**2. Understanding endometriosis**

### **2.1 Theories of endometriosis**

Endometriosis is an estrogen-dependent chronic inflammatory disease that primarily affects women of reproductive age and is defined as the presence of endometrial glands and stroma outside the uterine cavity [1].

The first reference of endometriosis appeared in 1690, when Daniel Shroen described the presence of "ulcers" disseminated in the pelvic cavity and that appeared only in women of reproductive age. From that date, different theories have been developed in an attempt to explain its origin; however, even today, endometriosis remains an enigmatic disease.

The pathogenesis of endometriosis can be included in six theories, which in turn can be subdivided into two groups depending on whether the implants come from the uterine or extrauterine endometrium [2].

#### *2.1.1 Origin of uterine endometriosis*

#### *2.1.1.1 Theory of retrograde menstruation*

It is the most widely accepted theory about the etiopathogenesis of endometriosis. It was proposed in 1920 by Sampson. Menstruation ascends from the ostium, passes through the tubes and empties into the peritoneal cavity, with the subsequent implantation and growth of the endometrial fragments deposited in the peritoneal cavity. They represent an autotransplant, in which normal endometrial tissue is transplanted to an ectopic location in the body. This theory explains the higher frequency of endometriosis and the anatomical distribution of the lesions. However, it is not able to demonstrate the presence of endometriosis outside the peritoneal cavity, the appearance in early puberty, newborns, women affected by the Mayer-Rokitansky-Küster-Hauser syndrome or in the male [2, 3].

## *2.1.1.2 Theory of hematogenous or lymphatic dissemination of endometrial tissue (Halban theory)*

It is based on the demonstration of ectopic endometrial tissue in locations distant from the uterus such as brain, lung, inguinal region, etc. However, on its own it is not capable of resolving its adhesion and progression capacity [2].

#### *2.1.2 Origin of extrauterine endometrial endometriosis*

#### *2.1.2.1 The theory of coelomic metaplasia (Meyer's theory)*

The peritoneum and endometrium appear to have a common embryological precursor that is the coelomic epithelium. Dioxins are proposed as a possible external chemical agent that acts as an endocrine disruptor. This theory could explain why although most women have some degree of retrograde menstruation only a small percentage have endometriosis, as well as the presence of the disease in the absence of menstruation [3–5].

**11**

*The Role of the Molecular Genetic Approach in the Pathogenesis of Endometriosis*

endometriotic lesions influenced by estrogen stimulation [7].

Stem cells in the circulation from bone marrow or from the basal layer of the endometrium could differentiate into endometrial tissue at different locations even from a distance. This theory would explain why women without endometriosis can have endometriosis, men with prostate cancer or after treatments can have high

Residual cells during embryonic development maintain the ability to develop

Endometriosis has a hereditary component. Susceptibility loci of the disease have been established in the 10q26 and 7p15 chromosomal regions. The endometrial cells need to adhere to each other and to the peritoneum, and integrins and cadherins participate in this process. Likewise, there is an upregulation of the antiapoptosis

Endometriosis is an epigenetic disease in which the steroidogenesis factor 1, the estrogen, and progesterone receptors are hypomethylated in the ectopic endometrium causing a greater estrogenic effect locally. The endometriotic implants express aromatase and dehydrogenase of 17B-hydroxysteroid type 1, which are the enzymes responsible for converting androstenedione to estrone and estrone to estradiol. At the same time, there is a deficit of 17B-hydroxysteroid type 2, an enzyme responsible for deactivating estrogen (passes estradiol to estrone), and estrogen receptors α and β are expressed differently, with a marked increase in β receptors, favoring all

Normal endometrial tissue does not express aromatase and produces abundant dehydrogenase and 17B-hydroxysteroid type 2 in response to progesterone, ensuring the attenuation of estrogenic effects at the endometrial level during the luteal phase. In endometriosis, there is a relative resistance to progesterone that prevents the attenuation of the estrogenic stimulus. Prostaglandin E2 is the most potent inducer of aromatase activity in endometrial stromal cells and acts through the PGE2 receptor. The estradiol produced by the increased activity of aromatase increases the production of PGE2 for the stimulation of cyclooxygenase 2 in the endometrial cells of the uterus, causing a positive feedback that accentuates the

