1. Corneal disease

A wide range of inherited conditions, both ocular and multisystemic,

are associated with abnormalities of corneal development, shape,

clarity or integrity.

Corneal size

Mean corneal diameter is around 10 mm at birth reaching a

maximum of 12.5 mm at 2 years. Abnormalities of corneal size

include microcornea (diameter <11 mm) and megalocornea

(diameter >13 mm). Microcornea is often associated with other

ocular abnormalities including microphthalmos (q.v.), coloboma

or cataract. Isolated megalocornea, in the absence of other ocular

abnormalities, is an uncommon, X-linked recessive condition of

benign prognosis. It is often initially diagnosed as congenital

glaucoma but may easily be distinguished by a high endothelial

cell count.

Corneal shape

Cornea plana is associated with non-progressive reduced corneal

curvatures. Increased corneal curvature is seen particularly in corneal

ectasias, a subgroup of the corneal dystrophies. Of these, keratoconus

is the most common. Isolated keratoconus affects 1:2–5000 of the

population and has a well-recognized genetic predisposition, although

the genetic etiology remains undefined. A small number of cases have

been shown to carry mutations in the ocular transcription factor VSX1.

The systemic associations include Down syndrome, Ehlers-Danlos

syndrome and Leber congenital amaurosis.

Anterior segment

Corneal abnormalities are a feature of many forms of the developmental

dysgenesis

anterior segment disorders. In Peters anomaly, a severe developmental

abnormality, corneal opacification (often central) is associated with

a defect in Descemet's membrane often with adhesions between

the cornea, the lens and/or iris. Hereditary forms are recognized

including defects of PAX6, PITX2, CYP1B1 and MAF. Aniridia may

be associated with abnormal peripheral corneal vascularization,

which is often progressive and may be troublesome in later life.

2

Genetics for Ophthalmologists

Corneal

The corneal dystrophies have been defined as inherited, bilateral,

dystrophies

slowly progressive disorders that alter corneal function in the

absence of inflammation and systemic sequelae. In most cases this

definition holds true, although exceptions can be found including

unilateral cases, those of rapid progression and those with systemic

associations. Traditionally, corneal dystrophies have been subdivided

according to the anatomical site at which the presumed defect

is located, i.e. anterior, stromal and posterior, or endothelial

dystrophies, with a remaining group of ectatic dystrophies. Such

morphological classification becomes increasingly cumbersome as

the genetic bases for these disorders are defined. It is now evident

that a remarkable number of supposedly distinct dystrophies share

a common molecular etiology (see Table 1). While a morphological

classification remains valid, this may prompt its re-evaluation.

Inborn errors of

A number of metabolic disorders are associated with corneal

metabolism

manifestations. These include Wilson disease or hepatolenticular

degeneration (Kayser-Fleischer ring), Fabry disease (vortex

keratopathy), mucopolysaccharidoses (corneal clouding) and

cystinosis (crystal deposition).

The following section includes a relatively small number of corneal

dystrophies and isolated corneal developmental abnormalities

for which the precise molecular defect has been defined. The

chromosomal location is known for a number of other, similar

conditions (see Table 2) and it is likely that this group will expand in

the near future as the genes that underlie other corneal dystrophies

are discovered.

Corneal disease

3

Condition

Exon

Mutation

Reis-Bucklers

4

R124L

Thiel-Behnke

12

R555Q

Granular

12

R555W

Avellino

4

R124H

Lattice type I

4

R124C

Condition

MIM

Inheritance

Chromosome

Gene

Meesmann

122100

AD

12q, 17q

K3, K12

Lisch

XL

Xq22.3

-

Cogan (Map-Dot Fingerprint)

121820

AD

-

-

Gelatinous drop-like

204870

AR

1p31

M1S1

Reis-Bucklers

121900

AD

5q31

BIGH3

Thiel-Behnke

121900

AD

5q31

BIGH3

Thiel-Behnke

602082

AD

10q24

-

Granular

121900

AD

5q31

BIGH3

Avellino

121900

AD

5q31

BIGH3

Macular

217800

AR

16q22

CHST6

Lattice type I

122200

AD

5q31

BIGH3

Lattice type II

105120

AD

9q34

Gelsolin

Lattice type IIIa

122200

AD

5q31

BIGH3

Schnyder crystalline

121800

AD

1p34.1–p36

-

Fuchs

136800

AD

-

-

Posterior polymorphous

122000

AD

20p11.2–q11.2

VSX1

Congenital hereditary endothelial

121700

AD

20cen

-

Congenital hereditary endothelial

217700

AR

20p13

-

4

Genetics for Ophthalmologists

MIM

217300; 603288 (KERA)

Clinical features

Both dominant and recessive forms are described, with the latter

being the more severe. Anterior segment abnormalities include

extreme hypermetropia (+10 D or more), a hazy corneal limbus

with stromal opacities and marked arcus juvenilis. The cornea is

thin with an indistinct sclerocorneal boundary. The two forms are

distinguished by a round thickened central opacity, approximately

5 mm in width, which is seen in most recessive cases but not those

of dominant inheritance. Iris malformations and iridocorneal

adhesions are more prevalent in the recessive form. Mild

microphthalmia may be present in the recessive form.

