The genetics of keratoconus

Novel in Vitro Model for Keratoconus Disease

Keratoconus is a disease where the cornea becomes cone-like due to structural thinning and ultimately leads to compromised corneal integrity and loss of vision. Currently, the therapeutic options are corrective lenses for early stages and surgery for advanced cases with no in vitro model available. In this study, we used human corneal fibroblasts (HCFs) and compared them to human Keratoconus fibroblasts (HKCs) cultured in a 3-dimensional (3D) model, in order to compare the expression and secretion of specific extracellular matrix (ECM) components. For four weeks, the cells were stimulated with a stable Vitamin C (VitC) derivative ± TGF-β1 or TGF-β3 (T1 and T3, respectively). After four weeks, HKCs stimulated with T1 and T3 were significantly thicker compared with Control (VitC only); however, HCF constructs were significantly thicker than HKCs under all conditions. Both cell types secreted copious amounts of type I and V collagens in their assembled, aligned collagen fibrils, which increased in the degree of alignment upon T3 stimulation. In contrast, only HKCs expressed high levels of corneal scarring markers, such as type III collagen, which was dramatically reduced with T3. HKCs expressed α-smooth muscle actin (SMA) under all conditions in contrast to HCFs, where T3 minimized SMA expression. Fast Fourier transform (FFT) data indicated that HKCs were more aligned when compared to HCFs, independent of treatments; however, HKC’s ECM showed the least degree of rotation. HKCs also secreted the most aligned type I collagen under T3 treatment, when compared to any condition and cell type. Overall, our model for Keratoconus disease studies is the first 3D in vitro tissue engineered model that can mimic the Keratoconus disease in vivo and may be a breakthrough in efforts to understand the progression of this disease.

Comparison of corneal thickness and curvature in myopic and keratoconic eyes of patients from French Caribbean Isles and continental France

PURPOSE

To evaluate the corneal thickness and curvature of myopic and patients with keratoconus from two countries.

METHODS

This cross-sectional study was conducted at Cabinet Opale, Fort de France, French West Indies and University Hospital of Bordeaux, France. Corneal thickness and curvature were assessed in 170 keratoconic eyes of 89 residents of the French Caribbean Islands (FCI) and 159 keratoconic eyes of 91 residents of the Aquitaine region of southwest France. A group of age-matched keratoconus-free patients who had been referred for refractive surgery owing to myopia (173 FCI [173 eyes; 87 individuals] and Aquitaine [181 eyes; 93 individuals]) were also assessed.

RESULTS

The mean age at keratoconus diagnosis was significantly higher among FCI than Aquitaine residents (p = 0.009). The mean keratometric (Km) reading was statistically higher for keratoconic FCI than Aquitaine patients, at 48.06 versus 46.21 diopters (p = 0.001). This difference was more pronounced among patients aged >40 years than those ≤40 years (p = 0.009). Patients with keratoconus showed no significant difference in mean central corneal thickness and thinnest corneal point values, irrespective of region. Myopic individuals from the FCI, however, had significantly lower mean central corneal thickness and thinnest corneal point measurements than Aquitaine myopics, irrespective of age group (p ≤ 0.0008).

CONCLUSION

The corneas of patients with keratoconus of African-Caribbean and Caucasian origins are of similar thickness. Myopic African-Caribbean patients referred for refractive surgery tend to present with thinner corneas than Caucasians.

The relation between keratoconus and plasma levels of MMP-2, zinc, and SOD

PURPOSE

To evaluate the levels of matrix metalloproteinase-2 (MMP-2), superoxide dismutase (SOD), and zinc in plasma taken from patients with keratoconus and to investigate the likely association between these factors and keratoconus.

METHODS

A total of 36 patients with keratoconus and 40 control subjects at the Department of Ophthalmology, Faculty of Medicine, Gaziosmanpaşa University, were included in the study. Plasma levels of zinc were determined with atomic absorption spectrometry for all the subjects. Measurements of plasma MMP-2 levels were performed by the enzyme-linked immunosorbent assay method. Total plasma (Cu/Zn and Mn) SOD activity was also determined photometrically.

RESULTS

The plasma concentrations of zinc and MMP-2 were significantly lower in patients with keratoconus than in the healthy controls (P < 0.001). Total plasma SOD levels were significantly higher in patients with keratoconus than in the healthy controls (P < 0.001).

CONCLUSIONS

We detected reduced plasma levels of zinc and MMP-2, and enhanced plasma levels of SOD in patients with keratoconus compared with the healthy subjects. The data presented provide insight into the potential role these molecules may play in the etiopathogenesis of this disease.

The thinnest, steepest, and maximum elevation corneal locations in noncontact and contact lens wearers in keratoconus

PURPOSE

To assess the relationship between the thinnest corneal location and the steepest and maximum elevation corneal locations in subjects with keratoconus and the effect of gas permeable contact lens wear on the location of these points.

METHODS

Sixty-one consecutive subjects (98 eyes) with keratoconus. Thirty-one (49 eyes) and 30 (49 eyes) subjects were gas permeable contact lens (CL-W) and non-contact lens wearers (N-CL), respectively. Thinnest, steepest, and maximum elevation corneal locations were evaluated from topographies collected with Pentacam Eye Scanner.

RESULTS

In the entire sample and in N-CL and CL-W, the thinnest location does not overlap with the steepest or maximum elevation corneal locations (all P > 0.05). The thinnest and maximum tangential curvature locations were found to be located further away from the geometric center of the cornea in CL-W versus N-CL (P < 0.05).

