Part of this write-up has been presented in my talk presented at SEUD 2025 in Prague on April 25, 2025.
Sun-Wei Guo
The pathogenesis of endometriosis, or its related disease, adenomyosis, has been traditionally viewed as “enigmatic”. Although Sampson’s retrograde menstruation theory 1 is the most widely accepted, the theory is nonetheless difficult to explain why there is a glaring gap between the nearly ubiquity of retrograde menstruation in women of reproductive age with patent fallopian tubes and the moderate prevalence (typically ~10%). The gap would be even more glaring if age-specific prevalence is considered. For adenomyosis, while the hypothesis of endometrial-myometrial interface disruption or EMID 2 is supported by ample epidemiological data indicating that the history of uterine procedures is a strong risk factor for developing adenomyosis 3, 4. It is also backed by solid animal experimentation, with the EMID procedure used to induce adenomyosis mimicking uterine procedures on humans 5, which is also independently validated using the same 6 or a different procedure of EMID 7. However, not all adenomyosis is caused by iatrogenic uterine procedures 2, 8. In view of the above, new hypotheses are needed to explain the pathogenesis of endometriosis and adenomyosis.
Heritability and missing heritability of endometriosis
Endometriosis has long been recognized to have a familial aggregation, with increased risk of endometriosis in first-degree relatives and sisters 9. This tendency of familial aggregation is often taken as an evidence for a genetic component in the pathogenesis of endometriosis. A frequently quoted measure of how much genetic or hereditary factors play a role in the pathogenesis of endometriosis is the 47% heritability 10, which, to many, implicates that nearly half of the causes for endometriosis is determined by heritable, and genetic susceptibility.
However, despite years of painstaking work involving tens of thousands of patients, a recent study reports that so far all the 42 loci identified only explain approximately 2% of disease variance 11, that is, more than 1-2/47=95.7% of the estimated heritability of endometriosis is unaccounted for, or missing. It is very likely that these 42 identified genetic loci represent low-hanging fruits, in the meaning that they likely have larger impact on the risk of developing endometriosis, and, as such, are much easier to identify. The task to identify the rest of the genetic loci is conceivably more difficult and challenging.
Shorter anogenital distance and endometriosis/adenomyosis
Defined as the distance between the anus and the genital tubercle, anogenital distance (AGD) is a sexually dimorphic feature, longer in male mammals but shorter in females 12, 13. In utero androgen levels affect the development of the perineal tissue and are negatively correlated with the AGD 14, which can serve as a life-long indicator of androgen, vis-à-vis estrogen, action in gestational weeks 8-14, which are known as the masculinization programming window 15. Hence, AGD represents a biomarker of the prenatal hormonal environment 16, 17, although AGD also can reflect the prenatal exposure to endocrine disruptors 18, 19 and androgens during the development of the reproductive system 20, 21.
Shorter AGD has been reported to be associated with a higher risk of endometriosis, especially deep endometriosis 12, 22-25. It is also associated with a higher risk of adenomyosis, but not uterine fibroids 26. In contrast, longer AGD is reported to be associated with polycystic ovarian syndrome (PCOS) 27-31. While a few studies failed find such a relationship 32, 33, a closer reading of the papers still suggests a shorter AGD in women with endometriosis.
Thus, shorter AGD is associated with a higher risk of developing endometriosis and/or adenomyosis, suggesting that in utero or perhaps perinatal exposure to higher levels of estrogens may increase the risk.
Developmental exposure to estrogens and the risk of endometriosis/adenomyosis
Voluminous research has demonstrated that fetus and the neonate are in a critical developmental period, and their reproductive tracks are exquisitely sensitive to endocrine-disrupting chemicals, especially xenobiotic estrogen34. For example,
prenatal exposure to bisphenol-A causes preferential epigenetic programing of estrogen response 35. Prenatal exposure to diethylstilbestrol (DES) results in lasting effect on the reproductive tract, including adenomyosis, in mice 36. In mice with neonatal exposure to DES, a plethora of genes are aberrantly expressed during the time of exposure, and some of them persist well into maturity and are associated with morphological and functional changes, and even pathology 37-40. Of relevant, neonatal exposure to bisphenol A causes adenomyosis in adult CD-1 mice 41, similar to tamoxifen-induced one 42. More remarkably, neonatal DES exposure apparently results in DNA methylation and gene specific changes in histone modification enzymes that are likely to cause changes in gene expression in adulthood37, 38, 43, 44.
These global epigenetic changes could account for long-term, persistent and likely profound transcriptional aberrations in the reproductive tract due to subsequent changes in their downstream genes 38, 45. In other words, these changes resulting from the adenomyosis-inducing procedure would likely to impact on fertility and possibly on response to drug treatment. Consequently, the results obtained based on this model are likely to attributable mostly from the lasting global epigenetic changes and subsequent transcriptomic changes within the uterus, not just from the presence of adenomyosis alone.
