Advances in Therapy

, Volume 36, Issue 9, pp 2205–2222 | Cite as

The Proteins of Keratoconus: a Literature Review Exploring Their Contribution to the Pathophysiology of the Disease

  • Eleftherios Loukovitis
  • Nikolaos Kozeis
  • Zisis Gatzioufas
  • Athina Kozei
  • Eleni Tsotridou
  • Maria Stoila
  • Spyros Koronis
  • Konstantinos Sfakianakis
  • Paris Tranos
  • Miltiadis Balidis
  • Zacharias Zachariadis
  • Dimitrios G. Mikropoulos
  • George Anogeianakis
  • Andreas Katsanos
  • Anastasios G. KonstasEmail author
Open Access



Keratoconus (KC) is a complex, genetically heterogeneous multifactorial degenerative disorder characterized by corneal ectasia and thinning. Its incidence is approximately 1/2000–1/50,000 in the general population. KC is associated with moderate to high myopia and irregular astigmatism, resulting in severe visual impairment. KC structural abnormalities primarily relate to the weakening of the corneal collagen. Their understanding is crucial and could contribute to effective management of the disease, such as with the aid of corneal cross-linking (CXL). The present article critically reviews the proteins involved in the pathophysiology of KC, with particular emphasis on the characteristics of collagen that pertain to CXL.


PubMed, MEDLINE, Google Scholar and GeneCards databases were screened for relevant articles published in English between January 2006 and June 2018. Keyword combinations of the words “keratoconus,” “risk factor(s),” “genetics,” “genes,” “genetic association(s),” “proteins”, “collagen” and “cornea’’ were used. In total, 272 articles were retrieved, reviewed and selected, with greater weight placed on more recently published evidence. Based on the reviewed literature, an attempt was made to tabulate the up- and down-regulation of genes involved in KC and their protein products and to delineate the mechanisms involved in CXL.


A total of 117 proteins and protein classes have been implicated in the pathogenesis and pathophysiology of KC. These have been tabulated in seven distinct tables according to their gene coding, their biochemistry and their metabolic control.


The pathogenesis and pathophysiology of KC remain enigmatic. Emerging evidence has improved our understanding of the molecular characteristics of KC and could further improve the success rate of CXL therapies.


Collagen Corneal biomechanical characteristics Corneal cross-linking CXL Ectasia Keratoconus proteins Ophthalmology 


Keratoconus (KC) is a corneal dystrophy that seriously affects the quality of life of KC patients [1]. Its prevalence ranges between 1/2000 and 1/50,000 in the general population. The condition is characterized by moderate to substantial visual impairment due to the development of corneal protrusion and thinning [2, 3, 4, 5, 6]. In KC, visual acuity decreases due to progressive myopia, irregular astigmatism and, often, corneal apical opacification [4, 6, 7]. The key underlying factor for these pathological changes is weakening of collagen tissue in corneal stroma [6, 8]. Although KC is generally a bilateral disorder [2, 9, 10], rare unilateral cases have also been described [11].


The present review is based on literature search that utilized the PubMed, MEDLINE, Google Scholar and GeneCards databases for articles related to KC. The keywords used were “keratoconus,” “risk factor(s),” “genetics,” “genes,” “genetic association(s),” “proteins,” “collagen” and “cornea’’ and all their relevant combinations. The search focused on articles written in English from January 2006 until June 2018. A total of 177 articles were identified and reviewed, and both their text and references were analyzed. The analysis of these references revealed an additional 95 relevant articles, which were also subsequently reviewed. The current article is based on previously conducted studies and does not contain studies on human subjects or animals performed by any of the authors; its aim is to summarize current knowledge on the proteins involved in KC. It also represents an attempt to map and categorize the proteins that are up- or down-regulated in KC according to their corneal stromal location.


Histopathology, Biochemistry and Biomechanics of KC Corneas

Histologically, KC is characterized by degradation of the basal membrane of the corneal epithelium, diminishing of the number and density of collagen fibrils, thinning of the corneal stroma, and keratocyte apoptosis with necrosis [2, 3, 4, 9]. Keratocyte apoptosis and necrosis involves the central anterior stroma and Bowman’s lamina and, typically, weakens the corneal tissue [12, 13]. Slit-lamp examination reveals subepithelial and anterior stromal scars [14] due to degeneration of epithelial basal cells, as well as fine folds in the posterior stroma at the corneal apex (Vogt’s striae). Endothelial damage is rarely visible. The presence of Fleisher’s ring, which is formed by the accumulation of hemosiderin around the base of the corneal cone [2, 15], is also frequent. In contrast, breaks of the Descemet membrane with subsequent leakage of aqueous humor into the corneal stroma, leading to corneal edema and decompensation (acute hydrops), are rare. Finally, evident corneal nerves together with a decrease in sub-basal nerve density are prominent features of advanced KC [2, 15, 16].

The stroma in KC is characterized by a decrease in the number of collagen lamellae and a reduction in the amounts of microfibrillar and fine granular material [17]. Changes are also observed in the arrangement of fibrils of the anterior stroma [15, 18], together with abnormal distribution of collagen fibers. This results in the decrease of corneal mechanical resistance [15, 18].

Overall, the thinning of the corneal stroma itself is associated with keratocyte apoptosis, alterations in the extracellular matrix (ECM), and changes in the activities of several enzymes [12, 19] that involve the activation of degrading enzymes and the promotion of cell death, both of which are mostly due to oxidative stress [20, 21]. Although the mechanisms of tissue breakdown remain obscure [21], over the last 25 years there has been strong evidence that collagens are susceptible to oxygen-free radical damage [20, 22]. In addition, collagen is the only protein susceptible to fragmentation by superoxide anions, a process during which small 4-hydroxyproline-containing-peptides are liberated [20]. In the presence of oxygen, hydroxyl radicals cleave collagen into small peptides; the cleavage is, apparently, specific to proline or 4-hydroxyproline residues [20]. In contrast, hydroxyl radicals, in the absence of oxygen, do not induce fragmentation of collagen molecules, but they trigger the polymerization of collagen through the formation of new cross-links such as dityrosine or disulfide bridges [20].

Thinning of the stroma and alterations of ECM in KC affect the biomechanical properties of the cornea. Comparisons of uniaxial tensile strength between KC and normal corneas have shown that maximum load and stress, maximum stiffness, and relative energy absorption were smaller in keratoconic corneas than in normal ones [2, 23]. Furthermore, load and stress values at corresponding strain values were smaller in keratoconic compared to normal corneas both for the initial (exponential) and for the linear parts of the curves [23]. In addition, no differences between KC and normal corneas have been detected in uronic acid or hydroxyproline concentrations [23]—an indication that the alteration of corneal biomechanical properties in KC are the result of collagen cleavage into smaller peptides [23]. This conclusion is further supported by the observation that the solubility of pepsin-treated collagen from KC corneas is greater than that from normal corneas—apparently due to the breakdown of KC corneal collagen into smaller peptides that has already occurred [23].


The amount of collagen, which is a major corneal protein [2, 24, 25], is reduced in KC [12, 26]. The decrease in collagen types I and III is related to alterations in the ECM basement membrane [26], while the decrease in collagen type IV is related to alterations in the basement membranes, of which collagen type IV is the major structural component [24, 27, 28]. In addition to the decrease in the amount of collagen in the cornea, the shape and transparency of the cornea also change due to re-orientation of the collagen molecules and fibrils [29, 30]. The combination of changes in collagen orientation and the reduction of total collagen contribute to corneal thinning in KC [30].

Type IV collagens are major structural components of basement membranes and consist of six proteins encoded by six genes (COL4A1–COL4A6). Interestingly, these genes are organized in pairs in a head-to-head conformation so that each gene pair shares a common promoter [31]. Thus, the COL4A6 gene is organized in a head-to-head conformation with the COL4A5 (alpha 5 type IV collagen) gene. Similarly, the gene pairs for COL4A1 and COL4A2 and for COL4A3 and COL4A4 are also conformed head-to-head [32]. This means that deletions in one member of the pair of genes extend into the other; it also means that alternative splicing results in multiple transcript variants encoding different isoforms [24, 33].

Collagen type IV α1/α2 chains have been reported in KC patients [34], while COL4A1 has been shown to be down-regulated in KC corneas [35]. The chromosomal loci of COL4A1 and COL4A2 are close to 13q32 [32], with the COL4A1 gene consisting of 52 exons and COL4A2 gene of 48 exons [28]. Mutations in COL4A3 and COL4A4 may be related to decreases in collagens I and III which contribute the increase of the KC risk [28]. It is noteworthy that there are several reports that COL4A3 has been found to be associated with KC in at least two European populations [28, 36]. Furthermore, genomic loci that differ between ethnic groups and are associated with variation in central corneal thinning [37, 38] are found in the genes that encode COL1A1, COL1A2, COL8A2 and COL5A1 [37, 38].

