Epigenetics.docx

Epigenetics Genetics is the study of biological processes and phenotypes and their functional molecules, encoded as heritable, gene sequence information. Epigenetics is the study of heritable gene expression and cellular phenotype changes that are not due to alteration in DNA nucleotide sequences^1...

Epigenetics Genetics is the study of biological processes and phenotypes and their functional molecules, encoded as heritable, gene sequence information. Epigenetics is the study of heritable gene expression and cellular phenotype changes that are not due to alteration in DNA nucleotide sequences^1^. For example, identical (monozygotic) twins have identical genome sequences, however one twin may develop cancer while the other does not^2^. This phenotypic difference between twins cannot be explained by DNA sequence but may be explained by a difference in their environment (for example nutrition). Cellular mechanisms that respond to the environment but do not alter DNA sequence are commonly classified as epigenetic^1,\ 3^. They include: DNA methylation, chromatin remodeling, histone modification, nucleosome positioning, noncoding RNA activity, and prion effects^†^. In recent decades the epigenetics field has been rapidly expanding, since its mechanisms are demonstrated to be integral in defining precise changes during normal development as well as maintenance of the final cell differentiation state4. The disruption of normal epigenetic mechanisms can lead to diseases such as cancer and Alzheimer's and the understanding of these mechanisms may hold the key for successful therapeutics in the future^.^ Schematic of epigenetic markers involved in chromatin formation: DNA methylation and histone acetylation. Jmol downloads from http://c4.cabrillo.edu/404/ PDB ID: [**1AOI **](http://www.rcsb.org/pdb/explore/explore.do?structureId=1aoi), [**Pubmed**](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=9305837) **Crystal structure of the nucleosome core particle at 2.8 A resolution.** Luger, K., Mader, A.W., Richmond, R.K., Sargent, D.F., Richmond, T.J. **Journal:** (1997) Nature **389:** 251-260 DNA is packaged inside the nucleus in nucleosomes. This structure shows a long piece of DNA wrapped around histone proteins to form a nucleosome The X-ray crystal structure of the nucleosome core particle of chromatin shows in atomic detail how the histone protein octamer is assembled and how 146 base pairs of DNA are organized into a superhelix around it. The histone proteins are perfectly designed for their jobs, so much so that histones are nearly identical in all non-bacterial organisms. Even slight modifications can be lethal. The surface of the histone octamer, shown on the left, is decorated with positively charged amino acids, shown with bright blue nitrogen atoms. These interact strongly with the negatively-charged phosphate groups on the DNA, shown at the right with bright yellow phosphorous and bright red oxygen atoms. This serves to glue the DNA strand to the protein core. This is no simple task. DNA is normally a long, straight molecule, but in nucleosomes the DNA must be forcably bent into these two tight circles. Specification Epigenetic control of gene expression in eukaryotes. Epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA. These changes are caused by changes in the environment that inhibit transcription by: increased methylation of the DNA or decreased acetylation of associated histones. The relevance of epigenetics on the development and treatment of disease, especially cancer. LEARNING OBJECTIVES - Describe the various epigenetic changes that can be made to DNA - Discuss how eukaryotic gene regulation occurs at the epigenetic level KEY POINTS - [DNA](https://www.boundless.com/definition/dna/) is packaged by wrapping around [histone](https://www.boundless.com/definition/histone/) [proteins](https://www.boundless.com/definition/protein/) into structures called [nucleosomes](https://www.boundless.com/definition/nucleosome/), which resemble beads on a string. - When DNA is to be transcribed, the nucleosomes can slide away from that region of DNA, opening it up to the [transcription](https://www.boundless.com/definition/transcription/) machinery of the [cell](https://www.boundless.com/definition/cell/). - Chemical modifications to either the histone proteins or the DNA itself signals whether or not a particular region of the [genome](https://www.boundless.com/definition/genome/) should be \"open\" or \"closed\" to the transcription machinery. - Modifications such as [acetylation](https://www.boundless.com/definition/acetylation/) or [methylation](https://www.boundless.com/definition/methylation/) of the histones can alter how tightly DNA is wrapped around them, while methylation of DNA changes how the DNA interacts with proteins, including the histone proteins that control access to the region. - This type of [genetic](https://www.boundless.com/definition/genetics/) regulation is called [epigenetic](https://www.boundless.com/definition/epigenetic/) regulation (\"above genetics\") as it does not change the [nucleotide](https://www.boundless.com/definition/nucleotide/) sequence of the DNA. TERMS [histone](https://www.boundless.com/definition/histone/) - any of various simple water-soluble proteins that are rich in the basic [amino acids](https://www.boundless.com/definition/amino-acid/) lysine and arginine and are complexed with DNA in the nucleosomes of eukaryotic chromatin [epigenetics](https://www.boundless.com/definition/epigenetics/) - the study of heritable changes caused by the activation and deactivation of [genes](https://www.boundless.com/definition/gene/) without any change in DNA sequence [nucleosome](https://www.boundless.com/definition/nucleosome/) - any of the subunits that repeat in chromatin; a coil of DNA surrounding a histone core Unlike simple [genetics](http://en.wikipedia.org/wiki/Genetics) based on changes to the DNA sequence (the [genotype](http://en.wikipedia.org/wiki/Genotype)), the changes in [gene expression](http://en.wikipedia.org/wiki/Gene_expression) or [cellular](http://en.wikipedia.org/wiki/Cell_(biology)) [phenotype](http://en.wikipedia.org/wiki/Phenotype) of epigenetics have other causes, One example of an epigenetic change in [eukaryotic](http://en.wikipedia.org/wiki/Eukaryotic) biology is the process of [cellular differentiation](http://en.wikipedia.org/wiki/Cellular_differentiation). During [morphogenesis](http://en.wikipedia.org/wiki/Morphogenesis), [totipotent](http://en.wikipedia.org/wiki/Totipotent) [stem cells](http://en.wikipedia.org/wiki/Stem_cells) become the various [pluripotent](http://en.wikipedia.org/wiki/Pluripotent) [cell lines](http://en.wikipedia.org/wiki/Cell_line) of the [embryo](http://en.wikipedia.org/wiki/Embryo), which in turn become fully differentiated cells. In other words, as a single fertilized egg cell -- the [zygote](http://en.wikipedia.org/wiki/Zygote) -- continues to [divide](http://en.wikipedia.org/wiki/Mitosis), the resulting daughter cells change into all the different cell types in an organism, including [neurons](http://en.wikipedia.org/wiki/Neurons), [muscle cells](http://en.wikipedia.org/wiki/Muscle_cells), [epithelium](http://en.wikipedia.org/wiki/Epithelium), [endothelium](http://en.wikipedia.org/wiki/Endothelium) of [blood vessels](http://en.wikipedia.org/wiki/Blood_vessels), etc., by activating some genes while inhibiting the expression of others. Epigenetic changes can modify the activation of certain genes, but not the sequence of [DNA](http://en.wikipedia.org/wiki/DNA). Additionally, the [chromatin](http://en.wikipedia.org/wiki/Chromatin) proteins associated with DNA may be activated or silenced. This is why the differentiated cells in a multi-cellular organism express only the genes that are necessary for their own activity. Epigenetic changes are preserved when cells divide. Most epigenetic changes only occur within the course of one individual organism\'s lifetime, but, if gene inactivation occurs in a sperm or egg cell that results in fertilization, then some epigenetic changes can be transferred to the next generation.[^\[21\]^](http://en.wikipedia.org/wiki/Epigenetics#cite_note-pmid17320501-21)  In the [Överkalix study](http://en.wikipedia.org/wiki/%C3%96verkalix_study), Marcus Pembrey and colleagues observed that the paternal (but not maternal) grandsons[^\[100\]^](http://en.wikipedia.org/wiki/Epigenetics#cite_note-paternal-grandson-100) of Swedish men who were exposed during preadolescence to famine in the 19th century were less likely to die of cardiovascular disease. If food was plentiful, then [diabetes](http://en.wikipedia.org/wiki/Diabetes) mortality in the grandchildren increased, suggesting that this was a transgenerational epigenetic inheritance.