Endometriosis pathophysiology is still enigmatic and far from being elucidated, as no single theory could explain all reported observations [17]. However, the constituent of genetic and epigenetic alterations in endometriosis is also a rapidly emerging research [18]. As proposed by Konickx et al., the genetic/epigenetic theory enhances the previously known theories of endometriosis by adding current knowledge of the cumulative genetic and epigenetic modifications transmitted at birth and acquired throughout life [17].
Accordingly, transcriptomic studies have shed light on the molecular basis of endometriosis through Genome-Wide Association Studies (GWAS). However, a shortcoming of recent GWAS is the relatively narrow approach, disregarding the potential impacts of long-range regulatory elements. Thus, linking GWAS with transcriptional analysis should help enlighten the essential genes associated with endometriosis [18]. Despite the massive data provided by different studies, constraints and challenges continue to affect the results, from the sample size to technical issues [19]. Combining larger samples from several studies is a promising approach to better comprehend the endometriosis-linked mechanism.
In the present study, we identified several endometriosis-associated features, including differentially expressed genes (DEGs), miRNA, and lncRNA. Expectedly, protein-coding genes accounted for the predominantly altered DEGs (~ 96,3%), of which 551 genes were common in at least four or more studies. As presented in Fig. 6A, the constructed network of DEGs (confidence score > 0.9) clearly differentiates the clusters of up and downregulated genes with comparable proportions. These results signify how gene alterations influence one another, resulting in intricate relationships to arouse and cease the essential process of endometriosis progression. These genes were linked to cellular and chemical responses and play a particular role in cell signaling and communication. Moreover, we pointed out that the alteration of extracellular components was primarily affected, and various molecular binding roles, such as cell adhesion, cytoskeletal protein, and kinase, marked the molecular function of gene ontology.
Intriguingly, we also found that Extracellular matrix (ECM) receptor interaction is the most enriched pathway, as reported by previous studies [20]. ECM components play a critical function in cellular networks. Klemmt et al. reported that the stromal cells of women with endometriosis display increased DNA synthesis, attachment, and proliferative capacity in response to soluble ECM components [21]. Accordingly, our findings also pinpoint the focal adhesion and cell cycle as the enriched pathways, which may explain the aftereffects of ECM alterations. Moreover, several integrin family genes were aberrated, including ITGA11, ITGA6, ITGB8, and ITGA8 (Table 3). It is known that integrins mediate the adhesion of cells to ECM components, such as collagen types I and IV, fibronectin, and laminin [22]. This environment was supported by our findings, of which COL4A3, COL6A2, LAMA4, LAMC2, and LAMA1 were altered in endometriosis patients (Table 3).
Given the growing evidence of interrelation between endometriosis and cancer, our results may confirm their shared connection. About 45 genes were associated with the pathway in cancer (Table 3), marking the highest number of hits of the KEGG pathway of studied DEGs. Similarly, Ni et al. reported that 571 DEGs overlapped between endometriosis and ovarian cancer [23], and several cancer mutations were harbored in 79% of endometriosis patients, as reported by Anglesio et al. [24]. However, our obtained hub genes may contradict the profound mechanism, as we found all downregulation of the following genes: CDK1, CCNB1, KIF11, CCNA2, BUB1B, DLGAP5, BUB1, TOP2A, ASPM, CEP55, CENPF, TPX2, CCNB2, KIFC, NCAPG (Fig. 6, Table 4). These genes control cell cycle machinery, and their unscheduled up-regulation or down-regulation can hinder the cell death sentence, leading to uncontrolled cell proliferation and malignant transformation [25]. Therefore, we suggest that certain inhibitory mechanisms and other intricate processes were involved in endometriosis progression.
Regarding the epigenetic roles and mechanisms, we further analyzed the shared miRNAs and lncRNAs from all datasets. MicroRNAs are highly stable non-coding RNAs that post-transcriptionally regulate gene expression, affecting cell death and proliferation, inflammation, and other pathological mechanism [26]. Among 16 identified miRNAs, the network of five miRNA-target with highest interaction were analyzed, namely miR-25, miR-503, miR-302b, miR-424 and miR-10a (Fig. 7). Previous study demonstrated a significant decrease in miR-25 in ectopic and eutopic endometrium, and miR-503 suppression were reported to regulate CD97 expression and related JAK2/STAT3 pathway [27, 28]. Moreover, a study by Lin et al. showed that miR-302 induces demethylation of overall genomic DNA, activating several transcription factors such as Oct4, Sox2, Nanog, and Lin28. miR-302 has also been known for its inhibitory roles in various cancers through cyclin D1 inhibition [29].
We identified 9 enriched lncRNA and their respective target, namely MALAT1, DLEU2, MSC-AS1, HAND-AS1, MIR99AHG, GATA2-AS1, LINC01140, KLF3-AS1, and MAGI2-AS3 (Fig. 8). lncRNAs, as well as miRNA, exhibit pivotal role in endometriosis etiology. GATA2-AS1 is a 2358 bp lncRNA that performed as a tumor suppressor for inhibit proliferation. This lncRNA is controlling HOXB4 and ALDH1A2 expression, which associated with greater invasiveness of ectopic implants in ovarian endometriosis [30]. On the other hand, study showed that HAND2-AS1 expression leads to an increase in DNA methylation of HAND2, and lncRNA MALAT1 were reported to inhibit apoptosis of endometrial stromal cells through miR-126-5p-CREB1 axis by activating PI3K-AKT pathway [31].
Given the drawbacks of the present study, the significant findings should be interpreted deliberately. First, the heterogeneity of individual donors in terms of their specific pathophysiological condition must generate the disparities between the expression pattern of the endometriosis and control group, necessitating experimental validation of our findings. In addition, limitations associated with the variability between studies, namely the experimental protocols (microarray or RNA-sequencing), tissue and endometriosis types, and timely sample collection (in regards to the menstrual cycle), can bias the obtained results. Therefore, a comparative experimental study with homogeneous clinical characteristics is required to validate the results. Notwithstanding, our in-depth bioinformatic analysis favorably integrates the transcriptomic data from publicly available studies, emphasizing molecular targets associated with endometriosis progression.