There are seven polymorphisms (M1327 V, V1516 V and F1644F in COL4A4, and P482S, P141L, D326Y and G895G in COL4A3) that are associated with KC under recessive, dominant or additive models [28, 39]. Finally, the expression of collagen types XII, XIII, XVIII and XV is altered in KC, although no relationships have been identified between mutations in these genes and their expression [40, 41]. It should be noted that collagen types XIII, XV, and XVIII are mainly expressed in basal corneal cells, and that these types are involved in the adhesion between corneal epithelial cells as well as between corneal epithelial cells and the basement membrane [35, 41].

Thus, changes in collagen have a vital role in the progression of KC. Ensuring collagen stability is the main target in corneal cross-linking (CXL), a relatively new, minimally invasive outpatient procedure used for the management of KC. Before the advent of CXL, there was no treatment to modify the underlying pathophysiology and arrest corneal ectasia. The introduction of this method to ophthalmology goes back to 1997 [42]. Subjective and objective results following this method seem to be promising. CXL is a photo-induced reaction, used to increase the rigidity of the corneal collagen and its resistance to ectasia, ensuring the mechanical and biochemical stability of the stromal tissue. Ultraviolet light A at 370 µm excites riboflavin (vitamin B2) which functions as a photosynthesizer. Free radicals and oxidizing substance cause the formation of new covalent bonds between collagen fibrils within the cornea which stabilize stroma [43].

Other Proteins Involved in KC

There are more than 1500 different peptides and proteins in the human tears, both of intra- and extracellular origin. Their anatomic location, the genes that code for them and their potential implication in KC are summarized in Tables 1, 2 and 3. They include cytokines and small molecules and are followed by lipids and metabolites [44, 45]. Among them, one can distinguish lactoferrin, secretory immunoglobulin A, tear lipocalin, lysozyme, lipophilin, proline-rich proteins and serum albumin, all of which help maintain the health of the ocular surface [44, 45, 46]. Since abnormal levels of enzymes and inflammatory molecules are found in KC patients [44, 47, 48], the deregulation of proteins has been linked to the thinning of the cornea [14, 49]. Changes in the structural integrity of the cornea including changes in the collagen content (e.g., the reduction in the number of collagen lamellae) [2, 9], and alterations in enzymes have been reported in KC patients [14, 49]. The biochemical abnormalities observed in the KC cornea (epithelium and stroma) include the increase in the level of degradative enzymes and the decrease of the protease inhibitors [50].
Table 1

Proteins (and their genes) of the corneal epithelium and their regulation in keratoconus




Gene [reference]


Gene [reference]

S100 calcium-binding protein A4 (S100A4)

S100A4 [44]

Calpain small subunit 1 (CAPNS1)

CAPNS1 [14, 44]


KRT [44]

FTH 1 (ferritin heavy chain protein 1)

FTH1 [14, 44]


GSN [44]

Annexin A2

ANXA2 [14, 44]

Alpha enolase (ENO1)

ENO1 [44]

Heat shock protein beta 1 (HSPB1)

HSPB1 [14, 44]

Keratin-5 (KRT5)

KRT5 [14, 44]


Annexin A8

ANXA8 [14, 44]


L-lactate dehydrogenase (LDH)

LDH [14, 44]


Serum albumin

ALB [14, 44]


SFRP1 (secreted frizzled-related protein 1)

SFRP1 [44]

Table 2

Proteins (and their genes) found in both epithelium and stroma and their regulation in keratoconus




Gene [reference]


Gene [reference]

Increased epithelial and stromal


Decreased epithelial and stromal


Keratin type I cytoskeletal 14 (KRT14)

KRT14 [14, 44]


TKT [14, 44]

Keratin type I cytoskeletal 16 (KRT16)

KRT16 [14, 44]

Pyruvate kinase

PKLR [14, 44]

Tubulin beta chain (TUBB)

TUBB [14, 44]

Stratifin (14-3-3 sigma isoform)

SFN [14, 44]

Lamin-A/C (LMNA)

LMNA [14, 44]

Phosphoglycerate kinase 1 (PGK 1)

PGK1 [14, 44]

S100 calcium-binding protein A4 (S100A4)

S100A4 [14, 44]

NAD (P)H dehydrogenase (quinone) 1

NQO1 [14, 44]

Heat shock cognate 71 kDa protein (HSPA8)

HSPA8 [14, 44]

Table 3

Proteins (and their genes) exclusive of the stroma and their regulation in keratoconus




Gene [reference]


Gene [reference]


VIM [14, 44]

β-actin (ACTB)

ACTB [14, 49, 71]

Keratocan (KTN)

KERA [14, 44]

TGF-beta (transforming growth factor beta) induced ig-h3 (Bigh3) protein

TGFBI [14, 44]


DNC [14, 44]

Meprin A-5 protein, and receptor protein-tyrosine phosphatase mu (MAM domain)

14, 44

Keratin 12

KRT12 [14, 44]


TF [14, 44]

Haptoglobin precursor (HP)

HP [14, 44]

HuR (human antigen R)

ELAVL1 [14, 49, 71]

Apolipoprotein A-IV precursor (APOA4)

APOA4 [14, 44]

Isoforms 2C2A of collagen alpha-2 (VI) chain

COL6A2 [14, 44]

ALDH3A1 (aldehyde dehydrogenase 3A1)

ALDH3A1 [14, 44]


Lipoprotein Gln

LPL [14, 44]



14, 44



TIMP3 [81, 142]



TIMP1 [14]

Recent clinical studies imply the existence of an inflammatory component in the pathogenesis of KC [47, 51]. This means that tissue degradation in KC involves the expression of pro-inflammatory cytokines, inflammatory mediators, cell adhesion molecules and matrix metalloproteinases (MMPs) [47, 51, 52]. Inflammation in KC is characterized by the presence of many inflammatory cells and markers such as IL-1, IL-6, MMP-9, TGF-β, and TNF-a [47, 51, 52, 53, 54]. Moreover, inflammatory factors like IL-4, IL-5, IL-6, IL-8, TNF-α, TNF-β, MMP-1, MMP-3, MMP-7, MMP-9, MMP-13 and cathepsins are increased in the tears of KC patients [47, 51, 52], a finding that suggests the deregulation of the underlying molecular pathways [55]. Table 4 summarizes the proteins that have been identified as definitive or potential biomarkers in KC together with the genes that are known to code for them.
Table 4

Proteins that serve as biomarkers in keratoconus and their genes


Gene [reference]


COL8A1 [40]


COL8A2 [40]

Basal corneal cells

 Collagen type XII

COL12A1 [35, 41]

 Collagen type XIII

COL13A1 [35, 41]

 Collagen type XVIII

COL18A1 [35, 41]

 Collagen type XV

COL15A1 [35, 41]


IL1A [119, 126, 127]


IL1B [119, 126, 127]

SPARC (secreted protein acidic and rich in cysteine)

SPARC [81, 82, 83]

Human leukocyte antigen



TIMP [68, 81, 142]

SLC4A11 (sodium bicarbonate transporter-like protein 11)

SLC4A11 [91, 92]

AQP5 (water channel protein aquaporin 5)

AQP5 [93]

Protein Changes in KC

Changes in proteins that might contribute to corneal thinning include the increase of degradative enzymes such as acid phosphatases, acid esterases and acid lipases [24]. In eyes with KC, the levels of enzymes such as cathepsin B and cathepsin G increase, while protease inhibitors like the a1 proteinase inhibitor (a1-PI) and the a2 macroglobulin decrease [14, 50]. Corneal thinning therefore is caused by the up-regulation of cellular proteases and the down-regulation of their inhibitors [14]. Both enzyme up-regulation and enzyme inhibitor down-regulation involve the mRNA as well as the protein levels of expression [56]. The increase of corneal proteinase activity is also involved in the pathophysiology of KC [50]. As a result, KC may develop because of defective regulation of the proteinase activity in the cornea [50]. Nitrotyrosine malonaldehyde, glutathione S transferase, and inducible nitric oxide synthase levels increase, while aldehyde dehydrogenase and superoxide dismutase levels decrease [14]; in addition, there is an accumulation of GAG (glycosaminoglycan) polyanions in keratoconic corneas [57], while analysis of the KC corneal proteome indicated a decrease in decorin, keratocan, lumican, and biglycan [58]. Keratan sulfate (KS) antigenicity appears to decrease in the central, thinned region of the keratoconic cornea [59], while the KS organization in corneas with advanced KC is markedly different from that in healthy corneas [60]. Interestingly, there are molecular alterations in proteoglycans in some connective tissue disorders that are associated with KC [61, 62].

Gross cystic disease fluid protein-15 (GCDFP15), a secretory glycoprotein, expressed in the proteome of human tear fluids [63], consists of 14 kDa [64] and is a potential biomarker for KC [63]. Analysis of the tear proteome shows that the key for the maintenance of a healthy corneal structure is the quality of tear fluids [65]. Three-dimensional cultures of human KC cells [66, 67] show an increase in oxidative stress levels when compared to normal human corneal fibroblast cultures [68]. Other known pathogenic factors are prolactin-induced protein (PIP) [69] which is involved in actin binding and the zinc-alpha-2-glycoprotein (AZGP1) [65]. The interplay between these two proteins (PIP-AZGP1) is apparently involved in KC pathogenesis [63].

