Oestrogen Receptor Gene Overexpression

Oestrogen receptor gene overexpression occurs when cells produce too much of the protein that responds to oestrogen hormones, affecting how genes are turned on and off in the body, with important implications for breast cancer development and treatment.

Table of contents

• What Are Oestrogen Receptors?
• How Oestrogen Receptors Control Gene Expression
• Overexpression in Breast Cancer
• Mechanisms That Regulate Receptor Expression
• Treatment Implications
• Related Conditions

What Are Oestrogen Receptors?

Oestrogen receptors are proteins inside cells that bind to oestrogen hormones and act as switches to turn genes on or off. These receptors are part of a larger family called nuclear receptors, which are proteins that regulate how our genes work^[1]. When oestrogen binds to these receptors, it triggers a series of events that control whether specific genes are activated or silenced.

There are two main types of oestrogen receptors in the body: ERα (estrogen receptor alpha) and ERβ (estrogen receptor beta). Both receptors share similar structures, with a part that binds to DNA and another part that binds to oestrogen. The DNA-binding region is nearly identical between the two types, with 97% similarity^[2]. Despite these similarities, ERα and ERβ can activate different genes and produce different effects in the body.

A third type of oestrogen receptor, called GPER1, sits in the cell membrane rather than inside the cell nucleus. This membrane receptor responds quickly to oestrogen and activates different cellular pathways^[5].

How Oestrogen Receptors Control Gene Expression

Oestrogen receptors control gene expression through two main pathways: direct and indirect mechanisms. In the direct pathway, when oestrogen binds to its receptor, the receptor moves into the cell’s nucleus and attaches directly to specific DNA sequences called oestrogen response elements (EREs). These sequences have a specific pattern (5′-AGGTCAnnnTGACCT-3′) that the receptor recognizes^[2].

Once attached to DNA, the oestrogen receptor doesn’t work alone. It recruits other proteins called coactivators in a specific order. Each coactivator that joins the complex causes structural changes that are necessary for the receptor to effectively turn genes on. Research has shown that when a coactivator called CARM1 joins the complex, it causes both chemical and structural changes that guide the next steps in gene activation^[11].

In the indirect pathway, oestrogen receptors can also control genes without directly binding to DNA. Instead, they interact with other proteins such as activating protein 1 (AP1) or nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB), which then regulate gene expression^[2].

Oestrogen-responsive genes include many that are involved in cell growth and division, such as cyclin D1, c-myc, and the progesterone receptor (PR)^[2]. This explains why oestrogen plays such an important role in tissues that need to grow and change, like the breast and uterus.

Overexpression in Breast Cancer

Approximately 70% to 75% of all breast cancers are characterized by the presence of oestrogen receptor, earning them the designation ER-positive (ER+) breast cancer^[3][6]. In these cancers, the oestrogen receptor continues to function as a gene-regulating protein, but it operates in an uncontrolled manner. This leads to cell division that is no longer regulated by normal control mechanisms, resulting in tumour growth.

The expression of oestrogen receptor in cancer is not simply a matter of having the receptor present or absent. The mechanisms that control receptor expression in tumours are complex and involve multiple layers of regulation. These include alternative splicing (where the same gene produces different protein versions), control by other proteins called transcription factors, chemical modifications to DNA that affect gene activity (epigenetic regulation), and control of how the receptor message is processed after the gene is read^[2].

In advanced breast cancer that has spread to other parts of the body (metastatic breast cancer), changes in the oestrogen receptor gene itself become common. Specifically, mutations (permanent changes in the DNA sequence) in the ESR1 gene (which codes for ERα) are found frequently in patients with advanced disease, especially in those who have been treated with drugs called aromatase inhibitors^[4]. These mutations are rare in untreated primary tumours but become more common as the cancer progresses and adapts to treatment.

The most common mutations occur at three specific locations in the receptor protein: at positions called leucine-536, tyrosine-537, and aspartate-538. Cancers with these mutations can grow without oestrogen being present, making them particularly difficult to treat^[4].

Mechanisms That Regulate Receptor Expression

The amount of oestrogen receptor produced in cells is tightly controlled through multiple mechanisms. Understanding these control systems helps explain why some cancers have high levels of receptor expression while others do not.

One important control mechanism involves proteins called transcription factors that can turn the oestrogen receptor gene on or off. Different transcription factors bind to the regulatory regions of the ESR1 gene and either promote or block the production of receptor protein^[2].

Epigenetic regulation represents another critical control layer. This includes chemical modifications to DNA and to the proteins around which DNA is wrapped, which can silence genes without changing the DNA sequence itself. In some breast cancers, the oestrogen receptor gene is silenced through these epigenetic mechanisms, resulting in ER-negative tumours^[2].

The stability and processing of the messenger RNA (the molecule that carries instructions from DNA to make proteins) also affects how much receptor protein is ultimately produced. Various molecules in the cell can either stabilize or destabilize this messenger RNA, controlling how much receptor is made from each copy of the gene^[2].

Recent research has revealed that another protein called HSF1 (heat shock factor 1) can influence oestrogen receptor expression. When HSF1 levels are modified, it impacts how much oestrogen receptor is produced and how cancer cells respond to anti-oestrogen treatments^[15].

Treatment Implications

The presence and level of oestrogen receptor expression has major implications for breast cancer treatment. ER-positive breast cancers are treated with endocrine therapies that either block oestrogen from binding to the receptor or reduce the amount of oestrogen in the body^[2].

These treatments include selective oestrogen receptor modulators (SERMs) like tamoxifen, which bind to the receptor but prevent it from activating genes. Other treatments, called aromatase inhibitors (AIs), reduce oestrogen production in the body. While these therapies have been successful in reducing breast cancer deaths, many patients eventually develop resistance to treatment^[2].

When oestrogen receptor mutations develop, they can make cancer cells less sensitive to standard treatments. Different mutations show varying responses to different types of anti-oestrogen drugs, meaning that knowing which specific mutation is present could help doctors choose the most effective treatment^[4].

Research has also identified patterns of gene expression that are associated with treatment resistance. Genes that are upregulated (more active) in resistant cancers include EZH2, WNT11, ITGB6, and TOP2A, while genes that are downregulated (less active) include SNAI2, ITPR1, PTEN, and WNT5A^[7]. Understanding these patterns may help identify patients at higher risk of treatment failure.

For cancers with oestrogen receptor mutations, newer drugs called selective oestrogen receptor degraders (SERDs) and other targeted therapies are being developed and tested. These aim to overcome the resistance caused by receptor mutations^[6].

Related Conditions

While oestrogen receptor overexpression is most commonly discussed in relation to breast cancer, oestrogen receptors also play roles in other cancers and conditions. Oestrogen has been associated with ovarian cancer and endometrial cancer, where it can promote tumour development^[2].

Interestingly, oestrogen may have protective effects in some organs. Some evidence suggests that oestrogen might have anti-cancer effects in liver cancer and colon cancer, though more research is needed to understand these mechanisms fully^[2].

Beyond cancer, oestrogen receptors are also important in brain health. They are expressed throughout the brain and have been implicated in learning, memory, and Alzheimer’s disease. Research suggests that oestrogen treatment might reduce the risk of Alzheimer’s disease in postmenopausal women, and that oestrogen receptors play roles in clearing proteins associated with the disease^[10].

• Breast tissue
• Ovaries
• Uterus (endometrium)
• Brain
• Bone
• Cardiovascular system
• Liver