Macular corneal dystrophy (MCD; MIM 217800) is an autosomal recessive hereditary disease in which progressive punctate opacities in the cornea result in bilateral loss of vision, eventually necessitating corneal transplantation. MCD is classified into two subtypes, type I and type II, defined by the respective absence and presence of sulphated keratan sulphate in the patient serum, although both types have clinically indistinguishable phenotypes. The gene responsible for MCD type I has been mapped to chromosome 16q22, and that responsible for MCD type II may involve the same locus. Here we identify a new carbohydrate sulphotransferase gene (CHST6), encoding an enzyme designated corneal N-acetylglucosamine-6-sulphotransferase (C-GlcNAc6ST), within the critical region of MCD type I. In MCD type I, we identified several mutations that may lead to inactivation of C-GlcNAc6ST within the coding region of CHST6. In MCD type II, we found large deletions and/or replacements caused by homologous recombination in the upstream region of CHST6. In situ hybridization analysis did not detect CHST6 transcripts in corneal epithelium in an MCD type II patient, suggesting that the mutations found in type II lead to loss of cornea-specific expression of CHST6.

Macular corneal dystrophy type I and type II are caused by distinct mutations in a new sulphotransferase gene.

The pertinent literature on inherited corneal diseases is reviewed in terms of the chromosomal localization and identification of the responsible genes. Disorders affecting the cornea have been mapped to human chromosome 1 (central crystalline corneal dystrophy, familial subepithelial corneal amyloidosis, early onset Fuchs dystrophy, posterior polymorphous corneal dystrophy), chromosome 4 (Bietti marginal crystalline dystrophy), chromosome 5 (lattice dystrophy types 1 and IIIA, granular corneal dystrophy types 1, 2 and 3, Thiel-Behnke corneal dystrophy), chromosome 9 (lattice dystrophy type II), chromosome 10 (Thiel-Behnke corneal dystrophy), chromosome 12 (Meesmann dystrophy), chromosome 16 (macular corneal dystrophy, fish eye disease, LCAT disease, tyrosinemia type II), chromosome 17 (Meesmann dystrophy, Stocker-Holt dystrophy), chromosome 20 (congenital hereditary endothelial corneal dystrophy types I and II, posterior polymorphous corneal dystrophy), chromosome 21 (autosomal dominant keratoconus) and the X chromosome (cornea verticillata, cornea farinata, deep filiform corneal dystrophy, keratosis follicularis spinulosa decalvans, Lisch corneal dystrophy). Mutations in nine genes (ARSC1, CHST6, COL8A2, GLA, GSN, KRT3, KRT12, M1S1and TGFBI [BIGH3]) account for some of the corneal diseases and three of them are associated with amyloid deposition in the cornea (GSN, M1S1, TGFBI) including most of the lattice corneal dystrophies (LCDs) [LCD types I, IA, II, IIIA, IIIB, IV, V, VI and VII] recognized by their lattice pattern of linear opacities. Genetic studies on inherited diseases affecting the cornea have provided insight into some of these disorders at a basic molecular level and it has become recognized that distinct clinicopathologic phenotypes can result from specific mutations in a particular gene, as well as some different mutations in the same gene. A molecular genetic understanding of inherited corneal diseases is leading to a better appreciation of the pathogenesis of these conditions and this knowledge has made it imperative to revise the classification of inherited corneal diseases.

The molecular genetics of the corneal dystrophies--current status.

Structural diversity of the sugar chains attached to proteins and lipids that arises from the variety of combinations of different monosaccharides, different types of linkages, branch formation and secondary modifications, such as sulfation, possesses a large amount of biological information. A number of proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates. Their sulfate groups provide a negative charge and play a role in a specific molecular recognition process. The sulfation of oligosaccharides is catalyzed by the Golgi-associated sulfotransferases. Recent success in molecular cloning of these sulfotransferases has brought a breakthrough in the understanding of biological function of sulfated oligosaccharides in a variety of contexts. Investigations on the relationship of sulfated oligosaccharides to human diseases including hereditary deficiency, cancer, inflammation, and infection may provide hints for curing disastrous diseases.

Sulfotransferases and sulfated oligosaccharides

Clinical and Histopathologic Features of Corneal Dystrophies in Japan

Monoclonal antibody against keratan sulfate (KS) was used for immunofluorescent staining of sections of human corneas from 8 normal eyes, 19 with keratoconus, 4 with pellucid marginal degeneration, 5 with primary macular corneal dystrophy, and 1 with recurrent macular corneal dystrophy. The anti-KS monoclonal antibody did not stain the corneas with primary macular corneal dystrophy, but stained ail other corneas to varying degrees. Staining intensity was weaker than normal in most keratoconus and pellucid marginal degeneration corneas, and was very weak in a case of macular corneal dystrophy that had recurred in a transplanted normal cornea. In several corneas with keratoconus, normal staining was seen at the periphery, and staining intensity decreased in the thinned central portion of the stroma. The decreased KS staining was not localized in stromal scar tissue found in the keratoconus and pellucid marginal degeneration corneas. Quantitation of relative staining intensity found keratoconus and pellucid marginal degeneration corneas to be 49% and 40% as intensely stained, respectively, as normal corneas, a statistically significant decrease (P < 0.01). Distribution of staining intensities of the keratoconus corneas demonstrated a single modality. These results are in agreement with findings of previous biochemical studies, which show reduction of highly sulfated keratan sulfate epitopes in corneas from keratoconus and pellucid marginal degeneration, and absence of sulfated keratan sulfate epitopes in macular corneal dystrophy., Invest Ophthalmol Vis Sci 31:419-428,1990 Keratoconus and pellucid marginal degeneration are disorders of the cornea involving progressive thinning of the stroma. 1 ' 2 In keratoconus, stromal thinning is central and accompanied by corneal ectasia, whereas in the less frequent condition of pellucid marginal degeneration, thinning occurs in the inferior peripheral stroma. 12 In both cases, disruption of Bowman's layer and stromal scarring occur. Stromal thinning may involve modification of stromal extracellular matrix metabolism, and even though no biochemical characteristic of the stroma has been linked to the etiology of these diseases, abnormalities in keratoconus have been identified in both protein and carbohydrate components of the stromal extracellular matrix. Work on this subject has been reviewed. 3

J.-M. Eugene

Papers

Immunohistochemical Evidence of Heterogeneity in Macular Corneal Dystrophy