to DNA hypermethylation and loss of HIC1 expression•RAR signaling has a fundamental role in initiating DNA demethylation at select loci
Hassan et al. report that DNA methylation and expression of Hypermethylated in Cancer 1 (Hic1) are regulated by the retinoic acid receptor (RAR) in a TDG dependent manner. Failure to recruit TDG and TETs may contribute to aberrant silencing of HIC1 in cancer.
Thymine DNA glycosylase (TDG) is a nuclear receptor coactivator that plays an essential role in the maintenance of epigenetic stability in cells. Here, we demonstrate that the conditional deletion of ...TDG in adult mice results in a male-predominant onset of hepatocellular carcinoma (HCC). TDG loss leads to a prediabetic state, as well as bile acid (BA) accumulation in the liver and serum of male mice. Consistent with these data, TDG deletion led to dysregulation of the farnesoid X receptor (FXR) and small heterodimer partner (SHP) regulatory cascade in the liver. FXR and SHP are tumor suppressors of HCC and play an essential role in BA and glucose homeostasis. These results indicate that TDG functions as a tumor suppressor of HCC by regulating a transcriptional program that protects against the development of glucose intolerance and BA accumulation in the liver.
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•The deletion of TDG in adult mice leads to late onset of hepatocellular carcinoma•Hepatocellular carcinoma resulting from a loss of TDG shows a male sex bias•TDG is essential for maintaining glucose and bile acid homeostasis in vivo•TDG interacts with and acts as a coactivator for FXR in the liver
TDG is a base excision repair protein that is essential for embryonic development. Hassan et al. show that the conditional deletion of TDG in adult mice causes dysregulation of FXR signaling and a loss of glucose and bile acid homeostasis. This leads to a late-onset development of hepatocellular carcinoma.
The Farnesoid X Receptor (FXR) belongs to the nuclear receptor superfamily and is an essential bile acid (BA) receptor that regulates the expression of genes involved in the metabolism of BAs. FXR ...protects the liver from BA overload, which is a major etiology of hepatocellular carcinoma. Herein, we investigated the changes in gene expression and chromatin accessibility in hepatocytes by performing RNA-seq in combination with the Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq) using a novel FXR knockout mouse model (
) generated through CRISPR/Cas9. Consistent with previous
knockout models, we found that
mice develop late-onset HCC associated with increased serum and hepatic BAs. FXR deletion was associated with a dramatic loss of chromatin accessibility, primarily at promoter-associated transcription factor binding sites. Importantly, several genes involved in BA biosynthesis and circadian rhythm were downregulated following loss of FXR, also displayed reduced chromatin accessibility at their promoter regions. Altogether, these findings suggest that FXR helps to maintain a transcriptionally active state by regulating chromatin accessibility through its binding and recruitment of transcription factors and coactivators.
The nuclear hormone receptors are DNA binding transcription factors that are regulated by binding of ligands, switching them from an inactive or repressive state to gene-activating functions. Recent ...evidence supports the hypothesis that many nuclear receptors switch, in a ligand-dependent manner, between binding of a multicomponent co-repressor complex containing histone deacetyltransferase activity, and binding of a co-activator complex containing factors with histone acetyltransferase activity that are further regulated by diverse signal transduction pathways. The identification of these limiting co-repressor and co-activator complexes and their specific interaction motifs, in concert with solution of the structures of the receptor ligand-binding domain in apo (empty) and ligand bound forms, indicates a common molecular mechanism by which these factors activate and repress gene transcription.
Transcriptional silencing by many transcription factors is mediated by the nuclear receptorcorepressor (N-CoR). The mechanism by which N-CoR represses basal transcription involves the direct or ...indirect recruitment of histone deacetylases (HDACs). We have isolated two multiprotein N-CoR complexes, designated N-CoR-1 and N-CoR-2, which possess histone deacetylase activity that is mediated by distinct HDACs. Based on Western blotting using antibodies against known subunits, the only HDAC found in the N-CoR-1 complex was HDAC3. In contrast, N-CoR-2 contained predominantly HDAC1 and HDAC2 as well as several other subunits that are found in the Sin3A·HDAC complex. Using mass spectrometry and Western blotting, we have identified several novel components of the N-CoR-1 complex including the SWI/SNF-related proteins BRG1, BAF 170, BAF 155, BAF 47/INI1, and the corepressor KAP-1 that is involved in silencing heterochromatin. Indirect immunofluorescence has revealed that both KAP-1 and N-CoR colocalize throughout the nucleus. These results suggest that N-CoR is found in distinct multiprotein complexes, which are involved in multiple pathways of transcriptional repression.
DNA repair in chromatin is subject to topological constraints, suggesting a requirement for chromatin modification and remodeling activities. Thymine DNA glycosylase (TDG) initiates repair of G/T and ...G/U mismatches, commonly associated with CpG islands, by removing thymine and uracil moieties. We report that TDG associates with transcriptional coactivators CBP and p300 and that the resulting complexes are competent for both the excision step of repair and histone acetylation. Furthermore, TDG stimulates CBP transcriptional activity in transfected cells and reciprocally serves as a substrate for CBP/p300 acetylation. Remarkably, this acetylation triggers release of CBP from DNA ternary complexes and also regulates recruitment of repair endonuclease APE. These observations reveal a potential regulatory role for protein acetylation in base mismatch repair and a role for CBP/p300 in maintaining genomic stability.
The estrogen receptor (ER) is a ligand-dependant transcription factor expressed in many breast cancers and is the target of many endocrine-based cancer therapies. Genome-wide studies have shown that ...the ER binds to gene-specific enhancer regions in response to β-estradiol (E2) which undergo transcription producing noncoding enhancer RNA (eRNA). While eRNAs are important for transcriptional activation of neighboring genes, the mechanism remains poorly understood.
Using ChIP-Seq we generate a global profile of thymine DNA glycosylase (TDG), an ER coactivator that plays an essential role in DNA demethylation, in response to E2 in the MCF7 breast cancer cell line. Remarkably, we found that in response to E2 TDG localized to enhancers which also recruit ERα, RNA Pol II and other coregulators and which are marked by histone modifications indicative of active enhancers. Importantly, depletion of TDG inhibits E2-mediated transcription of eRNAs and transcription of ER-target genes. Functionally, we find that TDG both sensitizes MCF7 cells to tamoxifen-mediated cytostasis and increases migration and invasion of MCF7 cells.
Taken together we find that TDG plays a central role in mediating transcription at a subset of enhancers and governs how MCF7 cells respond to both estrogenic and anti-estrogenic compounds and may be an effective therapeutic target.
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