Age of onset

Congenital

Epidemiology

Rare in the UK. Although prevalence is uncertain, the condition has

been described in autosomal dominant form in Cuba and autosomal

recessive form in Finland (carrier frequency estimated at 1:126 in

north-east Finland).

Inheritance

Autosomal dominant; autosomal recessive

Chromosomal location

12q21.3–q22 (autosomal recessive). Linkage analysis of a

dominant form in a Cuban kindred confirmed linkage to the region

of 12q implicated in recessive cornea plana. Finnish families with a

dominant form of cornea plana do not link to 12q, suggesting two

distinct dominant forms.

Gene

Keratocan (KERA) (recessive form)

Corneal disease

5

Diagnosis

Clinical examination

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Cornea plana

MIM

204870; 137290 (M1S1)

Clinical features

Amyloid accumulation beneath the epithelium produces whitish

deposits that are said to resemble a mulberry. These accumulations

lead to photophobia, discomfort and reduced visual acuity.

Histologically the amyloid deposits are seen above Bowman’s

layer and within the epithelium as well as within the stroma.

Age of onset

Amyloid begins to accumulate in the cornea during the first two

decades of life.

Epidemiology

1:300,000 (Japan). Rare but not unknown outside Japan.

Inheritance

Autosomal recessive

Chromosomal location

1p31

Gene

Membrane component, chromosome 1, surface marker 1 (M1S1)

Mutational spectrum

Four nonsense mutations described in a Japanese cohort of patients.

Effect of mutation

Protein truncating mutations result in the production of a shortened

amyloidogenic protein that accumulates within the tissues of the

anterior cornea. The function of the protein is unknown.

Diagnosis

GLDL is a clinical diagnosis. Genetic testing is not widely available

and does not alter clinical management. Molecular analysis of

patients with ‘gelatino-lattice’ dystrophy—in which there are

overlapping features of type I lattice dystrophy and GLDL—has

revealed a mutation in BIGH3 (R124C, see lattice dystrophy) but

none in M1S1. It is likely that this does not represent GLDL.

Corneal disease

7

8

Meesmann corneal dystrophy

Chromosomal location

17q12 (KRT3); 12q13 (KRT12)

Gene

Cornea-specific keratins: keratin 3 (KRT3) and keratin 12 (KRT12).

Mutational spectrum

Missense mutations within the helix-initiation or helix-termination

motifs of KRT3 or KRT12. The majority have been described in

KRT12, within the helix-initiation region.

Effect of mutation

Mutations act in a dominant negative manner leading to

defective epithelial cytoskeletal function and epithelial fragility.

Ultrastructural examination reveals cytoplasmic densities, which

are likely to represent tonofilament clumping as seen in other

dominant keratin disorders.

Keratin proteins are structurally important intermediate filaments

found in epithelia. The family of proteins is divided into type I (acidic,

K9–K21) and type II (neutral or basic, K1–K8) and their expression is

tissue-specific. Keratins 3 and 12 are coexpressed (paired) to form a

heterodimer which is specific to the corneal epithelium.

Several inherited epidermal diseases, such as epidermolysis bullosa

simplex, are caused by keratin mutations. Each protein contains

a helical domain flanked by a helix-initiation motif and a helix-

termination motif. These are highly conserved and thought to play

important roles in filament assembly and stability; they are

recognized as a mutation hotspot.

Diagnosis

Diagnosis is usually clear from history and slit-lamp examination.

Mutation analysis is available on a research basis only, and does

not alter clinical management.

Corneal disease

9

Chromosomal location

5q31

Gene

Beta-Ig-H3/transforming growth factor, beta-induced

(BIGH3/TGFB1).

Mutational spectrum

There is a tight genotype-phenotype correlation amongst mutations

in the BIGH3 gene (see granular dystrophy).

CDBI is caused by an R124L mutation in exon 4. A single

Sardinian family has been described with Reis-Bucklers

dystrophy, a trinucleotide deletion of exon 12 (∆F540).

