The Altered Gene Expression and Epigenetic Mechanisms in Breast Cancer

Among oncological diseases of women, breast cancer is a widespread disease. It should be noted that the disease is heterogeneous in many clinical and pathological features. The development of breast cancer is associated with many factors, and it is difficult to identify any specific risk factor. Thus studying and epigenetic mechanisms of genes associated with breast cancer, it is possible to effectively combine epigenetic therapy with the existing one in clinical practice (chemotherapy, hormonal therapy, etc.) and accordingly, the treatment of disease is significantly improved.

Among oncological diseases of women, breast cancer is a widespread disease. It should be noted that the disease is heterogeneous in many clinical and pathological features. The development of breast cancer is associated with many factors, and it is difficult to identify any specific risk factor. Thus studying and epigenetic mechanisms of genes associated with breast cancer, it is possible to effectively combine epigenetic therapy with the existing one in clinical practice (chemotherapy, hormonal therapy, etc.) and accordingly, the treatment of disease is significantly improved.

In general, oncological diseases are linked to numerous causative factors, including hormonal imbalance, the alteration of some clinic-laboratory characteristics genetics and environmental factors [1-12].

Genetic/epigenetic alteration contributes to non-oncological disease as well [13,14]. In general, among oncological diseases of women, breast cancer is a widespread disease. It is confirmed that the disease is heterogeneous regarding numerous clinical and pathological features. The development of breast cancer is associated with many factors, and it is difficult to identify any specific risk factor. Thus, a statement problem is a thorough study of the processes related to the disease, including the study of the molecular mechanisms associated with altered gene expression and the detection of relevant markers, which will make it possible to identify the problem at an early stage of the disease and development and implementation of effective measures. It should also be noted that the disease is a hormone-dependent tumor, so current changes in target genes can be due to both genetic and epigenetic features. Accordingly, against the background of complex changes occurring at the cellular and molecular level, the expression of many gene changes, which a change in the regulation of gene expression, DNA methylation, chromatin demodernization, etc can cause. In the recent period, especially research conducted in the direction of epigenetics, provide an opportunity to develop new approaches to the timely diagnosis and treatment of the disease. In particular, the study of epigenetic mechanisms provides an opportunity to develop combined strategies to treatment. Therefore, with a careful analysis of epigenetic mechanisms, it is possible to strengthen the personalized approach to the disease, identify the correct treatments, and ultimately significantly improve the patient's quality of life.

Etiology of BC

Breast Cancer (BC) is widespread in women and may develop in the breast lobules, tubes, or connective tissue. Although various criteria and classifications are possible, the most representative types of breast cancer can be divided into four types based on molecular subtypes with very different metastases, prognosis, and therapeutic methods [15]. These subtypes include Luminal A and B, characterized by Estrogen Receptors (ER) and/or Human Epidermal Growth Factor Receptor 2 (HER2). In addition, HER2-positive tumors form a subtype known as HER2-enriched, while Triple Negative Breast Cancer (TNBC) represents a subtype lacking ER, Progesterone Receptors (PR), and HER2.

The distinct molecular characteristics of these breast cancer subtypes arise from genetic and epigenetic changes. Epigenetic alterations are reversible modifications to genes that can influence gene expression levels, particularly in relation to cancer cell growth and spread. Common types of epigenetic changes include promoter methylation and modifications to histones, which are proteins that help package DNA.

Promoter methylation involves adding a methyl group to specific regions of DNA, which is carried out by DNA Methyltransferases (DNMT). Histone Acetyltransferases (HAT) activate histone acetylation, which adds an acetyl group to histones, while Histone Deacetylases (HDAC) remove acetyl groups. Histone Methyltransferases (HMT) and Histone Demethylases (HDM) are responsible for histone methylation. These modifications to DNA and histones play a role in regulating gene activity.

In the early stages of breast cancer development, higher levels of promoter methylation and modifications to chromatin structure can occur in certain genes. These epigenetic changes contribute to the initiation and progression of breast cancer [16,17].

Even with improvements in detecting breast cancer early and treating it successfully, some patients experience a progression of the disease to an advanced stage, and the exact reasons for this are not yet known. That's why it is crucial to explore new substances or molecules to gain a better understanding of how breast cancer progresses.

