{"paper_id":"ab2aa682-339d-48bd-90c0-0de41aad2f94","body_text":"Equine endometrosis is mainly characterized by periglandular and/or stromal endometrial\nfibrosis, with glandular alterations mostly within the fibrotic foci [ 1 ,  2 ].\nEndometrosis disrupts endometrial tissue structure and compromises its functional\ncapacity, resulting in embryo loss [ 1 , 2 , 3 ]. The\nseverity of endometrosis is evaluated histologically by examining the endometrium, with\na focus on the degree of periglandular fibrosis and inflammation. Based on this\nassessment, the endometrium is categorized into four stages (I, IIA, IIB, and III),\nusing the classification system established by Kenney and Doig [ 4 ]. With the increasing severity of endometrosis, the predicted rate\nof foaling a healthy offspring at term declines by as much as 90% in mares with the most\nsevere endometrosis [ 4 ]. Additionally, the\nprevalence of endometrosis rises with age, affecting as many as 93% of mares by the age\nof 20 [ 5 ,  6 ]. Infertility caused by endometrosis leads to significant economic losses in\nthe equine industry [ 3 ,  7 ,  8 ].\n\nDespite extensive research, the pathogenesis of endometrosis remains not fully\nunderstood. A hallmark of endometrosis is the excessive accumulation of extracellular\nmatrix (ECM), especially collagen type I (COL1) and COL3, accompanied by elevated\nexpression of α-smooth muscle actin (α-SMA), which indicates the activation of\nmyofibroblasts driving fibrosis [ 9 ]. It is also\ncharacterized by pathological alterations in the endometrial glands that include cystic\ndilatation. These changes disrupt the endometrial structure, impair the secretory\nfunction of uterine glands [ 2 ,  3 ,  10 ] and\nlimit nutrient exchange between the placenta and conceptus, further compromising\npregnancy maintenance [ 7 ,  11 ,  12 ]. At present, the only\nmeans of diagnosing endometrosis is through an endometrial biopsy, which represents the\ngold standard diagnostic for endometrial pathology [ 13 , 14 , 15 , 16 ]. This procedure provides the\nmost accurate assessment of fibrosis severity and inflammatory changes in the mare’s\nendometrium, enabling reliable predictions of future breeding potential [ 4 ,  17 ].\nAging and chronic endometritis are important contributors involved in the pathogenesis\nof endometrosis [ 7 ,  8 ,  18 ,  19 ]. Inflammatory cells, such as neutrophils, and proinflammatory mediators,\nlike cytokines, interleukins (IL), transforming growth factor β1 (TGF-β1), and\nprostaglandins (PGs) have been identified as exacerbating factors in fibrosis\nprogression, highlighting a complex interaction between the immune response and tissue\nremodeling in endometrosis [ 9 ,  10 ,  20 , 21 , 22 , 23 , 24 , 25 , 26 ]. In\nfact, in mare endometrium, neutrophils, which play an essential role in the immune\ndefense against pathogens, can form web-like structures, the neutrophil extracellular\ntraps (NETs), consisting of a complex DNA-protein structure [ 27 ]. The main desirable role of NETs is to entrap and kill\npathogens. Nevertheless, the continuous action of NETs, through their proteolytic\nenzymes (elastase, cathepsin G, myeloperoxidase), stimulated fibrogenesis in the\nendometrium, with increased COL deposition [ 28 ].\nIt has been suggested that NETs can promote fibrinogen cross-linking and the formation\nof physical adhesive structures through citrullinated histone H3 (H3Cit) modification in\nuterine adhesions in women [ 29 ].\nMatrix metalloproteinases, particularly MMP-2 and MMP-9, regulate COL degradation, while\ntheir activity is controlled by tissue inhibitors of metalloproteinases (TIMPs),\nespecially TIMP-1 and TIMP-2 [ 22 ,  30 ,  31 ].\nDisruption of the balance between MMPs and TIMPs contributes to the progression of\nfibrosis [ 28 ]. Study by Szóstek-Mioduchowska\n et al . [ 22 ] revealed that\nendometrial  MMP-2  and  MMP-9  mRNA transcription is\nupregulated in mares with mild to moderate endometrial fibrosis. Importantly, the\ntreatment with TGF-β1 increased  MMP-9  mRNA transcription in mare\nendometrial fibroblasts or epithelial cells [ 22 ].