Endometriosis

Abstract

Pelvic endometriosis is a complex syndrome characterized by an estrogen-dependent chronic inflammatory process that affects primarily pelvic tissues, including the ovaries. It is caused when shed endometrial tissue travels retrograde into the lower abdominal cavity. Endometriosis is the most common cause of chronic pelvic pain in women and is associated with infertility. The underlying pathologic mechanisms in the intracavitary endometrium and extrauterine endometriotic tissue involve defectively programmed endometrial mesenchymal progenitor/stem cells. Although endometriotic stromal cells, which compose the bulk of endometriotic lesions, do not carry somatic mutations, they demonstrate specific epigenetic abnormalities that alter expression of key transcription factors. For example, GATA-binding factor-6 overexpression transforms an endometrial stromal cell to an endometriotic phenotype, and steroidogenic factor-1 overexpression causes excessive production of estrogen, which drives inflammation via pathologically high levels of estrogen receptor-β. Progesterone receptor deficiency causes progesterone resistance. Populations of endometrial and endometriotic epithelial cells also harbor multiple cancer driver mutations, such as KRAS, which may be associated with the establishment of pelvic endometriosis or ovarian cancer. It is not known how interactions between epigenomically defective stromal cells and the mutated genes in epithelial cells contribute to the pathogenesis of endometriosis. Endometriosis-associated pelvic pain is managed by suppression of ovulatory menses and estrogen production, cyclooxygenase inhibitors, and surgical removal of pelvic lesions, and in vitro fertilization is frequently used to overcome infertility. Although novel targeted treatments are becoming available, as endometriosis pathophysiology is better understood, preventive approaches such as long-term ovulation suppression may play a critical role in the future.

Figure 1.

(a) Laparoscopy of the pelvis performed at the time of menstruation. Predictable cyclic ovulatory menses giving rise to repetitious episodes of retrograde travel of endometrial tissue and blood into the dependent portions of the pelvic cavity is the main cause of pelvic endometriosis. Not all women who experience retrograde menstruation, however, develop endometriosis. This suggests that a number of differences between the patients with endometriosis and disease-free women may account for this condition. These include increased quantities of menstrual tissue that reach the abdominal cavity because of outflow track obstruction or deeper separation of the functionalis layer from the basalis layer (see Fig. 6) and cellular and molecular defects in eutopic endometrial or peritoneal tissues of women with endometriosis. (b) Graphic depiction of retrograde flow of endometrial tissue fragments made of spindly stromal and cuboidal epithelial cells. (c and d) Menstrual tissue fragments may survive and grow on peritoneal or subperitoneal locations (peritoneal endometriosis) or may get deposited into the rectovaginal (RV) pouch during repetitious episodes of menstruation and remodel the neighboring vaginal, rectal, and cervical tissues via a chronic inflammatory process to give rise to a deep-infiltrating RV nodule. (e) The endometrial tissue fragments may populate the exposed lining of a follicular or corpus luteum cyst to eventually evolve into an endometrioma. [Adapted with permission from Bulun SE. Endometriosis. In: Strauss J, Barbieri R, eds. Yen & Jaffe’s Reproductive Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2019:609–642. Copyright © 2019 by Elsevier.]

Figure 2.

Laparoscopic views of pelvic endometriosis. (a) A raised superficial endometriotic implant on bowel serosa [visceral peritoneum]. (b) Deep-infiltrating endometriosis. A laparoscopic image sometimes described as “frozen pelvis” because of extensive endometriosis and diffuse tissue remodeling causing dense adhesions between the ovary, bowel (rectum), and the uterine peritoneum. White vesicular endometriotic lesions are visible in the delineated area that represents the upper tip of diffuse adhesions caused by endometriosis. A challenging dissection into this plane will eventually take the surgeon into the previously existing rectovaginal (RV) space now harboring a nodule composed of endometriotic tissue and surrounding fibrosis and allow the removal of this RV nodule [see Fig. 4b]. (c) Enlarged left ovary because of a large endometrioma buried in the normal tissue [see (d) and Fig. 4c]. (d) Dissection into the overly stretched normal white-tan ovarian cortical tissue. The fibrotic endometrioma cyst wall is grasped by a forceps. The suction apparatus was inserted into the cyst lumen to remove the thick chocolaty fluid composed of blood products. The surgeon will develop a plane between the normal ovarian tissue and cyst wall in an attempt to remove the cyst in its entirety.