CONCLUSION

The thinnest corneal location does not overlap with maximum axial and tangential curvatures or with the front and back elevation locations in keratoconus subjects. Contact lens wear does not affect this lack of overlapping.

Keratoconus in 18 pairs of twins

PURPOSE

To describe the concordance of keratoconus in 18 sets of twins.

METHODS

Thirteen monozygotic (MZ) and five dizygotic (DZ) pairs of twins were identified during an investigation of familial keratoconus. We used 16 forensic microsatellite markers to confirm the zygosity of same sex twins. Patients and available relatives were examined for signs of keratoconus using corneal topography. For each pair of twins, the severity of keratoconus in each eye was graded according to the steepest keratometry value and the average difference in score between the MZ and DZ twins compared.

RESULTS

All of the MZ twins and four of the five DZ twins were concordant for keratoconus but with differences in age of onset and severity of disease. The subjective age of onset of keratoconus tended to be earlier in the MZ twins (16.4 years, SD 4.66) than in the DZ twins (20.3 years, SD 7.55) (p=0.086). Additional relatives with keratoconus were identified in two (16%) of the families with MZ twins and in three (60%) of the families of DZ twins. The mean difference in severity scores was 1.4 (SD 1.73) for the MZ twins and 3.0 (SD 1.00) for the DZ twins (p=0.035).

CONCLUSION

This data provide evidence that the severity of keratoconus is more concordant in MZ than in DZ twins. The results support the currently accepted hypothesis of an important genetic contribution towards the pathogenesis of keratoconus, but suggest that there is also an environmental effect on the expression of disease.

The point mutation and polymorphism in keratoconus candidate gene TGFBI in Chinese population

OBJECTIVE

To understand the region point mutations and single nucleotide polymorphisms characteristic of keratoconus candidate gene in Chinese population, the TGFBI.

METHODS

Polymerase chain reaction-single strand conformation polymorphism and DNA direct sequencing were performed on blood samples from 30 cases of keratoconus patients and 30 normal controls. 17 exons from the coding region of TGFBI gene were examined for point mutations and single nucleotide polymorphisms.

RESULTS

Two types of base mutation were found in exon 12, which were both heterozygous. In 1 patient the site 535 showed GGA→TGA substitution, which was the change from glycine to stop codon (G535X). This was not found in all control cases. In 2 patients and 1 control case the site 540 showed TTT→TTC substitutions without changing of the coding for phenylalanine (F540F), suggesting for the polymorphism.

CONCLUSION

The candidate keratoconus gene TGFB1 showed genetic variation and mutation in keratoconus population. The gene might play a role in the development of keratoconus in Chinese population.

Collagen cross-linking: current status and future directions

Collagen cross-linking (CXL) using UVA light and riboflavin (vitamin B2) was introduced as a clinical application to stabilize the cornea by inducing cross-links within and between collagen fibers. CXL has been investigated extensively and has been shown clinically to arrest the progression of keratoconic or post-LASIK ectasia. With its minimal cost, simplicity, and proven positive clinical outcome, CXL can be regarded as a useful approach to reduce the number of penetrating keratoplasties performed. Small case series have also indicated that CXL is beneficial in corneal edema by reducing stromal swelling behavior and in keratitis by inhibiting pathogen growth. Despite these encouraging results, CXL remains a relatively new method that is potentially associated with complications. Aspects such as side effects and recurrence rates have still to be elucidated. In light of the growing interest in CXL, our paper summarizes present knowledge about this promising approach. We have intentionally endeavored to include the more relevant studies from the recent literature to provide an overview of the current status of CXL.

Novel mutation and three other sequence variants segregating with phenotype at keratoconus 13q32 susceptibility locus

Keratoconus (KTCN), a non-inflammatory corneal disorder characterized by stromal thinning, represents a major cause of corneal transplantations. Genetic and environmental factors have a role in the etiology of this complex disease. Previously reported linkage analysis revealed that chromosomal region 13q32 is likely to contain causative gene(s) for familial KTCN. Consequently, we have chosen eight positional candidate genes in this region: MBNL1, IPO5, FARP1, RNF113B, STK24, DOCK9, ZIC5 and ZIC2, and sequenced all of them in 51 individuals from Ecuadorian KTCN families and 105 matching controls. The mutation screening identified one mutation and three sequence variants showing 100% segregation under a dominant model with KTCN phenotype in one large Ecuadorian family. These substitutions were found in three different genes: c.2262A>C (p.Gln754His) and c.720+43A>G in DOCK9; c.2377-132A>C in IPO5 and c.1053+29G>C in STK24. PolyPhen analyses predicted that c.2262A>C (Gln754His) is possibly damaging for the protein function and structure. Our results suggest that c.2262A>C (p.Gln754His) mutation in DOCK9 may contribute to the KTCN phenotype in the large KTCN-014 family.