\nTranscriptomic analyses of endometrial samples collected during the anestrus phase from\nmares with or without endometrosis have provided valuable insights into the molecular\nbasis of this condition, apart from the influence of ovarian steroids [ 32 ]. Next-generation sequencing (NGS) analysis\nrevealed 55 differentially expressed genes (DEGs), with a predominant upregulation in\nfibrotic endometrial tissue [ 32 ]. These genes\nwere implicated in diverse biological processes, including metabolism (e.g., sulfur\nmetabolism via SELENBP1), steroidogenesis, mitochondrial function, and immune responses\n(e.g., by MMP7, PIGR, FCGRT) [ 32 ]. The FCGRT\nencodes the neonatal Fc receptor (FcRn) that protects immunoglobulin G (IgG) and albumin\nfrom catabolism, and IgG transport across the epithelium. It is involved in antigen\npresentation by antigen-presenting cells [ 33 ].\nThe biological mechanism of this receptor slows IgG destruction and is the primary\nmechanism for the extended persistence and bioavailability of IgG [ 34 ,  35 ]. Receptor FcRn is\nimplicated in aging-related adipose tissue fibrosis through the accumulation of IgG\n[ 36 ]. Notably, FCGRT has been previously\nassociated with female reproductive pathologies, showing expression in about 41.8% of\nendometrial cancer cases [ 37 ]. Therefore, it may\nsuggest a potential role in endometrosis and endometrosis and other reproductive\ndisorders.\nA complementary study by Szóstek-Mioduchowska  et al . [ 24 ] provided characterization of the transcriptomic\nprofile of the endometrium at the follicular phase of the estrous cycle in mares.\nDifferentially expressed genes were associated with immune activation, including\nleukocyte and macrophage infiltration, and elevated cytokine signaling, underscoring the\ncomplex interplay between metabolic and immune pathways in the progression of mild to\nmoderate endometrial fibrosis [ 24 ]. Using\nIngenuity Pathway Analysis, the authors identified key cytokines, including TGF-β1,\nIL-4, IL-13, and IL-17, as upstream regulators of DEGs related to cellular homeostasis,\nmetabolism, and fibrosis signaling pathways in the endometrium with endometrosis. These\ncytokines have been shown to regulate the expression of a disintegrin and\nmetalloproteinase with thrombospondin motif (ADAMTS) proteases in fibrotic endometrium\nand endometrial fibroblasts, suggesting their important role in the development of\nendometrosis. Dysregulated expression of ADAMTS has been linked to fibrosis in other\norgans, such as the kidney and lung [ 38 , 39 , 40 ].\nRecent results showed that IL-17A exerts an effect on the immunomodulatory properties of\nequine endometrial fibroblasts, thereby affecting the equilibrium between the deposition\nand degradation of the ECM [ 41 ].\nTranscriptomic analyses also depicted changes which occured in the mare myometrium\nduring endometrosis [ 42 ]. These transcriptomic\nalterations may indicate an impaired function of the contractile machinery, mechanisms\nregulating calcium influx and handling, as well as changes in ECM composition, leading\nto decreased contractile activity and structural changes in the myometrium of mares with\nendometrosis. Alongside these changes, vascular and lymphatic alterations may occur\nwithin the uterine wall, such as accumulation of lymphatic lacunae and smooth muscle\ndegradation [ 1 ,  43 ,  44 ]. Furthermore, our recent\nresearch demonstrated that mares suffering from endometrosis exhibited atypical\nmyometrial contractile responses to PGs and lysophosphatidic acid [ 45 ,  46 ].\nAlthough research has advanced, the molecular mechanisms underlying equine endometrosis\nremain incompletely understood. Recently, epigenetic regulation has emerged as a\ncritical modulatory factor influencing fibrosis development, offering promising avenues\nfor biomarkers and targeted therapies. Therefore, in this review, we focus on how\nepigenetic modifications could influence the progression of equine endometrosis ( Fig. 1 Fig. 1. The potential role of epigenetics (DNA methylation, histone modifications, and\nncRNA) in the processes related to the development of endometrial fibrosis in\nthe mare. ECM, extracellular matrix; COL1, collagen type I; FN, fibronectin;\nmiRNA, microRNA; lncRNA, long non-coding RNA; MMPs, Matrix Metalloproteinases;\nMMP-2, Matrix Metalloproteinase-2; MMP-9, Matrix Metalloproteinase-9; MMP-13,\nMatrix Metalloproteinase-13 (Biorender). ).