Altered gene expression and epigenetics in BC

A comprehensive definition of epigenetic events could be described as the adaptive modification of chromosomal regions to record, communicate, or maintain altered activity states. This definition encompasses both temporary modifications related to DNA repair or cell cycle stages and enduring changes that persist across multiple cell generations. It primarily pertains to chromosomes and genes, while implicitly excluding the potential influence of three-dimensional architectural templating of membrane systems and prions, unless they directly impact chromosome functioning [18].

Targeting changes in how genes are regulated in breast cancer is an exciting and evolving field that can potentially improve how we treat the disease. BC is influenced by complex pathways that control gene activity, known as epigenetic pathways. Combining epigenetic therapies with current treatments like chemotherapy and hormone therapy shows promise in improving outcomes for breast cancer patients. Since the effects of certain epigenetic therapies can be temporary and reversible, ongoing studies are being conducted to determine the best doses and schedules for these treatments [19].

Epigenetic processes have emerged as potential targets for altering breast cancer progression with positive outcomes. Epigenetic instability is known to play a crucial role in developing and advancing breast cancer. The ability to reverse the epigenetic changes associated with breast cancer is significant, as these changes can be extensive and reversible. Future investigations need to focus on maintaining the epigenetic balance in breast cancer [20].

MicroRNAs (miRNAs) are a type of small RNA molecules that play a role in controlling gene expression by either degrading RNA or interfering with translation. Abnormal expression of miRNAs has been observed in several types of cancer, and they are believed to have tumor-suppressing properties [21].

In general, different type of microRNA has key contribution to Breast Cancer’s development. According to a study TIMP-3 protein levels are negatively associated with miR-21 levels within pure invasive BCs. Moreover, Novel therapeutic strategies of miRNA-based agents may play an important role in breast cancer therapy [22-25].

Exploring the role of circular RNAs (circRNAs) in the progression of breast cancer through their interaction with miRNAs has been a topic of interest in breast cancer research. Among the various circRNAs associated with breast cancer, one particular circRNA, hsa_circ_001783, has shown significant relevance. Studies have revealed that higher expression levels of hsa_circ_001783 are associated with larger tumor burdens and poorer prognosis in breast cancer patients. Importantly, hsa_circ_001783 regulates breast cancer cells' proliferation and metastasis by acting as a sponge for a specific miRNA known as miR-200c-3p. This interaction plays a crucial role in the progression of breast cancer.

One such study examined the relationship between the expression of hsa_circ_001783 and pathological characteristics in breast cancer patients. The patients' primary tumors was classified in two portions in relation to their average hsa_circ_001783 expression. The findings revealed several significant correlations between hsa_circ_001783 expression and various tumor characteristics. The expression of hsa_circ_001783 was related with tumor size (p < 0.001), Lymph Node (LN) status (p < 0.001), TNM stage (p < 0.001), ER status (p = 0.02), PR status (p < 0.001), molecular subtype (TNBC vs. non-TNBC, p < 0.001), and Ki-67 index (p = 0.008). However, no significant associations were found with age, menopause, HER2 status, and histological grade [26].

The analysis using RNA FISH (Fluorescence in Situ Hybridization) showed that hsa_circ_001783 was amplified as compared to the non-cancerous tissue. Furthermore, the expression of hsa_circ_001783 was significantly higher in the ER-PR-HER2- (triple negative, TN) subtype compared to the luminal and HER2 amplification (HER2+) subtypes. This upregulation in Triple-Negative Breast Cancer (TNBC) was consistent with the results obtained from qPCR, which showed a 2.64-fold increase in hsa_circ_001783 expression (p < 0.0001) in TNBC tumor samples. Moreover, the study investigated the relationship between hsa_circ_001783 expression and the Ki-67 proliferation index. The results revealed a positive correlation between hsa_circ_001783 expression and the Ki-67 level in breast tumors (p = 0.009). Further analysis indicated that patients with higher levels of hsa_circ_001783 were more likely to experience disease recurrence and had poor disease-free survival (p < 0.001). Through analysis alone, it was determined that the level of expression of hsa_circ_001783 served as an independent factor in forecasting the prognosis of patients with breast cancer, with a Hazard Ratio (HR) of 9.114 (95% Confidence Interval [CI]: 2.428-34.206, p = 0.001) [26].