Effect of mutation

The R124L mutation is thought to cause abnormal folding of the

BIGH3 protein (see granular dystrophy).

Diagnosis

Slit-lamp examination may be sufficient for diagnosis, aided by

histopathological examination of the diseased cornea after surgery.

In CDBI there is accumulation of Masson trichrome positive material

above Bowman's layer, but may also be present in the anterior

stroma. The material is characterized by rod-shaped bodies on

electron microscopy and has led to the condition being called

‘superficial granular dystrophy’.

Since the different mutations causing the Bowman’s layer

dystrophies have clear phenotypic effects, confirmation of the

diagnosis by molecular testing may aid prediction of prognosis and

speed of progression.

Corneal disease

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MIM

602082 (type II)

Clinical features

CDBII usually presents with frequent recurrent erosions within the

first years of life. Bilateral superficial opacification in a honeycomb

pattern develops during early adult life but visual acuity is less

severely affected than in CDBI.

CDB type II or honeycomb dystrophy. Irregular, honeycombed opacification is seen

at the level of the Bowman’s layer.

Age of onset

CDBII presents with recurrent erosions in the first decade of life.

Chromosomal location

5q31 (BIGH3)

10q24 (see below)

Mutational spectrum

As mentioned previously, there is a tight genotype-phenotype

correlation amongst mutations in the BIGH3 gene (see granular

dystrophy).

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Bowman’s layer dystrophy type II

CDBII is caused by an R555Q mutation in exon 12. Confusion is

added by the description of a family with a superficial corneal

dystrophy (also called Thiel-Behnke dystrophy or CDBII) which is

linked to chromosome 10q24 and is not caused by mutations in the

BIGH3 gene. This suggests further, poorly delineated, heterogeneity

amongst the superficial Bowman’s layer dystrophies.

Effect of mutation

Mutations are thought to cause abnormal folding of the BIGH3

protein (see granular dystrophy).

Diagnosis

Slit-lamp examination may be sufficient for diagnosis.

Histopathological examination of the diseased cornea after surgery

will facilitate diagnosis. In CDBII, the Bowman’s layer is replaced

by a fibrous, paucicellular layer of variable thickness between the

epithelium and stroma. On electron microscopy this material

demonstrates the presence of short, twisted ‘curly fibers’.

Since the different mutations causing Bowman’s layer dystrophies

have clear phenotypic effects, confirmation of the diagnosis by

molecular testing may aid prediction of prognosis and speed of

progression.

Corneal disease

13

MIM

121900; 601692 (TGFBI)

Clinical features

Granular corneal dystrophy is characterized by the progressive

development of discrete, grayish-white opacities within the central

anterior corneal stroma. The condition is bilateral and symmetrical

and the intervening stroma remains clear. The number of opacities

increases with time and their position within the stroma deepens but

the limbal region of the cornea is spared. Two main forms of granular

dystrophy exist.

Classical granular dystrophy

Visual acuity often deteriorates during the third decade. Such a decline

continues to a point where penetrating keratoplasty is required, often

during the fifth decade of life. Corneal erosions are described in the

condition but do not represent a major symptom. While penetrating

keratoplasty is effective in improving vision the condition recurs,

presumably as a result of deposition of granular material into the graft

from the recipient’s epithelium. Histopathological examination of the

diseased cornea after surgery reveals anterior stromal opacities which

stain red with the Masson trichrome stain.

14

Granular corneal dystrophy

Corneal disease

15

Age of onset

In classical granular dystrophy, symptoms of photophobia are seen

within the first decade with visual acuity remaining good during

childhood. In the atypical form, symptoms may not present until

the third decade.

Mutational spectrum

Mutations in the BIGH3 gene show very strong genotype-phenotype

correlation. The classical form of granular corneal dystrophy is

caused by an arginine to tryptophan substitution of amino acid

555 (R555W). Atypical, or Avellino, dystrophy is caused by an

arginine to histidine substitution of residue 124 (R124H). A

third mutation (R124S) has also been described in a late-onset

form of granular dystrophy. The arginine residues at positions

124 and 555 are important in the development of a number of

corneal dystrophies.

Effect of mutation

Both mutations are thought to cause abnormal folding of the BIGH3

protein which results in abnormal aggregates or deposits of the protein

within the cornea. The BIGH3 protein is an extracellular matrix

molecule, which is induced by TGFb. The protein is widely expressed

and, in the cornea, is produced by the epithelium and stromal

keratocytes. It is thought to be important in the wound-healing

response although its exact function in the cornea is not yet defined.

The granular deposits are identical in both forms of granular

dystrophy (R555W and R124H). However, deposition of amyloid

material, as observed in lattice corneal dystrophy, is seen in some

cases of atypical granular dystrophy.