Different studies have shown the possibility that alterations in the immune system are responsible for the persistence of the ectopic endometrium, preventing immune mechanisms from eliminating endometrial cells within the peritoneal cavity [2]. These studies have shown a greater number of macrophages with altered function and alterations in the function of the natural killer cells, with a lower cytotoxic

*DOI: http://dx.doi.org/10.5772/intechopen.81598*

*2.1.2.2 Theory of endometrial stem cells*

*2.1.2.3 Theory of the Müllerian remains*

doses of estrogen [3, 4].

*2.1.3 Genetic predisposition*

*2.1.4 Hormone dependence*

*2.1.5 Resistance to progesterone*

*2.1.6 Immunological factors*

this a greater estrogenic environment [8–10].

estrogenic effects on the production of endometriosis [9].

gene BCL-2 [5–7].

*The Role of the Molecular Genetic Approach in the Pathogenesis of Endometriosis DOI: http://dx.doi.org/10.5772/intechopen.81598*

#### *2.1.2.2 Theory of endometrial stem cells*

*Molecular Bases of Endometriosis - The Integration Between Research and Clinical Practice*

with relevant findings on the definition and pathogenesis of endometriosis

**2. Understanding endometriosis**

endometriosis remains an enigmatic disease.

the uterine or extrauterine endometrium [2].

*2.1.1 Origin of uterine endometriosis*

*2.1.1.1 Theory of retrograde menstruation*

**2.1 Theories of endometriosis**

In this chapter, we will discuss some aspects of the molecular genetic approach,

Endometriosis is an estrogen-dependent chronic inflammatory disease that primarily affects women of reproductive age and is defined as the presence of

The first reference of endometriosis appeared in 1690, when Daniel Shroen described the presence of "ulcers" disseminated in the pelvic cavity and that appeared only in women of reproductive age. From that date, different theories have been developed in an attempt to explain its origin; however, even today,

The pathogenesis of endometriosis can be included in six theories, which in turn can be subdivided into two groups depending on whether the implants come from

It is the most widely accepted theory about the etiopathogenesis of endometriosis. It was proposed in 1920 by Sampson. Menstruation ascends from the ostium, passes through the tubes and empties into the peritoneal cavity, with the subsequent implantation and growth of the endometrial fragments deposited in the peritoneal cavity. They represent an autotransplant, in which normal endometrial tissue is transplanted to an ectopic location in the body. This theory explains the higher frequency of endometriosis and the anatomical distribution of the lesions. However, it is not able to demonstrate the presence of endometriosis outside the peritoneal cavity, the appearance in early puberty, newborns, women affected by the Mayer-Rokitansky-Küster-Hauser syndrome or in the

*2.1.1.2 Theory of hematogenous or lymphatic dissemination of endometrial tissue* 

is not capable of resolving its adhesion and progression capacity [2].

*2.1.2 Origin of extrauterine endometrial endometriosis*

*2.1.2.1 The theory of coelomic metaplasia (Meyer's theory)*

It is based on the demonstration of ectopic endometrial tissue in locations distant from the uterus such as brain, lung, inguinal region, etc. However, on its own it

The peritoneum and endometrium appear to have a common embryological precursor that is the coelomic epithelium. Dioxins are proposed as a possible external chemical agent that acts as an endocrine disruptor. This theory could explain why although most women have some degree of retrograde menstruation only a small percentage have endometriosis, as well as the presence of the disease in the absence

endometrial glands and stroma outside the uterine cavity [1].

**10**

of menstruation [3–5].

male [2, 3].

*(Halban theory)*

Stem cells in the circulation from bone marrow or from the basal layer of the endometrium could differentiate into endometrial tissue at different locations even from a distance. This theory would explain why women without endometriosis can have endometriosis, men with prostate cancer or after treatments can have high doses of estrogen [3, 4].