[^\[101\]^](http://en.wikipedia.org/wiki/Epigenetics#cite_note-pmid16391557-101) The opposite effect was observed for females---the paternal (but not maternal) granddaughters of women who experienced famine while in the womb (and therefore while their eggs were being formed) lived shorter lives on average.[^\[102\]^](http://en.wikipedia.org/wiki/Epigenetics#cite_note-102) #### DNA methylation in cancer\[(http://en.wikipedia.org/w/index.php?title=Epigenetics&action=edit&section=21)\] #### #### [DNA methylation](http://en.wikipedia.org/wiki/DNA_methylation) in vertebrates typically occurs at [CpG sites](http://en.wikipedia.org/wiki/CpG_site) (cytosine-phosphate-guanine sites, that is, where a [cytosine](http://en.wikipedia.org/wiki/Cytosine) is directly followed by a [guanine](http://en.wikipedia.org/wiki/Guanine) in the DNA sequence). This methylation results in the conversion of the cytosine to [5-methylcytosine](http://en.wikipedia.org/wiki/5-methylcytosine). The formation of Me-CpG is [catalyzed](http://en.wikipedia.org/wiki/Catalysis) by the enzyme [DNA methyltransferase](http://en.wikipedia.org/wiki/DNA_methyltransferase). Human DNA has about 80--90% of CpG sites methylated, but there are certain areas, known as [CpG islands](http://en.wikipedia.org/wiki/CpG_site#CpG_islands), that are GC-rich (high guanine and cytosine content, made up of about 65% CG residues), wherein none is methylated. These are associated with the [promoters](http://en.wikipedia.org/wiki/Promoter_(genetics)) of 56% of mammalian genes, including all [ubiquitously expressed genes](http://en.wikipedia.org/wiki/Housekeeping_gene). One to two percent of the human genome are CpG clusters, and there is an inverse relationship between CpG methylation and transcriptional activity. [DNA methylation](http://en.wikipedia.org/wiki/DNA_methylation) is an important regulator of gene transcription and a large body of evidence has demonstrated that aberrant DNA methylation is associated with unscheduled gene silencing, and the genes with high levels of 5-methylcytosine in their promoter region are transcriptionally silent. DNA methylation is essential during embryonic development, and in somatic cells, patterns of DNA methylation are in general transmitted to daughter cells with a high fidelity. Aberrant DNA methylation patterns have been associated with a large number of human malignancies and found in two distinct forms: hypermethylation and hypomethylation compared to normal tissue. **DNA Methylation & Cancer** Hypermethylation is one of the major epigenetic modifications that repress transcription via promoter region of tumour suppressor genes. Hypermethylation typically occurs at CpG islands in the promoter region and is associated with gene inactivation. Global hypomethylation has also been implicated in the development and progression of cancer through different mechanisms.  In [organic chemistry](http://en.wikipedia.org/wiki/Organic_chemistry), **acetyl** is a [functional group](http://en.wikipedia.org/wiki/Functional_group), the [acyl](http://en.wikipedia.org/wiki/Acyl) with [chemical formula](http://en.wikipedia.org/wiki/Chemical_formula) [C](http://en.wikipedia.org/wiki/Carbon)[O](http://en.wikipedia.org/wiki/Oxygen)C[H](http://en.wikipedia.org/wiki/Hydrogen)~3~. In [organic chemistry](http://en.wikipedia.org/wiki/Organic_chemistry), a **methyl group** is an [alkyl](http://en.wikipedia.org/wiki/Alkane) derived from [methane](http://en.wikipedia.org/wiki/Methane), containing one [carbon](http://en.wikipedia.org/wiki/Carbon) atom [bonded](http://en.wikipedia.org/wiki/Chemical_bond) to three [hydrogen](http://en.wikipedia.org/wiki/Hydrogen) atoms --- CH~3~.   Acetylation occurs as a co-translational and [post-translational modification](http://en.wikipedia.org/wiki/Post-translational_modification) of [proteins](http://en.wikipedia.org/wiki/Protein), for example, [histones](http://en.wikipedia.org/wiki/Histone), [p53](http://en.wikipedia.org/wiki/P53), and [tubulins](http://en.wikipedia.org/wiki/Tubulin). Among these proteins, [chromatin](http://en.wikipedia.org/wiki/Chromatin) proteins and metabolic enzymes are highly represented, indicating that acetylation has a considerable impact on [gene expression](http://en.wikipedia.org/wiki/Gene_expression) and [metabolism](http://en.wikipedia.org/wiki/Metabolism). In [bacteria](http://en.wikipedia.org/wiki/Bacteria), 90% of proteins involved in central metabolism of Salmonella enteric are acetylated. **Protein methylation** typically takes place on [arginine](http://en.wikipedia.org/wiki/Arginine) or [lysine](http://en.wikipedia.org/wiki/Lysine) [amino acid](http://en.wikipedia.org/wiki/Amino_acid) residues in the protein sequence.