{"paper_id":"064eee8b-7218-43c5-9484-5e6e6a6bd662","body_text":"The genome contains all the genetic information needed to develop and maintain a living organism. The transcriptome however represents the functional component of the genome. The GENCODE project has been proposed to annotate all the functional elements of the human genome. So far, 60,660 total human genes were identified, 19,962 of which are protein-coding genes, 17,958 are long non-coding RNA genes, and 7,569 are small non-coding RNA genes (Release 36, December 2020 available online:  https://www.gencodegenes.org/human/stats.html ). The old understanding of the genome mainly focused on the protein-coding genes since proteins play a major role in cellular functions. However, as researchers examine the functions of non-coding RNAs (ncRNAs), they have reached a new understanding of the substantial roles of ncRNAs. Perhaps that could explain the relatively larger proportion of ncRNA genes compared to protein-coding genes of the human genome.\nNon-coding RNAs can be classified bases on their functions into housekeeping RNAs and regulatory RNAs. Housekeeping RNAs are the most abundant among other types as they play an essential role in splicing and translation machineries such as ribosomal RNA (rRNA), transfer RNA (tRNA), small nuclear RNA (snRNA) and small nucleolar RNA (snoRNA) ( Uchida and Adams, 2019 ). On the other hand, regulatory RNAs are less abundant but they play an integral role in regulating gene expression ( Ferlita et al., 2018 ). Regulatory ncRNAs have been of great interest to the scientific community owing to their implications in human pathologies. Because ncRNAs can be detected in all biological fluids, they have been employed as non-invasive diagnostic and prognostic biomarkers for a wide array of human diseases ( Ferlita et al., 2018 ). The advent of ncRNAs in biomedical research led to the advancement of RNA-targeted therapies ( Crooke et al., 2018 ). The main classification of regulatory ncRNAs is based on their size specifically to distinguish between small ncRNAs from long ncRNAs. Small ncRNAs are smaller than 200 nucleotides (nt) whereas long ncRNAs are 200nt or longer transcripts ( Uchida and Adams, 2019 ). Among the best investigated small ncRNAs are microRNAs (miRNAs) and piwi-interacting RNAs (piRNAs). miRNAs range in size between 18 to 25nt long and they function mainly by suppressing mRNA translation by binding to the 3’ untranslated regions (3’ UTR) of the target transcript ( Ferlita et al., 2018 ). On the other hand, piRNAs are well-known for silencing non-coding transposable elements by means of epigenetic mechanisms such as DNA and histone methylation ( Ozata et al., 2019 ,  Nandi et al., 2016 ). Long ncRNAs (lncRNAs) can be classified based on their genomic location into intergenic and intragenic lncRNAs. Intragenic lncRNAs can be exonic or intronic lncRNAs, and exonic can be sense or antisense lncRNAs ( Ferlita et al., 2018 ). One of the remarkable features of lncRNAs is their ability to regulate gene expression through diverse mechanisms. Thus, lncRNAs can be further classified based on their mechanism of action into  cis -acting and  trans -acting lncRNAs. lncRNAs that act in  cis  regulate the chromatin structure and gene expression of neighboring genes through three proposed mechanisms: the recruitment of regulatory factors to the target gene locus, the transcription per se of the lncRNA independent of the transcript, and the functional DNA elements within the lncRNA promoter region independent of the transcript. On the other hand, lncRNAs that act in  trans  regulate the chromatin structure and gene expression of distant genes as well as post-transcriptional and post-translational regulation of target RNAs and proteins. Examples of lncRNAs that regulate the expression of their target genes include enhancer RNAs (eRNAs), dosage compensation lncRNAs, imprinted lncRNAs and competing endogenous RNAs (ceRNAs) ( Quinn and Chang, 2016 ,  Kopp and Mendell, 2018 ).\n\nThe female menstrual cycle is orchestrated by several hormones that are tightly regulated through the hypothalamic-pituitary-gonadal axis. However, the ovarian hormones estrogen (E2) and progesterone (P4) are the main players that directly act on the endometrium. The menstrual cycle can be divided into three main phases: the proliferative phase, the secretory phase and the menstrual phase. The human endometrium is composed of two layers: the basal layer and the functional layer. Only the functional layer is shed during menses while the basal layer stays intact throughout the lifetime of the reproductive cycle. Main endometrial cell types include epithelial cells that make up the uterine lining and uterine glands, stromal cells which make up the majority of the endometrium, and lastly endothelial cells that make up the vasculature. It is well established that P4 withdrawal initiates the inflammatory response leading to menstruation. Similarly, P4 withdrawal increases matrix metalloproteinases (MMPs) expression in the functional layer which induces the breakdown of the extracellular matrix (ECM) leading to tissue sloughing. Immune cells such as neutrophils and macrophages invade the endometrium following P4 withdrawal to promote tissue destruction and regulate the inflammatory response ( Maybin and Critchley, 2012 ,  Maybin and Critchley, 2015 ). Shortly after the first few days of menses, endometrial repair of the epithelium takes place. During the proliferative phase, all endometrial cell types grow exponentially to regenerate the functional layer after menses. Several growth factors play key role in vascular regeneration and cellular proliferation such are vascular endothelial growth factor (VEGF), stromal-derived growth factor-1 (SDF-1), and insulin-like growth factor-1 (IGF1). However, the ovarian E2 is the predominant hormone during the proliferative phase and the main trigger of cellular proliferation ( Maybin and Critchley, 2012 ,  Maybin and Critchley, 2015 ). E2 acts through the estrogen receptor (ER) which, upon ligand binding, translocate to the nucleus and activates several hundred target genes including the progesterone receptor gene (PGR). Thus, one of the well-recognized functions of E2/ER actions is priming the endometrium for P4 action. The secretory phase begins after ovulation when the corpus luteum produces ample amount of P4 along with E2. P4 acts via the progesterone receptor (PR) and counteracts the E2/ER action. Thus, P4 halts endometrial growth and induces epithelial and stromal cell differentiation to prepare the endometrium for embryo implantation ( Ashary et al., 2018 ). During this phase, the endometrium becomes receptive and the blastocyst can attach to the luminal epithelium during a narrow timeframe known as the window of implantation ( Ashary et al., 2018 ). Specific morphological and functional transformation of the luminal epithelium is critical to initiate this process such as reorganization of the apical epithelium and the development of pinopodes ( Aplin and Ruane, 2017 ). The embryonic trophoblast breaches the luminal epithelium to invade the endometrium. This invasion allows trophoblast to proliferate and come in direct contact with the endometrial stroma. In the mouse, trophoblast invasion induces stromal cell decidualization which transforms stromal cells into highly specialized secretory decidual cells ( Aplin and Ruane, 2017 ). In humans by contrast, decidualization of stromal cells occurs in response to the post-ovulatory rise of P4 independent of the conceptus ( Ramathal et al., 2010 ). However, stromal cell decidualization in both species is essential for proper implantation and placentation. Defective decidualization is associated with pregnancy disorders such as recurrent miscarriages and preeclampsia ( Garrido-Gomez et al., 2017 ,  Gellersen and Brosens, 2014 ). The decidua regulates trophoblast invasion to prevent excessive tissue breakdown mediated by trophoblastic factors such as MMPs ( Sharma et al., 2016 ). Moreover, the decidua provides the initial nourishment for the embryo before the development of the placenta and creates an optimal environment for implantation ( Ashary et al., 2018 ).\n\nEpigenetic modifications such as DNA methylation play a significant role in reproductive function. Imprinted genes are expressed unequally from one allele based on parent-of-origin methylation ( Paczkowski et al., 2015 ).  H19  is one of the first identified imprinted genes which was baffling at the time because  H19  can be transcribed but not translated, yet its deletion caused a significant phenotype of overgrowth ( Kallen et al., 2013 ,  Brannan et al., 1990 ,  Gabory et al., 2009 ,  Zhang and Tycko, 1992 ).  H19  yields a lncRNA that is transcribed exclusively from the maternal allele whereas the paternal allele is epigenetically silenced via methylation ( Nordin et al., 2014 ).  H19  belongs to a gene cluster that contains the paternally imprinted gene  IGF2  ( Kallen et al., 2013 ).  H19  gene is predominantly expressed during embryonic development and repressed after birth in most tissues except few organs including the uterus ( Adriaenssens et al., 1999 ). Several studies have shown that steroid hormones regulate the expression of  H19  in various hormone responsive tissues and cell lines ( Adriaenssens et al., 1999 ,  Ariel et al., 1997 ). In the endometrium,  H19  transcripts were detected in the stroma whereas glandular and luminal epithelium were devoid of  H19  expression ( Adriaenssens et al., 1999 ). One of the early studies that investigated the expression pattern of  H19  in human endometrial tissues found fluctuating levels of  H19  throughout the menstrual cycle ( Ariel et al., 1997 ).  H19  expression begins to increase before decidualization and peaks at the end of the secretory phase ( Ariel et al., 1997 ). Discrepancies among studies have been observed and could be owed to variable mice strains, study objectives and protocols. In one study using Swiss 3T3 mice,  H19  expression was predominantly regulated by E2 ( Adriaenssens et al., 1999 ). Thus,  H19  expression gradually increases in response to E2 during the proliferative phase and remains elevated throughout ovulation and early secretory phase then decreases to basal levels during diestrus ( Adriaenssens et al., 1999 ). Conversely, genome-wide analysis of lncRNAs in uterine tissue of Kunming White outbred mice have shown that  H19  expression was significantly upregulated during the window of implantation suggesting a major role of P4 in regulating the expression of  H19  ( Wang et al., 2017 ). The role of  H19  in endometrial function was emphasized by several findings that have shown aberrant  H19  expression is related to reproductive dysfunctions such as unexplained infertility and endometriosis ( Korucuoglu et al., 2010 ,  Ghazal et al., 2015 ). It is well established that  H19  lncRNA acts as a competing endogenous RNA (ceRNA) in several tissues including the endometrium. This mechanism suggests that  H19  lncRNA binds complementary miRNAs to prevent or diminish their actions. One of the well-studied miRNAs that bind  H19  is  let-7  miRNA ( Kallen et al., 2013 ). Therefore, higher  H19  expression can compete for  let-7  binding and inhibit  let-7  from targeting other mRNAs leading to higher expression of  let-7  targets and vice versa. A recent study investigating the role of  H19  in endometriosis found that human endometrial stromal cells (HESCs) express  H19  in response to E2 in vitro and this pathway is essential for HESCs proliferation due to its major role in regulating IGF signaling ( Ghazal et al., 2015 ). One of  let-7  targets is insulin-like growth factor 1 receptor ( IGF1R ) which was found downregulated in women with endometriosis due to significant downregulation of  H19  expression ( Ghazal et al., 2015 ). IGF1R is activated by IGF1 and IGF2 signaling which was shown to play a critical role in endometrial stromal cell proliferation and differentiation ( Ghazal et al., 2015 ).  