Keratoconus 1

A number sign (#) is used with this entry because of evidence that keratoconus-1 (KTCN1) is caused by heterozygous mutation in the VSX1 gene (605020) on chromosome 20p11.

Description

Keratoconus, the most common corneal dystrophy, is a bilateral, noninflammatory progressive corneal ectasia. Clinically, the cornea becomes progressively thin and conical, resulting in myopia, irregular astigmatism, and corneal scarring. The disease usually arises in the teenage years, eventually stabilizing in the third and fourth decades. The incidence of keratoconus is 1 in 2,000 in the general population; it occurs with no ethnic or gender preponderance, and causes significant visual impairment in young adults. No specific treatment exists except to replace the corneal tissue by surgery (corneal transplantation) when visual acuity can no longer be corrected by contact lenses (summary by Dash et al., 2006).

Ihalainen (1986) reviewed various conditions with which keratoconus is at times associated. Keratoconus is frequent in cases of amaurosis congenita of Leber (204000).

Genetic Heterogeneity of Keratoconus

Also see KTCN2 (608932), mapped to 16q22.3-q23.1; KTCN3 (608586), mapped to 3p14-q13; KTCN4 (609271), mapped to 2p24; KTCN5 (614622), mapped to 5q14.1-q21.3; KTCN6 (614623), mapped to 9q34; KTCN7 (614629), mapped to 13q32; KTCN8 (614628), mapped to 14q24; and KTCN9 (617928), caused by mutation in the TUBA3D gene (617878) on 2q21.

Clinical Features

In a study of large number of patients with keratoconus in Finland, Ihalainen (1986) found that symptoms usually began in young adults. Pregnancy seemed to precipitate keratoconus in some instances.

Nielsen et al. (2003) used gene microarrays to investigate differential gene expression in corneal epithelium from samples with and without keratoconus. Keratoconus epithelium appeared to be characterized by massive changes of the cytoskeleton, reduced extracellular matrix remodeling, altered transmembrane signaling, and modified cell-to-cell and cell-to-matrix interactions. Validation of gene expression with dChip analysis and real-time PCR indicated Gene Chip to be a valid technique for investigation of epithelium from single dissected corneal samples.

Dogru et al. (2003) reviewed the ocular surface disease in keratoconus. Keratoconus patients showed disorders of tear quality, lowered tear film breakup time (BUT), squamous metaplasia of the corneal epithelium, and goblet cell loss, all of which seemed to relate to the extent of keratoconus progression.

Li et al. (2004) examined 778 patients with keratoconus and found that 116 (14.9%) had clinically unilateral keratoconus at baseline. These 116 patients were followed for a period ranging from 6 months to 8 years. Approximately 50% of clinically normal fellow eyes progressed to keratoconus within 16 years. The greatest risk was during the first 6 years. Li et al. (2004) also described quantitative indices and qualitative patterns that might predict this progression.

Individuals with keratoconus are not candidates for LASIK (laser-assisted in situ keratomileusis) for correction of their myopia and/or astigmatism. Jabbur et al. (2001) described the clinical course and histopathology of an individual with suspected keratoconus who underwent bilateral simultaneous LASIK. She required penetrating keratoplasty due to progressively worsening vision from corneal ectasia after LASIK.

Classically, corneal allograft rejection was thought to be a Th1-mediated phenomenon. However, Th2-mediated allograft rejection has been reported in heart and kidney transplanted systems. Hargrave et al. (2003) reviewed the records of 84 consecutive patients who underwent penetrating keratoplasty for keratoconus. Because an association between keratoconus and atopic disease had been documented in the literature and had been considered significant since 1937, careful attention was paid to the clinical history of atopy (147050) in this study. Atopic patients have been shown to have a 'Th2 immune bias.' Of the 7 patients who rejected their corneal allografts, 4 had repeat penetrating keratoplasty. Of these 4 repeat corneal allografts, 3 showed eosinophilia when compared with rejected grafts in control (nonkeratoconic, nonatopic) patients. Atopic keratoconus patients had a mixed inflammatory cellular infiltrate in the rejected corneal tissue specimen with a significantly greater density of eosinophils compared with patients who did not have a preexisting Th2 bias. The histopathology was comparable to the authors' murine model of rejection in Th2 mice, characterized by a predominantly eosinophilic infiltrate when compared with wildtype (Th1) mice that had a predominantly mononuclear infiltrate in the rejected corneal graft bed.

Fuentes et al. (2015) noted that acute corneal hydrops, a condition characterized by marked corneal edema after a break in the Descemet membrane, typically affects young individuals with progressive disease and occurs in approximately 3% of patients with keratoconus. In data collected from 191 patients with advanced keratoconus during a minimum 24-month follow-up, the authors found that increased epithelial thickening, stromal thinning at the keratoconus cone, anterior hyperreflexives at the Bowman layer level, and the absence of stromal scarring were associated with a high risk of corneal hydrops. All 11 cases of corneal hydrops (5.8%) in their series occurred in young males.

