Overview
Over the past century, a number of corneal diseases have been documented with detailed family histories suggesting autosomal-dominant, autosomal-recessive, and X-linked recessive hereditary patterns. Modern genetic techniques such as whole-genome linkage analysis and gene sequencing have led to the discovery of specific gene mutations (genotypes) which correlate with specific disease presentations of clinical signs (phenotypes). For many of these clearly defined hereditary corneal dystrophies, the discovery of the underlying genetic mechanism has led to an understanding at the molecular level of the disease pathophysiology.
The hereditary corneal dystrophies subsequently described are, in order of the primary corneal layer most affected, epithelium, Bowman layer, stroma, Descemet’s membrane, and endothelium. Fuchs’ dystrophy is covered in another chapter. Some designations of the hereditary corneal dystrophies have recently been changed because of new histopathologic and genetic data suggesting distinct disease categories, such as corneal dystrophies of the Bowman layer type I and II, and this has clarified the differences between Reis–Bücklers and Thiel–Behnke dystrophies. Some dystrophies appear to have the same gene involved with slight differences in the clinical presentation. These similar hereditary corneal dystrophies have been grouped together with a mention of the historical reporting and similarities in clinical presentations, such as with Meesmann’s and Stocker–Holt dystrophies. Gene names are italicized. Where mutations are known to be causative of certain hereditary corneal dystrophies and result in amino acid changes at particular codons, the substitution of the wild type for the mutant amino acid will be given in full. In subsequent references, the mutation will be given as standard abbreviated designations. As an example, in the 124 codon of keratoepithelin (KE), a cysteine is substituted for arginine in lattice corneal dystrophy I (LCDI). Thereafter this mutation would be referred to as Arg124Cys.
Epithelial dystrophies
Meesmann corneal dystrophy (MCD) MIM 122100 (Stocker–Holt dystrophy)
Clinical background
MCD is characterized by numerous epithelial microcysts which can be noted in early childhood. The discrete, round cysts usually become more numerous with age. If in later years the microcysts erode the surface, affected individuals can become symptomatic with foreign-body sensation, photophobia, and decreased vision. Pameijer made the first clinical description of the disease in 1935, with Meesmann and Wilke describing the histologic features in 1939. In 1964, Kuwabara and Ciccarelli found aggregates of electron-dense material in corneal epithelial sheets studied by electron microscopy, termed “peculiar substance.” Stocker and Holt in 1955 reported families from Moravia who had microcysts apparent early in life, leading to decrease in vision, light sensitivity, and tearing. Irvine et al in 1997 reported mutations in the KRT3 and KRT12 genes cause MCD. Klintworth et al later identified a mutation in the KRT12 gene in a family with microcysts described by Stocker and Holt.
Pathology
Epithelial cells contain an intermediate filament cytoskeleton which protects against trauma. Keratins are expressed in pairs and keratin 3 and 12 are produced in the corneal anterior epithelium. Aggregation of the abnormal keratins occurs within the epithelium, resulting in microcysts. Environmental factors, such as wearing contact lenses, may contribute to epithelial fragility, worsening the disease and contributing to symptoms.
Pathophysiology
Mutations in keratin KRT3 and KRT12 genes have been demonstrated to be causative of MCD. Mutations have been reported as missense substitutions in the conserved helix initiation motif of KRT12 or in the helix termination motifs in KRT12 and KRT3 . These motifs are involved in the assembly of intermediate filaments. Mutations which occur in the helix boundary motifs of KRT5 and KRT14 are associated with the severe Dowling–Meara form of hereditary epidermolysis bullosa simplex. Interestingly, a thickened corneal epithelial basement membrane has been reported in epidermolysis bullosa disease ( Figure 4.1 ).