PIP expression is regulated by transforming growth factor-β (TGF-β), which has a vital role in corneal wound healing and is implicated in the pathophysiology of KC [65]. The TGF-β signaling pathway is a complex, multibranched signal transduction cascade that may modulate ECM, making it a potential contributing factor in KC pathogenesis [21]. However, increases of TGF-β2 levels in the aqueous humor of keratoconic patients are not confirmed by immunofluorescence studies [21].

The mRNA of human antigen R (HuR) is a very important post-transcriptional regulator of gene expression that is expressed in all proliferating cells [70]. The expression of mRNA and protein levels of β-actin and HuR are decreased on KC corneal stroma compared to normal corneas [14, 49, 71], indicating that the down-regulation of β-actin and HuR is the result of their mutual interaction [71].

KC is also associated with amyloid deposits [72, 73], and increased levels of specificity protein 1 (SP1) characterize KC corneas [74]. Species-specific tRNA processing (STP1) transcription factor is involved in tRNA maturation; STP1 transcription factor belongs to a specificity protein/Kruppel-like factor family, and it shares structural similarities and sequence homology with 25 other members of this family [75, 76]. The over-expression of SP1 is associated with neuro-degenerative diseases such as Huntington’s chorea [77] and tumorigenesis [78], while the promoter activity of the human α1-PI gene in corneal cells is suppressed by the up-regulation of Sp1 [79, 80].

Other proteins such as secreted protein acidic and rich in cysteine (SPARC), human leukocyte antigen, mitochondrial complex I genes and 2q21.3 with RAB3 GTPase-activating protein subunit 1 (catalytic) are also involved in KC [81, 82, 83]. Luminac, biclycan, keratocan and decorin decrease in KC corneas [84], whereas keratocytes express heparan sulfate proteoglycans [85, 86]. In addition, bone morphogenic protein 4 and the gene and protein expression of TGF are increased in KC corneas [21, 87].

The epithelial–endothelial IL-1 system is vital, not only for the organization of the cornea but also for its ability to respond to mechanical injuries and pathogen invasions, by provoking keratocyte apoptosis [88]. The up-regulation of IL-1 in KC induces keratocyte apoptosis [89] and manifests itself as an increase in IL-1 protein levels and a simultaneous rise in the number of IL-1 binding sites [87, 90]. The IL-1 family consists of pleiotropic cytokines. The genes that encode for the IL-1 cytokines are located on chromosome 2q14 [89] and their transcription is affected by polymorphisms located in this region [89]. Finally, cytokines IL-1α and IL-1β control the immune response, the pro-inflammatory response and hematopoiesis; their effects are modulated by the IL-1 receptor antagonist [91].

Sodium bicarbonate transporter-like protein 11 (SLC4A11) belongs to the SLC4 family of bicarbonate transporters [91] and may participate in KC pathogenesis, because its functional failure leads to keratocyte apoptosis [91, 92]. The SLC4A11 gene is located in the 20p13 chromosome and functions as an electrogenic, Na + coupled, borate co-transporter [91]. Also, the water channel protein, aquaporin 5, may be a marker for KC [93].

Some HLA antigens, including HLA-A26, B40 and DR9, have been linked to early onset KC [82]. In addition, keratoconic corneas manifest a decrease of IGF-2 transcripts and over-expression of IGF-binding proteins 3 and 5 [94]. Proteins such as dual-specificity phosphatases or mitogen-activated protein kinase phosphatases (MKPs) are activated by oxidative stress in KC [95]. The same proteins regulate the immune response and dephosphorylate tyrosine and threonine residues on MKPs [96].

Proteomic analyses of KC patient epithelia using two-dimensional gel electrophoresis followed by mass spectrometry [44] identified high over-expression of S100A4, cytokeratin and gelsolin in keratoconic epithelium [44], while alpha enolase was slightly up-regulated. Other studies, using the same method, suggest that beta actin and alpha enolase are slightly expressed in corneal wing and superficial epithelial cells of KC patients [44, 49]. It is interesting that proteins like gelsolin and cytokeratins are implicated in ocular (e.g., vitreoretinopathy) and non-ocular (e.g., cystic fibrosis, cancer, steatohepatitis) disorders [44, 97].

Corneal epithelial and stromal proteins are expressed differentially in KC [14, 44], and six epithelial proteins, keratin type I cytoskeletal 14, keratin type I cytoskeletal 16, tubulin beta chain, lamin-A/C, S100-A4 and heat shock cognate 71 kDa protein, were identified using a label-free nano-ESI-LC–MS/MS method to be increased in KC. Five other proteins, transketolase, pyruvate kinase, 14-3-3 sigma isoform, phosphoglycerate kinase 1 and nicotinamide adenine dinucleotide phosphate, and dehydrogenase (quinone) 1, are decreased [14, 44].

Stromal proteins, such as vimentin, keratocan, decorin, keratin 12, haptoglobin precursor, apolipoprotein A-IV precursor, aldehyde dehydrogenase 3A1 (ALDH3A1), lipoprotein Gln and prolipoprotein, are up-regulated in KC [14, 44]. while A-5 protein, TGF-beta (transforming growth factor beta) ig-h3 (Bigh3), receptor protein-tyrosine phosphatase-μ (MAM) domain-containing protein 2, serotransferrin, meprin, and isoforms 2C2A of collagen alpha-2 (VI) chain are down-regulated [14, 44]. Of the proteins of the corneal epithelium, keratin-5, annexin A8, L-lactate dehydrogenase and serum albumin are up-regulated [14, 44], and calpain small subunit 1, Ferritin heavy chain protein 1 (FTH 1), annexin A2 and heat shock protein beta 1 are down-regulated [14, 44].

Recently, the role of β-galactoside-binding proteins galectin (Gal)-1 and Gal-3 in patients with KC and postcorneal CXL treatment was investigated in vitro [98]. These proteins are associated with various cellular responses that involve inflammation [99]. In KC, increased levels of Gal-1 and Gal-3 were detected in conjunctival epithelial cells compared to controls [98]. In addition, keratocytes of KC patients were found to release significant amounts of Gal-1 in the stroma. In vitro, CXL promoted the release of Gal-1 from keratocytes but decreased the concentration of inflammatory biomarkers, such as IL-6, IL-8, MMP-2 and MMP-9 [98]. Overall, these results suggest that CXL exerts an immunosuppressive effect on keratocytes by inhibiting the release of MMPs and cytokines and increasing the levels of anti-inflammatory Gal-1 [98].

The FAS/FASLG Genes and Apoptosis

Apoptosis is considered the primary pathway of cell death in KC [13]. Diseases related to defective apoptotic mechanisms are associated with polymorphisms in the FAS [100] and FASLG [101] genes [102, 103]. The FAS-encoded protein belongs to the TNF-receptor superfamily and contains a death domain. It has a central role in the physiological regulation of programmed cell death, whereby its interaction with its ligand leads to the formation of a death-inducing signaling complex that includes the FADD (Fas-associated death domain protein) and caspases 8 and 10 and is responsible for apoptosis [100]. The FAS isoforms that lack the transmembrane domain may negatively regulate the apoptosis mediated by the full-length isoform. The FASLG gene also belongs to the TNF superfamily. It encodes a transmembrane protein which, when it binds to FAS, triggers apoptosis [101].

The FAS/FASLG system is expressed in the cornea and potentially has an important function in the physiology of the normal cornea, as well as the pathophysiology of corneal diseases [103, 104] through the modulation of keratocyte apoptosis following epithelial injury. Indeed, IL-1 stimulation induces corneal fibroblasts to produce apoptosis-associated FAS ligand. The fact that the same cells also express the FAS receptors makes them ideal candidates for inducing autocrine suicide of keratocytes in KC corneas. In addition, the development of KC is also characterized by an increase in the corneal sensitivity to apoptotic cytokines owing to abnormalities in the components of the FAS pathways [88]. This association between chronic keratocyte apoptosis and ongoing epithelial injury could be the link to previously unrecognized risk factors for KC, such as chronic eye rubbing, contact lens wear or atopic eye disease [105]. Finally, it is noteworthy that apoptosis is a common mechanism between KC and Fuchs corneal endothelial dystrophy  [13, 106].