Inheritance

Hamilton (1938) claimed that certain of his pedigrees in Tasmania strongly supported autosomal recessive inheritance.

Irregular autosomal dominant inheritance was suggested by Falls and Allen (1969), who observed affected aunt and niece. The mother, who presumably transmitted the trait, had astigmatism and other features the authors interpreted as forme fruste of keratoconus. They cited several instances of multigeneration involvement including the family of Staehli (1925) with transmission through 3 generations.

From study of a large series, Hallermann and Wilson (1977) favored multifactorial inheritance but could not exclude isolated instances of dominant or recessive inheritance.

Ihalainen (1986) found multiple cases in 19 of 101 families studied in the north of Finland and in 5 of 58 families in the south. Mean family size was 4.9 in the north as compared with 3.5 in the south. In 24 of 28 multiplex families the pattern of inheritance was autosomal dominant. The disorder was inherited from the mother in 15 cases and from the father in 9. Incomplete penetrance was indicated. Corneal transplant was carried out in 65 of the 144 patients coming from the area served by Oulu University Central Hospital in Finland. Among 212 patients, 63% were male.

Kennedy et al. (1986) found keratoconus in less than 6% of the relatives of affected probands.

Wang et al. (2000) conducted a family study to investigate genetic contributions to the development of keratoconus. The estimated prevalence in first-degree relatives was 3.34% (41/1,226), which is 15 to 67 times higher than that in the general population (0.23-0.05%). The correlation in sib and parent-offspring pairs (r = 0.30 and 0.22, respectively) was significantly greater than that in marital pairs (r = 0.14) and the latter was not significantly different from zero. Segregation analysis in 95 families did not reject a major gene model; the most parsimonious model was autosomal recessive inheritance.

Mapping

In patients with keratoconus-1 (KTCN1), Heon et al. (2002) identified mutations in the VSX1 gene (605020), which maps to chromosome 20p11.2.

Associations Pending Confirmation

Hamilton (1938) conducted studies of hereditary eye diseases in Tasmania where, in the coastal town of Burnie, keratoconus is present at a 5-fold increased incidence. Based on the assumption that individuals with keratoconus from this town are likely to be related through a founder effect, Fullerton et al. (2002) conducted a 10-cM interval genome scan on 6 patients of undefined genetic relationship and 1 affected sib pair to identify commonly shared chromosomal segments for the elucidation of candidate gene loci. Analysis of allele sharing revealed 4 markers on 3 chromosomes where all 8 individuals shared a common allele on at least 1 chromosome and 13 markers where all but 1 patient shared common alleles. No excess of allele sharing was observed at any marker tested on chromosome 21, a suggested candidate chromosome for keratoconus because of the occurrence of keratoconus with a 150-fold increased incidence in Down syndrome (Shapiro and France, 1985; van Allen et al., 1999). Further analysis of positive loci revealed suggestive association at 20q12, where significant deviation in frequency of the allele D20S119 was observed. The nearby candidate gene matrix metalloproteinase-9 (MMP9; 120361), which is located at 20q11.2-q13.1, was excluded.

Pathogenesis

Lema and Duran (2005) determined the levels of a panel of inflammatory molecules and matrix metalloproteinases in the tears of patients with keratoconus. Patients with keratoconus had significantly higher levels of IL6 (147620), TNFA (191160), and MMP9 (120361) than control subjects. The extent of the increase was associated with the severity of keratoconus. Lema and Duran (2005) suggested that the pathogenesis of keratoconus may involve chronic inflammatory events.

Atilano et al. (2005) found that keratoconus-affected corneas showed a trend of lower mtDNA-to-nDNA ratio than did control corneas, had decreased cytochrome c oxidase subunit I (MTCO1; 516030) in areas of corneal thinning, and had significantly increased numbers of mtDNA deletions compared to control corneas. Atilano et al. (2005) suggested that increased oxidative stress and altered integrity of mtDNA may be related to each other, contributing to keratoconus pathogenesis.

Kenney et al. (2005) found that keratoconus corneas exhibited a 2.20-fold increase in catalase (115500) mRNA and 1.8-fold increase in enzyme activity; a 1.5-fold increase in cathepsis V/L2 (603308) mRNA and abnormal protein distribution; and a 1.8-fold decrease in TIMP1 (305370) mRNA and a 2.8-fold decrease in protein compared with normal (physiologic) corneas. Kenney et al. (2005) concluded that keratoconus corneas had elevated levels of cathepsins V/L2, B (116810), and G (116830), which could stimulate hydrogen peroxide production, which, in turn, could upregulate catalase, an antioxidant enzyme. In addition, decreased TIMP1 and increased cathepsin V/L2 levels might play a role in the matrix degradation that is a hallmark of keratoconus corneas. These findings supported the hypothesis that keratoconus corneas undergo oxidative stress and tissue degradation.