Epithelial basement membrane corneal dystrophy (Cogan’s microcystic dystrophy; map-dot-fingerprint dystrophy) (EBMD) MIM 121820
Clinical background
In EBMD, reduplicated basement membrane is noted bilaterally in patterns of microcystic dots, map-like sheets, and fingerprint or horsetail lines. The map pattern is often described as grayish-white patches. The majority of patients are asymptomatic but some have painful recurrent erosions.
Vogt, in 1930, first described the condition which Cogan et al further characterized with a histopathological examination that clarified the microcystic nature of the dystrophy. Guerry noted in 1950 the fingerprint lines which later became associated with the dystrophy and also made the observation in 1965 of the map-like changes characteristic of the disease. In 1974 Krachmer and Laibson noted the hereditary pattern of the disease as autosomal dominant and most commonly affecting middle-aged and older adults. In 2006, Boutboul et al reported mutations in the TGFB1/BIGH3 gene in patients with EBMD.
Pathology
The different manifestations of EBMD, map, dot, and fingerprint, are all characterized by abnormal deposition of multilaminar basement membrane. Inverted basal cell layers, which continue to proliferate, cause the formation of the characteristic microcysts. The multilaminar basement lacks the adhesive strength of normal basement membrane and thus contributes to epithelial sloughing and the development of recurrent erosions.
Pathophysiology
Two point mutations in the TGFB1/BIGH3 gene were noted in patients with EBMD, resulting in a leucine to arginine shift at codon 509 in one pedigree, and arginine to serine at codon 666 in another pedigree. Mutations in TGFB1/BIGH3 cause a number of corneal dystrophies and are believed to result from alterations in the TGFB1/BIGH3 -encoded protein, keratoepithelin (KE). KE is secreted in the extracellular matrix and is believed to bind various collagens. The Leu509Arg and the Arg666Ser mutations have not been associated with other TGFB1/BIGH3 -associated dystrophies. The Leu509Arg and the Arg666Ser mutations could result in a misfolding of the protein, loss of function, and an increase in the epithelial extracellular matrix.
Band-shaped, whorled microcystic corneal dystrophy (Lisch corneal dystrophy)
Clinical background
Unilateral or bilateral gray intraepithelial opacities that are band-shaped and feathery, sometimes in a whorled pattern, characterize the disease. The microcysts are in a dense pattern as opposed to those noted in Meesmann’s dystrophy. No symptoms are associated with the condition.
The condition was first noted by Lisch et al in 1992. Linkage of the dystrophy to Xp22.3 was noted by Lisch et al in 2000, confirming that the disease is likely unrelated to Meesmann’s dystrophy, which has been associated with mutations of the KRT3 and KRT12 genes.
Pathology
The pathological mechanism involved in the disease remains unknown. However, histopathology demonstrates vacuolization of basal epithelial cells as opposed to the fibrillogranular or peculiar substance noted in Meesmann’s dystrophy. As yet the underlying genetic mechanism of the disease remains undetermined.
Bowman membrane dystrophies
Corneal dystrophy of the Bowman layer type I (CDBI) MIM 608470 (Reis–Bücklers dystrophy)
Clinical background
Corneal dystrophy of the Bowman layer type I (CDBI) is an extremely rare autosomal-dominant disease characterized by confluent geographic opacities in the Bowman layer. Patients typically have recurrent corneal erosions which can be quite painful. Vision loss can occur early and can be severe.
Reis described the disease in 1917 and Bücklers in 1949 provided further follow-up of Reis’ pedigree. Küchle et al, in 1995, proposed distinguishing Reis–Bücklers dystrophy from another anterior stromal dystrophy (Thiel–Behnke) with similar signs and symptoms by referring to them as CDBI and CDBII. Okada et al, in 1998, described a mutation in the TGFB1/BIGH3 gene encoding the protein keratoepithelin (KE), with an amino acid change of leucine for arginine at codon 124.
Pathology
CDBI is characterized by the destruction of Bowman’s layer with the deposition of granular band-shaped material and irregular epithelium. The deposits and irregular epithelia can be noted by light microscopy and electron microscopy. The staining patterns are similar to granular corneal dystrophy.
