ESR1
Introduction: Estrogens play a vital role in physiological processes involved in reproduction, sexual development, cardiovascular health, and bone integrity4. ESR1, a gene which encodes the estrogen-receptor alpha (ERα) is an important part of these processes. ERα is a nuclear steroid hormone receptor which acts as a transcription factor to regulate cell proliferation. It is also involved in estrogen signaling, which facilitates processes in the female reproductive system such as breast formation and ovulation. ERα is highly expressed in tissues such as the breast and ovaries, and this has been linked to carcinogenesis, or cancer formation5.
Gene Background: The ESR1 gene is located on chromosome 6 between positions 25.1 and 25.2, fixed on the 151,654,148 to 152,129,604 base pairs1.
Figure 1: ESR1 gene location- chromosome 6 between positions 25.1 and 25.21.
ESR1 is a large gene that spans 473 kb and has 8 exons. This gene is composed of three domains including an N-terminal domain also known as the AF-1 domain, a DNA-binding domain, and a C-terminal ligand-binding domain also known as the AF-2 domain. The AF-1 domain contains a ligand-independent transactivation function whereas the AF-2 domain contains a ligand-dependent transactivation function. Both the AF-1 and AF-2 domains are responsible for activating transcription. They accomplish this both in an independent fashion and also in a synergistical fashion, meaning that the overall effect is larger than the sum of individual effects4. The gene has been discovered to have 87 ornologues and some examples of these are chimpanzees, Rhesus monkeys, dogs, cows, mice, rats, chickens, zebrafish, and frogs3. ESR1 has been studied in animal models, specifically in mice in order to determine its function in certain human organs.
ERα is the transcription factor belonging to this gene and it is part of the nuclear receptor family. The nuclear receptor family is a class of proteins that play a role in sensing thyroid and steroid hormones10. There are several receptors part of this family and some examples of these include: the vitamin D receptor (PPP1R163), the thyroid receptor alpha (THRα), and the progesterone receptor (PR)10. The full length of ERα is 595 amino acids long. It contains two short isoforms: hERα-46 and hERα-36. These isoforms lack the AF-2 region of the three domains. There are 15 different transcripts of ERα and resulting from these transcripts are 47 different phenotypes1. There are several different splice variants associated with ESR1, but the full-length nature of most these variants has not been determined4. The 5’-untranslated region, or UTR has shown to differ in ERα when comparing most splice variants, but they do not differ in the coding sequence. ERα can form a heterodimer with another close receptor, ERβ. These two estrogen receptors have been shown to have very similar homology with the difference being in their N-terminal domains1. Together, they work to accomplish estrogen signaling in the cell.
Gene Effects: Estrogen signaling begins when the ESR1 gene encodes an estrogen receptor (ERα) which is responsible for many functions involved in DNA binding, hormone binding, and activating transcription. Estrogen is essential for sexual development and reproduction. Aside from this, estrogen plays a key role in biological processes in the cardiovascular, immune, musculoskeletal, and central nervous systems in both men and women. Because of this, ERα is widely spread throughout the body in many organs including the uterus and ovaries, male reproductive organs, mammary glands, bones, the heart, hypothalamus, pituitary gland, liver, lungs, kidneys, spleen, and adipose tissues2. In the cells, ESR1 has been found mostly localized in the vesicles of the cell and also localized in the nucleus1.
Figure 2: ESR1 localized in the vesicles and nucleus of the cell1.
There are many proteins made from ESR1, and these proteins can be put into protein classes of: cancer-related genes, disease related genes, FDA approved drug targets, nuclear receptors, predicted intracellular proteins, and transcription factors1. These proteins are expressed in tissues including the breast, cervix, uterus, endometrium, fallopian tube, and smooth muscle.
Figure 3: Expression of protein made by ESR1 in the smooth muscle, fallopian tubes, breast, vagina, cervix, and endometrium1.
mRNA is also made from ESR1 and is more widely expressed in tissues, as shown in the ‘RNA expression overview’ graph below.
Figure 4: Expression of RNA made by ESR1 in all of the tissues listed on the horizontal axis1.
When the structure of ESR1 is changed, variants are formed as a result. Of the 15 transcripts mentioned previously, many splice variations occur. Many of these splice variants are related to cancers associated with the reproductive system. Cancer occurs as a result of a gene mutation. Most mutations in ESR1 happen in the form of substitutions. These substitutions are usually missense mutations, in which one codon is switched for another causing a different amino acid to be formed in the resulting protein1. There are 47 different phenotypes for these variants. The most common substitution that occurs with ESR1 is an adenine (A) to guanine (G) substitution1.
