IL–13 contribution to EoE pathogenesis beyond Eotaxin–3 overproduction includes profound dysregulation of the epidermal differentiation complex (
EDC) gene expression
[35]. The epidermal differentiation complex (
EDC) is a gene complex on the human chromosome 1 in a locus
1q21. Genes residing in
EDC have similar, closely related functions and are essential for epithelial barrier formation and expressed during maturation of epithelial cells terminal differentiation
[36]. Across the human genome, the highest density of genes, expression of which is dysregulated in active EoE, is observed to occur in
EDC locus
[35]. Expression patterns in esophageal biopsy specimens of EoE patients show significant decreased expression or trends toward the decreased expression of most genes in the
EDC locus
[35]. Ex vivo response to IL–13 presents a similar downregulation of
EDC genes, including filaggrin (
FLG), involucrin (
IVL), and several small proline–rich repeat (
SPRR) family members (1A, 2D, 3, and 4)
[35].
FLG is expressed in the skin epidermis and epithelium of esophageal, nasal and oral mucous membranes.
FLG encodes progenitor protein Profilaggrin. During epithelial cell differentiation, Profilaggrin undergoes processing, and after proteolytic cleavage Filaggrin monomers are formed. Filaggrin is one of the essential structural proteins for stratified epithelial BFs. Filaggrin function is aggregation of keratin intermediate filaments during transformation of granular cells into flattened squamous cells, that compose the superficial layer of esophageal epithelium and essential for its BF, despite the fact that actual keratinization normally does not occur in esophagus, and human esophageal epithelium is stratified, squamous and nonkeratinized
[37]. In the case of
FLG downregulation, epithelial BF decreases, and exogenous allergens become able to penetrate epithelial barriers and activate immune responses. Loss of
FLG and other
EDC gene expression leads to defects in epidermal BF.
FLG and
IVL expression in EoE biopsies is decreased on gene and protein level
[38]. IL-13 also decreases levels of
FLG and
IVL mRNAs and proteins in ALI-cultured primary human esophageal epithelial cells (HEEC)
[38]. Furthermore,
FLG silencing with siRNA in ALI HEEC causes BF impairment; TEER and thickness of the cell layer was decreased in siRNA-transfected cultures, indicating alterations in cell proliferation and differentiation
[38]. It is remarkable that tight junction proteins (
CLDN1 and
CLDN4) have altered patterns of expression in
FLG-deficient cells, although the levels of the proteins are unchanged
[38].
2.6. The Role of the CAPN14 in the Development of EoE
CAPN14 is a cytosolic calcium–activated cysteine protease that was identified as an associated locus 2p23 in EoE genome-wide association studies (GWAS)
[39][40]. In comparison with other members of the
calpain family,
CAPN14 possesses a unique feature of its tissue-specific expression pattern.
CAPN14 is almost specifically expressed in the esophageal epithelium
[40]. Stimulation of EPC2 esophageal epithelial cells with IL-13 significantly upregulates
CAPN14 expression. IL-13 impact on calpain family gene expression is confined to
CAPN14 upregulation; other calpains except
CAPN14 are not induced by IL-13 in primary esophageal epithelial cell culture and in EPC2 ALI cultures
[40][41]. However, in specimens of esophageal biopsies, in addition to
CAPN14 upregulation,
CAPN3 level turned out to be significantly elevated. Conversely, expression of
CAPN7, CAPN5, CAPNS2 and
CAST (
calpastatin, endogenous calpain inhibitor) genes were downregulated
[40]. Throughout the calpain family and related genes,
CAPN14 reveals highest fold change.
The kinetics of IL-13-induced
CAPN14 expression are parallel to the induction of
CCL26 in EPC2 cells
[42]. Pronounced changes in an epigenetic signature are observed in the promoter region of
CAPN14 in response to IL–13 stimulation. The ChIP–seq detected a marked increase in H3 acetylation at the 27th lysine residue (
H3K27Ac)
[40][43] and H3 trimethylation at the 4th lysine residue (
H3K4me3)
[43] in the
CAPN14 promoter region near the TSS in IL–13–treated cells.