Pathophysiology
Mutations in the TGFB1/BIGH3 gene have been associated with a number of corneal dystrophies with varied phenotypes. The TGFB1/BIGH3 gene encodes the KE protein with position 124 as a “hot spot” for mutations. The increased severity of the disease in CDBI is believed to be related to the amino acid replaced at codon 124 with a leucine for an arginine. Leucine is hydrophobic and arginine is charged polar, a change which would result in a severe alteration in the KE protein. The Arg124Leu mutation is characterized by a nonamyloid-type deposition and appears not to affect abnormal proteolysis of KE. A summary of the genetics and pathogenesis of CDBI and CDBII and several other hereditary corneal dystrophies is given in Box 4.1 .
| TGFBI (5q31)-associated dystrophies | Exon | Amino acid change | Histopathology/deposit type |
|---|---|---|---|
| Corneal dystrophy of Bowman’s layer type I (CDB I) | 4 | Arg124Leu |
|
| Reis–Bücklers dystrophy | |||
| Corneal dystrophy of Bowman’s layer type II (CDB II) | 12 | Arg555Gln |
|
| Thiel–Behnke dystrophy | |||
| Honeycomb-shaped dystrophy | |||
| Granular dystrophy type I (GCD I) | 12 | Arg555Trp |
|
| Groenouw type I | |||
| Lattice corneal dystrophy I (LCD I) | 4 | Arg124Cys |
|
| Biber–Habb–Dimmer dystrophy | |||
| Combined granular lattice corneal dystrophy (CGLCD) | 4 | Arg124His |
|
| Avellino corneal dystrophy | |||
| Epithelial basement membrane corneal dystrophy (EBMD) | |||
| Cogan’s microcystic dystrophy |
|
|
|
| Map-dot-fingerprint dystrophy |
Corneal dystrophy of the Bowman layer type II (CBDII) MIM 602082 (Thiel–Behnke or honeycomb dystrophy)
Clinical background
CDBII (Thiel–Behnke) dystrophy is an autosomal-dominant disease that is more common than CDBI (Reis–Bücklers). The dystrophy is characterized clinically by honeycomb-shaped opacities occurring at the level of Bowman’s membrane. Vision is not usually as severely affected as it is in CDBI; however, patients often have recurrent erosions.
Thiel and Behnke described the condition in 1967 as an anterior stromal dystrophy distinct from Reis–Bücklers. Küchle et al proposed that Thiel–Behnke was indeed distinct from Reis–Bücklers, was more common, and had distinct histopathological features. They proposed that this disease be referred to as CDBII. Okada et al, in 1998, described a mutation in the TGFBI/BIGH3 gene resulting in an amino acid change in the KE protein, glycine for arginine at codon 555.
Pathology
In CDBII, the epithelium is usually irregular due to iron deposition. Bowman layer is either mostly or totally absent. Interposed fibrous tissue between the epithelium and the stroma is noted in an undulating or “sawtooth” pattern. On transmission electron microscopy, peculiar collagen filaments or “curly fibers” are found.
Pathophysiology
CDBII has been reported to be caused by mutations in TGFBI/BIGH3, resulting in substitution of glycine for arginine at codon 555 in the KE protein. This Arg555Gln mutation would be expected to alter the secondary structure of the KE protein and could result in the precipitation of the protein and the honeycomb pattern characteristic of the disease ( Figure 4.2 ).
Stromal dystrophies
Granular dystrophy type I (GCD1) MIM 1219000 (Groenouw type I)
Clinical background
The breadcrumb-type lesions of the dystrophy can become apparent in the first decade of life, and, as the disease progresses, the lesions become discrete corneal opacities, mostly in the central anterior cornea. With further progression the opacities coalesce but the peripheral cornea usually remains clear. Visual acuity is usually mildly affected, but patients who are homozygous for the Arg555Trp mutation are more likely to be more severely affected with symptoms at an earlier age. Epithelial erosions are common.