Figure 3.

The indispensable roles of ovulation and estrogen in endometriosis, highlighting the mechanisms of menstruation, tissue survival, inflammation and pain, and the targets of treatment in endometriosis. E2, estradiol; P, progesterone. [Adapted with permission from Bulun SE. Endometriosis. In: Strauss J, Barbieri R, eds. Yen & Jaffe’s Reproductive Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2019:609–642. Copyright © 2019 by Elsevier.]

Figure 4.

(a) Peritoneal endometriosis with fibrosis. (b) Rectovaginal nodule with extensive fibrosis and tissue remodeling surrounding islands of endometriotic stroma and occasional epithelial cells. (c) Sections of an ovarian endometrioma cyst. Note that the thickness of the endometrial lining (stroma and luminal epithelium) varies throughout the cyst wall, with foci of bleeding and macrophages containing blood pigment. The cyst wall is primarily composed of fibrotic tissue. (d and e) Higher-magnification pictures showing details of the ovarian endometrioma cyst wall from (c).

Figure 5.

Overview of the complex roles of retrograde menstruation, epigenetically defective endometrial stromal cells, the epithelial cells carrying mutations, DNA methylation, nuclear receptors, and inflammation in endometriosis. E2, estradiol. Endometrial tissue fragments may implant on pelvic peritoneal tissue surfaces or may get trapped in an ovarian inclusion cyst such as a hemorrhagic corpus luteum cyst.

Figure 6.

Possible interactions between spiral arterioles, somatic stem cells, and menstruation and the risk of endometriosis. Menstruation occurs after the functionalis layer of endometrium separates from the basalis and is expelled through the cervix or uterine tubes. This separation occurs in association with vasoconstriction and coagulation in the spiral arterioles giving rise to degradation of the extracellular matrix, hypoxia, and necrosis in the separated segment. The basalis layer may contain higher numbers of functional progenitor cells that will regenerate the functionalis layer during the next proliferative phase under the influence of estrogen. It is plausible to hypothesize that the separation between the two layers at a relatively superficial plane may lessen the likelihood of heavy bleeding or travel of stem cells into the pelvic cavity. Separation at a deeper plane, however, may favor the endometriosis phenotype. One can envision, therefore, that possible epigenetic abnormalities in the vascular system may affect the process of separation between the functionalis and basalis and thus affect the risk for endometriosis. The somatic stem cells per se may also be defective, or simply larger numbers of otherwise normal stem cells that travel into the abdominal cavity may increase the risk for endometriosis. (Adapted with permission from Bulun SE. Endometriosis. In: Strauss J, Barbieri R, eds. Yen & Jaffe’s Reproductive Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2019:609–642. Copyright © 2019 by Elsevier.)

Figure 7.