A genome-wide association study identifies a potential novel gene locus for keratoconus, one of the commonest causes for corneal transplantation in developed countries

Keratoconus is a condition in which the cornea progressively thins over time, and is a major cause for cornea transplantation. To identify keratoconus susceptibility regions, we performed a comprehensive genome-wide association study (GWAS) using a discovery and replication design. A discovery panel of 222 keratoconus Caucasian patients and 3324 Caucasian controls was genotyped using Illumina 370K beadchips. Further associated and fine-mapping single nucleotide polymorphisms (SNPs) (n= 4905) were genotyped in an independent replication case-control panel of 304 cases and 518 controls and a family panel of 307 subjects in 70 families. Logistic regression models implemented in PLINK were performed to test associations in case-control samples with and without principal component (PC) adjustments. Generalized estimation equation models accounting for familial correlations implemented in GWAF were used for association testing in families. No genome-wide associations were identified in the discovery GWAS panel. From the initial testing without adjustments for PCs, the top three SNPs located at 3p26 (rs6442925), 2q21.3 (rs4954218) and 19q13.3 (rs1428642) were identified with unadjusted P-values of 6.5 × 10(-8), 2.4 × 10(-7) and 3.1 × 10(-7), respectively. After adjustments for PCs, rs1428642 became the most significant through the genome with a P-value of 1.4 × 10(-6), while rs6442925 and rs4954218 were less significant (P= 1.9 × 10(-5) and 2.6 × 10(-4)). SNP rs4954218 was confirmed in two independent replication panels with P-values of 0.004 and 0.009, respectively. Meta-analysis revealed a highest association at rs4954218 with adjusted P= 1.6 × 10(-7) (unadjusted P= 1.2 × 10(-9)). These findings suggest SNP rs4954218, located near the RAB3GAP1 gene, previously reported to be associated with corneal malformation, is a potential susceptibility locus for keratoconus.

In vivo confocal laser-scanning microscopy to characterize wound repair in rabbit corneas after collagen cross-linking

BACKGROUND

Collagen cross-linking using the photosensitizer riboflavin combined with ultraviolet A light was developed to stiffen the cornea by increasing its mechanical and biochemical stability. Investigation of post-treatment events, such as wound healing, is important to evaluate possible risks and to optimize treatment protocols. This in vivo confocal laser-scanning microscopy study in rabbits was conducted to provide a quantitative and qualitative analysis of corneal wound repair over 16 weeks following collagen cross-linking.

METHODS

Six New Zealand White rabbits underwent riboflavin/ultraviolet A cross-linking. In vivo confocal laser-scanning microscopy using a Heidelberg Retina Tomograph equipped with a Rostock Cornea Module was performed preoperatively and at 2, 4, 8, 12 and 16 weeks postoperatively.

RESULTS

From 2 weeks onwards the epithelium demonstrated no abnormalities. Evidence of inflammation was visualized in the intermediate, basal cells and Bowman's membrane. Nerve fibre regeneration was first noted at 12 weeks. Keratocyte activation and hyperreflective extracellular matrix were observed consistently, but by 16 weeks keratocyte activation was diminished, and extracellular matrix resumed normal reflectivity. Cell density in the posterior stroma and endothelium regained preoperative values by 4 weeks, although anterior stroma keratocyte cell density was still reduced by about 10% at 16 weeks.

CONCLUSIONS

Complete qualitative and quantitative characterization of corneal wound repair was achieved by in vivo confocal laser-scanning microscopy over 16 weeks following collagen cross-linking in rabbits. In terms of assessing the ever-increasing range of cross-linking protocols, in vivo confocal laser-scanning microscopy may contribute to minimizing the number of experimental animals, because multiple examinations of the same cases are possible over time.

The morphologic characteristics of corneal nerves in advanced keratoconus as evaluated by acetylcholinesterase technique

PURPOSE

To study the morphologic characteristics of corneal nerves in patients with advanced keratoconus using the acetylcholinesterase technique in corneal whole mounts.

DESIGN

Prospective, observational case series.

METHODS

Fourteen corneal buttons from 14 keratoconic patients (9 males and 5 females; mean age, 34.3 years) who had undergone keratoplasty for advanced keratoconus and 6 corneal buttons from 6 normal corneas were included. Whole mounts were stained for acetylcholinesterase and were scanned with a novel digital pathology scanning microscope.

RESULTS

Seventy-one percent of keratoconic corneas demonstrated central stromal nerve changes, which included thickening, tortuosity, nerve spouting, and overgrowth. The nerve changes ranged from early to extensive and could be separated into 3 different grades. The central stromal nerves were abnormally thicker (18.9 ± 14.7 μm) than in controls (8.11 ± 3.31 μm; P < .001). The thickness of peripheral stromal nerves (12.6 ± 3.1 μm) was similar to that of controls (14.86 ± 5.60 μm; P = .072). Subbasal nerves showed changes in the form of loss of radial orientation and increased tortuosity, especially at the cone apex. At the cone base, a concentric arrangement of subbasal nerves was found in 43% of cases. Localized thickenings of subbasal nerves also were observed at their origin from the bulbous terminations of sub-Bowman nerves. The terminal bulbs, too, were enlarged. The mean diameter of the subbasal nerves in keratoconus (4.11 ± 0.60 μm) did not differ from that of the controls (4.0 ± 0.61 μm; P = .422).

CONCLUSIONS

This study provides additional histologic evidence of the involvement of corneal nerves in keratoconus and suggests further that they may play a role in the pathophysiologic factors and progression of the disease.

The genetics of keratoconus

Keratoconus (KTCN) is non-inflammatory thinning and anterior protrusion of the cornea that results in steepening and distortion of the cornea, altered refractive error, and decreased vision. Keratoconus is a complex condition of multifactorial etiology. Both genetic and environmental factors are associated with KTCN. Evidence of genetic etiology includes familial inheritance, discordance between dizygotic twins, and association with other known genetic disorders. Several loci responsible for a familial form of KTCN have been mapped; however, no mutations in any genes have been identified for any of these loci. This article focuses on the genetic aspects. In addition, bioinformatics methods applied in KTCN gene identification process are discussed.