Groenouw described a corneal dystrophy with autosomal-dominant inheritance that had large numbers of small, irregular discrete opacities in the central cornea. The larger opacities appear nodular, raise the epithelium, and give the corneal surface an irregular appearance – thus his designation of a “nodular degeneration.” Groenouw studied a small biopsy specimen from one of his patients and noted the material was positive with an acidophilic stain and was likely hyaline in nature. As opposed to the lattice dystrophies, which occur commonly in the Japanese population, GCD1 and the Arg555Trp mutation in the TGFB1 are rare in Japan.
Pathology
The distinct corneal opacities stain red with Masson trichrome and the noted rod-shaped bodies with discrete borders can be detected by electron microscopy.
Pathophysiology
A mutation in the TGFB1/BIGH3 gene that results in the substitution of tryptophan for arginine at codon 555, Arg555Trp, in the KE protein is responsible for the disease. The deposits in GCD1 are believed to be accumulations of mutant KE protein. The Arg555Trp mutant is associated with nonamyloid phenotypes as well as the other Arg555 mutant CDBII (Arg555Gln) ( Figure 4.3 ).
Lattice corneal dystrophy I MIM 122200 (Biber–Habb–Dimmer dystrophy)
Clinical background
The dystrophy, which is bilateral but can be asymmetric, usually begins late in the first or early in the second decade with progressive branching linear opacities. These linear arrays are mostly in the central cornea. As the dystrophy progresses, a generalized haziness develops in the central cornea while the peripheral cornea remains clear. Recurrent erosions occur early in the course of the disease. As the disease progresses the opacities can coalesce, with resultant declining vision, usually in the fourth to sixth decade.
Biber, in 1890, described this dystrophy as gitterige Keratitis, noting branching twig-like patterns with a clear peripheral cornea. Haab further described a lattice-like appearance and, along with Dimmer in 1889, recognized that the disease appeared inheritable. Seitelberger and Nemetz determined that lattice dystrophy was a localized amyloid degeneration. Munier et al in 1997 noted mutations in TGFB1/BIGH3, resulting in the substitution of cysteine for arginine at codon 124 in the encoded protein KE in patients with lattice dystrophy.
Pathology
Amyloid deposits, which stain positive with Congo red and periodic acid–Schiff, are found throughout the stroma. On electron microscopy, irregular deposits are noted interspersed among the collagen lamellae.
Pathophysiology
Mutations in TGFB1/BIGH3 gene, which encode KE proteins, are responsible for the protein amyloid deposits noted in the disease. Mutation “hot spots” have been found at the 124 codon position of the protein as multiple families with this mutation have been screened and identified. Haplotype analysis of these families demonstrates that these mutations have arisen independently and do not share a common ancestor. Amyloidogenesis in LCDI with the Arg124Cys mutation occurs with the accumulation of N-terminal fragments of KE. It is believed that amyloidogenesis in the Arg124Cys mutated cornea is associated with abnormal proteolysis of the protein. Because there is no other evidence of systemic amyloid deposition in patients with the Arg124Cys mutation, there are likely tissue-specific factors that lead to KE fragment aggregation. Evidence suggests that the Arg124Cys mutation in KE affects protein structure, resulting in increased beta sheet content. Korvatska et al have proposed that the Arg124Cys mutation abolishes a critical site of proteolysis of the KE protein that is essential for normal turnover of the protein ( Figure 4.4 ).
Lattice corneal dystrophy type II MIM 105120 (familial amyloid polyneuropathy type IV (Finnish or Meretoja type))
Clinical background
In this hereditary systemic amyloidosis, in the third decade lattice-type lines appear which are fewer in number than LCDI and begin in the periphery. The central cornea is spared until later when vision can be affected, usually mildly. If the disease is homozygous for the mutant gelsolin protein, disease onset is earlier. The corneal findings are part of a systemic amyloidosis which involves cranial nerves, causing nerve palsies and affecting the skin with lichen amyloidosis and cutis laxa, leading to frozen facial features. Corneal nerves may be affected, leading to an anesthetic cornea.