Other Genes Involved in KC Pathogenesis

KC has been associated with genes and proteins that regulate cellular and extracellular processes like wound healing, keratocyte proliferation, oxidative damage, differentiation, apoptosis and proteolysis [107, 108]. These genes are up-regulated in KC and myopic corneas [109]. In addition, KC patients present with abnormal gene expressions in at least some genes [44] identified using genome-wide scans, genome-wide association studies, studies of descent, and family-based linkage studies [110]. Additionally, more than 50 candidate genes have been excluded as contributors to KC development [111, 112]. Tables 5, 6 and 7 summarize the proteins that have been identified as targets of up- or down-regulation in KC as well as those genes that are known to code for them.
Table 5

Up-regulated or down-regulated proteins and their genes (if identified) in keratoconus

Up-regulated proteins

Gene [reference]

Down-regulated proteins

Gene [reference]

Collagen type XV

COL15 [28]


12, 26

Inflammatory mediators

47, 51, 52

Collagen type I, II, IV

COL1A1, COL1A2, COL2A1 [26]

Proinflammatory cytokines

47, 51, 52


COL4A1 [35]

Cell adhesion molecules

47, 51, 52

a1-PI (a1 proteinase inhibitor)

14, 28


47, 51, 52

a2 Macroglobulin

A2 M [14, 50]

Degradative enzymes


Inhibitors of cellular proteases


Acid phosphatases


Aldehyde dehydrogenase

ALDH [14]

Acid esterases


Superoxide dismutase

SOD [14]

Acid lipases



DCN [58, 84]

Cathepsin B

CTSB [14, 50]


KERA [58, 84]

Cathepsin G

CTSG [14, 50]

Lumican (LUM)

LUM [58, 84]

Cellular proteases



BGN [58, 84]

Nitrotyrosine malonaldehyde


KS (Keratan sulfate) antigenicity- central corneal

KERA [59]

Glutathione S transferase (GSTs)

GST [14]

Collagen type IV

COL4A1, COL4A2, COL4A3, COL4A4, COL4A5, COL4A6 [28]

iNOS (inducible nitric oxide synthase)

NOS1, NOS2, NOS3 [14]

IGF-2 transcripts

IGF2 [94]

GAG (glycosaminoglycan) polyanions


IGKC (immunoglobulin kappa chain)

IGKC [44]

(AZGP1) zinc-alpha-2-glycoprotein

AZGP1 [69]

Lactoferrin (LF)

LTF [44]

GCDFP15 (gross cystic disease fluid protein 15)-prolactin-inducible protein (PIP)

PIP [64, 69]


SP1 (specificity protein 1)

SP1 [73]


Amyloid deposits



BMP4 (bone morphogenic protein 4)

BMP4 [21, 86, 87]



TGFBI [21, 86, 87]


HSPGs (heparan sulfate proteoglycans)

HSPG2 [83, 85]



IL1α, IL1β [87, 89]


IGFBP (IGF binding proteins) 3 and 5



DUSPs (dual-specificity phosphatases)

(DUSP) [95]


MKPs (mitogen-activated protein kinase phosphatases)

(MAPK) [95, 96]



TNF [44, 47, 51, 52]


ANGPTL7 (angiopoietin-related protein 7)

ANGPTL7 [84]



TIMP3 [81, 142]



MMP2 [14]



MMP9 [55]

Table 6

Proteins (and their genes) associated with keratoconus but uncertain as to their role


Gene [reference]


HLA-A*26 [82]


HLA-B*40 [82]


HLA-DR*9 [82]


GSN [44, 97]


KRT [44, 97]

Table 7

Proteins (and their genes) found in keratoconus tears but uncertain as to their role


Gene [reference]


LCN [44]

Lysozyme C

LYZ [44]

Immunoglobulin alpha (IgA)

IgA [44]

Immunoglobulin kappa (IGKC)

IGKC [44]

Precursors to prolactin


KC is characterized by genetic heterogeneity [2, 4, 8, 112]. Many distinct genetic loci have been mapped for KC, but none has been confirmed as a KC-associated genetic factor [113, 114]. Some of those variants apparently are more penetrant or associated with a more severe outcome [112, 114]. These variants have been identified in diverse populations and localities. Thus, KC chromosomal loci 16q22.3–q23.1 [115] were identified in 20 small Finnish pedigrees [115]; locus 20q12 was identified in an isolated population in Tasmania, Australia [116]; loci 5q22.23–q24.2 were identified in a large Northern Irish family [117]; loci 5q14.3–q21.1 were identified from a large four-generation white family pedigree [118]; and loci 3p14–q13 were identified in an Italian two-generation autosomal dominant pedigree [119].

Other loci associated with KC have been mapped to chromosomes 20p11.21 (KTCN1) [120], 16q22.3–q23.1 (KTCN2) [115], 3p14–q13 (KTCN3) [119], 2p24 (KTCN4) [121], 1p36.23–36.21 [122], 4q31 [116, 123], 5q14.3–q21.1 [118], 5q21.2 [124], 5q31 [123], 5q32–q33 [124], 8q13.1–q21.11 [123], 8q24 and 9q34 [123], 12p12 [123], 13q32 [125], 14p11 [116, 123], 14q11.2 [124], 14q24.3 [123], 15q2.32 [124], 15q22.23–q24, 16q22-q23, 17q24, 20q12, 21q [115, 117, 126], and 17p13 [123, 127].

Generally, there are KC-associated single-nucleotide polymorphisms (SNPs), as in the cases of COL4A3 (collagen type IV, alpha 3) at 2q36–q37, COL4A4 (collagen type IV, alpha 4) at 2q35–q37, IL-1A and IL-1B (both members of the interleukin 1 cytokine family) [28, 112, 128, 129]. It should be noted that the protein encoded by IL-1A is a pleiotropic cytokine involved in various immune responses, and is proteolytically processed and released in response to cell injury, inducing apoptosis [130]. The protein encoded by IL-1B is an important mediator of the inflammatory response, and is involved in a variety of cellular activities, including cell proliferation, differentiation, and apoptosis [131].

Several other candidate genes have been proposed in KC, including SOD1 (superoxide dismutase 1 gene) (MIM 147450, locus 21q22.11), TGβI, DOCK9 (dedicator of cytokinesis 9 gene), which is located at 13q32, ZEB1, FLG [123, 132, 133, 134, 135, 136], LOX (lysyl oxidase) at 5q23.2, VSX1 (visual system homeobox-1 gene) (KTCN1, MIM605020, locus 20p11.2), TGF-β1, IL-1A (interleukin 1Α gene), IL-1B (interleukin 1B gene) at 2q14, IPO5, STK24 and HGF (hepatocyte growth factor) [89, 112, 120, 133, 134, 137, 138]. It is of interest that the protein encoded by FLG is an intermediate filament-associated protein that aggregates keratin intermediate filaments in the mammalian epidermis [139]. It is also of interest that the SOD1 gene, along with the CRB1 gene, have been implicated in Leber congenital amaurosis and Down syndrome, both of which are associated with KC [133, 140].

Genes VSX1, SOD1, IL-1B, COL4A3, COL4A4 and LOX are the most probable genetic substrates of KC [28, 112, 120, 133, 141], although the role of VSX1 in the pathogenesis of KC remains controversial [142]. The LOX gene encodes an enzyme that initiates the crosslinking of collagens and elastin by catalyzing oxidative deamination of the epsilon amino group in lysine residues of elastin and lysine and hydroxylisine residues of collagen [81]. Other genes that have been implicated in KC pathogenesis are the microRNA 184 (miR-184) gene which is positioned at 15q22–q25 [143], and the SPARC, MMP-2, MMP-9, COL6A1, COL8A1, and TIMP-3 [81, 112]. The TIMP-3 gene belongs to the tissue inhibitor of metalloproteinase gene family that are involved in ECM degradation [144]; mutations in these genes have been linked to Sorsby’s fundus dystrophy, an autosomal dominant disorder [144]. In addition, the MPDZ-NF1B (rs132183), FOXO1 (rs2721051), RXRA-COL5A1 (rs1536482), COL5A1 (rs7044529), FNDC3B (rs4894535) and BANP-ZNF469 [112, 145, 146] genes have been implicated in the pathogenesis of KC.

Finally, in KC patients from Poland, two SNPs in the RAD51 [147] gene have been genotyped [148]. The protein encoded by the RAD51 gene interacts with BRCA1 and BRCA2, which are important elements of the cellular response to DNA damage. BRCA2 regulates both the intracellular localization and the DNA-binding ability of this protein, and its inactivation is thought to lead to genomic instability and tumorigenesis [147].


The pathophysiology of keratoconus is multifactorial and still elusive. Differential expression of several corneal proteins in KC [14, 49] results in changes in the structural integrity of the cornea, its collagen content and its morphology [14, 49]. The biochemical abnormalities observed in corneal epithelium and stroma in KC include increased activity of degradative enzymes and reduced activity of protease inhibitors [50]. Corneal thinning, therefore, is probably caused by the up-regulation of cellular proteases and the down-regulation of their inhibitors [14]. The increase of proteinase activity [50] results in the induction of degradative process in the cornea [50]. Moreover, oxidative damage and keratocyte apoptosis seem to play an important role in the etiology of KC.

Exploring the proteomic changes in KC and analyzing its complex genetics will increase our understanding of its pathophysiology, and, most importantly, will potentially enable targeted genetic treatments in the future.




No funding or sponsorship was received for the publication of this article.


All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship for this article, take responsibility for the integrity of the work as a whole, and have given their approval for this version to be published.


Anastasios G. Konstas is a member of the journal’s Editorial Board. Eleftherios Loukovitis, Nikolaos Kozeis, Zisis Gatzioufas, Athina Kozei, Eleni Tsotridou, Maria Stoila, Spyros Koronis, Konstantinos Sfakianakis, Paris Tranos, Miltiadis Balidis, Zacharias Zachariadis, Dimitrios G. Mikropoulos, George Anogeianakis and Andreas Katsanos have nothing to disclose.