Igf1  null female mice are infertile due to multiple defects in the reproductive tract ( Baker et al., 1996 ). These findings indicate that infertility associated with endometriosis could be related to  H19  downregulation.\nAnother  let-7  target is one of the cell-extracellular matrix adhesion proteins known as integrin beta 3 ( ITGB3 ) ( He et al., 2019 ). Several studies have shown that ITGB3 plays a critical role in blastocyst adhesion and invasion ( He et al., 2019 ,  Zeng et al., 2017 ,  Liu et al., 2012 ). The transcript level of  ITGB3  as well as  H19  lncRNA were significantly downregulated in endometrial tissue obtained during the mid-luteal phase (window of implantation) from women with recurrent implantation failure ( Zeng et al., 2017 ). A recent study investigating the role of  H19  in trophoblast adhesion found a direct correlation between  H19  and  ITGB3  RNA and protein expression ( He et al., 2019 ). He D. and colleagues found that  H19  downregulation decreased  ITGB3  expression and impaired trophoblast adhesion and invasion in vitro ( He et al., 2019 ). Similar to early findings, the mechanism involves the upregulation of  let-7  miRNA ( He et al., 2019 ). Additionally,  H19  lncRNA,  ITGB3  transcripts, and ITGB3 protein levels were significantly downregulated in embryonic chorion tissue samples from women with spontaneous abortion ( He et al., 2019 ). These findings suggest that  H19  downregulation impacts several key regulators of endometrial receptivity and embryo implantation.\nIn an attempt to characterize the functional roles of lncRNAs in the endometrium, Liang and colleagues explored the role of long intergenic non-coding RNA (lincRNA)  LINC473  in HESCs decidualization ( Liang et al., 2016 ). The specific interest in  LINC473  was due to previously published studies on the molecular link between  LINC473  expression upon cAMP activation which play an integral role in HESCs decidualization ( Liang et al., 2016 ). They found that  LINC473  expression is regulated by cAMP/PKA signaling in HESCs independent of progesterone and estrogen ( Liang et al., 2016 ).  LINC473  promoter region contained a binding site for STAT3 at the transcription start site and STAT3 phosphorylation was required for  LINC473  induction ( Liang et al., 2016 ). STAT3 is a transcription factor and its expression in endometrial stroma increases remarkably during decidualization owing to its essential role during embryo implantation ( Lee et al., 2013 ).  LINC473  expression is required for proper HESCs decidualization as  LINC473  knockdown significantly reduced  PRL  and  IGFPB1  expression in vitro ( Liang et al., 2016 ). Moreover, several key players in endometrial decidualization and receptivity were significantly downregulated upon  LINC473  knockdown such as  PGR, FOXO1, HOXA10, HOXA11  and  WNT4  ( Liang et al., 2016 ). However,  CEBPB  expression, the predominant regulator of decidual marker  PRL , was not affected by  LINC473  knockdown suggesting a profound regulatory role of  LINC473  in decidualization independent of the cAMP/PKA pathway axis ( Liang et al., 2016 ). This study strongly suggests that  LINC473,  which expression is induced by cAMP/PKA signaling, mediates stromal cell decidualization through the transcriptional regulation of  PRL, IGFBP1, PGR, FOXO1, HOXA10, HOXA11  and  WNT4  but the mechanism of regulation is yet to be elucidated.\nHomeobox (Hox) genes play a critical role during early reproductive tract development as well as during embryo implantation ( Chau et al., 2002 ,  Gendron et al., 1997 ). In the human endometrium as well as in mice uterus,  HOXA10  and  HOXA11  expressions significantly increase during the window of implantation ( Gendron et al., 1997 ,  Chau et al., 2002 ). The disruption of either gene expression in humans or mice was associated with infertility due to implantation failure ( Chau et al., 2002 ,  Gendron et al., 1997 ). Therefore, a growing interest to identify upstream transcriptional regulators for  HOXA10  and  HOXA11  led to the recognition of  HOXA11  antisense lncRNA. The expression pattern of  HOXA11  antisense in the endometrium follows a cyclical pattern with respect to the menstrual cycle ( Chau et al., 2002 ). To the contrary of  HOXA11  expression,  HOXA11  antisense expression peaks during the late proliferative phase of the cycle ( Chau et al., 2002 ). Progesterone treatment had a significant impact on  HOXA11  antisense expression which is consistent with the natural increase in progesterone during the luteal phase of the cycle when  HOXA11  antisense expression is greatly diminished ( Chau et al., 2002 ). This study demonstrated that  HOXA11  antisense has an opposing effect on  HOXA11  expression. However,  Hoxa11  antisense RNA transfection into female mice during the luteal phase when  Hoxa11  expression is highest did not block  Hoxa11  translation or function ( Chau et al., 2002 ). The researchers speculated that  HOXA11  antisense could be competing for transcription in a mechanism known as transcriptional interference to block  HOXA11  transcription during the proliferative phase ( Chau et al., 2002 ). Following P4 rise during the luteal phase,  HOXA11  antisense is suppressed allowing for the increase in  HOXA11  expression which is critical for implantation.  