Background
Endometriosis is a common chronic gynecologic disease defined as presence of endometrial tissue outside the uterine cavity, primarily on pelvic peritoneum and ovary. Epidemiology studies reveal that endometriosis affects more than 10% of reproductive age women and possibly causes infertility [
1,
2]. The prevalence of endometriosis was 0.5-5% in fertile and 25-45% in infertile women [
3]. The mechanism underlying endometriosis development remains unclear, even though theories like implantation, altered immunity, and susceptible genetic factors have been proposed to explain the pathogenesis [
4‐
6]. Nevertheless, familial and identical twins studies have established the genetic predisposition to endometriosis development [
7].
Clinical manifestation of endometriosis is accompanied by angiogenesis and formation of cellular adhesion [
8,
9], possibly due to altered peritoneal environment and immune system [
10]. In endometriosis patients, changes in levels of growth factors, cytokines and oncofetal antigens may facilitate intraperitoneal endometrial growth and alter the peritoneal environment, which leads to disruption of normal pelvic organ architectures and infertility [
8]. For instance, interluekin-1 (IL-1), interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), nuclear factor-κB (NF-κB) and tumor necrosis factor-alpha (TNF-α) are among the major cytokines participating in regulation of immune system, angiogenesis, cell proliferation and tissue invasiveness during the formation of endometriosis [
11‐
14].
Mucins are high molecular weight glycoproteins with function of protecting and lubricating epithelial surface of respiratory, gastrointestinal and reproductive tracts [
15]. Mucins are also found to be expressed on activated lymphocytes, supporting the hypothesis that some mucin domains function as cytokines to mediate immune responses [
16,
17]. Early studies reported that mucin played important role in the progress of tumor invasion, which is influenced by their glycosylation status [
15,
18‐
20]. More recently, mucin1 (MUC1) has been reported to be linked to endometriosis and infertility [
21,
22], and mucin4 (
MUC4) gene polymorphisms were proved to be associated with endometriosis development as well [
23]. The human mucin2 (
MUC2) gene is located on chromosome 11p15.5, encoding one of the most common gel-forming secreted type of mucin [
24]. MUC2 expression was reported to be regulated by many endometriosis-related cytokines, such as IL-1β, IL-6, TNF-α, NF-Kappa B [
9,
25‐
27]. Abnormal increase of mucin2 (MUC2) expression was reported to be linked to intestinal and uterine cervix metaplasia progression [
28,
29].
However, there has been no study yet investigating the relationship between endometriosis and MUC2, while previous functional studies on MUC2 are more focused on its role in the gastrointestinal and respiratory tract [
30,
31].
The aim of this study was to investigate the possible association of MUC2 gene polymorphisms with the risk of endometriosis and endometriosis-related clinical symptoms in a Taiwanese population.
Discussion
Mucin proteins were known to be heavily glycosylated, on which oligosaccharide structures turned to be tumor-associated antigens and are essential for antibody recognition [
39‐
41]. Our data revealed two
MUC2 polymorphisms (rs10794288 and rs10902088) were associated with endometriosis development and the related infertility. Polymorphism at rs10902088 generates an amino-acid change Asn1149Lys, while rs10794288 is a silent substitution. Although Asn1149 is not a typical site for N-linked glycosylation, this substitution to positively charged residue may influence the glycosylation states of several neighboring serines and Asn1154, which is within a typical N-linked glycosylation tripeptide sequon Asn-Ile-Ser [
42]. Therefore, this polymorphism may alter the glycosylation status of MUC2, which may subsequently influence the interaction between MUC2 and host environments.
Endometriosis development is associated with altered inflammatory and immune responses, while clinical feature of endometriosis also mimics malignant reproductive disease, such as progressive invasion to adherent pelvic organ and recurrence abilities. Mucins are secreted by epithelium cells of reproductive tissues, generating the mucus of cervix and endometrium, which plays an important role in reproductive physiology. Impaired mucin secretion could impede spermatozoa migration, which may contribute to female infertility. Unlike the less consistent expression patterns of MUC1, MUC6 and MUC5AC in normal and cancer tissues, MUC2 levels were always measured low in normal endometrial and cervical tissue, and elevated MUC2 expressions were specifically found in various neoplastic lesions [
28,
29,
43‐
45]. However, expression patterns of MUC2 in ovarian tumor were heterogenic [
18]. Boman et al. reported that MUC2 were mainly present in benign and borderline ovarian tumor [
46], while Dong et al. showed that breast cancer patients with presence of MUC2 expression had shorter disease-free survival [
47]. We found that the minor allele of rs7103978 decreases the cognate codon frequency from 15.8‰ to 7.8‰ (Kazusa DNA Res. Inst.
http://www.kazusa.or.jp/codon/), which may increase the odds of premature translation termination and thus reduce MUC2 level. Therefore, our result suggested that expression of MUC2 may facilitate cell invasion or proliferation abilities. The observed association of
MUC2 polymorphisms and endometriosis may help us further elucidate the link between endometriosis and certain subtypes of ovarian cancer, if such genetic alterations were also present in the ovarian cancer patients.
Although endometriosis could cause pelvic adhesion and tubal occlusion which lead to infertility, some patients without anatomic disruption still had the problem of impaired fertilization. Possible mechanisms of endometriosis-related infertility include impaired folliculogenesis induced by abnormal immunological, chemical factors or toxins, poor oocyte quality, inhibited binding of spermatozoa to the zona-pellucida and impaired implantation of embryo. This phenomenon was correlated to changes of cytokines and growth factors in endometrium, follicular fluid and peritoneal fluid [
8]. Previous studies have already showed the positive association of endometriosis and polymorphism of cytokine genes [
7,
48]. Li et al. showed that macrophage induced IL-6 up-regulated the MUC1 but down-regulated MUC2 expression [
9]. Up-regulation of MUC1 was associated with implantation failure [
22]. IL-1 was also found to up-regulate MUC2 expression, and IL-1 was thought to regulate immune and inflammation response in endometrium and modulate extracellular matrix modeling of endometrium during menstruation and implantation [
49]. Moreover, previous studies demonstrated that IL-8, TNF-α and NF-κB tend to increase in the peritoneal fluid according to the severity of dysmenorrhea, extent pelvic adhesion and proliferation of endometrial stroma cells, and MUC2 expression could increase accordingly via activation of NF-κB pathway through these cytokines [
50‐
52]. Therefore, it is plausible that altered level of MUC2 could affect fertility as a downstream effecter that can further influence the secretion of mucus, sperm motility, oocyte quality and receptivity of endometrium.
Conclusions
In this study, our data revealed a significant association between MUC2 polymorphisms and endometriosis in a Taiwanese population. The results imply that MUC2 may play a role in the pathogenesis of endometriosis and endometriosis-related infertility, while the mechanisms underlying this phenomenon remain to be elucidated. As a major secreted form of mucins, MUC2 may have the ability to affect more surrounding tissues than the membrane-bound form of mucins. Since MUC2 is not as well investigated as some other mucins in reproductive organs, its molecular function in endometriosis and infertility is worth future study.
Acknowledgements
We appreciate the kind assistance from Ms Carmen Chan at China Medical University Hospital. This work was supported by the grant from China Medical University Hospital (DMR-99-091) and the China Medical University-Asia University Research Collaboration Fund (CMU98-asia-02), Taichung, Taiwan.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
CY-YC and YC: study design, execution and manuscript drafting; YC and C-MC: statistical analysis; CY-YC, W-CL and F-JT: patient collection; C-PC, S-CL and JJ-CS: critical discussion and manuscript editing. All authors read and approved the final manuscript.