Compliance with Ethics Guidelines

This article is based on previously conducted studies and does not contain any studies with human participants or animals performed by any of the authors.

Data Availability

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.


  1. 1.
    Kymes S, Walline J, Zadnik K, Sterling J, Gordon M. Changes in the quality-of-life of people with keratoconus. Am J Ophthalmol. 2008;145(4):611–7.Google Scholar
  2. 2.
    Rabinowitz Y. Keratoconus. Surv Ophthalmol. 1998;42(4):297–319.Google Scholar
  3. 3.
    Li X, Yang H, Rabinowitz Y. Longitudinal study of keratoconus progression. Exp Eye Res. 2007;85(4):502–7.Google Scholar
  4. 4.
    Rabinowitz Y. The genetics of keratoconus. Ophthalmol Clin North Am. 2003;16(4):607–20.Google Scholar
  5. 5.
    Abu-Amero KK, Al-Muammar AM, Kondkar AA. Genetics of keratoconus: where do we stand? J Ophthalmol. 2014;2014:1–11.Google Scholar
  6. 6.
    Vazirani J, Basu S. Keratoconus: current perspectives. Clin Ophthalmol. 2013;7:2019–30.Google Scholar
  7. 7.
    Sugar J, Macsai MS. What causes keratoconus? Cornea. 2012;31(6):716–9.Google Scholar
  8. 8.
    Edwards M, Mcghee CN, Dean S. The genetics of keratoconus. Clin Exp Ophthalmol. 2001;29(6):345–51.Google Scholar
  9. 9.
    Ambekar R, Toussaint K Jr, Wagoner Johnson A. The effect of keratoconus on the structural, mechanical, and optical properties of the cornea. J Mech Behav Biomed Mater. 2011;4(3):223–36.Google Scholar
  10. 10.
    Patel S. Characterisation of keratoconus. Br J Ophthalmol. 2011;95(6):759–60.Google Scholar
  11. 11.
    Krachmer J, Feder R, Belin M. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol. 1984;28(4):293–322.Google Scholar
  12. 12.
    Kenney MC, Nesburn AB, Burgeson RE, Butkowski RJ, Ljubimov AV. Abnormalities of the extracellular matrix in keratoconus corneas. Cornea. 1997;16(3):345–51.Google Scholar
  13. 13.
    Kaldawy RM, Wagner J, Ching S, Seigel GM. Evidence of apoptotic cell death in keratoconus. Cornea. 2002;21(2):206–9.Google Scholar
  14. 14.
    Joseph R, Srivastava O, Pfister R. Differential epithelial and stromal protein profiles in keratoconus and normal human corneas. Exp Eye Res. 2011;92(4):282–98.Google Scholar
  15. 15.
    Mocan MC, Yilmaz PT, Irkec M, Orhan M. The significance of Vogt’s striae in keratoconus as evaluated by in vivo confocal microscopy. Clin Exp Ophthalmol. 2008;36(4):329–34.Google Scholar
  16. 16.
    Sykakis E, Carley F, Irion L, Denton J, Hillarby M. An in depth analysis of histopathological characteristics found in keratoconus. Pathology. 2012;44(3):234–9.Google Scholar
  17. 17.
    Rong SS, Ma STU, Yu XT, Ma L, Chu WK, Chan TCY, et al. Genetic associations for keratoconus: a systematic review and meta-analysis. Sci Rep. 2017;7(1):4620.Google Scholar
  18. 18.
    Szczotka-Flynn L, Slaughter M, McMahon T, Barr J, Edrington T, Fink B, et al. Disease severity and family history in keratoconus. Br J Ophthalmol. 2008;92(8):1108–11.Google Scholar
  19. 19.
    Wilson SE. Role of apoptosis in wound healing in the cornea. Cornea. 2000;19(Suppl 3):S7–12.Google Scholar
  20. 20.
    Monboisse JC, Borel JP. Oxidative damage to collagen. EXS. 1992;62:323–7.Google Scholar
  21. 21.
    Engler C, Chakravarti S, Doyle J, Eberhart CG, Meng H, Stark WJ, et al. Transforming growth factor-β signaling pathway activation in keratoconus. Am J Ophthalmol. 2011;151(5):752–9.Google Scholar
  22. 22.
    Karamichos D, Hutcheon AEK, Rich CB, Trinkaus-Randall V, Asara JM, Zieske JD. In vitro model suggests oxidative stress involved in keratoconus disease. Sci Rep. 2014;4(1):4608.Google Scholar
  23. 23.
    Andreassen TT, Simonsen AH, Oxlund H. Biomechanical properties of keratoconus and normal corneas. Exp Eye Res. 1980;31(4):435–41.Google Scholar
  24. 24.
    Saravani R, Hasanian-Langroudi F, Validad M-H, Yari D, Bahari G, Faramarzi M, et al. Evaluation of possible relationship between COL4A4 gene polymorphisms and risk of keratoconus. Cornea. 2015;34(3):318–22.Google Scholar
  25. 25.
    Abalain JH, Dossou H, Colin J, Floch HH. Levels of collagen degradation products (telopeptides) in the tear film of patients with keratoconus. Cornea. 2000;19(4):474–6.Google Scholar
  26. 26.
    Critchfield JW, Calandra AJ, Nesburn AB, Kenney MC. Keratoconus: I. Biochemical studies. Exp Eye Res. 1988;46(6):953–63.Google Scholar
  27. 27.
    Nie X-C, Wang J-P, Zhu W, Xu X-Y, Xing Y-N, Yu M, et al. COL4A3 expression correlates with pathogenesis, pathologic behaviors, and prognosis of gastric carcinomas. Hum Pathol. 2013;44(1):77–86.Google Scholar
  28. 28.
    Stabuc-Silih M, Ravnik-Glavac M, Glavac D, Hawlina M, Strazisar M. Polymorphisms in COL4A3 and COL4A4 genes associated with keratoconus. Mol Vis. 2009;15:2848–60.Google Scholar
  29. 29.
    Maurice DM. The structure and transparency of the cornea. J of Physiol. 1957;136(2):263–86.Google Scholar
  30. 30.
    Meek KM, Tuft SJ, Huang Y, Gill PS, Hayes S, Newton RH, et al. Changes in collagen orientation and distribution in keratoconus corneas. Invest Opthal Vis Sci. 2005;46(6):1948–56.Google Scholar
  31. 31.
    Fischer G, Schmidt C, Opitz J, Cully Z, Kühn K, Pöschl E. Identification of a novel sequence element in the common promoter region of human collagen type IV genes, involved in the regulation of divergent transcription. Biochem J. 1993;292(3):687–95.Google Scholar
  32. 32.
    Khoshnoodi J, Pedchenko V, Hudson BG. Mammalian collagen IV. Microsc Res Tech. 2008;71(5):357–70.Google Scholar
  33. 33.
    Database G. COL4A6 Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  34. 34.
    Tuori AJ, Virtanen I, Aine E, Kalluri R, Miner J, Uusitalo HM. The immunohistochemical composition of corneal basement membrane in keratoconus. Curr Eye Res. 1997;16(8):792–801.Google Scholar
  35. 35.
    Stachs O, Bochert A, Gerber T, Koczan D, Thiessen HJ, Guthoff RF. The extracellular matrix structure in keratoconus. Ophthalmologe. 2004;101(4):384–9.Google Scholar
  36. 36.
    Kokolakis NS, Gazouli M, Chatziralli IP, Koutsandrea C, Gatzioufas Z, Peponis VG. Polymorphism analysis of COL4A3 and COL4A4 genes in Greek patients with keratoconus. Ophthalmic Genet. 2014;35(4):226–8.Google Scholar
  37. 37.
    Vitart V, Benčić G, Hayward C, Herman JŠ, Huffman J, Campbell S, et al. New loci associated with central cornea thickness include COL5A1, AKAP13 and AVGR8. Hum Mol Genet. 2010;19(21):4304–11.Google Scholar
  38. 38.
    Vithana EN, Aung T, Khor CC, Cornes BK, Tay W-T, Sim X, et al. Collagen-related genes influence the glaucoma risk factor, central corneal thickness. Hum Mol Genet. 2010;20(4):649–58.Google Scholar
  39. 39.
    Stabuc-Silih M, Strazisar M, Ravnik-Glavac M, Hawlina M, Glavac D. Genetics and clinical characteristics of keratoconus. Acta Dermatovenerol Alp Pannonica Adriat. 2010;19(2):3–10.Google Scholar
  40. 40.
    Määttä M, Väisänen T, Väisänen M-R, Pihlajaniemi T, Tervo T. Altered expression of type XIII collagen in keratoconus and scarred human cornea. Cornea. 2006;25(4):448–53.Google Scholar
  41. 41.
    Määttä M, Heljasvaara R, Sormunen R, Pihlajaniemi T, Autio-Harmainen H, Tervo T. Differential expression of collagen Types XVIII/endostatin and XV in normal, keratoconus, and scarred human corneas. Cornea. 2006;25(3):341–9.Google Scholar
  42. 42.
    Naderan M, Farjadnia M. Corneal cross-linking treatment of keratoconus. Oman J Ophthalmol. 2015;8(2):86.Google Scholar
  43. 43.
    Spoerl E, Mrochen M, Sliney D, Trokel S, Seiler T. Safety of UVA-riboflavin cross-linking of the cornea. Cornea. 2007;26(4):385–9.Google Scholar
  44. 44.
    Ghosh A, Ghosh A, Shetty R, Zhou L, Beuerman R. Proteomic and gene expression patterns of keratoconus. Indian J Ophthalmol. 2013;61(8):389–91.Google Scholar
  45. 45.
    Zhou L, Zhao SZ, Koh SK, Chen L, Vaz C, Tanavde V, et al. In-depth analysis of the human tear proteome. J Proteom. 2012;75(13):3877–85.Google Scholar
  46. 46.
    de Souza GA, Godoy LM, Mann M. Identification of 491 proteins in the tear fluid proteome reveals a large number of proteases and protease inhibitors. Genome Biol. 2006;7(8):R72.Google Scholar
  47. 47.
    Lema I, Duran J. Inflammatory molecules in the tears of patients with keratoconus. Ophthalmology. 2005;112(4):654–9.Google Scholar
  48. 48.
    Lema I, Durán JA, Ruiz C, Díez-Feijoo E, Acera A, Merayo J. Inflammatory response to contact lenses in patients with keratoconus compared with myopic subjects. Cornea. 2008;27(7):758–63.Google Scholar
  49. 49.
    Srivastava OP, Chandrasekaran D, Pfister RR. Molecular changes in selected epithelial proteins in human keratoconus corneas compared to normal corneas. Mol Vis. 2006;12:1615–25.Google Scholar
  50. 50.
    Zhou L, Sawaguchi S, Twining SS, Sugar J, Feder RS, Yue BY. Expression of degradative enzymes and protease inhibitors in corneas with keratoconus. Invest Ophthalmol Vis Sci. 1998;39(7):1117–24.Google Scholar
  51. 51.
    Lema I, Sobrino T, Duran JA, Brea D, Diez-Feijoo E. Subclinical keratoconus and inflammatory molecules from tears. Br J Ophthalmol. 2009;93(6):820–4.Google Scholar
  52. 52.
    Jun AS, Cope L, Speck C, Feng X, Lee S, Meng H, et al. Subnormal cytokine profile in the tear fluid of keratoconus patients. PLoS ONE. 2011;6(1).Google Scholar
  53. 53.
    Galvis V, Sherwin T, Tello A, Merayo J, Barrera R, Acera A. Keratoconus: an inflammatory disorder? Eye. 2015;29(7):843–59.Google Scholar