#### *2.1.2.3 Theory of the Müllerian remains*

Residual cells during embryonic development maintain the ability to develop endometriotic lesions influenced by estrogen stimulation [7].

### *2.1.3 Genetic predisposition*

Endometriosis has a hereditary component. Susceptibility loci of the disease have been established in the 10q26 and 7p15 chromosomal regions. The endometrial cells need to adhere to each other and to the peritoneum, and integrins and cadherins participate in this process. Likewise, there is an upregulation of the antiapoptosis gene BCL-2 [5–7].

#### *2.1.4 Hormone dependence*

Endometriosis is an epigenetic disease in which the steroidogenesis factor 1, the estrogen, and progesterone receptors are hypomethylated in the ectopic endometrium causing a greater estrogenic effect locally. The endometriotic implants express aromatase and dehydrogenase of 17B-hydroxysteroid type 1, which are the enzymes responsible for converting androstenedione to estrone and estrone to estradiol. At the same time, there is a deficit of 17B-hydroxysteroid type 2, an enzyme responsible for deactivating estrogen (passes estradiol to estrone), and estrogen receptors α and β are expressed differently, with a marked increase in β receptors, favoring all this a greater estrogenic environment [8–10].

#### *2.1.5 Resistance to progesterone*

Normal endometrial tissue does not express aromatase and produces abundant dehydrogenase and 17B-hydroxysteroid type 2 in response to progesterone, ensuring the attenuation of estrogenic effects at the endometrial level during the luteal phase. In endometriosis, there is a relative resistance to progesterone that prevents the attenuation of the estrogenic stimulus. Prostaglandin E2 is the most potent inducer of aromatase activity in endometrial stromal cells and acts through the PGE2 receptor. The estradiol produced by the increased activity of aromatase increases the production of PGE2 for the stimulation of cyclooxygenase 2 in the endometrial cells of the uterus, causing a positive feedback that accentuates the estrogenic effects on the production of endometriosis [9].

#### *2.1.6 Immunological factors*

Different studies have shown the possibility that alterations in the immune system are responsible for the persistence of the ectopic endometrium, preventing immune mechanisms from eliminating endometrial cells within the peritoneal cavity [2].

These studies have shown a greater number of macrophages with altered function and alterations in the function of the natural killer cells, with a lower cytotoxic activity, in turn an alteration in the humoral immunity to observe an increase in the concentration of IgG endometrial antibodies as well as IgG and IgA antibodies against endometrial and ovarian tissue.

#### **2.2 Pathogenesis of endometriosis**

Regardless of the mechanism that promotes endometriosis, it is a fact the presence of endometrial cells with the potential to be implanted in the receptors tissues, especially in the mesothelium of the pelvic cavity.

In the pathological process of endometriosis in the pelvic organs such as the ovary, the clinical consequence is the formation of chocolate cysts, which can be explained in different ways: (i) during each hormonal cycle, bleeding occurs in the endometriosis accumulation, with an inversion and invagination of the epithelium. At the same time, a cellular inflammatory process begins with adhesions to the surrounding peritoneum. (ii) Chocolate or endometriosis cysts affect the follicular cyst of the ovarian epithelium. (iii) A process of celomic/endometriosis metaplasia of the ovarian epithelium.

At the peritoneal level, especially in the rectovaginal septum, a natural evolution of the peritoneal endometriosis of the Douglas pouch may be caused by secondary infiltration of endometriosis emboli or metaplasia of embryonic/müllerian remains located in the rectovaginal septum [10].

A permissive peritoneal environment for the initiation and progression of endometriotic lesions may also be associated with the altered function of immune cells, together with local pelvic inflammatory processes that aid in the evasion of clearance by the immune system. In addition to the amount of menstrual endometrium reflux present in the peritoneal cavity, the altered secretion of immune factors, the formation of autoantibodies, impaired immune recognition, and the elimination of ectopic endometrial cells facilitate the initiation and progression of endometriosis. Laschke and Menger suggest that the "gut microbiome" or "microenvironment" could be crucial in the pathogenesis of endometriosis through the aberrant priming of immune responses [11].