Table 1  provides a summary of our current knowledge on the lncRNAs just discussed and their proposed role in the normal endometrium.\n\nEmbryo implantation and endometrial receptivity are fundamental for pregnancy establishment, and the impairment of one directly disrupts the other. Implantation failure following in vitro fertilization (IVF) is devastating. It is a costly procedure, yet the success rate is less than 60% ( Feng et al., 2018 ,  Xu et al., 2019a ). Mounting evidence suggest that the receptive endometrium as well as the implanting blastocyst exhibit a unique transcriptome signature that significantly differ from non-receptive endometria as well as non-implanting blastocysts ( Feng et al., 2018 ,  Xu et al., 2019a ,  Fan et al., 2017 ,  Wang et al., 2017 ,  Wang et al., 2014 ). A recent study revealed that human blastocysts obtained after IVF can secrete factors that regulate the RNA expression profile of human endometrial epithelial cells (HEEC) ( Takamura et al., 2020 ). HEEC form the lining of the endometrium and their optimal function is essential for blastocyst adhesion ( Takamura et al., 2020 ). The study objective was to assess the impact of the implanting blastocyst on endometrial receptivity, specifically HEEC functions ( Takamura et al., 2020 ). HEEC was incubated with media obtained from implanting and non-implanting embryos. Microarray analysis of HEEC revealed that a significant number of lncRNAs was differentially expressed in HEEC treated with media obtained from non-implanting embryos compared to implanting ones ( Takamura et al., 2020 ). One of the significantly mis-expressed lncRNA is the phosphatase and tensin homolog pseudogene 1 ( PTENP1 ) ( Takamura et al., 2020 ).  PTENP1  lncRNA was significantly downregulated in HEEC treated with non-implanting embryos compared to implanting ones. Most notably is the significant reduction of  PTENP1  lncRNA in endometrial tissue obtained from women with unexplained infertility during the late secretory phase compared to fertile women ( Takamura et al., 2020 ).  PTENP1  expression follows a cyclical pattern in human endometrial tissue throughout the menstrual cycle which peaks during the late secretory phase ( Takamura et al., 2020 ). Interestingly,  PTENP1  was shown to regulate the expression of  PTEN  gene by acting as a ceRNA that sponge miRNAs targeting  PTEN  transcripts leading to increase the bioavailability of  PTEN  ( Takamura et al., 2020 ). Although the role of PTEN in endometrial receptivity is not well understood, similar to  PTENP1, PTEN  expression follows a cyclical pattern in human endometrial tissue throughout the menstrual cycle ( Kayisli et al., 2004 ). The highest expression of  PTEN  was detected during the late secretory phase and early pregnancy ( Guzeloglu-Kayisli et al., 2003 ). These findings indicate that progesterone regulates the expression of  PTEN  in endometrial tissue which was confirmed in vitro ( Guzeloglu-Kayisli et al., 2003 ). Estrogen, on the other hand, increases PTEN phosphorylation which decreases PTEN activity indicating a vital role of E2 during the proliferative phase to suppress PTEN mediated apoptosis ( Guzeloglu-Kayisli et al., 2003 ). PTEN protein suppresses molecular pathways that mediate cell survival such as PI3K/AKT pathway and promotes proapoptotic factors. These findings suggest that PTEN actions mediate apoptosis during the late secretory phase to facilitate the cell death of endometrial epithelial and decidual cells allowing embryo implantation and invasion to take place ( Takamura et al., 2020 ,  Guzeloglu-Kayisli et al., 2003 ,  Kayisli et al., 2004 ).\nPreeclampsia is a pregnancy complication that yearly impacts 8 million pregnant women worldwide ( Garrido-Gomez et al., 2017 ). Preeclampsia is associated with shallow cytotrophoblast invasion, defective remodeling of spiral arteries leading to abnormal placentation ( Garrido-Gomez et al., 2017 ,  Fisher, 2015 ). Mounting evidence suggests that aberrant decidualization plays a key role in the pathophysiology of preeclampsia ( Garrido-Gomez et al., 2017 ,  Lv et al., 2018 ). Several key regulators of glucose metabolism have been implicated in endometrial decidualization ( Tsai et al., 2014 ). The inhibition of glucose metabolism pathway reduced decidualization significantly indicating an integral role of glucose metabolism in endometrial decidualization ( Tsai et al., 2014 ). A recent study investigating the role of lncRNA hexokinase 2 pseudogene 1 ( HK2P1 ) in severe preeclampsia uncovered a novel role of  HK2P1  in decidualization ( Lv et al., 2018 ). The expression of  HK2  and  HK2P1  was significantly reduced in women with severe preeclampsia suggesting a possible role of  HK2  and  HK2P1  downregulation the pathophysiology of preeclampsia ( Lv et al., 2018 ). Hexokinase 2 (HK2) phosphorylates glucose to glucose-6-phosphate which is the first rate-limiting step in glycolysis ( Lv et al., 2018 ). The expression of  HK2  and  HK2P1  is significantly upregulated during in vitro decidualization and the knockdown of either  HK2  or  HK2P1  reduced decidual markers greatly ( Lv et al., 2018 ). Interestingly, the knockdown of  HK2  resulted in  HK2P1  downregulation and vice versa ( Lv et al., 2018 ).  HK2  and  HK2P1  3’ UTRs share 93.13% homology suggesting a ceRNA type of regulation ( Lv et al., 2018 ). Among putative miRNAs targeting both  HK2  and  HK2P1, miR-6887-3p  was found upregulated in women with severe preeclampsia ( Lv et al., 2018 ). Moreover, the overexpression of  miR-6887-3p  in HESCs reduced  HK2  and  HK2P1  expression ( Lv et al., 2018 ). In vitro studies using luciferase assay revealed that  miR-6887-3p  binds specifically to  HK2  and  HK2P1  3’UTR region ( Lv et al., 2018 ). The overexpression of  miR-6887-3p  in HESCs inhibited the expression of decidualization markers and glucose utilization ( Lv et al., 2018 ). This study suggests that  HK2P1  lncRNA promotes the expression of  HK2  post-transcriptionally by acting as a ceRNA that buffers  miR-6887-3p  and prevents  HK2  downregulation.