Therefore, the study demonstrated significant correlations between hsa_circ_001783 expression and various pathological characteristics in breast cancer. The gene's expression was associated with tumor size, LN status, TNM stage, ER status, PR status, molecular subtype, and Ki-67 index. Additionally, hsa_circ_001783 showed remarkable over-expression in breast cancer tissue, particularly in the TNBC subtype. The findings also indicated that higher expression of hsa_circ_001783 was linked to increased proliferation potential, poorer disease-free survival, and higher risk of disease recurrence, emphasizing its potential as an independent prognostic factor in breast cancer patients [26].

Being a complex disease with diverse molecular subtypes and clinical outcomes, it has been the subject of research examining the correlation between LINC02273 gene expression and patient prognosis [27].

To explore this correlation, a study examined the expression of LINC02273 in a group of 55 breast cancer patients who had experienced lymph node metastasis. The results showed a significant increase in LINC02273 expression in the metastatic loci within lymph nodes compared to primary tumors (p < 0.001). In a larger breast cancer cohort (n = 319), tumors with positive lymph nodes exhibited significantly higher LINC02273 expression (p = 0.048). Moreover, patients with pN3 lymph node metastasis had higher LINC02273 expression in primary tumors compared to pN1 (p = 0.026) or pN0 patients (p = 0.026).

Furthermore, LINC02273 expression was found to be highly expressed in tumors of patients who later developed metastatic cancer. This observation suggests that LINC02273 may play a role in driving metastasis in breast cancer. The Recurrence-Free Survival (RFS) analysis indicated that high LINC02273 expression correlated with poor outcomes. Subgroup analysis revealed that patients with Estrogen Receptor (ER) and Progesterone Receptor (PR)-positive breast cancer and low LINC02273 expression showed significant benefits in terms of RFS. A similar trend was observed in the Triple-Negative Breast Cancer (TNBC) and HER2-positive groups, although statistical significance was not reached, likely due to smaller sample sizes. Analyses were performed to assess the predictive value of LINC02273 expression in the cohort. The results indicated that lymph node metastasis burden and high LINC02273 expression were prognostic predictors for poor RFS. It was confirmed that LINC02273 could serve as an independent prognostic factor in breast cancer (HR = 1.543, p = 0.045). In addition, the researchers utilized the bc-GenExMiner 3.0 database to explore the predictive role of LINC02273 for metastatic events in breast cancer. The analysis revealed that high LINC02273 expression was correlated with poor metastasis/recurrence-free survival (MR-free survival) (HR = 1.57, p = 0.017). In summary, the research findings indicated a notable rise in LINC02273 expression within lymph node metastases, establishing a direct link with unfavourable clinical outcomes in breast cancer. These findings suggest that LINC02273 may contribute to metastasis in breast cancer patients and highlight its potential as a prognostic marker for patient management.

The decreased expression of the DNAJC10 (ERDJ5) gene has been associated with reduced survival rates among breast cancer patients. DNAJC10, is known as an anti-cancer gene in several types of cancers. However, its significance and function in breast cancer remain unknown. The aim of one such study was to reveal the expression of DNAJC10 in breast cancer cells, both in vitro and in vivo. Furthermore, the researchers examined its potential as a biomarker and analyzed genetic and epigenetic alterations related to DNAJC10 in breast cancer. The results of the study revealed significantly lower levels of DNAJC10 mRNA expression in three out of four breast cancer cell lines compared to a non-tumorigenic mammary epithelial cell line called MCF 10A. Similarly, the protein expression of DNAJC10 was less common in samples of invasive ductal carcinoma (n = 121) compared to adjacent normal breast tissues (n = 32) (p < 0.0001). Moreover, the reduced mRNA expression of DNAJC10 was linked to poorer overall survival (n = 626) (p = 0.0096) and relapse-free survival (n = 1764) (p = 5.3e-12) in breast cancer patients. Point mutations and copy number variations in the DNAJC10 gene were not so common in samples. Additionally, no genetic alterations were identified in the promoter regions of DNAJC10 that were experimentally verified in breast cell lines. However, the CpG Island, a specific DNA sequence within the promoter regions of the DNAJC10 gene, was frequently found to be hypomethylated in breast cell lines. This suggests that factors other than promoter methylation may contribute to the decreased expression of DNAJC10 in breast cancer. Taking into account previous knowledge regarding the role of DNAJC10 in cancer development, the findings of this study indicate that DNAJC10 could serve as a potential diagnostic and prognostic biomarker and a potential tumor suppressor for breast cancer. Furthermore, the study suggests that epigenetic factors other than promoter methylation may be involved in downregulating DNAJC10 expression [28].