H3K27Ac and
H3K4me3 are highly enriched at active promoters near TSS and positively correlated to gene transcription, which is consistent with an increase in
CAPN14 transcriptional activity by RNA–seq
[40].
2.7. The Role of the POSTN in the Development of EoE
Moreover, DSG1 deficiency increases gene expression of the proinflammatory extracellular matrix molecule periostin (
POSTN)
[44].
POSTN is one of the markedly upregulated genes in EoE transcriptome (35-fold change), that encodes periostin—protein of extracellular matrix (ECM), that facilitates epithelial-mesenchymal transition (EMT), fibrotic remodeling and migration of certain cells to inflamed tissues. Periostin can directly enhance activated eosinophil adhesion via integrin αMβ2
[45], as well as increase keratinocyte production of thymic stromal lymphopoietin (
TSLP), a potent Th2–skewing cytokine
[46] that has been associated with EoE
[47][48]. It has been shown that treatment of esophageal epithelial cells with IL–13 induces
POSTN expression in this cell type
[49], as well as in bronchial epithelial cells
[50].
POSTN can induce epithelial–mesenchymal transition by increasing signaling through integrin
αV
β5 and epidermal growth factor receptor (
EGFR)
[51]. It is possible because epitheliocytes adopt a fibroblast-like phenotype due to induced loss of epithelial cell markers
[52]. Moreover, it has been demonstrated that
DSG1–dependent
EGFR signaling suppression promotes epithelial differentiation and reduces proliferative capacity
[53]. The epithelial–mesenchymal transition has been proposed to occur in EoE (
Figure 3)
[54][55].
Figure 3. The signaling pathways of the interleukin–13 receptor (IL13R), transforming growth factor beta (TGFβ) receptor, and epidermal growth factor receptor (ErbB) in esophagus keratinocyte and their alterations in eosinophilic esophagitis. The IL–13R receptor binds to its corresponding ligand, and heterodimerization occurs, enhancing Janus kinase (JAK) activity. Signaling molecules such as signal transducer and transcriptional activator (STAT) 6 and STAT3 can initiate transcription of target genes, including eotaxin–3. The effects of IL–13 are mediated by ErbB. ERBB2–Erbb2 interacting protein (ERBIN) negatively regulates TGFβ signaling. TGFβ mediates fibrosis by inducing fibrogenic target genes. Active TGFβ binds to its receptor to initiate SMAD–dependent and independent signaling. SMAD–dependent signaling regulates fibrogenic target genes such as α–smooth muscle actin, collagen, connective tissue growth factor, tissue metalloprotease inhibitor, and periostin. TYK2—non-receptor tyrosine-protein kinase, CRE—cAMP response element, CREB—cAMP response element-binding protein, CBP—CREB-binding protein, CoA—Coenzyme A, Ac—acyl group, H3K27ac—lysine acetylation at N-terminal position 27 of histone H3, CCL26—chemokine (C-C motif) ligand 26, CAPN14—calpain-14, POSTN—periostin, DSG1—desmoglein 1, SHOC2—Leucine-rich repeat protein SHOC-2, ErbB—receptor tyrosine-protein kinase ErbB, TGFβ—transforming growth factor β, MEK—mitogen-activated protein kinase kinase, ERK—extracellular signal-regulated kinase.
2.8. EoE-Associated Risk Genes
Loci, genes, and SNPs that have the most significant association with EoE on the basis of GWAS data are represented in Table 1 and Figure 4. Data were retrieved from the GWAS Catalog (EFO ID: EFO_0004232). The biological meaning and role in EoE predisposition and development of many genes, represented in the table, remain obscure. Further studies are needed to identify the influence of associated variations, genes, and their products, on EoE pathogenesis.
Figure 4. The position of the single nucleotide polymorphisms (SNP) with the most significant association with EoE from the genome-wide association studies (GWAS) data on the human genome.
Table 1. Genome–wide significant loci reported in GWAS.