\nEndometriosis is a common disease of the endometrium that affects women of reproductive age ( Nothnick, 2017 ). Common symptoms of endometriosis include pain, dysmenorrhea, and infertility in addition to a serious impairment of women’s ability to manage daily activities ( Nothnick, 2017 ). Endometriosis is characterized by the development of ectopic lesions of endometrial tissue in the pelvic cavity which can be further classified based on the location of the lesions into peritoneal endometriosis, ovarian endometriosis and deep infiltrating endometriosis ( Nothnick, 2017 ,  Donnez et al., 2018 ,  Konrad et al., 2020 ). The most commonly accepted theory of endometriosis etiology is retrograde menstruation which affects up to 90% of menstruating women but only causes the disease in 15% indicating that several factors contribute to the development of the disease ( Nothnick, 2017 ). Currently, laparoscopy is the main diagnostic procedure available which is invasive and requires general anesthesia in addition to the potential risks associated with a surgical procedure ( Nothnick et al., 2015 ). The demand for developing a safe and easy diagnostic test utilizing biomarkers of endometriosis is unequivocal. In an attempt to identify differentially expressed lncRNAs as diagnostic and prognostic biomarkers for endometriosis, several studies examined the role of lncRNAs in the pathogenesis of endometriosis.\nOne of the largest and recently published studies investigated the diagnostic and prognostic value of Urothelial carcinoma-associated-1 ( UCA1 ) lncRNA in ovarian endometriosis ( Huang et al., 2019a ).  UCA1  lncRNA was significantly downregulated in ectopic lesions compared to matched eutopic endometrial tissues of women with ovarian endometriosis ( Huang et al., 2019a ). Similarly, serum levels of  UCA1  was significantly lower in patients with ovarian endometriosis compared to healthy control. In addition, serum level of  UCA1  correlated with the progression of the disease ( Huang et al., 2019a ). The level of  UCA1  was lower in stage II compared to stage I, and stage IV was the lowest among other groups. Interestingly, the serum levels of  UCA1  lncRNA after laparoscopic treatment significantly increased ( Huang et al., 2019a ). Moreover, lower  UCA1  serum level was detected in patients with disease recurrence compared to patients who did not experience recurrence after two years follow up ( Huang et al., 2019a ). These findings suggest that serum  UCA1  lncRNA in ovarian endometriosis can provide a valuable diagnostic and prognostic biomarker that can benefit in early detection and treatment.\nMost recently, maternally expressed gene 3 (MEG3-210) lncRNA was found significantly downregulated in eutopic endometrial tissue from women with endometriosis compared to control group ( Liu et al., 2020 ).  MEG3  lncRNA is well characterized in the cancer research due to its tumor suppressive potential ( Ghafouri-Fard and Taheri, 2019 ).  MEG3  lncRNA was shown to regulate the expression of p53 and promote p53 transcriptional activity therefore  MEG3  was described as a tumor suppresser lncRNA ( Ghafouri-Fard and Taheri, 2019 ).  MEG3  acts as a chromatin-interacting lncRNA that regulates the expression of target genes through its interaction with the polycomb repressive complex 2 (PRC2) ( Ghafouri-Fard and Taheri, 2019 ). Additionally  MEG3  acts as a ceRNA that can sponge several miRNAs and regulate the expression of their target mRNAs ( Ghafouri-Fard and Taheri, 2019 ). Although Liu and colleagues investigated the role of  MEG3-210  in the pathogenesis of endometriosis,  MEG3-210  lncRNA mechanism of action was not explored ( Liu et al., 2020 ). Rather,  MEG3-210  downregulation was associated with several tumorigenic pathways such as the activation of mitogenic-activated protein kinases (MAPK) leading to increased migration and invasion of endometrial stromal cells in vitro ( Liu et al., 2020 ). Interestingly, they proposed that these mechanisms are mediated by Galectin-1 overexpression which they found interacting with  MEG3-210  but did not investigate Galectin-1 transcriptional regulation ( Liu et al., 2020 ). Galectins are glycan binding proteins that play a multitude of cellular functions due to their ability to modulate their target proteins and the signaling pathways they mediate ( Hisrich et al., 2020 ). Galectins can be secreted into the ECM and therefore can be detected in patients’ serum ( Johannes et al., 2018 ). Galectin-1 level was significantly higher in eutopic endometrial tissue as well as in serum samples from women with endometriosis ( Liu et al., 2020 ). Suggesting a possible clinical application of Galectin-1 as a diagnostic marker ( Liu et al., 2020 ). Further studies are needed to elucidate the functional role of  MEG3-210  in the transcriptional regulation of Galectine-1.\nMetastasis-associated lung adenocarcinoma transcript 1 ( MALAT1 ) lncRNA was significantly upregulated in ectopic lesions compared to matched eutopic endometrial tissue obtained from women with endometriosis as well as endometrial tissue obtained from women without endometriosis ( Liang et al., 2017 ,  Liu et al., 2019a ).  