Roles of endometriotic cell types with distinct abnormalities. Endometriotic stromal cells do not contain nonsynonymous base pair alterations (mutations) but display extensive epigenetic defects that regulate the expression or silencing of genes. Specific patterns of DNA methylation (●) or demethylation (○) give rise to the suppression or overproduction of specific proteins. In endometriotic stromal cells, GATA6, ERβ, and SF1 proteins are overproduced, whereas PR is suppressed, leading to progesterone resistance. These changes collectively cause the accumulation of inflammatory and tissue-remodeling substances, including PGE₂, E2, cytokines, and matrix metalloproteinases (MMPs). In contrast, portions of the eutopic endometrial or endometriotic epithelial cells accumulate tumor driver mutations that disrupt or change the function of critical proteins, including Kirsten rat sarcoma homolog (KRAS), phosphatidylinositol-3-kinase, catalytic α polypeptide (PIK3CA), AT-rich interactive domain-containing protein 1A (ARID1A), and numerous other oncogenes or tumor suppressors. Extraordinarily high concentrations of E2 and its metabolites in the ovary and stroma-derived inflammation may contribute to the accumulation of epithelial mutations. [Chronic inflammation is suspected to cause mutagenesis and carcinogenesis in other tissues such as Barrett’s esophagus and esophageal cancer (208).] The relative contributions of endometriotic stromal vs epithelial cells to the development of pelvic endometriosis are currently unknown. The effects of each cell type on the acquisition of genome-wide epigenetic defects or mutagenesis are also not known.

Figure 8.

Genome-wide DNA methylation differences between stromal cells isolated from eutopic endometrium and endometriosis. The methylated (●) or unmethylated (○) state of a cytosine nucleotide in the CpG (or CG) sequence may in part regulate how genes are expressed in the DNA of a cell. (a) Schematic diagram of CpGs depicts their genomic context relative to the nearest CpG island (top) or gene (bottom). Gene contexts of CpGs were described as within an “island” (brown); on the “shore,” defined as within 4 kb around the island (yellow); or in “open sea,” defined as at least 4 kb distal from an island, and relative to the nearest open reading frame within 1500 bp (TSS1500; purple) or 200 bp (TSS200; pink) of a transcription start site (TSS); in the 5′ untranslated region (UTR), the first exon of a transcript (green); in the body of the gene (orange) or the 3′UTR (red). (b) Pie charts show the distribution of the CpGs examined based on their genomic context for all probes retained from the original array (top), for all the probes identified as differentially methylated between endometriosis and endometrium (middle), and for all the differentially methylated probes that were matched to differentially expressed mRNAs (bottom; note that stacked bar graphs show the CpG island context broken down for each of the gene contexts). A large number (5423) of differentially methylated CpG islands in this array were linked to a differentially expressed gene. A further integrative analysis demonstrated that 403 genes associated with one or more differentially methylated CpGs are differentially expressed with possible functional consequences. “Transcription factors” as a category comprised the most important gene category, whereas the top biologic pathway was “blood vessel development.” [Reproduced from Dyson MT, Roqueiro D, Monsivais D, et al. Genome-wide DNA methylation analysis predicts an epigenetic switch for GATA factor expression in endometriosis. PLoS Genet. 2014;10(3):e1004158. Copyright © 2014 by Dyson et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.]

Figure 9.

An epigenetic switch from GATA2 to GATA6 in endometriosis. In endometrium, the GATA2 gene is characterized by a lack of DNA methylation (○). In contrast, endometriosis has hypermethylated CpGs throughout a CpG island and proximal sequences within the GATA2 promoter and throughout the gene. Several regions of the GATA6 gene are completely methylated in endometrium, whereas methylation was absent within three CpG islands and their shores within the body of the gene. As a result, GATA2 is expressed in healthy cells and upregulates several genes involved in decidualization; GATA2 may also maintain the expression of aldehyde dehydrogenase 1 family, member A2 (ALDH1A2), which is a key enzyme in retinoid metabolism. In endometriotic cells, aberrant methylation permits expression of GATA6. GATA6 regulates the expression of several genes involved in steroid metabolism, the nuclear steroid hormone receptors, and other GATA family members. Ectopic expression of GATA6 drives a pattern of gene expression similar to that seen in endometriotic tissues, essentially transforming healthy endometrium away from spontaneous decidualization and toward the disease phenotype. [Reproduced from Dyson MT, Roqueiro D, Monsivais D, et al. Genome-wide DNA methylation analysis predicts an epigenetic switch for GATA factor expression in endometriosis. PLoS Genet. 2014;10(3):e1004158. Copyright © 2014 by Dyson et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.]