EoE Risk Locus |
Mapped Gene |
Tag SNP |
The Strongest SNP Risk Allele |
p-Value |
OR |
Reference |
1p13.3 |
LINC02785 SLC25A24 |
rs2000260 |
A |
7 × 10−7 |
1.32 |
[40] |
1p32.2 |
LINC01767 PLPP3 |
rs11206830 |
? |
8 × 10−8 |
2.162 |
[40] |
1p36.12 |
KIF17 |
rs2296225 |
? |
1 × 10−7 |
1.626 |
[40] |
1p36.13 |
IFFO2 |
rs28530674 |
? |
3 × 10−7 |
1.826 |
[40] |
2p22.2 |
PRKD3 |
rs143457389 |
A |
3 × 10−16 |
1.77 |
[56] |
2 × 10−6 |
1.91 |
2p23.1 |
CAPN14 |
rs143457388 |
A |
3 × 10−16 |
1.77 |
[56] |
rs149864795 |
A |
5 × 10−10 |
2.216 |
[39] |
rs77569859 |
G |
3 × 10−10 |
1.98 |
[40] |
2q12.1 |
TMEM182 |
rs887992 |
C |
4 × 10−10 |
0.75 |
[56] |
3q22.1 |
CPNE4 |
rs554318837 |
C |
4 × 10−8 |
2.88 |
[56] |
3q26.32 |
? |
rs6799767 |
? |
4 × 10−7 |
1.49 |
[57] |
4q21.1 |
SHROOM3 |
rs13106227 |
? |
4 × 10−6 |
1.52 |
[47] |
rs1986734 |
? |
1 × 10−6 |
1.54 |
5q14.2 |
? |
rs1032757 |
T |
2 × 10−6 |
1.96 |
[47] |
5q22.1 |
TSLP |
rs3806932 |
? |
3 × 10−9 |
1.85 |
[47] |
rs3806933 |
G |
2 × 10−8 |
1.37 |
[40] |
TSLP WDR36 |
rs252716 |
C |
4 × 10−14 |
1.516 |
[39] |
WDR36 RPS3AP21 |
rs1438673 |
C |
1 × 10−13 |
1.43 |
[56] |
6 × 10−22 |
0.7 |
5q23.1 |
LINC02214 |
rs2055376 |
A |
7 × 10−8 |
2.3 |
[40] |
5q23.2 |
LINC02240 |
rs4240384 |
? |
2 × 10−7 |
1.4326648 |
[57] |
5q31.1 |
RAD50 |
rs2106984 |
A |
4 × 10−8 |
1.26 |
[56] |
6p11.2 |
GAPDHP15 |
rs9500256 |
? |
5 × 10−6 |
2.04 |
[47] |
6p21.33 |
SNHG32 NEU1 |
rs599707 |
? |
3 × 10−9 |
1.6920472 |
[57] |
6p22.3 |
BOLA2P3 |
rs1620996 |
T |
3 × 10−8 |
0.69 |
[56] |
7p13 |
URGCP-MRPS24 URGCP |
rs188483654 |
C |
9 × 10−9 |
5.68 |
[56] |
7p15.1 |
JAZF1 |
rs11495981 |
? |
9 × 10−7 |
1.308 |
[57] |
7q22.3 |
LARP1BP2 CCDC71L |
rs147307036 |
A |
1 × 10−8 |
8.04 |
[56] |
8p23.1 |
XKR6 |
rs2898261 |
C |
5 × 10−8 |
1.35 |
[40] |
8q22.2 |
MATN2 |
rs2513845 |
T |
7 × 10−9 |
4.18 |
[56] |
ERICH5 |
rs13278732 |
T |
6 × 10−6 |
1.31 |
[47] |
8q24.12 |
SNTB1 |
rs11989782 |
A |
7 × 10−6 |
1.53 |
[47] |
9p24.1 |
JAK2 |
rs62541556 |
T |
4 × 10−8 |
1.61 |
[56] |
10p11.21 |
CCNY |
rs191051238 |
C |
4 × 10−8 |
13.2 |
[56] |
10p12.31 |
MIR4675 |
rs11819199 |
G |
3 × 10−7 |
1.62 |
[40] |
10q21.1 |
PRKG1 |
rs185811602 |
T |
1 × 10−8 |
6.37 |
[56] |
10q23.1 |
LINC02650 |
rs2224865 |
G |
9 × 10−6 |
1.44 |
[47] |