MALAT1  acts as ceRNA that can sponge  miR-200c  and decrease its bioavailability ( Liang et al., 2017 ). As a result,  MALAT1  overexpression inhibits  miR-200c  from suppressing its target mRNAs such as Zinc finger E-box-binding homeobox 1 and 2 ( ZEB1/2 ) ( Liang et al., 2017 ). ZEB1 and ZEB2 transcription factors were found upregulated in ectopic lesions and were shown to suppress E-cadherin expression directly or indirectly ( Konrad et al., 2020 ). E-cadherin is an epithelial cell marker and a type of cell adhesion proteins that connect epithelial cells together. E-cadherin downregulation was described in epithelial-mesenchymal transition (EMT) pathway which is proposed to mediate endometrial cell migration and invasion leading to the formation of ectopic lesions ( Konrad et al., 2020 ). Several studies reported a significant reduction in E-cadherin expression in ectopic endometrial tissue compared to eutopic endometrium ( Konrad et al., 2020 ,  Lin et al., 2019 ). In vitro studies using HESCs confirmed the role of  MALAT1  and  miR-200c  in regulating ZEB1/2 and E-cadherin expression ( Liang et al., 2017 ). Moreover,  miR-200c  was shown to reduce endometriotic lesions volume in vivo using a rat model of endometriosis whereas  miR-200c  inhibitor had the opposite effect ( Liang et al., 2017 ). These findings demonstrate that  miR-200c  has a therapeutic potential, but further studies are needed to determine the safety and efficacy of  miR-200c  administration to reduce ectopic lesion proliferation and prevent future disease recurrence.\nAmong the hundreds of markedly upregulated lncRNAs in ectopic lesions compared to matched eutopic and normal endometria, few were investigated. Actin filament associated protein1- antisense RNA1 ( AFAP1-AS1 ) and  CCDC144NL-AS1  were shown to play a role in ectopic lesions migration and invasion in vitro ( Lin et al., 2019 ,  Zhang et al., 2018a ).  AFAP1-AS1  upregulation correlates with EMT markers such as high ZEB1 and low E-cadherin in addition to higher migration and invasion potentials compared to  AFAP1-AS1  downregulation in ectopic endometrial epithelial cells ( Lin et al., 2019 ). In vivo knockdown of  AFAP1-AS1  using xenograft in nude mice as a model of endometriosis resulted in significant reduction in tumor size ( Lin et al., 2019 ). On the other hand,  CCDC144NL-AS1  knockdown significantly decreased EMT marker vimentin and MMP9 expression in vitro ( Zhang et al., 2018a ). MMP9 expression was reported to be significantly elevated in the plasma and peritoneal fluid of patients with endometriosis ( Liu et al., 2016 ). However,  AFAP1-AS1  and  CCDC144NL-AS1  studies are preliminary and further studies are required to recognize the clinical applications of these interesting lncRNAs.\nAssessment and evaluation of  H19  lncRNA expression and potential role in endometriosis has been inconsistent which could be owing to diverse research protocols, objectives and patients’ diagnosis. The study by Ghazal and colleagues compared eutopic endometrial tissue from women with endometriosis to normal endometrial tissues and found  H19  lncRNA significantly downregulated in eutopic endometrium of women with endometriosis ( Ghazal et al., 2015 ). They hypothesized that E2 induces the expression of  H19  lncRNA during the proliferative phase, which by acting as a molecular sponge for  let-7  miRNA increases the level of  IGF1R  transcripts and promotes IGF1 signaling and stromal cell proliferation ( Ghazal et al., 2015 ). Although ectopic lesions samples were collected from the patients in this study, they were not examined for  H19  expression. However, the impact of  H19  downregulation on stromal cell proliferation provides a feasible explanation of the infertility associated with endometriosis. On the contrary, Liu and colleagues reported that  H19  lncRNA is significantly upregulated in ectopic lesions and eutopic endometrial tissues compared to healthy control ( Liu et al., 2019b ,  Xu et al., 2019b ). The knockdown of  H19  in ectopic endometrial cells resulted in a significant increase in several miRNAs;  miR-124-3p  was the most significant ( Liu et al., 2019b ). One of  miR-124-3p  putative targets was integrin beta-3 ( ITGB3 ) which was shown to play a critical role in cell adhesion and invasion ( He et al., 2019 ). Therefore, they hypothesized that the upregulation of  H19  in ectopic lesions promotes endometrial cell proliferation and invasion via increasing the bioavailability of  ITGB3  ( Liu et al., 2019b ). Although the in vitro assessment of  H19/miR-124-p3/ITGB3  provides compelling evidence of their role in cell proliferation and invasion, the expression level of  ITGB3  in ectopic lesions was not assessed. Thus, this study provides a supporting evidence that  H19  lncRNA play a critical role in endometrial cell proliferation despite the differences in mechanism of action. Xu and colleagues recruited infertile women with and without endometriosis to investigate the role of  H19  lncRNA in endometriosis ( Xu et al., 2019b ). Similar to the previous study, they found that  H19  expression is significantly upregulated in eutopic endometrial tissue compared to endometrial tissue without endometriosis ( Xu et al., 2019b ). This increased level of  H19  positively correlated with elevated levels of alpha smooth muscle actin ( ACTA2 ) expression although heterogeneity between patient samples was apparent ( Xu et al., 2019b ). They have shown that  H19  lncRNA acts as a ceRNA via  miR-216a-5p  competitive binding leading to increased  ACTA2  transcripts level ( Xu et al., 2019b ). Moreover, in vitro knockdown of  H19  as well as  ACTA2  significantly reduced endometrial stromal cells invasion and migration. Therefore, they hypothesized that  H19  lncRNA regulates stromal cell invasion and migration by increasing the bioavailability of  ACTA2  expression post-transcriptionally ( Xu et al., 2019b ). However, the role of  ACTA2  upregulation in relation to infertility was not examined although all patients suffered from infertility. After all,  H19  lncRNA studies were inconclusive but they have shown that  H19  lncRNA plays a critical role in the pathogenesis of endometriosis. A summary of our current knowledge on these lncRNAs and their proposed role in endometrial abnormalities associated with embryo implantation failure, preeclampsia and endometriosis is provided in  Table 2 .