Figure 10.

Estradiol production in endometriosis: roles of SF1 and GATA6. The endometriotic stromal cell is capable of converting cholesterol to estradiol via a number of enzymatic steps. Both transcription factors, SF1 and GATA6, are essential for the production of estradiol and mediate the effects of PGE₂ in stimulating these steroidogenic genes. SF1 is essential and sufficient for the expression of at least five steroidogenic genes, whereas SF1 and GATA6 together are essential and sufficient for the expression of three of these genes. The effects of SF1 or GATA6 on HSD17B1 in endometriosis are not known. The addition (ectopic expression) of both SF1 and GATA6 is required for a normal endometrial stromal cell to make estradiol from cholesterol. CYP11A1, side-chain cleavage enzyme; HSD3B2, 3β-hydroxysteroid dehydrogenase-2; HSD17B1, 17β-hydroxysteroid dehydrogenase type 1.

Figure 11.

Estrogen and PRs in endometriosis. Compared with endometrial stromal cells, ERβ levels are higher and ERα levels are lower in endometriotic stromal cells because of altered DNA methylation. The PR gene is also differentially methylated between these two cells types. It is likely that the severely elevated ratio of ERβ/ERα is in part responsible for the observed inhibition of ERα and PR expression in endometriotic stromal cells.

Figure 12.

ERβ action in endometriosis. ERβ induces RERG expression, whereas PGE₂, via protein kinase A (PKA), phosphorylates RERG. RERG phosphorylation is associated with its nuclear translocation. Estradiol- and PGE₂-mediated activation of RERG regulates endometriotic cell proliferation. Estradiol via ERβ also induces expression of serum and glucocorticoid–regulated kinase (SGK1), which is also activated by PGE₂ and oxidative stress. This leads to phosphorylation of FOXO3a, a proapoptotic factor, and enhanced cell survival. [Adapted from Monsivais D, Dyson MT, Yin P, et al. ER beta- and prostaglandin E₂-regulated pathways integrate cell proliferation via Ras-like and estrogen-regulated growth inhibitor in endometriosis. Mol Endocrinol. 2014;28(8):1304–1315.]

Figure 13.

Interactions of ERβ, TNF, SRC1, and IL-1β in endometriosis. ERβ plays a unique role in endometriotic tissue, where it interacts with the cytoplasmic apoptotic machinery and inflammasome complex to prevent TNF-induced cell death and enhance adhesion and proliferative activities of endometriotic tissues via SRC1 and IL-1β in this broad mechanistic pathway. This nongenomic action of ERβ has a predominant role in endometriosis progression. [Reproduced from Han SJ, Jung SY, Wu SP, Hawkins SM, Park MJ, Kyo S, Qin J, Lydon JP, Tsai SY, Tsai MJ, DeMayo FJ, O’Malley BW. Estrogen receptor β modulates apoptosis complexes and the inflammasome to drive the pathogenesis of endometriosis. Cell. 2015;163(4):960–974. doi: 10.1016/j.cell.2015.10.034. Copyright © 2015 by Elsevier, Inc.]

Figure 14.

Progesterone resistance: HSD17B2 deficiency in endometriotic epithelium. HSD17B2 is specifically expressed in eutopic endometrial epithelial cells during the secretory phase. This expression coincides with progesterone secretion by the corpus luteum. HSD17B2 converts the biologically potent estrogen, estradiol (E2), to estrogenically weak estrone (E1). In endometriotic tissue, however, this epithelial HSD17B2 is severely deficient, giving rise to E2 excess. These findings are suggestive that progesterone stimulates HSD17B2 in eutopic endometrium and that its deficiency in endometriosis is a consequence of progesterone resistance. [Reproduced from Zeitoun K, Takayama K, Sasano H, Suzuki T, Moghrabi N, Andersson S, Johns A, Meng L, Putman M, Carr B, Bulun SE. Deficient 17β-hydroxysteroid dehydrogenase type 2 expression in endometriosis: failure to metabolize 17β-estradiol. J Clin Endocrinol Metab. 1998;83(12):4474–4480.]