11p15.4 |
RHOG STIM1-AS1 |
rs147702004 |
T |
1 × 10−8 |
1.95 |
[56] |
11q13.4 |
SHANK2 |
rs182139615 |
T |
1 × 10−9 |
6.62 |
[56] |
11q13.5 |
EMSY |
rs61894547 |
T |
4 × 10−11 |
2.439 |
[39] |
T |
4 × 10−13 |
1.92 |
[56] |
T |
5 × 10−15 |
1.79 |
EMSY LINC02757 |
rs2155219 |
A |
4 × 10−7 |
1.37 |
[40] |
CAPN5 |
rs77301713 |
? |
1 × 10−7 |
2.22 |
[40] |
11q14.2 |
CCDC81 |
rs118086209 |
C |
2 × 10−7 |
2.19 |
[40] |
11q21 |
FAM76B |
rs1939875 |
T |
3 × 10−6 |
1.54 |
[47] |
12q13.3 |
STAT6 |
rs167769 |
T |
2 × 10−7 |
1.351 |
[39] |
T |
2 × 10−6 |
1.36 |
[47] |
13q12.13 |
WASF3 GPR12 |
rs146034499 |
A |
3 × 10−9 |
5.92 |
[56] |
14q12 |
LINC02588 |
rs8008716 |
G |
7 × 10−8 |
1.712 |
[39] |
15q13.3 |
LINC02352 KLF13 |
rs8041227 |
G |
6 × 10−12 |
1.52 |
[40] |
15q22.2 |
RORA |
rs2279293 |
G |
5 × 10−11 |
0.69 |
[56] |
15q22.33 |
SMAD3 |
rs56062135 |
T |
4 × 10−12 |
1.29 |
[56] |
16p13.13 |
CLEC16A |
rs35099084 |
C |
3 × 10−9 |
0.71 |
[56] |
T |
2 × 10−12 |
0.72 |
rs12924112 |
? |
1 × 10−7 |
1.310616 |
[57] |
16q24.1 |
MEAK7 |
rs371915 |
? |
2 × 10−8 |
1.9 |
[47] |
17q24.3 |
CALM2P1 |
rs6501384 |
T |
6 × 10−6 |
1.41 |
[47] |
17q25.3 |
CEP295NL TIMP2 |
rs3744790 |
? |
8 × 10−7 |
1.54 |
[40] |
18q12.1 |
DSG1 |
rs7236477 |
G |
7 × 10−6 |
2.22 |
[47] |
18q12.2 |
INO80C GALNT1 |
rs534845465 |
A |
2 × 10−8 |
5.78 |
[56] |
DCC |
rs9956738 |
? |
4 × 10−7 |
2.472 |
[40] |
19q13.11 |
ANKRD27 |
rs3815700 |
C |
2 × 10−9 |
1.618 |
[39] |
21q22.3 |
HSF2BP |
rs17004598 |
C |
1 × 10−7 |
2.57 |
[40] |
22q11.21 |
P2RX6 |
rs2075277 |
? |
9 × 10−7 |
1.544 |
[40] |
3. Conclusions
EoE pathogenesis is a complicated network of interactions and signaling between epithelial, mesenchymal, and immune cells on molecular and intercellular levels. Alterations produced by overactivation of some cytokine signaling pathways, e.g., IL–13 or TSLP, were evolved and observed in this review from the viewpoints of molecular, genetic, epigenetic, and transcriptomic changes. Despite the substantial amount of experimental data, the reliable and representative mechanism of EoE pathogenesis has yet to show itself, and so the place of EoE between mixed and non-IgE-mediated allergic disorders, between eosinophilic gastrointestinal disorders currently seems vague and unclear.