[^\[1\]^](http://en.wikipedia.org/wiki/Methylation#cite_note-isbn0-9747077-3-2.-1) Arginine can be methylated once or twice and Lysine can be methylated up to three times -- this is done under the action of methyltransferases. Protein methylation has been most studied in the [histones](http://en.wikipedia.org/wiki/Histone) which, when methylated can act [epigenetically](http://en.wikipedia.org/wiki/Epigenetics) to repress or activate gene expression. Protein methylation is one type of [post-translational modification](http://en.wikipedia.org/wiki/Post-translational_modification). **Histone acetylation and deacetylation** are the processes by which the [lysine](http://en.wikipedia.org/wiki/Lysine) residues within the [N-terminal](http://en.wikipedia.org/wiki/N-terminus) tail protruding from the [histone](http://en.wikipedia.org/wiki/Histone) core of the [nucleosome](http://en.wikipedia.org/wiki/Nucleosome) are [acetylated](http://en.wikipedia.org/wiki/Acetylate) and deacetylated as part of [gene regulation](http://en.wikipedia.org/wiki/Gene_regulation). Histone acetylation and deacetylation are essential parts of [gene regulation](http://en.wikipedia.org/wiki/Regulation_of_gene_expression). These reactions are typically catalysed by [enzymes](http://en.wikipedia.org/wiki/Enzyme) with \"[histone acetyltransferase](http://en.wikipedia.org/wiki/Histone_acetyltransferase)\" (HAT) or \"[histone deacetylase](http://en.wikipedia.org/wiki/Histone_deacetylase)\" (HDAC) activity. [Acetylation](http://en.wikipedia.org/wiki/Acetylation) is the process where an [acetyl](http://en.wikipedia.org/wiki/Acetyl) functional group is transferred from one molecule (e.g. [Acetyl-Coenzyme A](http://en.wikipedia.org/wiki/Acetyl-Coenzyme_A)) to another. Deacetylation is simply the reverse reaction where an acetyl group is removed from a molecule. Nucleosomes are portions of [double-stranded DNA (dsDNA)](http://en.wikipedia.org/wiki/DNA) that are wrapped around protein complexes called histone cores. These histone cores are composed of 8 subunits, two each of [H2A](http://en.wikipedia.org/wiki/Histone_H2A), [H2B](http://en.wikipedia.org/wiki/Histone_H2B), [H3](http://en.wikipedia.org/wiki/Histone_H3) and [H4](http://en.wikipedia.org/wiki/Histone_H4) histones. This protein complex forms a cylindrical shape that dsDNA wraps around with approximately 147 base pairs. Nucleosomes are formed as a beginning step for DNA compaction that also contributes to structural support as well as serves functional roles.[^\[2\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-Verdone-2) These functional roles are contributed by the tails of the histone subunits. The histone tails insert themselves in the[minor grooves](http://en.wikipedia.org/wiki/Nucleic_acid_double_helix) of the DNA and extend through the double helix,[^\[1\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-Molecular_Biology_of_the_Gene-1) which leaves them open for modifications involved in transcriptional activation.[^\[3\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-Kuo-3) Acetylation has been closely associated with increases in transcriptional activation while deacetylation has been linked with transcriptional deactivation. These reactions occur post-translation and are reversible.[^\[3\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-Kuo-3) The mechanism for acetylation and deacetylation takes place on the NH3+ groups of Lysine amino acid residues. These residues are located on the tails of histones that make up the nucleosome of packaged dsDNA.  Acetylation has the effect of changing the overall charge of the histone tail from positive to neutral. Nucleosome formation is dependent on the positive charges of the H4 histones and the negative charge on the surface of H2A histone fold domains. Acetylation of the histone tails disrupts this association, leading to weaker binding of the nucleosomal components.