Keratoconus: overview and update on treatment

Keratoconus is a non-inflammatory, progressive thinning process of the cornea. It is a relatively common disorder of unknown etiology that can involve each layer of the cornea and often leads to high myopia and astigmatism. Computer- assisted corneal topography devices are valuable diagnostic tools for the diagnosis of subclinical keratoconus and for tracking the progression of the disease. The traditional conservative management of keratoconus begins with spectacle correction and contact lenses. Several newer, more invasive, treatments are currently available, especially for contact lens-intolerant patients. Intrastromal corneal ring segments can be used to reshape the abnormal cornea to improve the topographic abnormalities and visual acuity. Phakic intraocular lenses such as iris-fixated, angle-supported, posterior chamber implantable collamer and toric lenses are additional valuable options for the correction of refractive error. Corneal cross-linking is a relatively new method of stiffening the cornea to halt the progression of the disease. The future management of keratoconus will most likely incorporate multiple treatment modalities, both simultaneous and sequential, for the prevention and treatment of this disease.

In vivo confocal microscopy of corneal nerves: analysis and clinical correlation

Corneal confocal microscopy is a growing technique for the study of the cornea at the cellular level, providing images comparable to ex vivo histochemical methods. In vivo confocal microscopy (IVCM) has an enormous potential, being a noninvasive procedure that images the living cornea, to study both its physiological and pathological states. Corneal nerves are of great interest to clinicians and scientists due to their important roles in regulating corneal sensation, epithelial integrity, proliferation, wound healing, and for their protective functions. IVCM enables the noninvasive examination of corneal nerves, allowing the study of nerve alterations in different ocular diseases, after corneal surgery, and in systemic diseases. To date, the correlation of sub-basal corneal nerves and their function has been studied in normal eyes, keratoconus, dry eye, contact lens wearers, and in neurotrophic keratopathy, among others. Further, the effect of corneal surgery on nerves has been studied, demonstrating the regenerative capacity of corneal nerves and the recovery of sensation. Moreover, IVCM has been applied in the diagnosis of peripheral diabetic neuropathy and the assessment of progression in this systemic disease. The purpose of this review is to describe the principles, applications, and clinical correlation of IVCM in the study of corneal nerves in different ocular and systemic diseases.

Pancorneal Contact Lens with a Toric Edge: A New Concept in Keratoconus

Purpose.

To investigate whether fitting a patient with keratoconus to a pancorneal toric rigid gas-permeable (RGP) contact lens leads to a change in corneal compression and improves the best-corrected visual acuity (BCVA) and keratometry values.

Methods.

Thirty eyes with keratoconus were fitted with a newly designed pancorneal toric RGP contact lens. Each patient was examined at the time of enrollment and after having used the new contact lens for at least 2 months. Corneal topography was performed both times.

Results.

A change in corneal compression was noticed in 23 eyes (77%). Following the use of the pancorneal toric RGP contact lens, the average BCVA improved significantly (p=0.007), with a mean BCVA of 0.63 (SD 0.15) before and 0.70 (SD 0.18) after using the toric contact lens. No significant changes were seen in the mean vertical and horizontal K-values or the mean E-values.

Conclusions.

While no significant measurable differences in K- and E-values were observed, fitting of a pancorneal toric RGP contact lens in keratoconus led to a marked improvement in visual acuity and a visible change in corneal compression.

Corneal cross-linking for the treatment of keratoconus in a patient with ipsilateral myelinated retinal nerve fiber layer

Keratoconus associated with myelinated retinal nerve fibers is not frequent and the relationship between the two pathologies is difficult to explain, therefore studies and further investigation are required. The etiology of each condition may suggest the role of genetic factors. Follow-up is important to evaluate the progression of keratoconus and myelination. Here we describe the unusual coexistence of keratoconus and ipsilateral myelinated retinal nerve fiber layer and, for the first time, the corneal cross-linking treatment in this condition.

Isolated orbital fibrous dysplasia associated with ipsilateral keratoconus

INTRODUCTION

Fibrous dysplasia is a primary orbital bone tumor, described as a benign disorder in which proliferation of fibrous tissue and osteoid replaces and distorts the bone from which it derives. Unilateral keratoconus is a rare entity. Herein, we report a case of an extensive ethmoidal fibrous dysplasia associated with ipsilateral keratoconus, and review the literature on the subject.

MATERIALS AND METHODS

A 22-year-old man presented with left painless proptosis evolving over 10 years. There was associated ipsilateral epiphora and gradual visual loss. On examination his visual acuity was 20/20 OD and CF OS. His left globe was displaced lateraly 12 mm, with 9 mm of proptosis. The extraocular movements were normal. Left nasolacrimal duct obstruction was noted. Clinical signs of keratoconus were present only in the left cornea. High resolution corneal topography confirmed unilateral keratoconus and a CT scan showed an ovoid mass with a well defined sclerotic margin arising from the left ethmoid sinus and invading the orbit.

RESULTS

The patient underwent resection of the lesion via a modified Lynch incision. Complete regression of proptosis was observed immediately after surgery. Histopathological examination revealed irregular trabeculae of woven bone in different levels of maturation in a moderately cellular fibrous matrix without nuclear atypia. Trabeculae were without osteoblastic rimming or osteoclastic resorption. There has been no recurrence over a 2-year follow-up period.