Figure 5: This figure shows that most (~41%) of substitution mutations occur between A to G)1.
Figure 6: This is a graph comparing the different types of mutation. A substantial amount of the mutations occur as missense substitutions1.
Several single nucleotide polymorphisms (SNPs) and variable-number tandem repeat (VNTR) polymorphisms have also been detected in ESR1 but not many have been studied in their health outcomes7. Two of the most widely studied SNPs are located on exon 1 of ESR1 and are named PvuII and Xbal. The T and C allele of the Pvull allele is referred to as the P and p allele and the A and G allele of the Xbal is referred to as the x and X allele, respectively7. The variant alleles are the X and P alleles. Currently, there is no explanation as to what biological pathways these alleles affect, but there is evidence that these SNPs influence the transcription of the ESR1 gene through altered transcription binding (a mechanism in which cis-regulatory variants affect gene expression). These polymorphisms can detect diseases in women like osteoporosis or cardiovascular diseases. Most studies indicate that the x and p alleles heighten the risk for breast cancer and osteoporosis, yet reduce the risk for endometriosis. ESR1 genes have also been found to be ethnic and race specific. An example of this is in Caucasian populations, the genotype associated with an increased risk of osteoporosis is pp and/or xx. But, in Asian populations the genotype associated with an increased risk of osteoporosis is PP and/or xx. Another widely studied polymorphism is a TA-variable number of tandem repeats (VNTR) in the promoter region that can have an impact on tissue-specific gene expression. It has been shown that lower repeat numbers results in a greater risk of endometriosis and premature ovarian dysfunction, lower bone mineral density, and a higher risk for osteoporotic fractures10.
Another example of a missense mutation in ESR1 causes estrogen resistance in metastatic estrogen receptor positive cancer cells, the most common type of breast cancer cells. ESR1 has been known to have mutations in the ligand-binding domain (AF-2), gene amplification, and gene translocation6. These 3 common mutations are most widely studied in breast cancer. This is because ERα is expressed in approximately 70% of all breast cancers. Estrogen receptor expression is one of the main features defined when determining tumor subtype and when discussing therapeutic options in treating breast cancer. In normal cells, when estrogen binds to the receptor, it induces changes in the ligand-binding domain, or AF-2 domain which allows the estrogen-estrogen receptor complex to bind specific DNA sequences. While these conformational changes are happening, coactivator and corepressor proteins play the role in regulating transcription of the estrogen-responsive genes which are important in numerous physiological and pathological processes, including carcinogenesis and tumor progression. In mutated cells, the AF-2 domain is removed and this results in constitutive activation of the estrogen receptor (ERα). This in turn causes promoted growth of the cancer cells, resulting in a very undesirable mutation. This mutation also causes estrogen receptor therapy resistance. Selective estrogen receptor downregulators (SERDs) aid in the treatment of breast cancer by stopping estrogen from entering the cell. Because the mutated cells no longer have the AF-2 domain, it is constitutively activating and SERDs have no effect on the cell6. Several other genome wide association studies have been performed on ESR1 in order to to see if any variant is shown to be associated with a trait. Some findings in phenotypes for different variants include susceptibility to chronic myeloid leukemia, alcohol dependence, and sudden cardiac arrest in patients with coronary artery disease4.
Recent research: New strategies to overcome these diseases caused by mutation in ESR1 are ongoing in current research. In a study published in 2015, scientists located mutations in the ESR1 gene in a significant number of patients with estrogen receptor positive breast cancer. They created laboratory models of breast cancer, then tested the models and observed that the mutations caused estrogen receptor therapy resistance to drugs used in cancer therapy4. A more recent study published in 2018 added to this study by finding another effect of these mutations. They found that the mutations not only cause resistance to estrogen blocking drugs, they also cause cancer cells to metastasize, or spread into other organs. These findings can play a role in future perspectives of creating a strategy to effectively treat mutations in breast cancer cells9. Scientists have also developed a highly sensitive blood test to detect if a person with breast cancer has become resistant to treatment by looking at mutations in the ESR1 gene6. Another way to determine the effectiveness of estrogen-receptor treatment is to use serial positron emission tomography (PET) scans, using a special estrogen-containing isotope called 18F-flouroestradiol (FES)5. The PET scans aid in visualising the endocrine-therapy in vivo activity. This technology makes it possible for future drug developments to measure the effectiveness at specific targets in the cell. These two methods also remove the need for invasive biopsies and it could give an early warning of treatment failure. Also, as the clinical trial of drugs that target ESR1 mutations are developed, it helps to elect the most fitting treatment for people with advanced breast cancer. It has been shown in several studies that epigenetic events like methylation and deacetylation are involved in the mechanisms that regulate promoter transcription. Scientists have found that by silencing the promoter region of ESR1 through methylation, expression of the estrogen receptor protein is affected. High methylation results in estrogen negative receptors, making the person resistant to hormone therapy. These epigenetic markers may also be able to help anticancer therapy8.