\nEndometrial cancer (EC) is a common tumor of the female reproductive system. There are several classifications of EC based on the cellular characteristics and molecular features of the tumor biopsy; most commonly known are type I and type II. Type II EC, also known as non-endometrioid endometrial carcinoma (NEEC), occurs exclusively in post-menopausal women which accounts for 20% of all EC cases ( Treeck et al., 2020 ,  Smolle et al., 2015 ). NEEC can be further classified into serous, clear-cell EC and carcinosarcoma ( Treeck et al., 2020 ). Type I EC, also known as endometrioid endometrial carcinoma (EEC), arise from endometrial glands and accounts for 80% of EC cases ( Smolle et al., 2015 ). EEC is characterized by elevated estrogen level, endometrial hyperplasia, and frequent mutation in the tumor suppresser phosphatase and tensin homolog ( PTEN)  gene ( Smolle et al., 2015 ). The expression of  PTEN  transcript is regulated by several mechanisms including upstream regulators, epigenetic modulators, and post-transcriptional regulators such as miRNAs and lncRNAs. Several studies have shown that  PTEN  transcript expression can be regulated by several lncRNAs including the pseudogene  PTENP1,  which shares 95% homology with the  PTEN  gene 3’ UTR ( Poliseno et al., 2010 ,  Xin et al., 2015 ). The sense lncRNA acts as a ceRNA that buffers miRNAs targeting  PTEN  and therefore prevents  PTEN  silencing ( Tay et al., 2011 ). Overexpression studies of the  PTENP1  3’ UTR resulted in significant increase in  PTEN  transcripts level ( Poliseno et al., 2010 ). Suggesting a tumor suppressive feature of  PTENP1  pseudogene. Examination of the  PTEN  gene methylation in endometrial samples obtained from women with endometrial hyperplasia and EC revealed that  PTEN  gene is not methylated ( Kovalenko et al., 2018 ). On the other hand,  PTENP1  pseudogene was highly methylated in over 70% of endometrial hyperplasia and 70% of EC biopsies ( Kovalenko et al., 2018 ). These findings suggest that  PTEN  expression could be compromised by unrestrained miRNA activity as a result of  PTENP1  pseudogene silencing. Other lncRNAs that regulate  PTEN  expression and were implicated in EC include Fer-1-like protein 4 ( FER1L4 ),  RP11-395G23.3,  and  LA16c-313D11.11  ( Qiao and Li, 2016 ,  Xin et al., 2015 ).  FER1L4  expression was found significantly downregulated in EC tissue compared to adjacent normal endometrial tissue ( Qiao and Li, 2016 ). The overexpression of  FER1L4  lncRNA in vitro promotes the expression of  PTEN  and inhibits the activation of AKT pathway ( Qiao and Li, 2016 ). Similarly, lncRNAs  RP11-395G23.3  and  LA16c-313D11.11  were found downregulated in EC compared to control group ( Xin et al., 2015 ). lncRNAs  RP11-395G23.3  and  LA16c-313D11.11  were confirmed targets of  miR-205-5p  which directly regulates  PTEN  expression ( Xin et al., 2015 ). The expression level of  miR-205-5p  was found significantly upregulated in EC and associated with poor prognosis ( Xin et al., 2015 ). Therefore, the lncRNAs  RP11-395G23.3  and  LA16c-313D11.11  act as ceRNA similar to  PTENP1 .\nMounting evidence suggest that endometrial hyperplasia observed in type I EC is due to unopposed estrogen activity ( Kim and Chapman-Davis, 2010 ). It is well established that progesterone antagonizes estrogen mediated actions and inhibits endometrial cell proliferation. Nuclear enriched abundant transcript 1 ( NEAT1 ) lncRNA was shown to be negatively regulated by progesterone in endometrial cancer cell lines (Ishikawa) ( Huang et al., 2019b ). Several studies have reported that  NEAT1  is overexpressed in EC tissue samples compared to adjacent tissue ( Wang et al., 2019 ,  Li et al., 2016 ). Elevated  NEAT1  expression in EC promotes cell proliferation, migration, and invasion through various pathways.  NEAT1  lncRNA acts as a ceRNA for several miRNAs. For instance,  miR-146b-5p  was confirmed to target lymphoid enhancing factor 1 ( LEF1 ) as well as  NEAT1  expression in EC cell lines ( Huang et al., 2019b ).  LEF1  encodes for a transcription factor and a component of the Wnt/β-catenin signaling pathway that activates the transcription of oncogenic genes such as  c-MYC  ( Huang et al., 2019b ).  NEAT1  overexpression was shown to be positively correlated with  LEF1  upregulation and  miR-146b-5p  downregulation indicating that  NEAT1 , by acting as a molecular sponge for  miR-146b-5p , increases the bioavailability of  LEF1  leading to the activation of Wnt/β-catenin pathway ( Huang et al., 2019b ).  NEAT1  was also shown to sponge  miR-144-3p  leading to the upregulation of its target transcripts ( Wang et al., 2019 ).  EZH2 , one of  miR-144-3p  targets that was implicated in endometrial cancer, is a histone methyltransferase that modulates the chromatin structure at the promoter region of target genes ( Wang et al., 2019 ). Several recent studies have shown that  EZH2  transcripts and protein levels were significantly upregulated in type I EC tissue samples compared to matched normal endometrial tissue ( Krill et al., 2020 ,  Oki et al., 2017 ).\nH19  lncRNA was reported in several studies as a hormone responsive lncRNA irrespective of the variable findings ( Adriaenssens et al., 1999 ,  Ariel et al., 1997 ,  Ghazal et al., 2015 ).  