The missing link
From the above, we can see that on one hand, prenatal and/or neonatal exposure to estrogen or xenobiotic estrogens causes long-last epigenetic changes in the female reproductive tract, increasing the risk of developing adenomyosis (which, unlike endometriosis, can arise spontaneously, but rarely 46 and mostly in aged rodents 47-50). On the other, shorter AGD, which may result from exposure to elevated estrogens in utero, is associated with endometriosis and adenomyosis. Therefore, these data may suggest that prenatal, and perhaps neonatal as well, exposure to increased levels of estrogen may increase the risk of endometriosis and adenomyosis.
Here the question is: What causes the elevated estrogen levels, either in utero or neonatal, in the first place?
This missing but critical piece in this puzzle can be found from a talk by Professor Paola Vigano at the recent SEUD annual conference held in Prague. She reports that her team detected a significantly elevated estrogen concentration in the umbilical cord blood from delivery women with previously diagnosed endometriosis.
Phenotypic imprinting
Professor Vigano’s finding seems to provide a critical piece of evidence linking AGD-endometriosis/adenomyosis association and animal studies showing the association of prenatal/neonatal exposure to estrogen/xenobiotic estrogens and adenomyosis. Thus, it can be hypothesized that a phenotypic imprinting mechanism exists, in which women with endometriosis and/or adenomyosis have an elevated levels of estrogens in their cord blood when pregnant, and possibly in their milk after delivery. This would establish a prenatal and neonatal environment with elevated estrogen levels to their female offspring, and, as such, increase the risk of developing endometriosis and/or adenomyosis.
Similar to genetic imprinting, which is essentially an epigenetic mechanism, this increase in risk may also be mediated by long-lasting epigenetic changes in the reproductive tract in female offspring brought about by prenatal and neonatal exposure to elevated levels of estrogen. So the name of phenotypic imprinting.
Thus, the tendency of familial aggregation in endometriosis, and possibly in adenomyosis as well, can be explained by this phenotypic imprinting.
Very importantly, this hypothesis is falsifiable. By carefully designing animal experiments and human studies, this hypothesis can be rigorously tested.
Acknowledgement
I would like to thank Professor Paola Vigano for sharing her finding during a personal communication and her inspiring presentation at SEUD 2025 in Prague.
References
[1] Sampson JA. Peritoneal endometriosis due to the menstrual dissemination of endometrial tissue into the peritoneal cavity. . Am J Obstet Gynecol. 1927; 14: 422-69.
[2] Guo SW. The Pathogenesis of Adenomyosis vis-a-vis Endometriosis. J Clin Med. 2020; 9(2).
[3] Levgur M, Abadi MA, Tucker A. Adenomyosis: symptoms, histology, and pregnancy terminations. Obstet Gynecol. 2000; 95(5): 688-91.
[4] Curtis KM, Hillis SD, Marchbanks PA, Peterson HB. Disruption of the endometrial-myometrial border during pregnancy as a risk factor for adenomyosis. Am J Obstet Gynecol. 2002; 187(3): 543-4.
[5] Hao M, Liu X, Guo SW. Adenomyosis in mice resulting from mechanically or thermally induced endometrial-myometrial interface disruption and its possible prevention. Reprod Biomed Online. 2020; 41(5): 925-42.
[6] Zheng H, Liu M, Su Q, Li H, Wang F. Impaired fertility and perinatal outcomes in adenomyosis: insights from a novel murine model and uterine gene profile alterations during implantations. Am J Obstet Gynecol. 2025.
[7] Elsherbini M, Koga K, Hiraoka T, et al. Establishment of a novel mouse model of adenomyosis suitable for longitudinal and quantitative analysis and perinatal outcome studies. Sci Rep. 2022; 12(1): 17515.
[8] Kishi Y, Suginami H, Kuramori R, Yabuta M, Suginami R, Taniguchi F. Four subtypes of adenomyosis assessed by magnetic resonance imaging and their specification. Am J Obstet Gynecol. 2012; 207(2): 114 e1-7.
[9] Bischoff FZ, Simpson JL. Heritability and molecular genetic studies of endometriosis. Hum Reprod Update. 2000; 6(1): 37-44.
[10] Saha R, Pettersson HJ, Svedberg P, et al. Heritability of endometriosis. Fertil Steril. 2015; 104(4): 947-52.
[11] Rahmioglu N, Mortlock S, Ghiasi M, et al. The genetic basis of endometriosis and comorbidity with other pain and inflammatory conditions. Nat Genet. 2023; 55(3): 423-36.