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Granular corneal dystrophy

Atypical granular dystrophy – a teenage girl with severe granular-like corneal

dystrophy. She is homozygous for the R124H mutation. Recurrences within the

grafts were frequent and severe.

Diagnosis

Slit-lamp examination may be sufficient for diagnosis. Since the

different mutations have clear phenotypic effects, confirmation of the

diagnosis by molecular testing may aid the prediction of prognosis.

While granular dystrophy is autosomal dominant, the condition is

strictly semi-dominant: homozygous patients within consanguineous

families (i.e. those with two affected parents) show a severe and

early-onset form of the disease which shows rapid progression and

marked, early visual loss. In these cases the disorder recurs rapidly

within grafted tissue (see above).

Corneal disease

17

MIM

122200; 601692 (TGFBI)

Clinical features

LCD is characterized by the development of anterior stromal

opacities. In LCDI these are gray, linear and fine, situated mainly

within the central cornea. The intervening cornea remains clear

initially but becomes progressively hazy. As in granular dystrophy,

the opacities do not extend to the limbus. Erosions may begin early,

even in childhood, while visual acuity is usually normal until early

adulthood. Grafting is usually required from the third decade.

Recurrence within the graft can lead to further visual deterioration.

The histologic findings are of congophilic deposits that have the

characteristics of amyloid protein.

In some forms of autosomal dominant LCD, termed lattice corneal

dystrophy type IIIa, the onset of symptoms is delayed until the

fifth/sixth decade when there is visual deterioration and development

of recurrent erosions. Examination demonstrates the appearance of

sparse, thick rope-like lattice lines which are often asymmetrical

unlike those of LCDI. Histological examination is indistinguishable.

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Lattice corneal dystrophy type I

LCD type IIIa. Stromal lattice lines are said to be thicker in late onset forms of

isolated LCD.

Age of onset

First decade of life in LCDI; fourth/fifth decades in LCDIIIa.

Mutational spectrum

To date, all analyzed forms of early-onset LCD (LCDI) have been

caused by a single mutation (R124C) within exon 4 of the BIGH3

gene. Amongst later-onset forms of isolated lattice dystrophy a

broader range of missense mutations exist, usually in the later exons

of the gene. This explains at least some of the clinical, interfamilial

heterogeneity seen amongst patients with isolated lattice dystrophy.

BIGH3 mutations underlying dominant, late-onset forms of LCD

have not been found to have any geographic or racial bias.

Effect of mutation

As with the other BIGH3-related dystrophies it is hypothesized that

there is abnormal folding of BIGH3 which has amyloidogenic

potential and aggregates within the cornea. Amyloid deposits in

corneas from patients with lattice dystrophy have been shown on

immunohistochemical analysis to co-localize with BIGH3.

Diagnosis

Slit-lamp examination and histologic examination of corneal buttons.

Mutation analysis can facilitate the determination of prognosis.

Corneal disease

19

MIM

105120; 137350 (Gelsolin)

Clinical features

This is one of the inherited systemic amyloidoses and is

characterized by corneal lattice dystrophy and cranial neuropathy.

20

Lattice corneal dystrophy type II

Neurological

Abnormalities are common. Cranial neuropathy (especially of the

facial nerve), peripheral polyneuropathy (mainly affecting vibration

and touch senses) and minor autonomic dysfunction are frequent.

Facial paralysis is progressive although extraocular muscles are not

affected and there is no ptosis. Amyloid deposition is widespread

and can also cause cardiac and renal symptomatology.

Age of onset

Slit-lamp examination may reveal subtle lattice lines from the fourth

decade onwards. At this stage mild neurological abnormalities, such

as corneal hypoesthesia and facial paresis, may be detected. There

may then be evidence of dermal changes in the face, particularly in

the brow and lower lip.

Inheritance

Autosomal dominant

Chromosomal location

9q34

Gene

Gelsolin

Mutational spectrum

Two missense mutations of residue 187 (Asp187Asn and Asp187Tyr).

Effect of mutation

Gelsolin is part of the extracellular actin-scavenger system which

prevents the toxic effects of actin release into the extracellular

space during necrosis. It is required by those cell types involved

in mediating responses to stress and apoptosis. If transfected into

mammalian cultured cells, the pathogenic substitutions result in

the secretion of an aberrant polypeptide which contains an amyloid-

forming sequence.

Corneal disease

21

Diagnosis

Amyloidosis V is one of the differential diagnoses of late-onset

lattice dystrophy. Although it is of higher frequency in Finland,

the diagnosis should not be dismissed in other parts of the world.