The initial investigation aimed to examine whether the expression of NUPR1 varies during the development of resistance to tamoxifen in breast cancer cells. Tamoxifen-resistant (TamR) clones were generated from three breast cancer cell lines (MCF-7, T-47D, and BT-474) by exposing them to 2 μM Tamoxifen for over 15 months. Initially, the viability of MCF-7 cells, sensitive to estrogen, significantly decreased upon Tamoxifen treatment compared to the control. Subsequently, analysis revealed that the mRNA level of NUPR1 was higher in TamR cells than control cells. Additionally, elevated protein levels of NUPR1 were observed in MCF-7TamR, T-47DTamR, and BT-474TamR cells compared to MCF-10A cells, which have low NUPR1 expression. However, Tamoxifen treatment did not significantly affect NUPR1 protein levels in MDA-MB-231 cells, which are Tamoxifen-resistant. The findings suggest that NUPR1 potentially contributes to the emergence of Tamoxifen resistance during development in Estrogen Receptor Alpha (ERα)-positive breast cancer cell lines following 15 months of treatment. A tissue array consisting of primary invasive ductal carcinoma samples from human breast tissues (n = 133) was utilized to further investigate. Immunohistochemistry (IHC) analysis demonstrated strong nuclear NUPR1 staining in 30% of breast tumor tissues, whereas adjacent noncancerous breast tissues showed no staining. Immunocytochemistry analysis confirmed the specificity of the NUPR1 antibody by showing reduced NUPR1 staining in NUPR1-depleted MCF-7TamR cells. Moreover, the level of NUPR1 protein was significantly associated with post-surgery survival time. Among the 133 breast cancer patients, those with low NUPR1 staining scores (n = 91) had a median survival of 95.4 months, whereas those with high NUPR1 scores (n = 42) had a median survival of 68.1 months. This difference was statistically significant (p = 0.0021), indicating that high NUPR1 protein levels predicted lower survival rates (hazard ratio 6.998, CI 4.128–11.862). The NUPR1 staining scores did not correlate significantly with age, TNM, or hormone status. Overall, these findings suggest that high expression of NUPR1 is linked to lower overall survival rates in breast cancer patients [29].

In order to investigate the clinical relevance of the results, the researchers conducted an analysis using human breast tumor samples and adjacent breast tissues. They utilized Immunohistochemistry (IHC) to examine the expression of two proteins, namely CUL4B and ER-α36, in these samples. The findings demonstrated that CUL4B was primarily localized in the nuclei and showed increased expression in 24 out of 30 tumor samples (80%) compared to their matched adjacent non-tumor tissues. Similarly, ER-α36 levels were higher in 26 out of 30 tumor samples (86.7%) compared to the corresponding non-tumor tissues.

Importantly, a significant positive correlation was discovered between the expression of CUL4B and ER-α36 (r = 0.6628, p < 0.001). To further validate this correlation, the researchers conducted an additional analysis using western blotting on 38 breast cancer and paired non-cancerous tissue specimens. The results indicated that CUL4B was upregulated in 24 out of 38 cancer tissues (63%) and positively correlated with the levels of ER-α36.

Furthermore, the researchers examined the levels of miR-32-5p in the paired breast specimens using RT-qPCR. Consistent with the findings from breast cancer cell lines, a significant inverse correlation was observed between the expression of CUL4B protein and the levels of miR-32-5p RNA. Additionally, the levels of miR-32-5p RNA showed a negative correlation with the expression of ER-α36 protein. Collectively, the results revealed a strong positive correlation between the expression of CUL4B and ER-α36 and a negative correlation between the expression of CUL4B and miR-32-5p. These findings were consistent in both clinical breast tumor samples and breast cancer cell lines. The statistical analysis confirmed the significance of the correlation between CUL4B and ER-α36 (r = 0.6628, p < 0.001), indicating a robust association between the two proteins. Similarly, the correlation between CUL4B and miR-32-5p was also significant (p < 0.001), suggesting an inverse relationship between them [30].

A research study was conducted on women with advanced hormone-resistant or Triple-Negative Breast Cancer (TNBC) to test the effectiveness of a treatment combining 5-azacitidine (AZA) and entinostat. The patients in the study were given AZA for specific days within a 28-day cycle, along with entinostat on different days of the cycle. If there was progression of the disease, the patients were given the option to continue the epigenetic therapy with the addition of endocrine therapy (optional continuation phase).