CONCLUSION

To our knowledge, the coexistence of unilateral isolated craniofacial fibrous dysplasia with ipsilateral keratoconus has not been reported so far.

The role of crumbs genes in the vertebrate cornea

PURPOSE

To evaluate the role of crumbs genes and related epithelial polarity loci in the vertebrate cornea.

METHODS

The authors used histologic analysis and electron microscopy to evaluate the corneas of zebrafish mutant for a crumbs locus oko meduzy (ome) and in mutants of four other loci, nagie oko (nok), heart and soul (has), mosaic eyes (moe), and ncad (formerly glass onion), that function in the same or related genetic pathways. In parallel, they performed an evaluation of corneas in human carriers of a crumbs gene, CRB1, and mutations using topography and biomicroscopy. The expression of the CRB1 gene in the normal human cornea was examined by polymerase chain reaction (PCR) and immunohistochemical staining.

RESULTS

The corneas of zebrafish mutants display severe abnormalities of the epithelial and stromal layers. The epithelial cells do not properly adhere to each other, and fluid-filled spaces form between them. In addition, the layering of the corneal stroma is poorly formed or absent. The corneas of human carriers of CRB1 mutations display shape deviations compared with what has been observed in normal individuals. A PCR product of the correct size was obtained from normal human corneal samples. Sequence analyses confirmed its identity to be the human CRB1 gene. Immunohistochemical staining using anti-CRB1 yielded positive brown deposits in the human cornea.

CONCLUSIONS

crumbs genes play a role in the differentiation of the vertebrate cornea. Corneal defects associated with crumbs gene mutations are very severe in the zebrafish model and, in comparison, appear clinically less pronounced in the human eye.

Are proteinases the reason for keratoconus?

PURPOSE

Keratoconus is a degenerating disease of the eye that results in an irregularly-shaped cornea. The etiology of the disease is unknown and the prognosis is difficult due to the variability in outcome. Keratoconus has been associated with eye rubbing, atopy, contact lens wear, as well as genetic conditions, such as Down's syndrome, Ehlers-Danlos syndrome, and Marfan's syndrome. Thinning of the cornea in keratoconus has been well studied and is documented to occur as a result of degradation of corneal collagen. The reason for this tissue degradation is unknown but has been hypothesized to be linked with proteases.

METHODS

This study used a literature search to review the role of proteases and inflammatory molecules in the aetiology of keratoconus.

RESULTS

Early studies demonstrated elevated levels of collagenolytic and gelatinolytic activities in laboratory cultures of keratoconic corneas. Matrix metalloproteinases (MMPs) are a family of zinc-dependent proteins that include collagenases and gelatinases. MMPs levels are altered in keratoconus corneas compared to normal corneas and the level of tissue inhibitor of metalloproteinases-1 (TIMP-1) is decreased in keratoconic corneas. Recent studies have demonstrated the involvement of Cathepsin B, G, and K in keratoconus. Although thought to be a non-inflammatory disease, inflammatory molecules, such as interleukins and tumor necrosis factor have been shown to be elevated in keratoconus, and these inflammatory molecules may mediate production and activation of proteases.

CONCLUSIONS

Proteases may be implicated in keratoconus. An in-depth investigation of these proteases may help in better understanding the course of the disease.

Longterm follow-up of penetrating keratoplasty for keratoconus

PURPOSE

This study aims to describe the current visual and refractive status of patients who underwent penetrating keratoplasty (PK) for keratoconus > 20 years ago and to report on the current status of their grafts.

METHODS

A total of 138 eyes in 103 patients were grafted for keratoconus between August 1968 and December 1985. Patients who had not undergone retransplantation were invited to attend a clinical examination. Forty-eight patients (with 61 grafts) accepted the invitation and were examined.

RESULTS

The average length of time since PK was 26.9 years (standard deviation [SD] 4.2 years, range 20.8-38.0 years, n = 61). The average graft age at examination was 82.1 years (SD 19.9 years, range 41-115 years). A total of 80% (49 of 61 grafts) of the examined eyes had a clear graft and 46% (28 of 61 eyes) had best spectacle-corrected visual acuity (BSCVA) > or = 0.5. The mean endothelial cell density per mm(2) was 894 (SD 4.6, range 470-1775). The mean central corneal thickness of the clear grafts was 0.565 mm (SD 0.048 mm).

CONCLUSIONS

Penetrating keratoplasty for keratoconus has a good longterm prognosis; half of the eyes examined in this study had BSCVA > or = 0.5 at > 20 years after surgery.

Absence of pathogenic mutations in VSX1 and SOD1 genes in patients with keratoconus

PURPOSE

Keratoconus (KC) is a bilateral, noninflammatory, and progressive corneal ectasia that occurs mostly as a sporadic disorder, but it has long been recognized that a significant minority of patients also exhibit a family history. In recent years, several candidate genes, including VSX1 and SOD1, have been proposed and some disease-causing mutations have been identified.

METHODS

To investigate the role of the 2 genes in 113 Slovenian patients with sporadic and familial KC, the complete coding region with corresponding intronic sequences was analyzed. The same regions of both genes were also checked in 100 healthy blood donors. We also checked the relation of 627+23G>A polymorphism in the VSX1 gene with the hereditary form of the disease.