Figure 15.

Paracrine stromal–epithelial interactions for progesterone and retinoid action in endometrium and endometriosis. Deficient genes and pathways in endometriosis are indicated by arrows and dotted lines. Blood vessels deliver progesterone (P4) to endometrial stromal cells, which express PR; activation leads to the production of paracrine factors, including RA. RA acts in a paracrine manner to stimulate differentiation and oppose estradiol (E2)-dependent proliferation in endometrial epithelial cells. Moreover, RA stimulates HSD17B2, which converts biologically active E2 to estrogenically weak estrone (E1). Endometriotic stromal cells express lower levels of PR, which leads to lower RA formation. As a result, paracrine signaling to neighboring epithelial cells is lost, and these cells do not differentiate or express HSD17B2, leading to excess E2. The mechanisms supporting retinoid transport between endometrial stromal and epithelial cells are not fully understood. The retinoid cell surface receptor stimulated by RA-6 (STRA6) in endometrial stromal cells binds retinol binding protein (RBP)–bound retinol in the circulation for cellular uptake of retinol. Retinol is converted to RA and transported to nuclear RARs by the shuttling protein cellular RA binding protein-2 (CRABP2). RA-bound RAR stimulates endometrial stromal cell differentiation and apoptosis. PR induces stromal STRA6 and CRABP2 expression. In endometriosis, this pathway is disrupted by a deficiency of PR, STRA6, and CRABP2 as abnormal expression of the RA-metabolizing enzymes CYP26B1 and CYP26A1 in stromal and epithelial endometrial cells. 4OH-RA, 4-hydroxy-RA. [Reproduced from Kim JJ, Kurita T, Bulun SE. Progesterone action in endometrial cancer, endometriosis, uterine fibroids, and breast cancer. Endocrine Rev. 2013;34(1):130–162.]

Figure 16.

Summary of key estrogen-dependent mechanisms in endometriosis. Pathologically decreased methylation of the SF1, GATA6, and ERβ genes causes extremely high levels of their proteins in endometriotic stromal cells. SF1 together with GATA6 coordinately mediate the PGE₂-induced expression of many genes in the estrogen synthetic cascade to produce estradiol from cholesterol. In addition to decreased methylation of its gene promoter, ERβ expression is high also due to a stimulatory effect from GATA6. Estradiol and ERβ induce COX2 expression and PGE₂ production in endometrial cells. Moreover, PGE₂per se and IL-1β also induce COX2 expression and PGE₂ production in endometrial cells. ERβ and GATA6 suppress ERα and PR, leading deficiencies of RA production and the HSD17B2 enzyme. The end result is extremely high local concentrations of estradiol and PGE₂ in endometriotic tissue; both of these molecules are key inducers of inflammation and pain in endometriosis. This pathway is clinically relevant because its disruption by an aromatase inhibitor or a COX inhibitor (e.g., a nonsteroidal anti-inflammatory drug) reduces the extent of disease or pelvic pain.

Figure 17.

A vision of the future of endometriosis treatment. Human iPS cells are currently being tested for the treatment of a number of human conditions. The genomes of the biopsied skin or bone marrow cells are reprogrammed via the addition of four stem cell factors (OCT4, SOX2, KLF4, and MYC) to generate iPS cells. It was recently demonstrated that these pluripotent stem cells can be differentiated to endometrial stromal cells (ESCs) under defined molecular conditions (207). In the future, the differentiation protocol may be optimized to produce appropriately progesterone-responsive and healthy ESCs, which can be used to replace epigenetically defective and progesterone-resistant stromal cells in the intrauterine endometrial tissue of a woman with endometriosis.