#### **2.3 Local microenvironment, exosomes**

The endometrial lesions are composed of the same structural units as the lining of the uterus, the endometrium. This glandular epithelium is positive for cytokeratin and is apparently composed of two cell types, namely, positive E-cadherin and very few negative E-cadherin cells. The endometriotic stromal cells express mesenchymal markers such as vimentin and THY-1 and can be distinguished from the surrounding fibroblasts by, for example, expression of the CD10 metallo-endopeptidase membrane (common acute lymphocytic leukemia antigen).

The adhesive, proliferative, and invasive properties of the endometriosis conjunctive tissue, as well as the cellular functions of this epithelium can be related to the components of its extracellular matrix [13, 14].

Thus, in the pathogenesis and progression of endometriosis, the local microenvironment is vital to understand how endometriosis cells adapt to the control mechanisms of their host, escaping from immunological detections. The cell-stromal intercommunication through paracrine, hormonal, and angiogenic messengers is vital for the perpetuity of endometriosis tissue [15–17].

In a model by Hull et al., comparing the microarray data obtained from a xenotransplant model and eutopic versus ectopic endometrial paired samples, they identified alterations in four pathways: cell injury (ubiquitin/proteasome), inflammation (NFκB), tissue remodeling (TGF-β), and cell proliferation (KRAS). There is

**13**

various lesions [22, 23].

*The Role of the Molecular Genetic Approach in the Pathogenesis of Endometriosis*

thus an extensive metabolic reprogramming and the acquisition of changes similar to cancer that are reflected in an increase in the capacity of penetration and cellular penetration, a reduced apoptotic potential and an altered immune function [16]. The local microenvironment could also influence by altering gene expression through, for example, epigenetic changes (DNA methylation), histone modifications, and miRNA. The molecular networks associated with endometriosis are regulated by miRNA at the posttranscriptional level. In fact, 22 miRNAs aberrantly expressed in endometriotic lesions have been identified. In addition to differentially expressed miRNAs, the altered DNA methylation pattern occurs during the onset and/or progression of endometriosis. Since endometriosis is an estrogen-dependent but progesterone-resistant condition, it is not surprising that its respective pro-

In addition to the epigenetic modifications, endometriotic cells also present chromosomal abnormalities and instability that could alter gene expression by loss or mutations of DNA sequences expressed as alterations in the signaling pathways

Exosomes containing ectonucleotidase could contribute to the progression of endometriosis and the local suppression of immune responses by regulating

They can also exert enhanced angiogenic effects. It is likely that the endometrial exosomes can be directed retrogradely in the pelvic cavity or can be detached by the menstrual cells and influence the fate of the ectopic cells. Thus, in the control of the local microenvironment, exosomes could be an important factor in allowing a temporary endometriotic lesion to establish a sufficient blood supply to grow and survive in the ectopic site. The endometrial exosomes of women with endometriosis act in an autocrine, paracrine, and endocrine fashion but in turn may play a role in

Diaphonia within the local microenvironments through the exosomes can represent the union intersection where the different theories about the pathogenesis of this entity converge. They could, through their ability to send information between tissue strains, to induce changes such as metaplasias, tissue remodeling, and even represent a mechanism of regulation/alteration in signal transduction [18].

Ectopic endometrial cells have been little investigated, mainly due to the rare availability of endometriotic tissue required for cell culture and the limited number of cells, particularly those of epithelial phenotype. The proposed in vitro cell models have their limitations since the endometriotic lesions are histologically complex and contain both glandular and stromal elements. Therefore, cell lines immortalized with a cell type, which normally exhibit characteristics of undifferentiated cells, do not accurately represent the situation in vivo. Thus, to evaluate the endometriosis phenotype, the cultures are prepared in general from biopsies of the

Among epithelial markers, cytokeratin expression remains one of the most specific characteristics of endometriosis cells. More than 40% of the cells were