  54. 54.
    Duran JA, Lema I. Inflammatory markers in keratoconus. Invest Ophthalmol Vis Sci. 2003;44:1314.Google Scholar
  55. 55.
    Shetty R, Sathyanarayanamoorthy A, Ramachandra RA, Arora V, Ghosh A, Srivatsa SR. Attenuation of lysyl oxidase and collagen gene expression in keratoconus patient corneal epithelium corresponds to disease severity. Mol Vis. 2015;21:12–25.Google Scholar
  56. 56.
    Whitelock RB, Fukuchi T, Zhou L, Twining SS, Sugar J, Feder RS, et al. Cathepsin G, acid phosphatase, and α1-proteinase inhibitor messenger RNA levels in keratoconus corneas. Invest Ophthalmol Vis Sci. 1997;38(2):529–34.Google Scholar
  57. 57.
    Yue BY, Sugar J, Schrode K. Histochemical studies of keratoconus. Curr Eye Res. 1988;7(1):81–6.Google Scholar
  58. 58.
    García B, García-Suárez O, Merayo-Lloves J, Alcalde I, Alfonso JF, Cueto LFV, et al. Differential expression of proteoglycans by corneal stromal cells in keratoconus. Invest Opthalmol Vis Sci. 2016;57(6):2618–28.Google Scholar
  59. 59.
    Funderburgh JL, Funderburgh ML, Rodrigues MM, Krachmer JH, Conrad GW. Altered antigenicity of keratan sulfate proteoglycan in selected corneal diseases. Invest Ophthalmol Vis Sci. 1990;31(3):419–28.Google Scholar
  60. 60.
    Akhtar S, Bron AJ, Hayes AJ, Meek KM, Caterson B. Role of keratan sulphate (sulphated poly-N-acetyllactosamine repeats) in keratoconic cornea, histochemical, and ultrastructural analysis. Graefes Arch Clin Exp Ophthalmol. 2010;249(3):413–20.Google Scholar
  61. 61.
    Ameye L, Young MF. Mice deficient in small leucine-rich proteoglycans: novel in vivo models for osteoporosis, osteoarthritis, Ehlers-Danlos syndrome, muscular dystrophy, and corneal diseases. Glycobiology. 2002;12(9):107–16.Google Scholar
  62. 62.
    Sarathchandra P, Cassella JP, Ali SY. Ultrastructural localization of proteoglycans in bone in osteogenesis imperfecta as demonstrated by Cuprolinic Blue staining. J Bone Miner Metabol. 2002;20(5):288–93.Google Scholar
  63. 63.
    Koo B-S, Lee D-Y, Ha H-S, Kim J-C, Kim C-W. comparative analysis of the tear protein expression in blepharitis patients using two-dimensional electrophoresis. J Proteom Res. 2005;4(3):719–24.Google Scholar
  64. 64.
    Gallo A, Martini D, Sernissi F, Giacomelli C, Pepe P, Rossi C, et al. Gross cystic disease fluid protein-15(GCDFP-15)/prolactin-inducible protein (PIP) as functional salivary biomarker for primary Sjögren’s syndrome. J Genet Syndr Gene Ther. 2013;04(04).Google Scholar
  65. 65.
    Priyadarsini S, Hjortdal J, Sarker-Nag A, Sejersen H, Asara JM, Karamichos D. Gross cystic disease fluid protein-15/prolactin-inducible protein as a biomarker for keratoconus disease. PLoS ONE. 2014;9(11).Google Scholar
  66. 66.
    Karamichos D, Zareian R, Guo X, Hutcheon A, Ruberti J, Zieske J. Novel in vitro model for keratoconus disease. J Funct Biomater. 2012;3(4):760–75.Google Scholar
  67. 67.
    Karamichos D, Hutcheon A, Zieske J. Reversal of fibrosis by TGF-β3 in a 3D in vitro model. Exp Eye Res. 2014;124:31–6.Google Scholar
  68. 68.
    Kenney MC, Chwa M, Atilano SR, Tran A, Carballo M, Saghizadeh M, et al. Increased Levels of catalase and cathepsin V/L2 but decreased TIMP-1 in keratoconus corneas: evidence that oxidative stress plays a role in this disorder. Invest Opthalmol Vis Sci. 2005;46(3):823–32.Google Scholar
  69. 69.
    Database G. PIP Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  70. 70.
    Fan XC. Overexpression of HuR, a nuclear-cytoplasmic shuttling protein, increases the invivo stability of ARE-containing mRNAs. EMBO J. 1998;17(12):3448–60.Google Scholar
  71. 71.
    Joseph R, Srivastava OP, Pfister RR. Downregulation of β-Actin Gene and Human Antigen R in Human Keratoconus. Invest Opthalmol Vis Sci. 2012;53(7):4032–41.Google Scholar
  72. 72.
    Mcpherson SD, Kiffney GT, Freed CC. Corneal amyloidosis. Am J Ophthalmol. 1966;62(6):1025–33.Google Scholar
  73. 73.
    Stern GA, Knapp A, Hood CI. Corneal amyloidosis associated with keratoconus. Ophthalmology. 1988;95(1):52–5.Google Scholar
  74. 74.
    Whitelock R, Li Y, Zhou L, Sugar J, Yue BY. Expression of transcription factors in keratoconus, a cornea-thinning disease. Biochem Biophys Res Commun. 1997;235(1):253–8.Google Scholar
  75. 75.
    Cook T, Gebelein B, Urrutia R. Sp1 and its likes: biochemical and functional predictions for a growing family of zinc finger transcription factors. Ann NY Acad Sci. 1999;880:94–102.Google Scholar
  76. 76.
    Suske G, Bruford E, Philipsen S. Mammalian SP/KLF transcription factors: bring in the family. Genomics. 2005;85(5):551–6.Google Scholar
  77. 77.
    Qiu Z, Norflus F, Singh B, Swindell MK, Buzescu R, Bejarano M, et al. Sp1 Is up-regulated in cellular and transgenic models of huntington disease, and its reduction is neuroprotective. J Biol Chem. 2006;281(24):16672–80.Google Scholar
  78. 78.
    Safe S, Abdelrahim M. Sp transcription factor family and its role in cancer. Eur J Cancer. 2005;41(16):2438–48.Google Scholar
  79. 79.
    Li Y, Zhou L, Twining SS, Sugar J, Yue BYJT. Involvement of Sp1 elements in the promoter activity of the α1-proteinase inhibitor gene. J Biol Chem. 1998;273(16):9959–65.Google Scholar
  80. 80.
    Maruyama Y, Wang X, Li Y, Sugar J. Yue BYJT Involvement of Sp1 elements in the promoter activity of genes affected in keratoconus. Invest Ophthalmol Vis Sci. 2001;42(9):1980–5.Google Scholar
  81. 81.
    De Bonis P, Laborante A, Pizzicoli C, Stallone R, Barbano R, Longo C. Mutational screening of VSX1, SPARC, SOD1, LOX, and TIMP3 in keratoconus. Mol Vis. 2011;17:2482–94.Google Scholar
  82. 82.
    Adachi W, Mitsuishi Y, Terai K, Nakayama C, Hyakutake Y, Yokoyama J, et al. The association of HLA with young-onset keratoconus in Japan. Am J Ophthalmol. 2002;133(4):557–9.Google Scholar
  83. 83.
    Pathak D, Nayak B, Singh M, Sharma N, Tandon R, Sinha R, et al. Mitochondrial complex 1 gene analysis in keratoconus. Mol Vis. 2011;17:1514–25.Google Scholar
  84. 84.
    Chaerkady R, Shao H, Scott S-G, Pandey A, Jun AS, Chakravarti S. The keratoconus corneal proteome: loss of epithelial integrity and stromal degeneration. J Proteom. 2013;87:122–31.Google Scholar
  85. 85.
    Esko JD, Lindahl U. Molecular diversity of heparan sulfate. J Clin Invest. 2001;108(2):169–73.Google Scholar
  86. 86.
    Whitelock JM, Iozzo RV. heparan sulfate: a complex polymer charged with biological activity. ChemInform. 2005;36(42):2745–64.Google Scholar
  87. 87.
    Zhou L, Yue BYJT, Twining SS, Sugar J, Feder RS. Expression of wound healing and stress-related proteins in keratoconus corneas. Curr Eye Res. 1996;15(11):1124–31.Google Scholar
  88. 88.
    Kim W-J, Rabinowitz YS, Meisler DM, Wilson SE. Keratocyte apoptosis associated with keratoconus. Exp Eye Res. 1999;69(5):475–81.Google Scholar
  89. 89.
    Wang Y, Wei W, Zhang C, Zhang X, Liu M, Zhu X, et al. Association of interleukin-1 gene single nucleotide polymorphisms with keratoconus in chinese han population. Curr Eye Res. 2016;41:630–5.Google Scholar
  90. 90.
    Fabre EJ, Bureau J, Pouliquen Y, Lorans G. Binding sites for human interleukin 1 α, gamma interferon and tumor necrosis factor on cultured fibroblasts of normal cornea and keratoconus. Curr Eye Res. 1991;10(7):585–92.Google Scholar
  91. 91.
    Nowak DM, Karolak JA, Kubiak J, Gut M, Pitarque JA, Molinari A, et al. Substitution at IL1RN and deletion at SLC4A11 segregating with phenotype in familial keratoconus. Invest Opthalmol Vis Sci. 2013;54(3):2207–15.Google Scholar
  92. 92.
    Liu J, Seet L-F, Koh LW, Venkatraman A, Venkataraman D, Mohan RR, et al. Depletion of SLC4A11 causes cell death by apoptosis in an immortalized human corneal endothelial cell line. Invest Opthalmol Vis Sci. 2012;53(7):3270–9.Google Scholar
  93. 93.
    Garfias Y, Navas A, Perez-Cano HJ, Quevedo J, Villalvazo L, Zenteno JC. Comparative expression analysis of aquaporin-5 (AQP5) in keratoconic and healthy corneas. Mol Vis. 2008;14:756–61.Google Scholar
  94. 94.
    Cheung IM, Mcghee CN, Sherwin T. Deficient repair regulatory response to injury in keratoconic stromal cells. Clin Exp Optom. 2013;97(3):234–9.Google Scholar
  95. 95.
    Schweikl H, Hiller K-A, Eckhardt A, Bolay C, Spagnuolo G, Stempfl T, et al. Differential gene expression involved in oxidative stress response caused by triethylene glycol dimethacrylate. Biomaterials. 2008;29(10):1377–87.Google Scholar
  96. 96.
    Arthur JSC, Ley SC. Mitogen-activated protein kinases in innate immunity. Nat Rev Immunol. 2013;13(9):679–92.Google Scholar
  97. 97.
    Brouillard F, Fritsch J, Edelman A, Ollero M. Contribution of proteomics to the study of the role of cytokeratins in disease and physiopathology. Proteom Clin Appl. 2008;2(2):264–85.Google Scholar
  98. 98.
    Andrade FEC, Covre JL, Ramos L, et al. Evaluation of galectin-1 and galectin-3 as prospective biomarkers in keratoconus. Br J Ophthalmol. 2018;102(5):700–7.Google Scholar
  99. 99.
    Leffler H, Carlsson S, Hedlund M, Qian Y, Poirier F. Introduction to galectins. Glycoconj J. 2002;19(7–9):433–40.Google Scholar
  100. 100.
    Database G. FAS Gene (Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  101. 101.
    Database G. FASLG Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  102. 102.