Summary: The ESR1 gene is a large gene which includes three domains involved in activating transcription. The transcription factor belonging to this gene is known as ERα. ERα is part of the nuclear receptor family and it is involved in sensing thyroid and steroid hormones. Most splice variants in ESR1 tend to differ in their 5’-untranslated (UTR) region of the gene, but not in the coding sequence. These variants make functional mRNA and protein expressed in tissues such as the uterus, smooth muscle, fallopian tubes, breast, vagina, cervix, and endometrium. Many variants related to ESR1 are causes of certain cancers. Single polymorphisms are one of the most common mutations in ESR1. SNPs influence the transcription of the ESR1 gene through altered transcription binding and can detect diseases in women like osteoporosis or cardiovascular diseases. To overcome mutations in ESR1 which cause breast cancer, researchers are currently experimenting with different types of detection options. Non-invasive procedures have been developed in order to determine if the cancer has become resistant to estrogen-receptor therapy. Scientists have also found epigenetic markers that relate to discovering anticancer therapies. Further advancing knowledge about ESR1 and its receptor, ERα will aid in determining a better understanding of estrogen, estrogen receptors, and their relation to disease.
Bibliography
1. “Cell Atlas - ESR1.” The Human Protein Atlas, 2018, www.proteinatlas.org/ENSG00000091831-ESR1/cell.
2. Cosmic. “ESR1 Gene - COSMIC.” NRG3 Gene - Somatic Mutations in Cancer, 2018, cancer.sanger.ac.uk/cosmic/gene/analysis?ln=ESR1.
3. Database, Gene. “ESR1 Gene(Protein Coding).” GeneCards, 2018, www.genecards.org/cgi-bin/carddisp.pl?gene=ESR1.
4. “ESTROGEN RECEPTOR 1; ESR1.” OMIM, 2018, www.omim.org/entry/133430.
5. Fred Hutchinson Cancer Research Center. "PET scans confirm effectiveness of estrogen-blocking drugs in breast cancer patients." ScienceDaily. ScienceDaily, 18 August 2011. .
6. G. Schiavon, et al. Analysis of ESR1 mutation in circulating tumor DNA demonstrates evolution during therapy for metastatic breast cancer. Science Translational Medicine, 2015; 7 (313): 313ra182 DOI: 10.1126/scitranslmed.aac7551
7. Heldring, Nina, et. al. “Estrogen Receptors: How Do They Signal and What Are Their Targets.” American Journal of Physiology-Endocrinology and Metabolism, 2017, www.physiology.org/doi/10.1152/physrev.00026.2006.
8. Joaquina Martínez-Galán, et al. “ESR1 Gene Promoter Region Methylation in Free Circulating DNA and Its Correlation with Estrogen Receptor Protein Expression in Tumor Tissue in Breast Cancer Patients.” BMC Cancer, BioMed Central, 4 Feb. 2014, bmccancer.biomedcentral.com/articles/10.1186/1471-2407-14-59.
9. Rinath Jeselsohn, et al. Allele-Specific Chromatin Recruitment and Therapeutic Vulnerabilities of ESR1 Activating Mutations. Cancer Cell, 2018; 33 (2): 173 DOI: 10.1016/j.ccell.2018.01.004
10. Reinert, Tomas, et al. “Clinical Implications of ESR1 Mutations in Hormone Receptor-Positive Advanced
Breast Cancer.” Frontiers in Oncology, U.S. National Library of Medicine, 2017, www.ncbi.nlm.nih.gov/pmc/articles/PMC5350138/.