[^\[1\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-Molecular_Biology_of_the_Gene-1) By doing this, the DNA is more accessible and leads to more transcription factors being able to reach the DNA. **Histone Acetylation & Cancer** Due to the regulatory role during transcription of epigenetic modifications in genes, it is not surprising that changes in [epigenetic](http://en.wikipedia.org/wiki/Epigenetic) markers, such as acetylation, can contribute to cancer development. HDACs expression and activity in [tumor](http://en.wikipedia.org/wiki/Tumor) cells is very different from normal cells. The [overexpression](http://en.wikipedia.org/wiki/Overexpression) and increased activity of HDACs has been shown to be characteristic of [tumorigenesis](http://en.wikipedia.org/wiki/Carcinogenesis) and [metastasis](http://en.wikipedia.org/wiki/Metastasis), suggesting an important regulatory role of histone deacetylation on oncogene expression.[^\[28\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-28) One of the examples is the regulation role of histone acetylation/deacetylation in P300 and CBP, both of which contribute to [oncogenesis](http://en.wikipedia.org/wiki/Carcinogenesis).[^\[29\]^](http://en.wikipedia.org/wiki/Histone_acetylation_and_deacetylation#cite_note-29) Approved in 2006 by the U.S. [Food and Drug Administration](http://en.wikipedia.org/wiki/Food_and_Drug_Administration) (FDA), [Vorinostat](http://en.wikipedia.org/wiki/Vorinostat) represents a new category for anticancer drugs that are in development. Vorinostat targets histone acetylation mechanisms and can effectively inhibit abnormal chromatin remodeling in cancerous cells. C:\\Users\\Robert George\\Desktop\\New Bio Images\\Normal-cancer-epigenome.png Co-translational modification is the process of covalently altering one or more [amino acids](http://en.wikipedia.org/wiki/Amino_acid) in a protein at the same time as its [mRNA](http://en.wikipedia.org/wiki/Messenger_RNA) is being translated on polyribosomes;[^\[4\]^](http://en.wikipedia.org/wiki/Acetylation#cite_note-4) and [post-translational modification](http://en.wikipedia.org/wiki/Post-translational_modification) includes protein [phosphorylation](http://en.wikipedia.org/wiki/Phosphorylation), [glycosylation](http://en.wikipedia.org/wiki/Glycosylation), [ubiquitination](http://en.wikipedia.org/wiki/Ubiquitination), [nitrosylation](http://en.wikipedia.org/wiki/Nitrosylation), [methylation](http://en.wikipedia.org/wiki/Methylation), acetylation, [lipidation](http://en.wikipedia.org/wiki/Lipidation) and [proteolysis](http://en.wikipedia.org/wiki/Proteolysis) before becoming the mature protein product The human genome encodes over 20,000 genes; each of the 23 pairs of human chromosomes encodes thousands of genes. The DNA in the nucleus is precisely wound, folded, and compacted into chromosomes so that it will fit into the nucleus. It is also organized so that specific segments can be accessed as needed by a specific cell type. The first level of organization, or packing, is the winding of DNA strands around histone proteins. Histones package and order DNA into structural units called nucleosome [complexes](https://www.boundless.com/definition/complex/), which can control the access of proteins to the DNA regions. Under the [electron](https://www.boundless.com/definition/electron/) microscope, this winding of DNA around histone proteins to form nucleosomes looks like small beads on a string. These beads (histone proteins) can move along the string (DNA) and change the structure of the [molecule](https://www.boundless.com/definition/molecule/). If DNA encoding a specific gene is to be transcribed into RNA, the nucleosomes surrounding that region of DNA can slide down the DNA to open that specific chromosomal region and allow for the transcriptional machinery (RNA [polymerase](https://www.boundless.com/definition/polymerase/)) to initiate transcription. Nucleosomes can move to open the chromosome structure to expose a segment of DNA, but do so in a very controlled manner.  How the histone proteins move is dependent on signals found on both the histone proteins and on the DNA. These signals are tags, or modifications, added to histone proteins and DNA that tell the histones if a chromosomal region should be open or closed. These tags are not permanent, but may be added or removed as needed. They are chemical modifications (phosphate, methyl, or acetyl groups) that are attached to specific amino acids in the protein or to the nucleotides of the DNA. The tags do not alter the DNA base sequence, but they do alter how tightly wound the DNA is around the histone proteins. DNA is a negatively-charged molecule; therefore, changes in the charge of the histone