    Vij N, Roberts L, Joyce S, Chakravarti S. Lumican suppresses cell proliferation and aids Fas-Fas ligand mediated apoptosis: implications in the cornea. Exp Eye Res. 2004;78(5):957–71.Google Scholar
  103. 103.
    Hasby EA, Saad HA. Immunohistochemical expression of fas ligand (FasL) and neprilysin (neutral endopeptidase/CD10) in keratoconus. Int Ophthalmol. 2012;33(2):125–31.Google Scholar
  104. 104.
    Mohan RR, Liang Q, Kim W-J, Helena MC, Baerveldt F, Wilson SE. Apoptosis in the cornea: further characterization of fas/fas ligand system. Exp Eye Res. 1997;65(4):575–89.Google Scholar
  105. 105.
    Buddi R, Lin B, Atilano SR, Zorapapel NC. Kenney MC Brown DJ Evidence of oxidative stress in human corneal diseases. J Histochem Cytochem. 2002;50(3):341–51.Google Scholar
  106. 106.
    Borderie VM, Baudrimont M, Vallée A, Ereau TL, Gray F. Laroche L Corneal endothelial cell apoptosis in patients with Fuchs’ dystrophy. Invest Ophthalmol Vis Sci. 2000;41(9):2501–5.Google Scholar
  107. 107.
    Romero-Jiménez M, Santodomingo-Rubido J, Wolffsohn JS. Keratoconus: a review. Contact Lens Anterior Eye. 2010;33(4):157–66.Google Scholar
  108. 108.
    Sherwin T, Brookes NH. Morphological changes in keratoconus: pathology or pathogenesis. Clin Exp Ophthalmol. 2004;32(2):211–7.Google Scholar
  109. 109.
    Nielsen K, Birkenkamp-Demtro¨der K, Ehlers N, Orntoft TF. Identification of differentially expressed genes in keratoconus epithelium analyzed on microarrays. Invest Opthalmol Vis Sci. 2003;44(6):2466–76.Google Scholar
  110. 110.
    Nielsen K, Hjortdal J, Pihlmann M, Corydon TJ. Update on the keratoconus genetics. Acta Ophthalmol. 2012;91(2):106–13.Google Scholar
  111. 111.
    Dash DP, Silvestri G, Hughes AE. Fine mapping of the keratoconus with cataract locus on chromosome 15q and candidate gene analysis. Mol Vis. 2006;12:499–505.Google Scholar
  112. 112.
    Loukovitis E, Sfakianakis K, Syrmakesi P, Tsotridou E, Orfanidou M, Bakaloudi DR, et al. Genetic aspects of keratoconus: a literature review exploring potential genetic contributions and possible genetic relationships with comorbidities. Ophthalmol Ther. 2018;7(2):263–92.Google Scholar
  113. 113.
    Ihalainen A. Clinical and epidemiological features of keratoconus genetic and external factors in the pathogenesis of the disease. Acta Ophthalmol. 1986;178:1–64.Google Scholar
  114. 114.
    Davitson AE, Hayes S, Hardcastle AJ, Tuft SJ. The pathogenesis of keratoconus. Eye (Lond). 2014;28(2):189–95.Google Scholar
  115. 115.
    Tyynismaa H, Sistonen P, Tuupanen S, Tervo T, Dammert A, Latvala T. A locus for autosomal dominant keratoconus: linkage to 16q22.3-q23.1 in Finnish families. Invest Ophthalmol Vis Sci. 2002;43(10):3160–4.Google Scholar
  116. 116.
    Fullerton J, Paprocki P, Foote S, Mackey DA, Williamson R, Forrest S. Identity-by-descent approach to gene localisation in eight individuals affected by keratoconus from north-west Tasmania, Australia. Hum Genet. 2002;110(5):462–70.Google Scholar
  117. 117.
    Hughes AE, Dash DP, Jackson AJ, Frazer DG, Silvestri G. Familial keratoconus with cataract: linkage to the long arm of chromosome 15 and exclusion of candidate genes. Invest Opthalmol Vis Sci. 2003;44(12):5063–6.Google Scholar
  118. 118.
    Tang YG, Rabinowitz YS, Taylor KD, Li X, Hu M, Picornell Y, et al. Genomewide linkage scan in a multigeneration Caucasian pedigree identifies a novel locus for keratoconus on chromosome 5q14.3-q21.1. Genet Med. 2005;7(6):397–405.Google Scholar
  119. 119.
    Brancati F. A locus for autosomal dominant keratoconus maps to human chromosome 3p14-q13. J Med Genet. 2004;41(3):188–92.Google Scholar
  120. 120.
    Heon E. VSX1: a gene for posterior polymorphous dystrophy and keratoconus. Hum Mol Genet. 2002;11(9):1029–36.Google Scholar
  121. 121.
    Hutchings H. Identification of a new locus for isolated familial keratoconus at 2p24. J Med Genet. 2005;42(1):88–94.Google Scholar
  122. 122.
    Burdon KP, Coster DJ, Charlesworth JC, Mills RA, Laurie KJ, Giunta C, et al. Apparent autosomal dominant keratoconus in a large Australian pedigree accounted for by digenic inheritance of two novel loci. Hum Genet. 2008;124(4):379–86.Google Scholar
  123. 123.
    Li X, Rabinowitz YS, Tang YG, Picornell Y, Taylor KD, Hu M, et al. Two-stage genome-wide linkage scan in keratoconus sib pair families. Invest Opthalmol Vis Sci. 2006;47(9):3791–5.Google Scholar
  124. 124.
    Bisceglia L, Bonis PD, Pizzicoli C, Fischetti L, Laborante A, Perna MD, et al. Linkage analysis in keratoconus: replication of locus 5q21.2 and identification of other suggestive loci. Invest Opthalmol Vis Sci. 2009;50(3):1081–6.Google Scholar
  125. 125.
    Gajecka M, Radhakrishna U, Winters D, Nath SK, Rydzanicz M, Ratnamala U, et al. Localization of a gene for keratoconus to a 5.6-Mb interval on 13q32. Invest Opthalmol Vis Sci. 2009;50(4):1531–9.Google Scholar
  126. 126.
    Rabinowitz YS, Maumenee IH, Lundergan MK, Puffenberger E, Zhu D, Antonarakis S, et al. Molecular genetic analysis in autosomal dominant keratoconus. Cornea. 1992;11(4):302–8.Google Scholar
  127. 127.
    Rabinowitz YS, Zu H, Yang Y, Wang Y, Rotter J, Pulst S. Keratoconus: non-parametric linkage analysis suggests a gene locus near to the centromere on chromosome 21. Invest Ophthalmol Vis Sci. 1999;40(4):S564.Google Scholar
  128. 128.
    Kim SH, Mok JW, Kim HS, Joo CK. Association of 31T>C and 511 C>T polymorphisms in the interleukin 1 beta (IL1B) promoter in Korean keratoconus patients. Mol Vis. 2008;14:2109–16.Google Scholar
  129. 129.
    Burdon KP, Vincent AL. Insights into keratoconus from a genetic perspective. Clin Exp Optom. 2013;96(2):146–54.Google Scholar
  130. 130.
    Database G. IL1A Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  131. 131.
    Database G. IL1B Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  132. 132.
    Czugala M, Karolak JA, Nowak DM, Polakowski P, Pitarque J, Molinari A, et al. Novel mutation and three other sequence variants segregating with phenotype at keratoconus 13q32 susceptibility locus. Eur J Hum Genet. 2011Feb;20(4):389–97.Google Scholar
  133. 133.
    Udar N, Atilano SR, Brown DJ, Holguin B, Small K, Nesburn AB, et al. SOD1: a candidate gene for keratoconus. Invest Ophthalmol Vis Sci. 2006;47(8):3345–51.Google Scholar
  134. 134.
    Guan T, Liu C, Ma Z, Ding S. The point mutation and polymorphism in keratoconus candidate gene TGFBI in Chinese population. Gene. 2012;503(1):137–9.Google Scholar
  135. 135.
    Droitcourt C, Touboul D, Ged C, Ezzedine K, Cario-Andre M, de Verneuil H. A prospective study of filaggrin null mutations in keratoconus patients with or without atopic disorders. Dermatology. 2011;222(4):336–41.Google Scholar
  136. 136.
    Muszynska DLJ, Dash D, Heon E, Hughes A, Willoughby C. Identification and characterization of a novel missense homeodomain mutation in ZEB1 resulting in keratoconus. Invest Ophthalmol Vis Sci. 2011;52:1077.Google Scholar
  137. 137.
    Burdon KP, Macgregor S, Bykhovskaya Y, Javadiyan S, Li X, Laurie KJ, et al. Association of polymorphisms in the hepatocyte growth factor gene promoter with keratoconus. Invest Ophthalmol Vis Sci. 2011;52(11):8514–9.Google Scholar
  138. 138.
    Hasanian-Langroudi F, Saravani R, Validad M-H, Bahari G, Yari D. Association of lysyl oxidase (LOX) polymorphisms with the risk of keratoconus in an iranian population. Ophthalmic Genet. 2014;36(4):309–14.Google Scholar
  139. 139.
    Database G. FLG Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  140. 140.
    Mcmahon TT, Kim LS, Fishman GA, Stone EM, Zhao XC, Yee RW, et al. Crb1gene mutations are associated with keratoconus in patients with leber congenital amaurosis. Invest Ophthalmol Vis Sci. 2009;50(7):3185–7.Google Scholar
  141. 141.
    Bykhovskaya Y, Li X, Epifantseva I, Haritunians T, Siscovick D, Aldave A, et al. Variation in the lysyl oxidase (LOX) gene is associated with keratoconus in family-based and case-control studies. Invest Ophthalmol Vis Sci. 2012;53(7):4152–7.Google Scholar
  142. 142.
    Dash DP, George S, Oprey D, Burns D, Nabili S, Donnelly U, et al. Mutational screening of VSX1 in keratoconus patients from the European population. Eye. 2009;24(6):1085–92.Google Scholar
  143. 143.
    Hughes AE, Bradley DT, Campbell M, Lechner J, Dash DP, Simpson DA, et al. Mutation altering the miR-184 seed region causes familial keratoconus with cataract. Am J Hum Genet. 2011;89(5):628–33.Google Scholar
  144. 144.
    Database G. TIMP3 Gene (Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  145. 145.
    Gajecka M, Nowak D. The genetics of keratoconus. Middle East Afr J Ophthalmol. 2011;18(1):2–6.Google Scholar
  146. 146.
    Lu Y, Vitart V, Burdon KP, Khor CC, Bykhovskaya Y, Mirshahi A. Genome-wide association analyses identify multiple loci associated with central corneal thickness and keratoconus. Nat Genet. 2013;45(2):155–63.Google Scholar
  147. 147.
    Database G. RAD51 Gene(Protein Coding) [Internet]. GeneCards is a searchable, integrative database that provides comprehensive, user-friendly information on all annotated and predicted human genes. Available from: Accessed 16 July 2019.
  148. 148.
    Synowiec E, Wojcik KA, Izdebska J, Binczyk E, Blasiak J, Szaflik J, et al. Polymorphisms of the homologous recombination geneRAD51in keratoconus and fuchs endothelial corneal dystrophy. Dis Markers. 2013;35:353–62.Google Scholar