Because matrix degrading enzymes could potentially influence keratoconus progression, Matthews et al. (2007) studied the effects of TIMP1 and TIMP3 (188826) on stromal cell viability. Overexpression of TIMP3 induced apoptosis in corneal stromal cell cultures. Upregulated TIMP1 production or the addition of exogenous TIMP1 protein prevented stromal cell overgrowth, changed stromal cell morphology, and reduced the extent of TIMP3 induced apoptosis. Localized relative concentrations of TIMP1/TIMP3 could thus determine whether cells remained viable or became apoptotic. Matthews et al. (2007) concluded that this might be relevant to keratoconus because significantly more apoptotic cells were identified in the anterior stroma of keratoconic corneas than in normal corneas and the majority of the TIMP1 and TIMP3 producing stromal cells were located in that region.

Shetty et al. (2015) studied the expression of select genes associated with corneal structure in a large cohort of patients with keratoconus (90 eyes) compared with patients undergoing photorefractive keratectomy who did not have keratoconus (52 eyes). Shetty et al. (2015) observed a significant reduction in lysyl oxidase (LOX; 153455) transcript levels in KTCN corneal epithelia, and LOX activity in KCTN tears correlated with disease severity. Collagen transcript levels (COL1A1, 120150; COL4A1, 120130) were also reduced in KCTN, whereas MMP9 (120361) transcript levels were upregulated and correlated with disease severity. IL6 (147620) transcript levels were moderately increased in KCTN patients. Immunohistochemistry demonstrated a reduction in the protein expression levels of LOX in the epithelium and COL4A1 in the basement membrane of KCTN patients (27 eyes) compared to healthy donor corneas (15 eyes). Shetty et al. (2015) concluded that the structural deformity of the KCTN cornea may be dependent on reduced expression of collagens and LOX, as well as on the concomitant increased expression of MMP9.

Molecular Genetics

Heon et al. (2002) analyzed the VSX1 gene in 63 patients with keratoconus (see KTCN1, 148300) and identified missense mutations in 2 probands (R166W, 605020.0001 and L159M, 605020.0003). They also screened VSX1 in 22 patients with posterior polymorphous corneal dystrophy (PPCD; see 122000), and identified a different missense mutation in 4 sibs (G160D; 605020.0002). The pathogenicity of 2 of these variants G160D and L159M, were later called into question.

Bisceglia et al. (2005) evaluated the role of the VSX1 gene in a series of 80 keratoconus-affected Italian subjects. They found 3 previously described missense changes (see, e.g., 605020.0002) and a novel mutation (605020.0005) in 7 of 80 unrelated patients (8.7%); they also found 2 previously undescribed intronic polymorphisms. The authors concluded that the VSX1 gene plays an important role in a significant proportion of patients affected by keratoconus inherited as an autosomal dominant trait with variable expressivity and incomplete penetrance.

In a case-control panel of 77 sporadic keratoconus patients and 71 controls and a keratoconus family panel involving 444 individuals from 75 families, Tang et al. (2008) screened for 3 keratoconus-associated VSX1 mutations, L159M, R166W, and H244R. The R166W and H244R variants were not found in the case-control panel, and L159M was detected in heterozygosity in 1 control. In the family panel, R166W was not found; L159M was detected in 5 individuals, 3 affected and 2 unaffected, and H244R was detected in 3 individuals, 2 affected and 1 unaffected. Tang et al. (2008) concluded that their results did not support a role for variation in the VSX1 gene in the pathogenesis of keratoconus.

Dash et al. (2010) analyzed the entire coding region, intron-exon junctions, and 5- and 3-prime UTR of the VSX1 gene in 66 unrelated patients with keratoconus, including 27 familial cases and 39 sporadic cases. The G160D change (605020.0002), previously detected in a family with posterior polymorphous corneal dystrophy (PPCD1; 122000) and in a family with keratoconus, was identified in 2 sporadic keratoconus patients and not found in 100 controls; however, other variants that were found did not segregate with disease and/or did not demonstrate pathogenicity. Dash et al. (2010) concluded that VSX1 plays a minor role in keratoconus pathogenesis.

Stabuc-Silih et al. (2010) analyzed the coding regions and intron-exon junctions of the VSX1 gene in 113 unrelated Slovenian patients with keratoconus, but identified no disease-causing mutations; they concluded that other genetic factors are involved in the development of keratoconus.