will change how tightly wound the DNA molecule will be. When unmodified, the histone proteins have a large positive charge; by adding chemical modifications, such as acetyl groups, the charge becomes less positive. The DNA molecule itself can also be modified. This occurs within very specific regions called CpG islands. These are stretches with a high [frequency](https://www.boundless.com/definition/frequency/) of cytosine and guanine dinucleotide DNA pairs (CG) found in the [promoter](https://www.boundless.com/definition/promoter/) regions of genes. When this configuration exists, the cytosine member of the pair can be methylated (a methyl group is added). This modification changes how the DNA interacts with proteins, including the histone proteins that control access to the region. Highly-methylated (hypermethylated) DNA regions with deacetylated histones are tightly coiled and transcriptionally inactive. These changes to DNA are inherited from parent to offspring, such that while the DNA sequence is not altered, the pattern of [gene expression](https://www.boundless.com/definition/gene-expression/) is passed to the next generation. This type of gene regulation is called epigenetic regulation. Epigenetics means \"above genetics.\" The changes that occur to the histone proteins and DNA do not alter the nucleotide sequence and are not permanent. Instead, these changes are temporary (although they often persist through multiple rounds of cell division) and alter the chromosomal structure (open or closed) as needed. A gene can be turned on or off depending upon the location and modifications to the histone proteins and DNA. If a gene is to be transcribed, the histone proteins and DNA are modified surrounding the chromosomal region encoding that gene. This opens the chromosomal region to allow access for [RNA polymerase](https://www.boundless.com/definition/rna-polymerase/) and other proteins, called [transcription factors](https://www.boundless.com/definition/transcription-factor/), to bind to the promoter region, located just upstream of the gene, and initiate transcription. If a gene is to remain turned off, or silenced, the histone proteins and DNA have different modifications that signal a closed chromosomal configuration. In this closed configuration, the RNA polymerase and transcription factors do not have access to the DNA and transcription cannot occur. Source: Boundless. "Epigenetic Control: Regulating Access to Genes within the Chromosome." Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 16 Dec. 2014 from  Epigenetics In the clinic, the estrogen receptor (ESR) and more precisely the estrogen receptor α (ESR1α) is an important prognostic disease marker. Approximately two-thirds of breast cancers are ESR1-positive. The binding of estrogen to the ESR1 is not only a key regulator for the physiological growth and differentiation of the mammary gland, it is also a key element in the malignant progression of breast cancer, i.e. the growth of ESR1 expressed breast cancer cells is stimulated by estrogen, which in turn makes it accessible to endocrine treatment strategies, while breast cancers that do not express ESR1 exhibit a primary resistance to endocrine treatment. Therefore, the presence of ESR1 correlates with increased disease-free survival and a better prognosis when compared to ESR1-negative breast cancers. While at the time of diagnosis up to one-third of breast cancers are ESR1 negative, quite a few cancers that are initially ESR1 positive lose the ESR1 during the course of tumor progression and are therefore no longer responsive to endocrine therapy designed to block ESR1 function.Tumor growth is estrogen independent in approximately one-third of all breast cancers, which makes these patients unresponsive to hormonal treatment. This unresponsiveness to hormonal treatment may be explained through the absence of the estrogen receptor alpha (ESR1). The ESR1 gene re-expression through epigenetic modulators such as DNA methyltransferase inhibitors and/or histone deacetylase inhibitors restores tamoxifen sensitivity in ESR1 negative breast cancer cell lines and opens new treatment horizons in patients who were previously associated with a poor prognosis. Ribavirin and analogs could pave the way to novel translational research projects that aim to restore ESR1 gene re-expression and thus the susceptibility to tamoxifen-based endocrine treatment strategies.