Thus, we could be at the dawn of a brave new world in which the “root treatment” would potentially forestall the genesis of endometriosis for good, sparing millions of women worldwide from pain and suffering, the dashed dream of having a family, endless distress, fear of uncertainty of surgery and recurrence, lost productivity, school absenteeism, strained relationships and emotional, social and financial tolls and alleviating enormous economic burden to the society.
2. Aberrations in Eutopic Endometrium: Cause or Consequence of Endometriosis?
The presence of various molecular and cellular aberrations in the endometrium of women with endometriosis has been extensively reported. Searching PubMed using the phrase “endometriosis and (endometrial or endometrium)“ yielded 7109 papers (accessed on 1 February 2023), published since 2000 when aberrations reported before were comprehensively reviewed by Vantier et al.
[8]. Indeed, expression profiling studies typically report tens or hundreds of genes, miRNAs, or long non-coding RNAs differentially expressed in the endometrium between women with endometriosis and without
[17][18][19][20][21][22][23]. The evaluation of changes in DNA methylome yielded similar findings
[24][25]. A comprehensive review, published in 2016, of published endometrial biomarkers of endometriosis involving 2729 participants concluded that only “17βHSD2, IL-1R2, caldesmon (a binding protein capable of regulating actomyosin contraction), and a number of neural markers (VIP, CGRP, SP, NPY and combination of VIP, PGP 9.5 and SP) showed promising evidence of diagnostic accuracy, but there was insufficient or poor quality evidence for any clinical recommendations.”
[26].
Thus, considering the sheer number and type of aberrations, it may not be entirely informative to tally these aberrations, especially since different studies often yield conflicting results and because such differences vanish under refined grouping
[27].
The first such aberration is the increased nerve fiber density in the endometrium. First reported in 2006
[28], it was subsequently consistently observed by the same research team
[29][30], which went on to demonstrate, in a double-blinded study, an impressive specificity and sensitivity of 83% and 98%, respectively, in diagnosing endometriosis
[31]. More remarkably, this finding was independently corroborated by a Belgian team, reporting a nearly perfect 95% sensitivity, 100% specificity and 97.5% accuracy in predicting the presence of minimal–mild endometriosis using just three neural markers
[32].
Unfortunately, subsequent studies failed to replicate the relationship between endometriosis and increased nerve fiber density in the endometrium. Instead, they found that endometrial innervation is pain-dependent, rather than endometriosis specific
[33][34]. Indeed, several later studies found either no such difference
[35] or unacceptable sensitivity and specificity (32% and 46%, respectively, as reported in
[36], and 64% and 50%, respectively, as reported in
[37]). Consistent with the observation that the endometrial hyperinnervation is pain-, but not endometriosis-related, the estimates of sensitivity and specificity are the lowest when subjects who complained of pelvic pains were included
[36].
Thus, 16 years after its first report, endometrial hyperinnervation as quantitated by the nerve fiber density in endometrium has not become a diagnostic tool for endometriosis as of today. In essence, this example illustrates that an endometrial aberration may be linked with a non-specific symptom, such as dysmenorrhea or pelvic pain, which may be shared by many disorders, including endometriosis, but not endometriosis exclusively, as shown in
Figure 1A.
Figure 1. Possible scenarios in which endometrial aberrations do not cause endometriosis exclusively. (A) In this scenario, endometrial aberration A may be associated with a particular symptom, which is not specific to endometriosis. (B) Endometrial aberration B could lead to multiple conditions, including endometriosis. (C) Endometrial aberrations C1 and C2 are the consequences of endometriosis, where C1 and C2 are induced by different lesions of possibly different subtypes and/or their proximity to uterus. (D) Endometrial aberration D, along with endometriosis, is the result of a third, unknown factor. Lines without arrows indicate an association. The directional arrows indicate the causal relationship.
The second aberration is the gene or protein expression levels of PGRs. Progesterone is known to be a critical sex steroid hormone for the endometrium, and it also exerts a therapeutic effect on ectopic endometrium. Progesterone, as well as progestins, mediates their effects through two isoforms of PGR, namely PR-A and PR-B. Both PR-A and PR-B are encoded by the same gene, PGR, and, as such, both isoforms are identical in sequence, except PR-A is short of 164 amino acids at the N terminus, since both are transcribed from the same gene via two independent specific promoter regions and translation start sites
[38]. Since progestins constitute an important class of drugs in the limited armamentarium of medical treatment of endometriosis, and since progesterone resistance (manifesting itself clinically as refractoriness to progestin treatment) is well-documented in endometriosis, PGR expression in both eutopic and ectopic endometrium is of major interest in endometriosis
[39].
In endometriotic lesions, the reduced expression of both PR-A and PR-B, especially the latter, has been well-documented
[40][41][42]. In fact, promoter hypermethylation of PR-B, which is responsible for its silence in endometriosis as well as in adenomyosis, has been reported as early as 2006
[43][44], and is likely caused by persistent inflammation
[45]. Suppressed expression of PR-B in endometriosis apparently endows enhanced proliferative propensity
[46].