RESULTS

No disease-causing mutations were identified in either gene. We did discover a significant association of 627+23G>A polymorphism distribution (VSX1) with unrelated patients diagnosed with the hereditary form of KC.

CONCLUSION

The absence of pathogenic mutations in our large number of unrelated patients with KC indicates that other genetic factors are involved in the development of this disorder.

Keratoconus in a cynomolgus monkey

In a seven-year-old male cynomolgus monkey, erythema of the upper eyelid and forehead and corneal opacity, edema and conical protrusion in the eye were observed. At necropsy, ophthalmological and serological examinations revealed binocular corneal opacity and conical protrusion and a high IgE level, respectively. Thinning of the epithelium and stroma of the cornea were noted histopathologically. At the center of the corneal epithelium, the number of epithelial cells was reduced, their cytoplasm was poorer and the basal cells were flatter than at the periphery. Bowman's membrane was folded with partial loss or breakage. Collagen fibers were compacted or disarranged, and the keratocytes were increased in the stroma, with focal pyknosis or loss of the endothelium and folding of Descemet's membrane. Electron microscopical examination revealed atrophy of the corneal epithelial basal cells. This is the first report of a case of keratoconus in a cynomolgus monkey.

2008 Sir Norman McAlister Gregg Lecture: 150 years of practical observations on the conical cornea--what have we learned?

The first detailed descriptions of keratoconus were published exactly 150 years ago in the original work of Dr John Nottingham, bringing a degree of clarity to a previously confusing clinical phenomenon--further supported by observations of other contemporaries in the field such as Sir William Bowman. However, it would be another 100 years before knowledge of keratoconus would grow substantially; indeed, our current level of understanding is primarily a result of extensive clinical and laboratory research conducted over the last 50 years--particularly based upon the enormous technological advances of the last two decades. Large clinical studies have confirmed that keratoconus is a non-inflammatory corneal disease with central or paracentral corneal thinning, which exhibits progressive corneal steepening and protrusion that typically results in increasing regular and thereafter irregular astigmatism. Ultimately, disease progression may lead to corneal scarring, corneal hydrops and loss of best spectacle-corrected visual acuity. Although visual rehabilitation may be effected by expert contact lens fitting, 20% of subjects may require corneal transplantation. This Gregg lecture provides a highly referenced, wide-ranging overview of both historical and contemporary aspects of keratoconus, including diagnostic, phenotypic and prognostic factors revealed by large clinical studies, critical diagnostic advances enabled by Placido and slit-scanning computerized corneal topography, the emerging roles of higher order aberration wave-front analysis and corneal hysteresis in delineating early and subclinical keratoconus, inheritance and genetic predisposition to keratoconus, corneal microstructural changes unveiled by in vivo confocal microscopy, unifying theories to explain associations between keratoconus, atopy, eye rubbing and keratocyte apoptosis, and surgical options for keratoconus, such as corneal transplantation, intrastromal ring segments, collagen cross-linking and keratocyte transplantation. However, 150 years along the path our knowledge of keratoconus remains incomplete, but technological advances should enable us to put together the final pieces of the jigsaw in the foreseeable future.

Localization of a gene for keratoconus to a 5.6-Mb interval on 13q32

PURPOSE

Keratoconus (KTCN) is a noninflammatory thinning and anterior protrusion of the cornea that results in steepening and distortion of the cornea, altered refractive powers, and reduced visual acuity. Several loci responsible for a familial form of KTCN have been mapped, however; no mutations in any genes have been identified for any of these loci. There is also evidence that VSX1 and SOD1 may be involved in the etiology of KTCN. The purpose of this study was to verify the available data and to identify a new keratoconus susceptibility locus.

METHODS

KTCN without other ocular or systemic features was diagnosed in 18 families. VSX1 and SOD1 sequencing was performed on affected individuals and control subjects. Genomewide linkage analysis was then performed in all families using polymorphic microsatellite markers with an average spacing of 5 cM. Next, single-nucleotide polymorphism (SNP) arrays, fluorescence in situ hybridization (FISH) analysis, and a comparative genomic hybridization array were used in one family to assess a candidate region on 13q32.

RESULTS

All previously reported KTCN loci were excluded. VSX1 and SOD1 were sequenced, and no potentially functional variants were found. One KTCN family yielded a maximum multipoint parametric LOD score of 4.1 and multipoint nonparametric linkage (NPL) LOD score of 3.2. Multipoint linkage and haplotype analysis narrowed the locus to a 5.6-Mb region between the SNPs rs9516572 and rs3825523 on 13q32.

CONCLUSIONS

The results exclude VSX1 and SOD1 as potential disease-causing genes in these families and localize a novel gene for keratoconus to a 5.6-Mb interval on 13q32.

VSX1 gene variants are associated with keratoconus in unrelated Korean patients

Keratoconus is a bilateral ectatic disorder characterized by the central thinning of corneal tissue leading to visual impairment. To investigate the possibility of visual system homeobox 1 (VSXI) as a candidate susceptibility gene for Korean patients with keratoconus, we performed a mutation screening of the VSXI gene in 249 unrelated patients with keratoconus and 208 control subjects without the ocular disorder. We found two heterozygous novel missense mutations in exon 2: N151S and G160V. The G160V mutation was identified in 13 keratoconus patients (5.3%), and the N151S mutation was found in only one keratoconus patient (0.4%). We also detected three synonymous polymorphisms and four intragenic polymorphisms. The IVS1-11*a allele was associated with a significantly increased risk of keratoconus in Korean patients [3.6 vs. 0.5%, p = 0.001, odds ratio (OR) = 7.76, 95% confidence interval (CI) 1.989-30.241). Other polymorphisms did not show an association with keratoconus risk. Our data is the first reported VSX1 mutation screening in Korean keratoconus patients. We detected two novel missense mutations and one intragenic polymorphism in the VSX1 gene, which show a strong statistical association with unrelated keratoconus patients. Consequently, our study suggests that VSX1 gene variants seem to be significant genetic variants for keratoconus predisposition in unrelated Korean patients.