\nThe potential role of epigenetics (DNA methylation, histone modifications, and\nncRNA) in the processes related to the development of endometrial fibrosis in\nthe mare. ECM, extracellular matrix; COL1, collagen type I; FN, fibronectin;\nmiRNA, microRNA; lncRNA, long non-coding RNA; MMPs, Matrix Metalloproteinases;\nMMP-2, Matrix Metalloproteinase-2; MMP-9, Matrix Metalloproteinase-9; MMP-13,\nMatrix Metalloproteinase-13 (Biorender).\n\nEpigenetic modifications regulate gene expression without altering the DNA sequence,\nmainly through DNA methylation, histone modifications, and the action of non-coding RNAs\n(ncRNAs), such as microRNAs (miRNAs) and long non-coding RNA (lncRNAs) [ 47 , 48 , 49 ]. These changes affect chromatin accessibility,\ninfluencing transcription factor binding, regulating gene expression and thereby\ncontrolling biological processes [ 48 ]. Unlike\ngenetic mutations, epigenetic alterations are dynamic and reversible, making them\npromising targets for therapeutic intervention [ 48 ,  49 ].\nEpigenetic alterations have been implicated in numerous disorders, including cancer\n[ 50 ] and fibrosis [ 51 , 52 , 53 ]. In fibrosis, persistent activation of myofibroblasts results\nfrom their failure to revert to quiescence, accompanied by extensive gene expression\nchanges driven by chromatin remodeling [ 54 ,  55 ]. Fibroblast heterogeneity partially explains why\nonly certain fibroblasts exposed to fibrogenic stimuli differentiate into\nmyofibroblasts, characterized by α-SMA expression [ 56 ]. Multiple genes regulate this differentiation, and epigenetic mechanisms\nregulate these signaling pathways in an organ- and disease-specific manner [ 57 , 58 , 59 ]. Although epigenetics is a major determinant of\ncellular phenotype and myofibroblast function, our understanding of these processes\nremains incomplete in the context of equine endometrosis.\n\nMethylation of DNA is a key epigenetic mechanism involving the addition of methyl group\nonto the C5 position of cytosine residues within CpG dinucleotides, primarily occurring\nin CpG islands often located in gene promoters [ 60 ]. This modification is catalyzed by DNA methyltransferases (DNMTs),\nincluding the maintenance enzyme DNMT1 and the  de novo \nmethyltransferases DNMT3A and DNMT3B [ 61 ,  62 ]. The ten-eleven translocation (TET) protein\nfamily, including TET1, TET2, and TET3, catalyzes the oxidation of 5-methylcytosine\n(5mC) to 5-hydroxymethylcytosine (5hmC). This reaction enables the selective reversal of\nDNA methylation at specific genomic sites. Consequently, TET enzymes play a crucial role\nin the active DNA demethylation [ 63 ].\nPatterns of DNA methylation are pivotal regulators of gene expression, influencing\ndiverse biological processes [ 64 ,  65 ]. Functionally, DNA methylation is crucial for\ndevelopment and cell differentiation [ 66 ,  67 ], regulating the switching of genes between\nactive and inactive states [ 68 ]. Methylation of\ngene promoters typically represses transcription, leading to gene silencing [ 69 ]. While DNA hypermethylation tends to suppress\ngenes, hypomethylation can increase their expression [ 70 ,  71 ]. However, methylation does not\nalways result in gene expression changes [ 72 ].\nChronic inflammation and fibrosis might be associated with aberrant DNA methylation\n[ 73 ,  74 ], by maintaining the activated myofibroblast phenotype responsible for\npathological ECM protein overproduction [ 75 ]. DNA\nmethylation plays a critical role in fibroblast activation and their differentiation\ninto myofibroblasts, driving the characteristic ECM accumulation in fibrotic scar tissue\nand multiple fibrotic organ systems [ 48 ,  76 ]. Hypermethylation suppresses the expression of\ngenes encoding antifibrotic factors, while DNMTs inhibition reduces fibrosis in fibrotic\ndiseases of the kidney, idiopathic pulmonary fibrosis and in systemic sclerosis [ 77 ,  78 ,  79 ].