H19  was reported as a tumor suppressor and an oncogene in several cancer types ( Matouk et al., 2016 ). Nonetheless, several studies reported that H19 is upregulated in EC tissue samples indicating an oncogenic role of  H19  in EC ( Zhao et al., 2017 ,  Zhang et al., 2018b ,  Zhu et al., 2019 ). In vitro knockdown of  H19  was shown to decrease the migration and invasion of EC cell lines ( Zhao et al., 2017 ). Another study found that  H19  acts as a ceRNA to increase the bioavailability of  HOXA10  ( Zhang et al., 2018b ). HOXA10 is a well-known transcription factor that plays a critical role in endometrial receptivity but the role of HOXA10 in EC was not clear ( Wang et al., 2018 ,  Li et al., 2015a ). Recently,  H19  upregulation in EC was shown to promote hypoxia inducible factor-1α ( HIF-1α ) expression by competing with  miR-20b-5p  which targets  HIF-1α  ( Zhu et al., 2019 ). Hypoxia is a critical cue to induce angiogenesis and tissue repair in regenerating tissues such as the endometrium ( Maybin et al., 2018 ). However, cancer cells exploit this mechanism to promote cell survival and metastasis ( Matouk et al., 2016 ).  H19  overexpression leading to  HIF-1α  upregulation was shown to promote EC cell proliferation, and migration through EMT ( Zhu et al., 2019 ).\nMost recently, several lncRNAs were shown to act as oncogenes by promoting the expression of other oncogenes such as  c-MYC  and  c-MET . Colon cancer associated transcript-1 ( CCAT1)  lncRNA was found significantly upregulated in endometrial tissues of type I EC patients as well as EC cell lines ( Treeck et al., 2020 ).  CCAT1  may act as an oncogene owing to its ability to sponge several tumor suppressor miRNAs such as  let-7  ( Deng et al., 2015 ).  CCAT1  overexpression in hepatocellular carcinoma cells lines was shown to decrease  let-7  expression leading to the upregulation of  let-7  targets such as  c-MYC  and  HMGA2  expression ( Deng et al., 2015 ). Small nucleolar RNA host gene 8 ( SNHG8 ) was also upregulated in patients with EC and correlated with poor prognosis ( Yang et al., 2018 ).  SNHG8  acts as a molecular sponge for  miR-152  which was shown to target  c-MET  ( Yang et al., 2018 ). Mesenchymal-epithelial transition receptor (c-MET), also known as hepatocyte growth factor receptor (HGFR), is a tyrosine kinase receptor which promotes tumorigenesis when activated aberrantly. The overexpression of  c-MET  is correlated with poor prognosis in several cancer types including EC ( Li et al., 2015b ,  Moosavi et al., 2019 ,  Liang and Wang, 2020 ).\nOther lncRNAs promoted EC proliferation and invasion by directly interacting with tumorigenic factors such as EZH2, mTOR, VEGFA. These lncRNA were found significantly upregulated in type I EC as well as several EC cell lines: prostate cancer associated transcript 1 ( PCAT1),  deleted in lymphocytic leukemia 1 ( DLEU1 ), and testis developmental related gene 1 ( TDRG1 ). Although  PCAT1  expression among EC patients was heterogenous, lower PCAT1 expression correlated with higher survival rate suggesting that  PCAT1  expression can serve as a prognosis biomarker of EC ( Zhang et al., 2020 ). The overexpression of  PCAT1  lncRNA was associated with E-cadherin downregulation and EZH2 upregulation suggesting that  PCAT1  promotes EC invasion via EMT ( Zhang et al., 2020 ).  PCAT1  lncRNA was shown to interact with EZH2 via chromatin immunoprecipitation (ChIP) assay indicating that  PCAT1  may act through chromatin remodeling mediated by EZH2 ( Zhang et al., 2020 ).  DLEU1  lncRNA was shown to interact directly with mTOR leading to the activation of PI3K/AKT mTOR pathway and enhanced tumorigenesis ( Du et al., 2018 ). Downregulation of  DLEU1  in vitro inhibited cell proliferation and migration as well as decreased mTOR expression and downstream effectors ( Du et al., 2018 ).  TDRG1  lncRNA overexpression promoted EC tumorigenicity by directly interacting with the angiogenic factor vascular endothelial growth factor A (VEGFA) ( Chen et al., 2018 ).\nSeveral lncRNAs such as  NEAT1 , antisense lncRNA  AC002454.1 ,  LINC01279 , lncRNA  TC0101441 ,  MALAT1 , ovarian adenocarcinoma amplified lncRNA ( OVAL ), HOX transcript antisense intergenic RNA ( HOTAIR ), and steroid receptor RNA activator ( SRA ) were not discussed in this review given that there are recent review articles about lncRNAs in endometriosis and endometrial cancer ( Smolle et al., 2015 ,  Wang et al., 2020 ).  Table 3  provides a summary of our current knowledge on the lncRNAs just discussed, including their regulation and proposed role in endometrial cancer.\n\nSeveral research efforts carried out genome-wide analysis of differentially expressed lncRNA in tissues obtained from the endometrium from women with endometrial pathologies compared to control group. Hundreds of lncRNAs are differentially expressed but the role of each is not well defined. For instance, RNA sequencing analysis of human endometrial tissue obtained from normally cycling women during the proliferative and secretory phases, specifically the window of implantation, reported 516 differentially expressed lncRNAs ( Sigurgeirsson et al., 2017 ). Nuclear enriched abundant transcript 1 ( NEAT1 ) was one of the most differentially expressed lncRNAs ( Sigurgeirsson et al., 2017 ). However, identification of differentially expressed lncRNAs is not enough to understand their functional roles in the menstrual cycle. Further characterization studies are needed to understand the physiological and functional roles of these cycle-specific lncRNAs in the endometrium. Current research endeavor must focus on identifying biomarkers that can improve the success rate of embryo implantation after IVF procedures as well as the diagnosis and prognosis of endometrial pathologies. It is evident as we further our understanding on the regulation and role of lncRNAs in the endometrium, we will deepen our understanding on normal and abnormal endometrial physiology which will lead to new diagnostic and therapeutics based upon the biology of these regulatory RNAs.","source_license":"CC0","license_restricted":false}