In Search of Molecular Mechanisms in Endometriosis

Endometriosis is one of the most common causes of chronic pelvic pain and infertility. It affects 10% of women of reproductive age, and the incidence increases to 35%–50% in infertile women (1, 2). The economic impact of this disease in the United States is profound, with an estimated yearly cost for diagnosis and treatment exceeding $22 billion (3). Endometriosis likely contributes significantly to other common disorders, including irritable bowel syndrome, interstitial cystitis, and chronic fatigue syndromes (2, 4). The initiation of endometriosis is difficult to evaluate because the disease has usually been prevalent for 8–11 years at the time of clinical diagnosis (2, 5). Surgical removal of lesions and hormonal suppression are the current gold standards of therapy, but both approaches are associated with various side effects and a high incidence of relapse. Therefore, identification of mechanisms involved in the early pathogenesis of endometriosis and noninvasive diagnosis and strategic therapies for treatment are critical. A good experimental animal model is required for the research to elucidate pathogenic mechanism and to develop new therapeutic drugs. Model organisms often provide a surrogate for experiments that cannot be performed in humans. Endometriosis spontaneously occurs only in humans and some nonhuman primates. Nonhuman primates have been extensively used for the investigation of endometriosis (6,–8). However, the very high costs of animal handling limit the use of monkeys as an experimental model. Therefore, endometriosis models have been surgically induced in small animals, such as rabbit, rat, and mouse (9,–14). Animal models based on laboratory mice have been widely used for endometriosis research. Extensive molecular, cellular, and genetic data can be obtained from experiments in endometriosis models. An elegant study in this issue of Endocrinology by Heard et al (15) develops a mouse endometriosis model to address whether ablation of Kruppel-like factor 9 (KLF9) is causative to ectopic lesion establishment. KLF9 plays a critical role in uterine function. Dysregulation of KLF9 is associated with subfertility, uterine hypoplasia, leiomyoma, endometrial adenocarcinoma, endometriosis, and defects in the timing of parturition (16,–20). KLF9 acts as a progesterone receptor (PGR) coregulator (21, 22) and as a regulator of estrogen receptor-α (ESR1) signaling (23, 24). The expression of KLF9 is decreased in eutopic endometrium from women with endometriosis and infertile women (17, 18, 25). Despite observations of its dysregulation in human endometriosis, an etiological role for KLF9 in ectopic lesion establishment and underlying mechanisms has remained elusive. This animal model clearly demonstrates that a deficiency in endometrial KLF9 expression is causal to ectopic lesion establishment. Progesterone resistance is a hallmark of endometriosis. Progesterone resistance described that a subset of progesterone-dependent genes in the eutopic secretory endometrium is deregulated in endometriosis patients (24,–26). Impaired gene expression in eutopic endometrium of patients with endometriosis occurs throughout the entire cycle, including the proliferative phase, although the most extensive changes were found in early-secretory endometrium (25). Many of the deregulated genes identified during this phase of the cycle were known progesterone targets, and the overall pattern of aberrant gene expression suggested a prolongation of the proliferative phenotype after ovulation (25). Therefore, the distinct expression levels of PGR target genes between wild-type and Klf9 null lesions support the transcriptional deregulation of PGR signaling with Klf9 loss. Several studies have demonstrated the functional importance of the Hedgehog (Hh) pathway in maintaining progesterone sensitivity in pregnancy and endometrial function by direct PGR regulation of the Hh ligand, Indian hedgehog (27, 28). Importantly, Indian hedgehog (27) and KLF9 (16) both display decreased expression in eutopic endometria of women with endometriosis. The study by Heard et al (15) found that a molecular link between KLF9 and the expression of several components of the Notch and Hedgehog signaling pathways in ectopic lesions. Both signaling pathways are critical for the proliferation and maintenance of endometrial cells as well as adult stem cells (29, 30). Stem cells are thought to contribute to the cyclical regeneration of the endometrium (31, 32) and have been linked to the development of endometriosis (29,–31, 33). Although KLF9 has not been linked specifically to endometrial stem cell biology and pathology, levels of Notch ligand Jagged 2 (Jag2) transcript and activated Notch intracellular domain and multiple components of the Hedgehog signaling pathway are increased in Klf9 null than in wild-type lesions. Whether KLF9 is a direct regulator of Notch and Hedgehog signaling or functions as an indirect mediator through estrogen receptor (ESR) and PGR requires further study. PGR and ESR signaling is so tightly coupled to KLF9 that loss of KLF9 expression in ectopic lesions is associated with significant alterations in expression ratios of ESR2 and estrogen receptor-α isoforms; substantial loss of PGR expression; and deregulation of expression of a subset of PGR target genes. These data provide further support for the role of KLF9 as a unique bridge between molecular signaling and cell turnover. It is interesting to speculate that integrated key molecules involved in steroid hormone (ESR, PGR), Notch, and Hh signaling pathways with proliferation and apoptotic status that together may mediate the pathological steps involved in lesion establishment with loss of KLF9 transcriptional control. Therefore, this outstanding work could become the keystone of a new paradigm because the authors suggest the endometrial KLF9 deficiency promotes endometriotic lesion establishment by the coincident deregulation of Notch, Hedgehog, and steroid receptor-regulated pathways. Progress in our understanding of the etiology and pathophysiology of endometriosis and potential therapeutic interventions by targeted pharmacological agents has been hampered due, in part, to the lack of defined molecular mechanisms and animal models. This model will motivate the translation of animal models for biomarker identification as well as the development of new therapeutic approaches for endometriosis.