\nThis epigenetic dysregulation is evident across hepatic [ 52 ], pulmonary [ 80 ,  81 ], renal [ 82 ], and cardiac fibrosis [ 83 ,  84 ]. Beyond fibrosis, aberrant DNA methylation is\nimplicated in cancers, autoimmune, and neurodevelopmental diseases [ 50 ,  85 ]. In\nuterine pathologies, like human endometriosis and leiomyomas, methylation abnormalities\naffect endometrial function and fertility [ 86 , 87 , 88 ].\nIt has been demonstrated that DNA methylotransferases, especially DNMT1 and DNMT3A, are\ncritical in promoting liver fibrosis. For instance, elevated levels of DNMT1 and DNMT3A\ncause DNA methylation and silencing of key antifibrotic genes like\n PPARγ . Consequently, this activates profibrotic pathways (e.g.,\nTGF-β1/SMAD and Wnt/β-catenin), driving excessive COL deposition. Using DNMT inhibition\nor knockout models demonstrates that DNMT1 or DNMT3A deficiency reduces hepatic stellate\ncells proliferation and the synthesis of COL. These findings highlight DNMTs as key\nepigenetic drivers of liver fibrosis, suggesting that targeting them could reverse\naberrant DNA methylation and mitigate liver fibrogenesis [ 89 , 90 , 91 , 92 ].\nIn the mare endometrium, epigenetic regulation, particularly DNA methylation, seems to\nmodulate transcriptional networks that regulate ECM deposition during the development\nand progression of endometrosis [ 6 ,  25 ,  93 ,  94 ]. Alpoim-Moreira  et al . [ 6 ] conducted studies to assess the epigenetic\ninvolvement in mare endometrosis. According to this study, the positive correlation\nbetween  DNMT3B  and  COL3A1  mRNA transcription was\nobserved in fibrotic category III endometrium, pointing to a potential epigenetic role.\nThe hypermethylation of  MMP2  and  MMP9 , but not of\n COL1A1  genes, occurred simultaneously with a decrease in their mRNA\nlevels, with endometrial fibrosis, suggesting that this hypermethylation is responsible\nfor repressing their transcription. The results show that processess related to\nendometrosis may be epigenetically modulated by anti-fibrotic genes\n( MMP2  and  MMP9 ) inhibition, rather than fibrotic\ngenes activation and therefore, might be promising targets for therapeutic use. [ 25 ].\nTransforming growth factor-β1 is a key profibrotic cytokine involved in fibrosis\ndevelopment in various tissues, including the equine endometrium [ 10 ,  22 ,  95 ]. It stimulates ECM production and promotes the proliferation of\nendometrial fibroblasts [ 10 ]. Transforming growth\nfactor-β1 signaling has been shown to enhance the activity of DNMTs, thereby linking it\nto the epigenetic modification of fibrosis-related genes in primary lung fibroblasts\n[ 96 ]. Furthermore, TGF-β1 induces the\ndifferentiation of fibroblasts to myofibroblasts, a process involving epigenomic\nremodeling and changes in gene expression, during pulmonary fibrosis [ 97 ]. A recent work has shown that  in\nvitro  TGF-β1 treatment upregulated  DNMT3A  mRNA\ntranscription and COL secretion in mare endometrial fibroblasts [ 94 ]. Inhibiting DNA methylation with 5-aza-2′-deoxycytidine\n(decitabine) selectively reduced mRNA transcription and secretion of COL without\naffecting  α-SMA  mRNA transcription. This selective effect of decitabine\non COL reduction in TGF-β1-treated fibroblasts suggests that DNA methylation may\nspecifically regulate COL synthesis during the development of endometrosis [ 94 ].\nA decrease in the contractile function of the myometrium may cause insufficient\nclearance of the uterine cavity, which can lead to persistent endometritis and\npotentially endometrosis in mares [ 7 ]. While\nepigenetic regulation of myometrial gene expression has been studied in humans [ 98 ], there is limited information on the expression\nof DNMTs and TETs in the myometrium of mares. Recently, the mRNA transcription of these\nenzymes and DNA methylation of the  OXT receptor  ( OXTR )\ngene in the myometrium of mares with endometrosis were analyzed [ 99 ]. Myometrial  OXTR  mRNA transcription and DNA\nmethylation in its promoter region showed no differences between endometrial categories.\nHowever, the expression of  DNMTs  and  TETs  in the\nmyometrium was found to be dependent on the severity of endometrosis and the phase of\nthe estrous cycle, suggesting a potential regulatory role in DNA methylation of\nmyometrial gene expression [ 99 ].