Currently, the worst prognosisof cancer is breast cancer that has spread or metastasized into other sitesduring diagnosis16. Toprevent this metastasis (a spread of cancer to different body parts of where ithad started), millions of lives would be saved. However, the actual progress onthis front means to think about ways in which the metastasis inhibitors aretested within the clinical trials (S58)16.
There are approximately1.3 million women in the world that are being diagnosed every year with breastcancer16. This makes it the second common cancer behind lung cancer16.
The emergence of the robust awareness-raising efforts, e.g., “the pinkribbon campaign” has caused a rise in the public profile of breast cancer16.
Estrogen is the primary hormone in stimulating growth and the development ofbreast cancer7. The epigenome is described as a group of thechemical compounds which inform the genomes what to do18. Human genomes have complete assemblies of theDNA (deoxyribonucleic acids) approximately compromising of 3 billion base pairsmaking each of the individuals unique18. The DNA preservesinstructions for the building of proteins which then carry a range of functionswithin the cell. Epigenomic compounds usually are attached to the DNA and canchange the functions containing “marked” genomes. The marks don’talter sequences of DNA. However, they adjust ways the cells use instructions ofthe DNA18.
Marks normally are passed from cell to cell when celldivision taking place18. Also passing from a generation to nextgeneration18. Epigenetics is referred as the heritablechanges in the gene expressions, occurring without changes in the nucleotide sequence5.Changes include the chemical modifications of DNA by DNA methylation or thechanges of proteins, which closely link with the DNA, for example, histones,which bind and compact the DNA in chromatin packages5. Epigeneticmodifications are enzymatic and as such can be reversed/controlled byepigenetic inhibitors3. The epigenome is the epigenetic state of acell. It is transmitted from the parent to the daughter cells which maintainthe specific epigenotype in the cell lineage15.
Gene inactivation orgene silencing of a tumor suppressor gene can not only be achieved throughmutations. It can be due to a result of inhibition or the translocation ofcomponents of the transcription machinery15. A mechanism which leadsto the inhibition of transcription is gene silencing via epigenetic changes,e.
g., acquisition of promoter methylation which introduces alterations withinthe chromatin structure15. The alterations of epigenetics are anessential factor in the development of cancers1.
Changes in DNAmethylation are a factor in the early events of the progress in tumors. Theyhave emerged as a hallmark of many cancer types including breast cancer2.These are also involved in the development of cancer and have a role in itsdevelopment and its progression, as exemplified by breast cancer4.
Environmentalfactors and also lifestyle can cause a change within epigenomes. These includelifestyle factors and also the environmental which include diet, smoking eventhe infectious diseases capable of exposing to an individual to pressures whichprompt the response of chemicals18. Most cancers are due to thechanges within genomes or epigenomes but can be due to both of these.Alterations within epigenomes are capable of switching genes on or offinvolving the cell growth or immune responses. Changes of these can cause theuncontrolled growth, hallmarks of the cancers or the failure of the immunesystem in destroying the tumours18. Chromatin is a highly ordered structurecontaining nucleosomal repeats linked through linker DNA14. Thechromatin complex comprises of histones, DNA, and also the non-histone proteinsthat are condensed in the nucleoprotein complex14. There are twodifferent conformational organizations chromatin normally acquire.
Theseinclude the heterochromatin (densely compact and transcription is inactive), andthe euchromatin (decondensed and transcription is active)14.Histones are small proteins which consist of a glomerular domain. They alsohave a charged NH2 terminus (N-terminal tail) which is flexible andprotrudes from the nucleosomes14. The mechanisms for controlling thetranscription of genes are crucial for cell differentiation, cell survival, andcell proliferation. When deregulated, this process can lead to malignancydevelopment14.
The main determinants of the chromatin structure arethe histone acetylation and the DNA methylation. The chromatin structure is themain regulator of gene transcription10. Within the nucleus, double-stranded DNA iscompacted and then organized in chromosomes3. DNA normally iswrapped around the histone protein-complexes to form a large nucleosomalstructure order3. The structure of chromatin is shown in figure 1.
The methylation of DNA happenswithin the compact chromatin, influenced by a range of changes within thehistone structures15. The chromatin is the nucleoprotein complexwhich compromises of the repeated units as nucleosomes. The single nucleosomeconsists of two turns.
The first one is the DNA wrapped around the core histone octamerwhich consists of the H1A, H2B, H3 and the H4 histones15. The secondis when genes are involved in DNA repair, cell cycle control, apoptosis(programmed cell death), angiogenesis and cell-to-cell interaction that areusually influenced by the hypermethylation of CpG-island promoters15.These are also involved in the development of cancer15. Figure 1 | The Structures ofChromatin adapted from 17.
Nucleosomes,the essential subunits of the chromatin, compromising of the octameric core histonesincluding the two H2A-H2B dimers and also the H3-H4 tetramer, wrapped on thetwo superhelical turns of the DNA leading to the protein having a DNA ratio of1:117. A rise in the acetylation of histones promoteschromatin structures to open more which is linked to gene activation. Thehistone methyltransferases (HMTs) mediate histone methylation8.Histone methylation events lead to changes in chromatin, alteration ofepigenetic control of gene expressions, e.g. activation or repression of thegenes involved in DNA repair, cell cycle and cancer progression8. Themechanisms which play a role in altering breast cancer include DNAhypermethylation, the DNA hypomethylation, the histone hyperacetylation, thehistone demethylation also the histone methylation7.