Longitudinal study of keratoconus progression

To determine if differences in topographic progression between unaffected keratoconus relatives and normal controls can predict factors associated with the development of keratoconus in a longitudinal study. We recruited 369 unaffected keratoconus relatives and 119 normal controls in Los Angeles. Both eyes of subjects were examined at baseline clinically and by quantitative videokeratography and at a period ranging from 1 year to 8 years. Progression to keratoconus was evaluated by quantitative videokeratography variables. Unaffected relatives had higher Central K (CK), I-S and KISA values and were younger than normal controls (CK: 44.70 vs 44.01, P<0.01; I-S: 0.76 vs 0.58, P<0.01, KISA: 29.97 vs 23.89, P=0.02; age: 34.8 vs 41.0, P<0.01) at baseline. All three indices significantly increased with age, and CK and KISA values were associated with a positive family history for keratoconus (P<0.001 for CK and P=0.05 for KISA), however, the two groups were not statistically different in progression of keratoconus. After grouping unaffected relatives as high risk (age< or = 30 or Central K > or = 47.2 and I-S> or =1.2 or KISA> or = 60) and low risk (age>30 and Central K<47.2 and I-S<1.2 and KISA< 60), relatives in the high risk group had a greater increase in CK and I-S values than those in the low risk group (CK: P=0.009; I-S: P<0.001), which indicated that there were significantly different rates of progression between two groups. Unaffected relatives had higher videokeratography indices than normal controls, but overall they did not progress to keratoconus quicker than normal controls. However, relatives in the high risk group may have a greater risk of progression to keratoconus.

Topographic Indications of Emerging Keratoconus in Teenage New Zealanders

PURPOSE

To screen a population of teenagers for emerging topographic signs of keratoconus (KC), with particular reference to ethnicity.

METHODS

Corneal topography, visual acuity, and an environmental risk factor analysis were included in a screening program designed to identify early indications of potential KC in teenagers. Two schools from central North Island, New Zealand, were invited to participate: one with predominantly Maori/Polynesian students and the other with students of mainly European descent.

RESULTS

A total of 198 Maori/Polynesian, 16.8 +/- 1.05 years of age, and 243 European students, 16.2 +/- 1.3 years of age, participated in the study. Corneal topography suggestive strongly of KC was evident in 3 Maori/Polynesian [1.2%; 95% confidence interval (CI): 0.3-4.3] and no European students (0%; 95% CI, 0%-1.5%). Anomalous corneal topography reminiscent of emerging KC in at least 1 eye were found in 19% (95% CI, 15.6%-22.9%). A significant difference (P = 0.0014) in the percentage of suspected KC was found between European students (12.9%; 95% CI, 9.2%-17.9%) and Maori/Polynesian students (26.9%; 95% CI, 21.0%-33.7%). In multivariate analyses, the significant independent predictors of probable or suspected KC included being Maori/Polynesian [odds ratio (OR) = 2.1; 95% CI, 1.25-3.54; P = 0.0052], increasing age (OR = 1.4; 95% CI, 1.10-1.80; P = 0.0067), and a history of hayfever (OR = 2.0; 95% CI, 1.16-3.59; P = 0.013). Topographical indicators that were associated significantly with suspected KC included central keratometry (P < 0.0001), astigmatism (P = 0.014), and inferior-superior asymmetry (P < 0.0001).

CONCLUSIONS

Although only a longitudinal study will determine the proportion of true KC candidates, these data provide evidence of ethnicity-related differences in corneal topography in teenage New Zealanders.

A New Method for Grading the Severity of Keratoconus

PURPOSE

To define a new method for grading severity of keratoconus, the Keratoconus Severity Score (KSS).

METHODS

A rationale for grading keratoconus severity was developed using common clinical markers plus 2 corneal topographic indices, creating a 0 to 5 severity score. An initial test set of 1012 eyes, including normal eyes, eyes with abnormal corneal and topographic findings but not keratoconus, and eyes with keratoconus having a wide range of severity, was used to determine cutpoints for the KSS. Validation set 1, comprising data from 128 eyes, was assigned a KSS and compared with a clinician's ranking of severity termed the "gold standard" to determine if the scale fairly represented how a clinician would grade disease severity. kappa statistics, sensitivity, and specificity were calculated. A program was developed to automate the determination of the score. This was tested against a manual assignment of KSS in 2121 (validation set 2) eyes from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study, as well as normal eyes and abnormal eyes without keratoconus. Ten percent of eyes underwent repeat manual assignment of KSS to determine the variability of manual assignment of a score.