\n\nOther than DNA methylation, additional epigenetic mechanisms have attracted significant\nattention in the study of equine endometrial fibrosis. Non-coding RNAs, such as miRNAs\nor lncRNAs, are considered an epigenetic mechanism. They play a significant role in\nregulating gene expression without altering the DNA sequence. MicroRNAs are small ncRNA\nmolecules that are approximately 18 to 23 nucleotides long and serve as key\npost-transcriptional regulators of gene expression [ 100 ,  101 ]. These molecules bind to\nthe 3′ untranslated regions (UTRs) of target messenger RNAs (mRNAs), leading to\ntranslational repression or mRNA degradation [ 102 ]. A single miRNA can target and modulate hundreds of different mRNAs,\nwhile individual mRNAs can be co-regulated by multiple miRNAs. This complex network\nallows for post-transcriptional control of gene expression [ 96 ,  103 ,  104 ]. These short RNAs, transcribed from non-coding\ngenes, engage in diverse cellular processes beyond gene silencing. They participate in\ndiverse molecular processes, including heterochromatin formation and modulation of DNA\nmethylation patterns, thereby affecting gene expression at multiple regulatory levels\n[ 49 ,  105 ]. Through mechanisms such as mRNA cleavage, destabilization, or\ntranslational repression, miRNAs regulate critical cellular functions [ 100 ].\nNumerous biological processes, including cell proliferation, differentiation, apoptosis,\nmetabolism, angiogenesis, immune response, and aging, are influenced by miRNAs [ 106 , 107 , 108 ]. Importantly, they play\nessential roles in reproductive processes, regulating the expression of genes involved\nin steroidogenesis, gametogenesis, fertilization, embryonic development, endometrial\nreceptivity, implantation, and placentation [ 109 , 110 , 111 ]. This places miRNAs as potential epigenetic regulators,\ninfluencing both physiological and pathological states, including equine endometrial\nfibrosis.\nThe role of miRNAs in the pathogenesis of various diseases, including tissue injury and\nfibrotic disorders affecting the liver, heart, skin, kidney, and lungs, has been\nincreasingly recognized [ 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 ],\nemphasizing miRNAs as critical fibrosis regulators and potential therapeutic targets.\nAltered miRNA expression patterns have been linked to cancer, cardiovascular disease,\ninflammatory conditions, infertility, miscarriage, and gynaecological diseases,\nincluding endometriosis and intrauterine adhesions in women [ 120 , 121 , 122 , 123 ].\nIn different types of fibrosis, miRNAs either promote or suppress fibrotic pathways\ndepending on their specific gene targets [ 112 ,\n 124 ,  125 ]. Their remarkable tissue specificity and stability make miRNA promising\nbiomarkers and therapeutic targets, with the advantage of being detectable in biological\nfluids, enabling non-invasive diagnostic approaches [ 107 ,  126 ,  127 ]. In fibrogenesis, miRNAs regulate the expression of genes\ninvolved in cellular processes, including inflammation, ECM metabolism, and fibroblast\nactivation [ 100 ,  128 ,  129 ].\nIn equine endometrosis, miRNAs seem to play an important role in modulating the\nexpression of genes related to fibrotic remodeling [ 93 ,  130 ,  131 ]. Wójtowicz and collaborators [ 130 ] provided the first comprehensive miRNA expression profile (miRNAome) of\nthe mare endometrium across different endometrium categories, identifying miRNA\nalteration correlated with endometrosis progression. Specifically, miRNA expression\nchanges were evident at distinct endometrium types: one miRNA (novel-eca-miR-42) was\ndownregulated in mild fibrosis (category IIA  vs . I); 26 miRNAs were\naltered (14 upregulated, 12 downregulated) in moderate fibrosis (category IIB\n vs . I); and five miRNAs changed (two upregulated, three\ndownregulated) in severe fibrosis (category III  vs . I). Functional\nanalysis indicated these miRNAs potentially influence key pathways implicated in\nfibrosis development, including focal adhesion, ECM-receptor interaction, Hippo, and\nPI3K-Akt signaling pathways [ 130 ]. Notably, it\nwas found that TGF-β1 affected the expression of novel-eca-miR-42 in cultured\nendometrial fibroblasts [ 130 ]. Furthermore,\nchanges in the expression of miRNAs related to the regulation of genes of\n OXT  signaling pathways could also disrupt uterine contractility and\nestrous cycle regulation, contributing to infertility associated with endometrosis.