De Bonis et al. (2011) analyzed the VSX1 gene in 222 unrelated Italian probands with keratoconus and reviewed previously published results. De Bonis et al. (2011) found 1 novel and 3 previously identified VSX1 missense variants in 6 keratoconus patients (see, e.g., 605020.0002 and 605020.0005), none of which had been found in controls. They concluded that VSX1 has a possible pathogenic role in keratoconus, although in a small number of patients.

Associations Pending Confirmation

In 15 unrelated probands with keratoconus, Udar et al. (2006) analyzed the candidate gene SOD1 (147450) and found a heterozygous splice site variant (IVS2+50del7) in 2 probands. The 7-bp deletion segregated with disease in 1 family, being present in an affected father and daughter and absent from 3 unaffected family members; DNA was not available from the other proband's family members. The variant was not found in 312 control chromosomes or in the ALS (see 105400) database either as a mutation or polymorphism. Analysis of the daughter's RNA showed that in addition to wildtype, 2 other SOD1 transcripts were expressed: 1 lacking all of exon 2, and 1 lacking all of exons 2 and 3. Udar et al. (2006) concluded that further studies would be required to determine whether a causal relationship existed between the splice variants and the keratoconus phenotype.

Stabuc-Silih et al. (2010) analyzed the coding regions and intron-exon junctions of the SOD1, COL4A3 (120070), and COL4A4 (120131) genes in 113 unrelated Slovenian patients with keratoconus, but identified no disease-causing mutations in any of the genes. However, 1 polymorphism in COL4A3 showed significant association with keratoconus (D326Y; odds ratio, 14.703 for the 976G allele) as well as 2 polymorphisms in COL4A4, M1327V (OR, 0.3969 for 3979A) and F1644F (OR, 1.751 for 4932C). Stabuc-Silih et al. (2010) concluded that other genetic factors are involved in the development of keratoconus.

De Bonis et al. (2011) analyzed the SOD1 and SPARC (182120) genes in 302 unrelated Italian probands with keratoconus, 80 of whom were previously studied by Bisceglia et al. (2005). The 7-bp deletion in intron 2, previously found in keratoconus patients by Udar et al. (2006), was identified in 2 sporadic patients and was not found in 200 controls. Six missense variants in the SPARC gene were detected in 1 familial and 5 sporadic cases, respectively; none was found in 200 controls, but the variant in the familial case did not segregate with disease in the family, and no relatives of the sporadic patients were available for study. De Bonis et al. (2011) concluded that the role played by SOD1 and SPARC in keratoconus was not definitively clarified.

Al-Muammar et al. (2015) sequenced the entire coding region, exon-intron boundaries, and intron 2 encompassing the previously reported 7-bp deletion in the SOD1 gene in 55 Saudi patients with clinically confirmed keratoconus and identified no pathogenic mutations.

Lechner et al. (2014) analyzed the ZNF469 gene (612078) in 112 European probands with keratoconus and in 96 unaffected and unrelated European individuals, and found significant enrichment of potentially pathogenic ZNF469 alleles in the keratoconus patients compared to controls (p = 0.00102; odds ratio, 13.6; relative risk, 12.0). The authors noted that the allele frequency differences showed that the rare ZNF469 alleles were not in linkage disequilibrium with a known common variant strongly associated with corneal thickness (but not keratoconus), located within a 53-kb linkage disequilibrium block 117 kb from the 5-prime end of ZNF46. Lechner et al. (2014) stated that the enrichment of rare potentially pathogenic ZNF469 alleles in 12.5% of keratoconus patients represented a significant mutational load and highlighted ZNF469 as the most significant genetic factor responsible for keratoconus yet reported.

Exclusion Studies

De Bonis et al. (2011) analyzed the LOX (153455) and TIMP3 (188826) genes in 302 unrelated Italian probands with keratoconus, 80 of whom were previously studied by Bisceglia et al. (2005), and did not find any disease-causing variants.

Animal Model

Tachibana et al. (2002) established an inbred line of spontaneous mutant mice with keratoconus-affected corneas (SKC mice). The SKC mouse cornea resembled corneas of human eyes with keratoconus: both corneas were conical and showed similar changes, including apoptosis of keratocytes and increased expression of Fos protein (164810). The SKC mouse phenotype was transmitted in an autosomal recessive manner, but it was observed almost exclusively in males. Female mice showed the phenotype when injected with testosterone, whereas male incidence of the phenotype diminished drastically when the mice were castrated. Linkage analysis localized a predisposition locus to a major histocompatibility complex (MHC) region on mouse chromosome 17 that includes the gene encoding 'sex-limited protein,' or Slp. The authors proposed that the SKC mouse may be a potential model for a subset of human keratoconus.