The studies by Connolly RM, et al. [31], main goal was to determine the Objective Response Rate (ORR) in each group of patients. The study's results showed that among the 27 women with hormone-resistant disease, only one patient had a partial response, resulting in an ORR of 4% (95% confidence interval: 0–19%). In the group of 13 women with TNBC, there were no partial responses observed. However, one additional partial response was observed among 12 patients in the optional continuation phase for the hormone-resistant group. Tumor samples were collected before and after treatment, and approximately 58% of the samples were paired. Around 50% of the post-treatment biopsies in the hormone-resistant group showed either up- or down-regulation of ER, while this observation was not seen in the TNBC group. In conclusion, the patients tolerated the combination of epigenetic therapy using AZA and entinostat. The results from the optional continuation phase suggest that some women may still benefit from epigenetic therapy and/or the reintroduction of endocrine therapy after disease progression. Further research is necessary to investigate this potential benefit in more detail.

Furthermore, it was discovered that the level of ZDHHC22 expression in breast cancer (BrCa) is related to the status of Estrogen Receptors (ER) due to a process called hypermethylation, which affects the gene's activity. Experiments involving manipulating the function of ZDHHC22 showed that it significantly hindered the growth of BrCa cells in the laboratory and living organisms. ZDHHC22 is an enzyme that transfers a fatty acid called palmitate to other proteins, and its ability to fight against tumors in BrCa depends on its enzymatic activity. Mechanistic investigations revealed that ZDHHC22 reduces the stability of a protein called mTOR by adding palmitate to it, inhibiting the activity of mTOR's downstream targets. Moreover, the expression of ZDHHC22 was found to be negatively linked to mTOR in BrCa cells that were resistant to tamoxifen, a common drug used in endocrine therapy. Restoring ZDHHC22 expression could improve the sensitivity of these tamoxifen-resistant BrCa cells to treatment. To summarize, this study provides insights into the expression pattern and role of ZDHHC22 in BrCa. It demonstrates how ZDHHC22 modifies mTOR through palmitoylation, leading to a decrease in the growth of malignant cells and resistance to endocrine therapy. These findings expand our understanding of how the ZDHHC protein family regulates the development and progression of cancer and offer a foundation for future exploration of targeted drugs that affect palmitoylation to overcome endocrine therapy resistance in breast tumors [32].

In a study that looked at previous data, researchers examined ESR1 mutations in stored blood samples from two clinical trials. The first trial, called SoFEA, compared the effectiveness of two treatments (exemestane and fulvestrant) in patients who had previously responded well to nonsteroidal AI medication. The second trial, called PALOMA3, compared two treatments (fulvestrant plus placebo and fulvestrant plus palbociclib) in patients whose condition worsened after previous hormonal therapy. The researchers used a method called multiplex digital polymerase chain reaction to analyze the ESR1 mutations.

In the SoFEA trial, they found ESR1 mutations in 39.1% of patients (63 out of 161). Among those with mutations, 49.1% (27 out of 55) had multiple mutations, and the detection of mutations was not affected by delays in processing the stored blood samples. Patients with ESR1 mutations had better Progression-Free Survival (PFS) when treated with fulvestrant compared to exemestane. Specifically, out of the patients who took fulvestrant, 45 had ESR1 mutations and 18 had wild-type ESR1. The analysis showed that the risk of progression or worsening of the disease was lower in patients with ESR1 mutations who received fulvestrant (HR of 0.52; 95% CI of 0.30 to 0.92; p = .02). On the other hand, patients with wild-type ESR1 had similar PFS regardless of whether they received exemestane or fulvestrant (HR of 1.07; 95% CI of 0.68 to 1.67; p = .77).

In the PALOMA3 trial, ESR1 mutations were found in the plasma of 25.3% of patients (91 out of 360). Among those with mutations, 28.6% (26 out of 91) had multiple mutations, and these mutations were associated with acquired resistance to previous AI treatment. The combination of fulvestrant and palbociclib improved PFS compared to fulvestrant plus placebo in both patients with ESR1 mutations and those without. For patients with ESR1 mutations, the HR was 0.43 (95% CI of 0.25 to 0.74; p = 0.002), indicating a lower risk of disease progression or worsening. Similarly, for patients with wild-type ESR1, the HR was 0.49 (95% CI of 0.35 to 0.70; p < 0.001), indicating a lower risk as well.