Meretoja described in 1969 a family with systemic amyloidosis and a lattice type dystrophy. Klintworth recognized the corneal clinical findings as different from LCDI and termed this lattice dystrophy LCDII. Paunio et al described a mutation in the GSN gene, which encodes the protein gelsolin, in affected patients with Finnish-type familial amyloidosis. Most cases have a Finnish origin but families with the disease have been identified in Japan, Portugal, Czech Republic, and Denmark. Amyloid positivity for antigelsolin antibody, along with genetic testing, can confirm the diagnosis. The associated systemic findings for LCDII and several other hereditary corneal dystrophies are given in Box 4.2 .
| Corneal dystrophies | Associated systemic diseases/symptoms |
|---|---|
| Lattice corneal dystrophy type II (LCDII) |
|
| Familial amyloid polyneuropathy type IV: Finnish or Meretoja type | |
| Schnyder crystalline corneal dystrophy (SCCD) |
|
| Pre-Descemet dystrophy with ichthyosis (XLRI) |
|
| Harboyan syndrome congenital dystrophy and perceptive deafness (CDPPD) | Sensorineural deafness |
| Posterior polymorphous dystrophy (PPCD, PPMD) |
|
Pathology
Gelsolin is an actin-modulating protein that is expressed in most tissues. The amyloid deposits in LCDII consist of gelsolin fragments which coalesce underneath the corneal epithelium and the anterior stroma. There is a mostly continuous deposition of this amyloid beneath Bowman’s layer. Less amyloid deposition occurs in LCDII than in LCDI.
Pathophysiology
A substitution of asparagine for aspartic acid at codon 187 in the GSN gene encodes a mutated gelsolin protein. The accumulated gelsolin protein fragments are responsible for the amyloid deposits ( Figure 4.5 ).
Combined granular-lattice dystrophy (CGLCD) OMIM 607541 (Avellino corneal dystrophy)
Clinical background
The dystrophy becomes manifest in the second decade. By biomicroscopy, it has discrete gray-white opacities in the superficial to anterior one-third of the stroma. Intervening stroma can be hazy and linear opacities can be observed, while the periphery is clear. The disease progression is slower than in GCD or LCDI and vision is usually not severely affected. Corneal erosions are less common than with GCD.
In 1988, Folberg et al presented four patients from three families with clinical features similar to granular dystrophy but with histopathologic features similar to lattice dystrophy (LCDI) with fusiform stromal deposits of amyloid. In addition, deposits that appear morphologically similar to what is noted in GCD did not react with the usual histochemical stains. Folberg et al traced the ancestry of these families to Avellino, Italy; hence in some literature the disease is referred to as Avellino corneal dystrophy. The disease has been noted in many countries, particularly in Japan.
Pathology
In CGLCD granular deposits are noted in the anterior third of the stroma. Amyloid can be detected in some granular deposits. Typical fusiform deposits, identified as amyloid, are noted deep to granular deposits. CGLCD is associated with a mutation in the TGFB1/BIGH3 gene resulting in a substitution of histidine for arginine at codon 124, Arg124His, in the KE protein. Patients homozygous for the Arg124His mutation have much more severe disease.
Pathophysiology
The Arg124His mutation in the KE protein had mostly nonamyloid inclusions. The accumulation of the pathologic KE also occurred with abnormal proteolysis of the protein. A unique 66-kDa KE protein was noted in CGLCD and could be responsible for the deposits found in the disease ( Figure 4.6 ).
Gelatinous drop-like corneal dystrophy (GDLD) MIM 204870 (primary familial subepithelial corneal amyloidosis)
Clinical background
This dystrophy is characterized by severe corneal amyloidosis which can lead to marked visual impairment. At an early stage of the disease, whitish-yellow subepithelial and nodular lesions are noted centrally. As the lesions coalesce, a “mulberry” appearance with a whitish-yellow color occupies the central cornea. Ide et al have classified these different clinical presentations as band keratopathy type, stromal opacity type, kumquat-like type, and typical mulberry type.