Copyright information

© The Author(s) 2019

Open AccessThis article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (, which permits any noncommercial use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors and Affiliations

  • Eleftherios Loukovitis
    • 1
    • 2
  • Nikolaos Kozeis
    • 2
  • Zisis Gatzioufas
    • 3
  • Athina Kozei
    • 2
    • 4
  • Eleni Tsotridou
    • 2
    • 5
  • Maria Stoila
    • 2
    • 5
  • Spyros Koronis
    • 2
  • Konstantinos Sfakianakis
    • 6
  • Paris Tranos
    • 2
  • Miltiadis Balidis
    • 2
  • Zacharias Zachariadis
    • 2
  • Dimitrios G. Mikropoulos
    • 7
  • George Anogeianakis
    • 2
    • 8
  • Andreas Katsanos
    • 9
  • Anastasios G. Konstas
    • 7
    Email author
  1. 1.Hellenic Army Medical CorpsThessalonikiGreece
  2. 2.Ophthalmica Eye InstituteThessalonikiGreece
  3. 3.Augenklinik, Hornhaut, Katarakt und Refractive ChirurgieUniversitaetsspital BaselBaselSwitzerland
  4. 4.School of PharmacologyUniversity of NicosiaNicosiaCyprus
  5. 5.Faculty of MedicineAristotle University of ThessalonikiThessalonikiGreece
  6. 6.Division of Surgical Anatomy, Laboratory of Anatomy, Medical SchoolDemocritus University of Thrace, University CampusAlexandroupolisGreece
  7. 7.1st and 3rd University Departments of OphthalmologyAristotle UniversityThessalonikiGreece
  8. 8.Association for Training in Biomedical TechnologyThessalonikiGreece
  9. 9.Department of OphthalmologyUniversity of IoanninaIoanninaGreece

Personalised recommendations