Patients should be screened for potential complications: from an

ophthalmic viewpoint screening for glaucoma should be instituted.

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Lattice corneal dystrophy type II

MIM

217800; 605294 (CHST6)

Clinical features

Macular corneal dystrophy is characterized by a diffuse corneal

stromal clouding and reduction of corneal thickness by about

one-third. The opacity is initially superficial but deepens with time

and results in progressive visual deterioration. Recurrent erosions

do not occur. Unlike the granular and lattice dystrophies, corneal

clouding does not spare the limbal region. On examination there

are ill-defined grayish-white stromal opacities between which

the intervening stroma is hazy. After penetrating keratoplasty,

histological examination of corneal buttons shows staining of

abnormal deposits with alcian blue demonstrating the presence

of lakes of glycosaminoglycans within the stromal matrix.

A. Opacification reaches the limbus.

B. Recurrence within the grafts is rare.

Age of onset

Corneal opacities may be present from the first decade of life. Visual

deterioration is variable and penetrating keratoplasty is usually

indicated from the third and fourth decades. Recurrence in the graft

is exceptional and is not sight-threatening.

Inheritance

Autosomal recessive

Corneal disease

23

Chromosomal location

16q22

Gene

Carbohydrate sulfotransferase 6 (CHST6), also known as corneal

N-acetylglucosamine-6-sulfotransferase.

Mutational spectrum

Type I MCDC is characterized by absent sulfated keratan sulfate

in serum. Inactivating mutations of CHST6, including missense

frameshift and deletion mutations, are described.

Type II MCDC is characterized by the presence of sulfated keratan

sulfate, in serum. Large rearrangements in the 5´ region upstream

of CHST6 have been defined.

Effect of mutation

CHST6 is thought to be important in the production of sulfated keratan

sulfate, which is essential for the maintenance of corneal clarity. It is

hypothesized that mutations in type I MCDC result in the inactivation

of CHST6. In type II MCDC, loss of tissue-specific regulatory elements

are thought to abolish CHST6 expression in the cornea.

Diagnosis

MCDC is diagnosed clinically. Genetic testing is available on a

research basis only but does not generally alter clinical

management; delineation of type I or type II MCDC is not important

clinically or for genetic counselling.

24

Macular corneal dystrophy

Including: Posterior polymorphous dystrophy (PPCD)

MIM

136800 (FECD); 122000 (PPCD)

Clinical features

FECD is one of the most common indications for corneal

transplantation (up to 19%) in developed countries. Symptoms

of painful visual loss result from corneal decompensation. The

development in the central cornea of focal wart-like guttae arising

from Descemet’s membrane, which is thickened by abnormal

collagenous deposition. There is reduced endothelial function and

cell density as well as cellular pleomorphism.

PPCD is characterized by formation of blister-like lesions within

the corneal endothelium or by regions of endothelial basement

membrane thickening with associated corneal edema. There is

replacement of the normal amitotic endothelial cells by epithelial-

like cells that possess abundant intermediate filaments,

desmosomes and microvilli. The endothelium becomes multilayered

and the abnormally proliferating cells may extend outwards from the

cornea over the trabecular meshwork to cause glaucoma. In this

regard PPCD resembles iridocorneal endothelial (ICE) syndrome.

Age of onset

In FECD signs may be present from the 4th decade of life onwards.

PPCD although variable in both penetrance and expressivity usually

presents earlier and may be symptomatic from childhood.

Inheritance

FECD is usually sporadic although this may be a reflection of its late

onset. Highly penetrant dominant forms are described. PPCD is

inherited in an autosomal dominant manner.

Chromosomal location

FECD: 1p34.3–p32 (COL8A2)

PPCD: 20p11.2–q11.2 (VSX1)

Corneal disease

25

Gene

Collagen type VIII, alpha 2 (COL8A2)

Mutational spectrum

Missense substitutions of the X position of the Gly-X-Y triplet of

the collagenous triple helical domain of the α2 chain of type VIII

collagen have been described in families with early-onset and

classical Fuchs’ dystrophy as well as in PPCD. Mutations were

found in <10% of patients with FECD.

Effect of mutation

Type VIII collagen is a member of the short chain collagen-like

family of proteins that also includes type X collagen. It comprises

two α-chains, α1(VIII) and α2(VIII). Type VIII collagen is a major

component of the hexagonal lattice of Descemet’s membrane. It

is thought that mutations disrupt the stability of supramolecular

assembly. Type VIII collagen is abnormally deposited in the corneas

in both FECD and PPCD.

Diagnosis

Clinical

26

Fuchs’ endothelial corneal dystrophy