Endometriosis cells release extracellular vesicles, such as exosomes and microvesicles, composed of various types of plasma membranes and origin of endosomal membrane, which are an alternative source for intercellular communication as they contain, for example, miRNAs with target genes in signaling pathways connected to the embryo-endometrial interface or enzymes and are capable of modulating cellular responses, for example, survival, differentiation, or modulation of immunogenic

*DOI: http://dx.doi.org/10.5772/intechopen.81598*

moter regions are affected accordingly [17].

in endometriotic cells (regulatory proteins) [8, 12, 13].

the manifestation of endometriosis as a disease.

**2.4 Phenotype and cytogenetics of endometriosis**

responses, important for endometriosis during implantation [18–23].

extracellular ATP and increasing extracellular adenosine levels [17].

#### *The Role of the Molecular Genetic Approach in the Pathogenesis of Endometriosis DOI: http://dx.doi.org/10.5772/intechopen.81598*

thus an extensive metabolic reprogramming and the acquisition of changes similar to cancer that are reflected in an increase in the capacity of penetration and cellular penetration, a reduced apoptotic potential and an altered immune function [16].

The local microenvironment could also influence by altering gene expression through, for example, epigenetic changes (DNA methylation), histone modifications, and miRNA. The molecular networks associated with endometriosis are regulated by miRNA at the posttranscriptional level. In fact, 22 miRNAs aberrantly expressed in endometriotic lesions have been identified. In addition to differentially expressed miRNAs, the altered DNA methylation pattern occurs during the onset and/or progression of endometriosis. Since endometriosis is an estrogen-dependent but progesterone-resistant condition, it is not surprising that its respective promoter regions are affected accordingly [17].

In addition to the epigenetic modifications, endometriotic cells also present chromosomal abnormalities and instability that could alter gene expression by loss or mutations of DNA sequences expressed as alterations in the signaling pathways in endometriotic cells (regulatory proteins) [8, 12, 13].

Endometriosis cells release extracellular vesicles, such as exosomes and microvesicles, composed of various types of plasma membranes and origin of endosomal membrane, which are an alternative source for intercellular communication as they contain, for example, miRNAs with target genes in signaling pathways connected to the embryo-endometrial interface or enzymes and are capable of modulating cellular responses, for example, survival, differentiation, or modulation of immunogenic responses, important for endometriosis during implantation [18–23].

Exosomes containing ectonucleotidase could contribute to the progression of endometriosis and the local suppression of immune responses by regulating extracellular ATP and increasing extracellular adenosine levels [17].

They can also exert enhanced angiogenic effects. It is likely that the endometrial exosomes can be directed retrogradely in the pelvic cavity or can be detached by the menstrual cells and influence the fate of the ectopic cells. Thus, in the control of the local microenvironment, exosomes could be an important factor in allowing a temporary endometriotic lesion to establish a sufficient blood supply to grow and survive in the ectopic site. The endometrial exosomes of women with endometriosis act in an autocrine, paracrine, and endocrine fashion but in turn may play a role in the manifestation of endometriosis as a disease.

Diaphonia within the local microenvironments through the exosomes can represent the union intersection where the different theories about the pathogenesis of this entity converge. They could, through their ability to send information between tissue strains, to induce changes such as metaplasias, tissue remodeling, and even represent a mechanism of regulation/alteration in signal transduction [18].

#### **2.4 Phenotype and cytogenetics of endometriosis**

Ectopic endometrial cells have been little investigated, mainly due to the rare availability of endometriotic tissue required for cell culture and the limited number of cells, particularly those of epithelial phenotype. The proposed in vitro cell models have their limitations since the endometriotic lesions are histologically complex and contain both glandular and stromal elements. Therefore, cell lines immortalized with a cell type, which normally exhibit characteristics of undifferentiated cells, do not accurately represent the situation in vivo. Thus, to evaluate the endometriosis phenotype, the cultures are prepared in general from biopsies of the various lesions [22, 23].

