Let-7e expression was significantly reduced in both the severe endometriosis group (−32.7051) and the mild endometriosis group (−4.255), whereas let-7f was significantly reduced only in the severe endometriosis group. The average threshold cycles of let-7a, let-7d, miR-150, miR-322, and miR-222 were greater than the defined cut-off value (default 35) in all groups, making this fold-change erroneous and uninterpretable (Figs 2a and 3a). The patterns of expression of different genes across different patients were grouped into distinct clusters (Fig. 4), in which genes in the same cluster were assumed to be potentially functionally related or to be influenced by a common upstream factor (miR-130a). Such a cluster structure is used to aid the elucidation of regulatory networks of miR-130a.
MicroRNAs are a new class of endogenous small noncoding RNAs that can pair with sites in 3′ untranslated regions in the mRNAs of protein-coding genes to downregulate their expression 14, and are associated with a variety of human diseases, including cancer, heart failure, vascular disease, diabetes, etc. In the last several years, miRNA has been investigated extensively, and it is now recognized that miRNA plays a key role in regulating fundamental cellular processes 12. Blood miRNAs have been considered as promising novel biomarkers 15,16. The results of this study clearly uphold the basic hypothesis that miRNAs are present in blood and represent useful clinical biomarkers. We have shown that the blood levels of miR-130a can serve as a marker for endometriosis. This result is easily understood as miR-130a is shown to be significantly increased to around 4 fold in the mild endometriosis cases and 17-fold in the severe cases. The correlation of miR-130a with the different grades of endometriosis may at least partly a role in the progression of this condition. Endometriosis is characterized by angiogenesis, and new tissue formation and proliferation. MiR-130a is located on chromosome 11 and the expression of this miRNA was found to be rapidly upregulated under high serum culture conditions 17.
Interestingly, miR-130a has a proangiogenic function. It downregulates (at the post-transcriptional level) the antiangiogenic homeobox genes, that is, GAX and HoxA5 and functionally antagonizes its inhibitory effects on endothelial cell proliferation, migration, and tube formation 17, thereby facilitating the angiogenic process.
The proangiogenic properties of miR-130a may affect the transition of an undifferentiated stem cell to a differentiated phenotype, which plays a critical role in the pathogenesis of endometriosis. The origins of the mesenchymal cells participating in tissue repair and pathological processes are poorly understood. Oosterlynck et al. 24 found that the transforming growth factor β is upregulated in the peritoneal fluid of women with endometriosis, and several studies have identified this growth factor as a central component in molecular signaling networks and mediate a switching from an initially ischemic and inflammatory environment that causes tissue damage and necrosis to a ‘healing’ milieu that promotes cellular proliferation and tissue remodeling during endometriotic lesion development 21,25–27. Translation of this factor is repressed by miR-21 28, which is downregulated in the blood of mild cases in association with overexpression of miR-130a, leading to an enhancement in its activity in the transformation of peritoneal stem cell into endometriotic-like cells. Also, miR-21 was found to be upregulated in eutopic endometrium throughout the menstrual cycle in severe versus mild endometriosis 29. Most miRNAs that were downregulated in the blood of mild cases represent a cluster of miRNAs responsible for the regulation of several physiological and pathological conditions in endometriosis 30. These miRNAs are upregulated in the severe group in association with an increase in the expression of miR-130a, which may reflect their role in inducing endometriosis. The Let-7 family is a regulator of cell cycle, proliferation, and apoptosis 31. Let-7e was significantly downregulated in both the severe and the mild groups compared with the control group. This downregulation was exponentially proportionate to the severity of endometriosis, which may be a clue of its regulatory role in the severity of endometriosis.
Thus, miR-130a appears to be a potent regulator of gene expression in endometriosis (Fig. 5), raising the prospect of using blood miRNAs as biomarkers and therapeutic tools in endometriosis. In the future, it would be conceivably beneficial to examine the function of miR-130a in the initiation and progression of endometriosis in a large number of cases.
miR-130a is a potent regulator of gene expression in endometriosis, leading to enhanced transformation of mesenchymal stem cell into endometriotic-like cells. Furthermore, the blood levels of miR-130a may serve as indicators for endometriosis. It also explains the pathophysiology of endometriosis. This may open up an exciting new avenue for targeted anti-angiogenesis therapy for such a devastating disease.
The authors acknowledge the major contributions of Tamer Taha, Hitham Badran, Morad Selim, Mohamed Abd-Elaty, Mazen Abd Elrasheid, and Ehab Nabeil during the planning phase of this study and for kindly providing samples. The authors also would like to thank Mohamed Reda and Zaynab Algammal or their assistance in sample preparation.
There are no conflicts of interest.
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