\nFurthermore, Wong  et al . [ 131 ]\ndemonstrated an upregulation of pro-fibrotic miRNAs (miR-17, miR-21, miR-433) in\nendometrial stromal cells during the follicular phase of the estrous cycle after the\ntreatment with TGF-β1 and cytokine combinations. Nevertheless, these miRNAs were either\nunchanged or downregulated during the mid-luteal phase of the estrous cycle. In\ncontrast, anti-fibrotic miRNAs (miR-26a, miR-29b/c, miR-145, miR-378, miR-488) were\npredominantly overexpressed in the mid-luteal phase, but not in the follicular phase of\nthe estrous cycle. A comparable expression pattern was observed in extracellular\nvesicles secreted by those stromal cells, with elevated pro-fibrotic miRNAs in the\nfollicular phase and anti-fibrotic miRNAs in the luteal phase of the estrous cycle. It\nwas shown that pro-inflammatory cytokines enhanced TGF-β1 signaling in endometrial\nstromal cells derived from the follicular phase, thereby promoting the expression of\ngenes involved in ECM, MMPs imbalance, estrogen receptor downregulation, and\nupregulation of pro-fibrotic factors. Conversely, the luteal phase appears to exert\nprotective effects through increased anti-fibrotic miRNAs, reduced SMAD2\nphosphorylation, and decreased fibrosis-related gene expression [ 131 ].\nLong non-coding RNAs are RNA molecules longer than 200 nucleotides that are not\ntranslated. LncRNAs are predominantly transcribed by RNA polymerase II and exhibit\ndiverse functions at the molecular level. Long non-coding RNAs regulate the expression\nof different genes based on their cellular location, affecting multiple molecular\nmechanisms, including chromatin modification, transcriptional regulation, and\npost-transcriptional regulation. Due to their versatility, they play a crucial role in\nnearly all physiological processes, including cellular development, differentiation, and\ngrowth [ 132 ,  133 ]. A growing number of studies emphasize the important role of lncRNAs in\nthe development of fibrosis in the liver, heart or kidney [ 134 ,  135 ].\nCurrently, little is known about lncRNAs in endometrosis in mares. The transcriptomic\nanalysis of myometrium from mares with endometrosis revealed 119, 116, and 86\ndifferentially expressed lncRNAs in the myometria of mares with endometrium categories\nIIA, IIB, and III, respectively, compared to category I [ 42 ]. These data indicate that lncRNA expression in mare myometrium changes\nwith increasing endometrosis severity. Long non-coding RNAs may contribute to myometrial\ndysfunction in endometrosis, such as disrupted contractility. However, these hypotheses\nrequire experimental validation. Furthermore, Wójtowicz  et al . [ 136 ] demonstrated that IL-4 altered lncRNA\nexpression in mare endometrial cultured fibroblasts isolated from endometria with or\nwithout endometrosis. Interleukin 4 treatment altered the expression of 143 and 135\nlncRNAs in fibroblasts derived from endometrium without and with endometrosis,\nrespectively. Gene Ontology (GO) functional enrichment analysis of co-expressed\ntrans-target genes predicted to be regulated by these differentially expressed lncRNAs\nrevealed enrichment in processes related to inflammatory response, ECM organization,\ntissue homeostasis, and tissue remodeling in cells without endometrosis. In contrast, in\nfibroblasts derived from endometria with endometrosis, enriched processes included not\nonly these pathways but also response to wounding, regulation of metallopeptidase\nactivity, and cellular response to fibroblast growth factor stimulus. The results of\nthis study indicate that IL-4 can induce a lncRNA-mediated transcriptional response in\nequine endometrial cultured fibroblasts, and that this response is dependent on the\ncellular origin of the fibroblasts. However, further research is required to investigate\nthe role of lncRNA in processes associated with fibrosis development in the\nendometrium.