Among epithelial markers, cytokeratin expression remains one of the most specific characteristics of endometriosis cells. More than 40% of the cells were

*Molecular Bases of Endometriosis - The Integration Between Research and Clinical Practice*

against endometrial and ovarian tissue.

especially in the mesothelium of the pelvic cavity.

**2.2 Pathogenesis of endometriosis**

of the ovarian epithelium.

of immune responses [11].

located in the rectovaginal septum [10].

**2.3 Local microenvironment, exosomes**

activity, in turn an alteration in the humoral immunity to observe an increase in the concentration of IgG endometrial antibodies as well as IgG and IgA antibodies

Regardless of the mechanism that promotes endometriosis, it is a fact the presence of endometrial cells with the potential to be implanted in the receptors tissues,

In the pathological process of endometriosis in the pelvic organs such as the ovary, the clinical consequence is the formation of chocolate cysts, which can be explained in different ways: (i) during each hormonal cycle, bleeding occurs in the endometriosis accumulation, with an inversion and invagination of the epithelium. At the same time, a cellular inflammatory process begins with adhesions to the surrounding peritoneum. (ii) Chocolate or endometriosis cysts affect the follicular cyst of the ovarian epithelium. (iii) A process of celomic/endometriosis metaplasia

At the peritoneal level, especially in the rectovaginal septum, a natural evolution of the peritoneal endometriosis of the Douglas pouch may be caused by secondary infiltration of endometriosis emboli or metaplasia of embryonic/müllerian remains

A permissive peritoneal environment for the initiation and progression of endometriotic lesions may also be associated with the altered function of immune cells, together with local pelvic inflammatory processes that aid in the evasion of clearance by the immune system. In addition to the amount of menstrual endometrium reflux present in the peritoneal cavity, the altered secretion of immune factors, the formation of autoantibodies, impaired immune recognition, and the elimination of ectopic endometrial cells facilitate the initiation and progression of endometriosis. Laschke and Menger suggest that the "gut microbiome" or "microenvironment" could be crucial in the pathogenesis of endometriosis through the aberrant priming

The endometrial lesions are composed of the same structural units as the lining

of the uterus, the endometrium. This glandular epithelium is positive for cytokeratin and is apparently composed of two cell types, namely, positive E-cadherin and very few negative E-cadherin cells. The endometriotic stromal cells express mesenchymal markers such as vimentin and THY-1 and can be distinguished from the surrounding fibroblasts by, for example, expression of the CD10 metallo-endo-

The adhesive, proliferative, and invasive properties of the endometriosis conjunctive tissue, as well as the cellular functions of this epithelium can be related

In a model by Hull et al., comparing the microarray data obtained from a xenotransplant model and eutopic versus ectopic endometrial paired samples, they identified alterations in four pathways: cell injury (ubiquitin/proteasome), inflammation (NFκB), tissue remodeling (TGF-β), and cell proliferation (KRAS). There is

Thus, in the pathogenesis and progression of endometriosis, the local microenvironment is vital to understand how endometriosis cells adapt to the control mechanisms of their host, escaping from immunological detections. The cell-stromal intercommunication through paracrine, hormonal, and angiogenic messengers

peptidase membrane (common acute lymphocytic leukemia antigen).

to the components of its extracellular matrix [13, 14].

is vital for the perpetuity of endometriosis tissue [15–17].

**12**

immunoreactive with the anti-progesterone receptor (PR) antibody which shows a brown nuclear stain produced by the diaminobenzidine colorimetric reaction. Less than 15% are immunoreactive with the anti-androgen receptor antibody. Cytokeratins such as vimentin were expressed in endometriotic stromal cells. Therefore, most of the phenotypic characteristics of the normal endometrium are conserved in the endometriosis cells.

The existence of genomic aberrations in the tissues of endometriosis is probably related to genes involved in the development of the disease. Most genetic changes occur as germ line defects that can result in a hereditary predisposition to the development of endometriosis with a genetic basis for endometriosis. The prevalence of the disease is higher in first-degree relatives of affected women than in the general population [21–23].