In eutopic endometrium, the tendency for progesterone resistance has been also well-established
[17][18]. Published data predominantly show PGR, especially PR-B, expression is either lost or reduced in the eutopic endometrium from women with endometriosis
[42][47][48][49][50][51][52]. There is evidence for PR-B promoter hypermethylation, which may account for its loss
[43][53]. However, negative reports also have been reported
[54][55][56]. These discrepancies have been attributed to possible differences in experimental methods, endometriosis subtypes and cell types analyzed, and resolution of PGR isoforms
[57]. It is also attributble to “patchy” endometrium
[58], i.e., endometrial heterogeneity within the same patient.
However, reduced PGR expression, or even PR-B promoter hypermethylation in the endometrium is not exclusively confined to patients with endometriosis. Endometrial PR-B expression also is reduced in patients with adenomyosis
[59], and its reduction may also be attributed to its promoter hypermethylation
[44]. In poorly differentiated endometrial cancer, PR-B expression is reduced as well
[60]. In women wearing a levonorgestrel-releasing intrauterine system (LNG-IUS), total PGR and PR-B staining in the endometrium are significantly reduced
[61]. Therefore, reduced endometrial PGR or PR-B expression is not a sufficient condition for endometriosis. This scenario, called “one-to-many”, is depicted in
Figure 1B.
The last example is promoter hypermethylation of HOXA10 in eutopic endometrium from women with endometriosis. First reported in 2005
[62], it was later independently confirmed in baboon and mouse models of endometriosis
[63][64], as well as in humans
[65][66]. The silencing of endometrial HOXA10 because of promoter hypermethylation is associated with decreased fertility, implantation defects and/or the reproductive wastage seen in certain disease states that affect the female reproductive tract
[67]. However, promoter hypermethylation of HOXA10 in the endometrium turns out to occur not only in patients with endometriosis but also those with Asherman’s syndrome, intramural and submucosal uterine myoma, as well as endometrial polyps
[67]. The endometrium of women wearing intrauterine devices also carry HOXA10 methylation
[68]. Therefore, again, this molecular aberration is not exclusively confined to patients with endometriosis, and, as such, does not qualify as a sufficient condition (
Figure 1C).
Through the above three examples, it can be concluded that many molecular/cellular aberrations in the endometrium of women with endometriosis are either not universal to all patients with endometriosis (as in endometrial hyperinnervation restricted to those complaining of pain), or not exclusively confined to those with endometriosis. In other words, these aberrations are not a sufficient condition, and in some cases not even a necessary condition. Most importantly, however, since these aberrations are found, post hoc, in the endometrium from women who have been already diagnosed with endometriosis, there is no way to tell whether these aberrations are the cause, or merely the consequences, of endometriosis. Of course, for a complex disease such as endometriosis, a linear cause-and-effect relationship seldom exists during the entire course of the disease progression. Many factors could initially be the consequence of the disease but later become part of the causal complex. However, to truly qualify for “endometrial determinism”, the endometrial aberration must occur first, endowing the aberration-carrying menstrual debris the ability to invade, adhere to ectopic sites and then evade immune surveillance, detection and clearance, and cause symptoms. That is, it predisposes its bearer to endometriosis. Therefore, two additional requirements are needed to establish “endometrial determinism” (
Figure 1C,D). In scenario C (the consequence), the endometrial aberration is the consequence of endometriosis, and the aberration may vary depending on the subtype of endometriotic lesion and its proximity to the uterus, as reviewed below. In scenario D (the third party), both endometrial aberration and endometriosis could be caused by a third party, which could be linked to in utero exposure of high levels of estrogens
[69][70] or some inborn errors or other extraneous factors yet to be identified.
When exploring the hypothesis that endometriosis originates from the endometrium harboring molecular aberrations, it should be considered that differences between ectopic and the eutopic endometrium could be secondary to the vast differences in the microenvironment existing between the two locations. The ectopic endometrium in endometriosis is influenced by the peritoneal fluid, or by local microenvironments in the case of ovarian endometrioma
[71]. In turn, ectopic endometrium could influence gene expression in the eutopic endometrium, since endometrial tissue implanted at sites proximal and distal to the uterus in the mouse model of endometriosis alters gene expression in the eutopic endometrium. Furthermore, the changes in gene expression differ in relation to the site of implantation
[72].
In addition to aberrations in eutopic endometrium, emerging data also indicate that endometriosis may cause systemic changes. In mice with induced endometriosis, dysregulated hepatic metabolic genes have been demonstrated
[73]. This may be responsible for the low body mass index observed in women with endometriosis
[74]. Recently, it has been reported that endometriosis promotes atherosclerosis in mice
[75].