Nakaizumi first reported this rare dystrophy in a Japanese patient in 1914. The disease occurs in about one in 300,000 of the general population in Japan with scattered reports in other countries and is inherited as an autosomal-recessive disorder. Tsujikawa et al in 1999 found GDLD to be a result of a mutation in the M1S1 gene.
Pathology
GDLD is an autosomal-recessive disorder with mutations in the M1S1 gene localized to chromosome 1p. The commonest mutation resulted in a glutamine replaced with a stop at codon 118. Sixteen of 20 members of the families studied were homozygous for the Q118X mutation. All alleles studied carried the disease haplotype which strongly suggested that the Q118X mutation is the major mutation in the Japanese GDLD patients. Other nonsense and frameshift mutations have been noted in the M1S1 gene.
Pathophysiology
The function of the M1S1 protein is not understood. The M1S1 Q118X mutation and other mutations predict a truncated protein with loss of function or aggregation of the M1S1 protein. Cells transfected with the truncated M1S1 protein demonstrate aggregate perinuclear cytoplasmic bodies, supporting the possibility that an aggregation of protein leads to the formation of amyloid deposits and is responsible for the disease ( Figure 4.7 ).
Macular corneal dystrophy (MCD) MIM 217800 (Groenouw type II)
Clinical background
MCD is characterized by progressive bilateral corneal clouding beginning in the first decade with grayish opacities and poorly defined borders. The opacities start centrally and can extend throughout the stroma, leading in most cases to corneal thinning. The diffuse opaque spotty clouding is initially noted in the superficial central cornea and spreads peripherally and into deeper stroma with age. The endothelium and Descemet’s membrane can be affected with the development of guttae. Severe visual impairment can occur as early as the age of 40. The disease is rare except in Iceland.
Groenouw described the characteristics of MCD in his original report of corneal nodular dystrophies along with the clinical findings of granular corneal dystrophy. The two diseases have been referred to as Groenouw type II and Groenouw type I, respectively. Jones and Zimmerman demonstrated accumulation of acid mucopolysaccharide and Klintworth and Vogel found that MCD is an inherited storage disorder of mucopolysaccharide in corneal fibroblasts in 1964. Hassell et al, in 1980, found that failure to synthesize a mature keratan sulfate proteoglycan was responsible for the disease. Akama et al, in 2000, found that the carbohydrate sulfotransferase gene ( CHST6 ), encoding an enzyme designated corneal N -acetylglucosamine-6-sulfotransferase, was responsible for MCD I and II.
Studies of mutations in this gene in multiple populations have demonstrated marked heterogeneity with many different missense mutations, deletions, and insertions.
In the diagnostic workup of MCD, the dystrophy has been divided into three subtypes (MCD type I, IA, and II) based on the immunoreactivity of the patient’s serum and cornea to an antibody to sulfated keratan sulfate. MCD I has no reactivity of the antibody to serum or the cornea. In MCD IA, antigenicity is missing in the serum and cornea but can be detected in keratocytes. MCD II has reactivity in the cornea and in the serum.
Pathology
Sulfation of polylactosamine, the nonsulfated precursor to keratan sulfate, is critical to obtaining proper hydration of the stroma and maintaining corneal clarity. The CHST6 gene encodes the enzyme N -acetyl glucosamine-6-sulfotransferase which catalyzes the sulfation of polylactosamine of the keratan sulfate containing proteoglycans in the cornea.
Pathophysiology
It is yet unknown how the various mutations in the CHST6 gene cause disease. However, due to the high degree of mutational heterogeneity found in patients with this disease and this gene, it is believed that loss of function with deficient enzyme activity is responsible for the dystrophy ( Figure 4.8 ).