Based on these findings, analyzing ESR1 mutations in plasma after disease progression following previous AI therapy could help guide the choice of further hormonal treatment [33].

In a study, it was discovered that a protein called SALL2 is reduced during the use of tamoxifen treatment. This was observed by examining 9 pairs of breast cancer tissues: the original tumors before tamoxifen treatment and the resistant tissues that developed later in the same individuals. To investigate further, the researchers conducted experiments in the laboratory and living organisms. The results demonstrated that SALL2 plays a crucial role in controlling the expression of the Estrogen Receptor (ER), which is important in breast cancer. Additionally, it was found that breast cancer patients with high levels of methylation in the SALL2 gene had a shorter period of time without disease recurrence when treated with tamoxifen. However, when the researchers restored the levels of SALL2 using a specific inhibitor, they observed increased sensitivity to tamoxifen therapy in breast cancer cells with high levels of SALL2 methylation and either high or low levels of methylation in the ER gene (ESR1).

Therefore, these findings provide valuable insights into the function of SALL2 in regulating the estrogen receptor and suggest the possibility of using SALL2 methylation status as a clinical marker to categorize breast cancer patients. This marker could help identify those likely to benefit from a combined treatment involving tamoxifen and a DNMT inhibitor, a drug that targets DNA methylation.

The researchers also discovered that SALL2 messenger RNA (mRNA) levels were significantly associated with the levels of ESR1 in breast cancer tissues with low levels of methylation in the ER gene (p = 0.005, r = 0.420; n = 44). However, this association was not observed in breast cancer tissues with high levels of methylation in the ER gene (p = 0.201, r = 0.130; n = 99), which are also negative for the estrogen receptor. Importantly, when SALL2 was overexpressed, meaning its levels were artificially increased, it resulted in a significant increase in ER mRNA in breast cancer cells with low levels of methylation in the ER gene (specifically, BT-20 cells). However, this overexpression had no effect on ER expression in breast cancer cells with high levels of methylation in the ER gene (specifically, BT-549 and MDA-MB-231 cells). Furthermore, when SALL2 was overexpressed in BT-20 cells but not in BT-549 and MDA-MB-231 cells, it led to a significantly improved response to tamoxifen treatment.

These additional findings further support the idea that SALL2 activates the expression of the estrogen receptor and enhances the response to tamoxifen treatment in breast cancer [34].

SR-4835 is a drug that can be taken orally, and it specifically targets a group of proteins called CDK12 and CDK13. It has been tested against a large number of different kinases (a type of protein) and has shown to have a strong effect on CDK12 and CDK13 without affecting many other proteins. One important thing to note is that SR-4835 has shown great potential in fighting a type of breast cancer known as TNBC (triple-negative breast cancer). It has been found to work well against TNBC both in cell-based experiments and in living organisms. Additionally, when used in combination with other standard treatments for TNBC like cisplatin, irinotecan, and olaparib, SR-4835 has been found to enhance their anti-cancer effects.

The way SR-4835 works to fight cancer is by causing stress responses within the cancer cells. One of these responses is a significant change in the DDR (DNA damage response), which is a process that helps repair damaged DNA in our cells. Inhibition of CDK12 and CDK13 by SR-4835 disrupts this DDR process and decreases the expression of important proteins involved in DNA damage repair. This disruption triggers an alternative mechanism called intragenic poly(A) usage, which results in the rapid accumulation of DNA lesions (damages) and ultimately causes the cancer cells to undergo apoptosis (cell death). It is worth mentioning that these effects have also been observed in models of TNBC that were developed from patient-derived tumor tissues and were resistant to conventional therapies. In these models, combining SR-4835 with cisplatin or irinotecan has been shown to increase DNA damage, induce apoptosis, and lead to the regression of tumors [35].

According to the literature, it is that along with genetics, changes in epigenetic mechanisms play a leading role in the development of breast cancer. In the early stages of tumor development, promoter methylation and changes in chromatin structure can occur in some target genes (Table 1). Thus, altered epigenetic mechanisms generally significantly determine the initiation and progression of the tumor process in the mammary gland. Even if breast cancer is detected in early stages and treated successfully, the disease still can progress. Therefore, careful study of the progression of breast cancer is critical. In particular, by studying the expression and epigenetic mechanisms of genes associated with breast cancer, it is possible to effectively combine epigenetic therapy with the existing one in clinical practice (chemotherapy, hormonal therapy, etc.) and accordingly, the treatment of disease is significantly improved.

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