[12] Mendiola J, Sanchez-Ferrer ML, Jimenez-Velazquez R, et al. Endometriomas and deep infiltrating endometriosis in adulthood are strongly associated with anogenital distance, a biomarker for prenatal hormonal environment. Hum Reprod. 2016; 31(10): 2377-83.
[13] Garcia-Penarrubia P, Ruiz-Alcaraz AJ, Martinez-Esparza M, Marin P, Machado-Linde F. Hypothetical roadmap towards endometriosis: prenatal endocrine-disrupting chemical pollutant exposure, anogenital distance, gut-genital microbiota and subclinical infections. Hum Reprod Update. 2020; 26(2): 214-46.
[14] Schwartz CL, Christiansen S, Vinggaard AM, Axelstad M, Hass U, Svingen T. Anogenital distance as a toxicological or clinical marker for fetal androgen action and risk for reproductive disorders. Arch Toxicol. 2019; 93(2): 253-72.
[15] Welsh M, Saunders PT, Fisken M, et al. Identification in rats of a programming window for reproductive tract masculinization, disruption of which leads to hypospadias and cryptorchidism. J Clin Invest. 2008; 118(4): 1479-90.
[16] Greenham LW, Greenham V. Sexing mouse pups. Lab Anim. 1977; 11(3): 181-4.
[17] Kurzrock EA, Jegatheesan P, Cunha GR, Baskin LS. Urethral development in the fetal rabbit and induction of hypospadias: a model for human development. J Urol. 2000; 164(5): 1786-92.
[18] Swan SH, Sathyanarayana S, Barrett ES, et al. First trimester phthalate exposure and anogenital distance in newborns. Hum Reprod. 2015; 30(4): 963-72.
[19] Bornehag CG, Carlstedt F, Jonsson BA, et al. Prenatal phthalate exposures and anogenital distance in Swedish boys. Environ Health Perspect. 2015; 123(1): 101-7.
[20] Dean A, Sharpe RM. Clinical review: Anogenital distance or digit length ratio as measures of fetal androgen exposure: relationship to male reproductive development and its disorders. J Clin Endocrinol Metab. 2013; 98(6): 2230-8.
[21] Jain VG, Singal AK. Shorter anogenital distance correlates with undescended testis: a detailed genital anthropometric analysis in human newborns. Hum Reprod. 2013; 28(9): 2343-9.
[22] Crestani A, Abdel Wahab C, Arfi A, et al. A short anogenital distance on MRI is a marker of endometriosis. Hum Reprod Open. 2021; 2021(1): hoab003.
[23] Sanchez-Ferrer ML, Jimenez-Velazquez R, Mendiola J, et al. Accuracy of anogenital distance and anti-Mullerian hormone in the diagnosis of endometriosis without surgery. Int J Gynaecol Obstet. 2019; 144(1): 90-6.
[24] Sanchez-Ferrer ML, Mendiola J, Jimenez-Velazquez R, et al. Investigation of anogenital distance as a diagnostic tool in endometriosis. Reprod Biomed Online. 34(4): 375-82.
[25] Peters HE, Laeven CHC, Trimbos C, et al. Anthropometric biomarkers for abnormal prenatal reproductive hormone exposure in women with Mayer-Rokitanksy-Kuster-Hauser syndrome, polycystic ovary syndrome, and endometriosis. Fertil Steril. 2020; 114(6): 1297-305.
[26] Liu X, Ding D, Shen M, Yan D, Guo SW. Shorter Anogenital Distance in Women with Ovarian Endometriomas and Adenomyosis, but Not Uterine Leiomyomas. Biomedicines. 2023; 11(10).
[27] Sanchez-Ferrer ML, Mendiola J, Hernandez-Penalver AI, et al. Presence of polycystic ovary syndrome is associated with longer anogenital distance in adult Mediterranean women. Hum Reprod. 2017; 32(11): 2315-23.
[28] Wu Y, Zhong G, Chen S, Zheng C, Liao D, Xie M. Polycystic ovary syndrome is associated with anogenital distance, a marker of prenatal androgen exposure. Hum Reprod. 2017; 32(4): 937-43.
[29] Hernandez-Penalver AI, Sanchez-Ferrer ML, Mendiola J, et al. Assessment of anogenital distance as a diagnostic tool in polycystic ovary syndrome. Reprod Biomed Online. 2018; 37(6): 741-9.
[30] Prieto-Sanchez MT, Hernandez-Penalver AI, Sanchez-Ferrer ML, Mendiola J, Torres-Cantero AM. Anogenital distance and anti-Mullerian hormone combined improves the diagnosis of polycystic ovary syndrome. Hum Fertil (Camb). 2022; 25(2): 274-82.
[31] Simsir C, Pekcan MK, Aksoy RT, et al. The ratio of anterior anogenital distance to posterior anogenital distance: A novel-biomarker for polycystic ovary syndrome. J Chin Med Assoc. 2019; 82(10): 782-6.