RESULTS

From initial assessments, the KSS used 2 corneal topography indices: average corneal power and root mean square (RMS) error for higher-order Zernike terms derived from the first corneal surface wavefront. Clinical signs including Vogt striae, Fleischer rings, and corneal scarring were also included. Last, a manual interpretation of the map pattern was included. Validation set 1 yielded a kappa statistic of 0.904, with sensitivities ranging from 0.64 to 1.00 and specificities ranging from 0.93 to 0.98. The sensitivity and specificity for determining nonkeratoconus from keratoconus were both 1.00. Validation set 2 showed kappa statistics of 0.94 and 0.95 for right and left eyes, respectively. Test-retest analysis yielded kappa statistics of 0.84 and 0.83 for right and left eyes, respectively.

CONCLUSION

A simple and reliable grading system for keratoconus was developed that can be largely automated. Such a grading scheme could be useful in genetic studies for a complex trait such as keratoconus requiring a quantitative measure of disease presence and severity.

The variable expression of keratoconus within monozygotic twins: Dundee University Scottish Keratoconus Study (DUSKS)

INTRODUCTION

As part of a longitudinal observational prospective study (Dundee University Scottish Keratoconus Study, DUSKS) a caucasian female subject was identified with keratoconus affecting both corneas. Of interest was that the subject's mother was a monozygotic twin, however, the mother was reportedly unaffected by keratoconus, although her sister (the subject's aunt) had been diagnosed with keratoconus.

METHOD

Computer video-keratoscopy (CVK) was undertaken was on this family to clarify this apparent discordance.

RESULTS

CVK demonstrated the variable expression of the keratoconic phenotype, by identifying evidence of subclinical keratoconus in the 'unaffected' twin.

CONCLUSION

So far only one set of monozygotic twins with keratoconus have been reported within the UK, this paper reports a second. Since CVK has been available the majority agree on concordance of keratoconus expression but not all, including this study. If monozygotic twin studies demonstrate heterogeneity of keratoconus this may at least explain the contradictory data that exists within the literature and why the inheritance pattern of keratoconus is still unclear in 2005.

Quantitative analysis of corneal microstructure in keratoconus utilising in vivo confocal microscopy

PURPOSE

To establish and quantify the in vivo confocal microscopic features of moderate to advanced keratoconus.

METHODS

Nineteen keratoconus subjects were catergorised using Orbscan-derived corneal apex power and pachymetry as exhibiting moderate (n=7) and advanced (n=12) keratoconus. Control subjects included 23 noncontact lens wearers (Group A) and 15 contact lens wearers (Group B). All subjects underwent Confoscan slit scanning in vivo confocal microscopy.

RESULTS

Compared with Group A (4912+/-434 cells/mm(2)), basal epithelial density was significantly lower in both moderate (4592+/-414 cells/mm(2), P<0.05) and advanced keratoconus (4530+/-596 cells/mm(2), P=0.01). In comparison to Group A (761+/-118 cells/mm(2)), anterior stroma keratocyte density was significantly greater in both moderate keratoconus (883+/-111 cells/mm(2), P=0.001) and advanced keratoconus (952+/-122 cells/mm(2), P<0.001). Compared to Group A (504+/-80 cells/mm(2)) posterior stroma keratocyte density was also significantly greater in advanced keratoconus (599+/-97 cells/mm(2), P<0.001) and posterior stromal keratocyte density appeared to increase with increasing severity of keratoconus (P<0.05). However, comparing control Groups A and B, contact lens wear per se, was associated with significantly reduced (P=0.000) keratocyte density in the anterior stroma (609+/-66 cells/mm(2)) and demonstrated a trend (P=0.056) in the posterior stroma (470+/-63 cells/mm(2)). Keratoconic corneas (429+/-72 microm) were significantly thinner than control Groups A (508+/-77 mm) and B (495+/-80 microm). The presence of keratoconus did not affect the endothelial cell density (P=0.54).

CONCLUSION

In vivo confocal microscopy can provide insight into the microstructural changes that occur in keratoconus.

Genomewide linkage scan in a multigeneration Caucasian pedigree identifies a novel locus for keratoconus on chromosome 5q14.3-q21.1

PURPOSE

Keratoconus is a corneal dystrophy with an incidence of 1 in 2000 and a leading cause for cornea transplantation in Western developed countries. Both clinical observations and segregation analyses suggest a major role for genes in its pathogenesis. It is genetically heterogeneous, most commonly sporadic, but inherited patterns with recessive or dominant modes have also been reported. We studied a four-generation autosomal-dominant pedigree to identify disease loci for keratoconus.

METHODS

A two-stage genome-wide scan was applied to 27 family members. First linkage analysis was performed with 343 microsatellite markers along the 22 autosomal chromosomes at approximately 10 cM density. This was followed by fine mapping at approximately 2 cM density, in regions suggestive of linkage. Multipoint linkage analysis was performed using GeneHunter2.

RESULTS

Evidence of suggestive linkage from the initial scan was observed at the 82 to 112 cM region of chromosome 5q14.1-q21.3 with a maximum lod score (LOD) of 3.48 (penetrance = 0.5). Fine mapping by testing an additional 11 microsatellite markers at 1 to 3 cM intervals revealed a narrower and higher peak (99-119 cM) with LOD 3.53. By analysis of the recombination of haplotypes, the putative locus of keratoconus was further narrowed to a 6 cM region (8.2 Mbp physical distance) between markers D5S2499 and D5S495.

CONCLUSION

These results indicate a promising new locus for keratoconus in this pedigree. Because of the heterogeneous nature of keratoconus, this locus may be specific to familial autosomal-dominant keratoconus. Nevertheless, the identification of this locus may provide new insights into the pathogenesis of keratoconus.