The three DNAmethyltransferases that are active include DNA (cytosine-5)-methyltransferase 1(DNMT1), DNA(cytosine-5)-methyltransferase 3A (DNMT3a), and DNA (cytosine-5-)-methyltransferase3 (DNMT3b). Expression of DNMT1 and DNMTb is often upregulated in breast cancercells8. The DNMTs interact directly with the histone deacetylases(HDACs) and with proteins of the methyl-CpG binding domain (MBD) family (afamily of proteins located close to the promoter region to form the repressivetranscription complexes)8. Histone modification and chromatinstructure are affected by the molecular interplay between histoneacetyltransferases (HAT) and the Histone deacetylases (HDAC)8.
The methylation of DNA is usually linked tonormal development and growth. An alteration in the pattern of the DNAmethylation is linked to the development of breast cancer, the metastasis, andthe progression3. Breast cancer is linked to hypermethylation oftumor suppressor genes and the hypomethylation of oncogenes (genes involved incausing cancer)3.
The epigenetic changes within cancers include thehistone modification, the DNA methylation, and assays. Histones have a role inshape maintenance of the chromatin structure11. A range of post-translation changes can occurat the N-terminal tail of histone proteins, leading to a change in conformationwithin the chromatin, affecting the transcription of vital genes includingtumor suppressors11.
Transcriptional events can be affected by acetylation, methylation orphosphorylation of target proteins11. HDACs are a member of theprotein complex which is responsible for recruiting transcription factors tothe promoter region of the genes, including tumor suppressor genes11.The regulation acetylation status is the specific cell cycle regulatoryproteins11. The higher order structure is defined as the assembly ofnucleosomes which assume the reproducibility conformation within the 3D space.The common chromatin structure is mitotic/meiotic chromosome where DNA iscompacted in 10,000 to 20,000 fold. The metaphase chromosome compromises of theshapes, the banding patterns and also the locations of the particular genes1. Transcription factors (TFs) are the mainproteins that are involved in regulating gene transcription.
They bindspecifically at the cis-regulatory region of DNA. In breast cancer, the threemain TFs are ER?, FOXA1, and GATA3. TFs contribute to regulating geneexpressions that are linked to estrogen dependent tumor growth2.Within breast tumors, activated fibroblasts (Cancer-Associated Fibroblasts) arepredominantly of stromal cell type. CAFs also express myofibroblasts9.CAFs include; ?-smooth muscle actin, vimentin, neuron-glial antigen-2 alsofibroblast specific protein 1, which can be used as markers. A range of growthfactors is then secreted9.
DNA methylation is described as gene silencingthat happens due to DNA methylation at a promoter region of the gene11.Four bases; adenine, guanine, cytosine, and thymine are the building blocks ofthe genetic makeup. A methyl group (–CH3) is added to the pyrimidinering of a cytosine, by DNA methyltransferases (DNMTs), to form themethylcytosine (DNA methylation). Thesehave a vital role in hyper-methylation of tumor suppressor genes11. DNA methylation processes occur only at thecytosines preceding guanine within DNA sequences known as CpG dinucleotides11. CpG dinucleotides, existing in the genome,can be profoundly methylated impeding the gene transcription. When many CpG dinucleotides are found in theregion of gene promoters, these are referred to as CpG islands11. A normal tissue of CpG islands isunmethylated, for gene transcription to occur.
Within cancer, there is abnormalDNA methylation of CpG islands that obstructs the transcription of vital genessuch as tumor suppressor genes11. The role of DNA methylation inbreast cancer has been well characterized by its epigenetic implications. These usually occur within the cancer cell8.The hypermethylated CpG dinucleotides that are in the promoter region have avital role regulating the individual genes which promote carcinogenesis8.
The mediation of DNA methylation takes place through the action of DNAmethyltransferases (DNMTs). The DNA methylation takes place at a cytosine baselocated at the 5′ to guanosine, in a sequence known as the CpG dinucleotide5. The regions of CpG are shown in cancer andnormal cellsfigure 2.
Figure 2 | Diagram shows theDNA methylation within cancer and healthy cells (adapted from 13 l The CpGislands indicated as “c” within the promoter region are activelytranscribed within normal cells and are un-methylated, permitting transcriptionactivities (indicated by green arrow). The CpG islands elsewhere in the genesand the intergenic space are methylated (indicated as “M”). The red arrow(deleted with an “X”) indicates transcription which is repressed. However,within cancer cells, the reverse holds true13.
Epigenetics field is thrilling and evolves areasof investigation within the cancer research and also therapeutics11.Possibilities of HDAC inhibitors to overcome the resistance of hormonal therapyin patients with advanced breast cancer is highlighted currently. However, aconfirmatory trial is required before information can be used within clinics11.We can identify the robust biomarkers by forecasting response to HDACinhibitors. However, confirmation is necessary for clinical trials in thefuture11.