[32] Buggio L, Somigliana E, Sergenti G, Ottolini F, Dridi D, Vercellini P. Anogenital Distance and Endometriosis: Results of a Case-Control Study. Reprod Sci. 2022; 29(12): 3508-15.
[33] Khan KN, Fujishita A, Hiraki K, et al. Lack of association between the length of anogenital distance and vaginal pH in women with endometriosis. J Obstet Gynaecol Res. 2024; 50(12): 2327-37.
[34] Yamashita S. Expression of estrogen-regulated genes during development in the mouse uterus exposed to diethylstilbestrol neonatally. Curr Pharm Des. 2006; 12(12): 1505-20.
[35] Jorgensen EM, Alderman MH, 3rd, Taylor HS. Preferential epigenetic programming of estrogen response after in utero xenoestrogen (bisphenol-A) exposure. FASEB J. 2016; 30(9): 3194-201.
[36] Huseby RA, Thurlow S. Effects of prenatal exposure of mice to “low-dose” diethylstilbestrol and the development of adenomyosis associated with evidence of hyperprolactinemia. Am J Obstet Gynecol. 1982; 144(8): 939-49.
[37] Newbold RR, Jefferson WN, Grissom SF, Padilla-Banks E, Snyder RJ, Lobenhofer EK. Developmental exposure to diethylstilbestrol alters uterine gene expression that may be associated with uterine neoplasia later in life. Mol Carcinog. 2007; 46(9): 783-96.
[38] Jefferson WN, Chevalier DM, Phelps JY, et al. Persistently altered epigenetic marks in the mouse uterus after neonatal estrogen exposure. Mol Endocrinol. 2013; 27(10): 1666-77.
[39] Yin Y, Lin C, Veith GM, Chen H, Dhandha M, Ma L. Neonatal diethylstilbestrol exposure alters the metabolic profile of uterine epithelial cells. Dis Model Mech. 2012; 5(6): 870-80.
[40] Suen AA, Jefferson WN, Wood CE, Padilla-Banks E, Bae-Jump VL, Williams CJ. SIX1 Oncoprotein as a Biomarker in a Model of Hormonal Carcinogenesis and in Human Endometrial Cancer. Mol Cancer Res. 2016; 14(9): 849-58.
[41] Newbold RR, Jefferson WN, Padilla-Banks E. Long-term adverse effects of neonatal exposure to bisphenol A on the murine female reproductive tract. Reprod Toxicol. 2007; 24(2): 253-8.
[42] Parrott E, Butterworth M, Green A, White IN, Greaves P. Adenomyosis–a result of disordered stromal differentiation. Am J Pathol. 2001; 159(2): 623-30.
[43] Li S, Hansman R, Newbold R, Davis B, McLachlan JA, Barrett JC. Neonatal diethylstilbestrol exposure induces persistent elevation of c-fos expression and hypomethylation in its exon-4 in mouse uterus. Mol Carcinog. 2003; 38(2): 78-84.
[44] Tang WY, Newbold R, Mardilovich K, et al. Persistent hypomethylation in the promoter of nucleosomal binding protein 1 (Nsbp1) correlates with overexpression of Nsbp1 in mouse uteri neonatally exposed to diethylstilbestrol or genistein. Endocrinology. 2008; 149(12): 5922-31.
[45] Bredfeldt TG, Greathouse KL, Safe SH, Hung MC, Bedford MT, Walker CL. Xenoestrogen-induced regulation of EZH2 and histone methylation via estrogen receptor signaling to PI3K/AKT. Mol Endocrinol. 2010; 24(5): 993-1006.
[46] Wang X, Benagiano G, Liu X, Guo SW. Unveiling the Pathogenesis of Adenomyosis through Animal Models. J Clin Med. 2022; 11(6).
[47] Walker BE. Uterine tumors in old female mice exposed prenatally to diethylstilbestrol. J Natl Cancer Inst. 1983; 70(3): 477-84.
[48] Huseby RA, Soares MJ, Talamantes F. Ectopic pituitary grafts in mice: hormone levels, effects on fertility, and the development of adenomyosis uteri, prolactinomas, and mammary carcinomas. Endocrinology. 1985; 116(4): 1440-8.
[49] Singtripop T, Mori T, Shiraishi K, Park MK, Kawashima S. Age-related changes in gonadotropin, prolactin and growth hormone levels with reference to the development of uterine adenomyosis in female SHN mice. In Vivo. 1993; 7(2): 147-50.
[50] Danilovich N, Roy I, Sairam MR. Emergence of uterine pathology during accelerated biological aging in FSH receptor-haploinsufficient mice. Endocrinology. 2002; 143(9): 3618-27.
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