\n\nHistone modifications, including acetylation, methylation, phosphorylation, and\nubiquitylation, regulate chromatin structure and transcription [ 137 ,  138 ]. Histone\nacetylation generally promotes gene transcription by loosening the chromatin structure,\nmaking DNA more accessible. In contrast, histone methylation can either activate or\nrepress gene expression depending on the specific amino acid residues modified and the\nbiological context in which these modifications occur [ 139 ]. Emerging evidence shows that histone modifications act as a central\nregulator in the pathogenesis of fibrotic disorders by controlling the gene\ntranscription patterns that promote fibroblast activation and ECM deposition in renal\nand cardiac fibrosis [ 140 ]. The involvement of\nhistone methylation represents a conserved mechanism across multiple organ\nsystems—including cardiac, renal, and pulmonary tissues [ 141 , 142 , 143 , 144 ].\nFor instance, the upregulation of lysine demethylase 5B (KDM5B) in cardiac fibrosis has\nbeen shown to promote ECM accumulation, suggesting a promising target for therapeutic\nintervention [ 145 ]. In renal fibrosis, histone\nmodifications are associated with the activation of profibrotic signaling pathways,\nunderscoring the significance of epigenetic regulation in kidney disease [ 146 ]. Research in human endometriosis and fibrosis\nindicated that histone hypoacetylation (H3K9) and repressive marks (H3K27me3) silence\nkey genes essential for endometrial function (e.g., HOXA10, ESR1, MMPs). Overexpression\nof histone deacetylases (HDACs) contributes to chromatin condensation and fibrotic gene\nsilencing [ 147 ]. Despite the recognized\nimportance of histone modifications in fibrosis and endometrial diseases in other\nspecies, the role of histone modifications in equine endometrosis remains an open area\nfor future research. Our  in vitro  study showed that histone\ndeacetylation inhibitors such as suberoylanilide hydroxamic acid (SAHA) and valproic\nacid (VPA) could impair COL release from mare endometrial fibroblasts stimulated with\nTGF-β1 [ 148 ].\n\nA better understanding of the epigenetic mechanisms involved in the development and\nprogression of endometrial fibrosis could help to reveal the mechanisms behind the\nfibrotic process. Epigenetic alterations, such as aberrant DNA methylation, ncRNA\ndysregulation, or histone modification, drive persistent fibroblast activation and\nexcessive ECM accumulation, driving the development of endometrial fibrosis.\nImportantly, the reversible nature of epigenetic changes offers promising opportunities\nfor targeted therapies, including DNMT inhibitors, miRNA modulators, and histone\ndeacetylase (HDAC) inhibitors that are currently under investigation. Despite extensive\nresearch, there is an absence of effective treatments for mare endometrial fibrosis.\nThis emphasises the urgent need for novel strategies that could be derived from\nepigenetic mechanisms. Understanding epigenetic contributions can not only advance\nscientific knowledge but also potentially improve reproductive efficiency in mare with\nendometrosis, offering significant economic benefits for the breeding industry.\nAdditionally, current results highlighting epigenetics involvement in endometrosis\npathogenesis make them promising candidates for the identification of markers of\nendometrosis progression [ 94 ,  130 ,  148 ].\nSpecific epigenetic changes could serve early indicators of disease and therapeutic\ntargets. For example, epigenetic research could enable the development of diagnostic\ntests using altered DNA methylation patterns or lncRNAs for the detection of early\nconditions. The growing recognition of the importance of the epigenetic signature, as\nboth diagnostic markers and therapeutic targets, may open new possibilities for the\nmanagement and treatment of equine endometrosis. Moreover, a better understanding of the\nepigenetic mechanisms is expected to lead to more accurate diagnoses and improved\ntherapeutic strategies to restore fertility and uterine tissue function in mares. This\nintegrative approach aims to translate molecular insights into effective clinical\ntreatments for equine endometrosis.\n\nThe authors have nothing to declare.","source_license":"public-domain-us","license_restricted":false}