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Build up of keratan sulphate and βig-h3 protein in the cornea causes opacity in Maroteaux-Lamy syndrome (MLS) type B, as shown for the first time by researchers in Cardiff, UK. MLS is one of the mucopolysaccharide diseases. A deficiency in a lysosomal enzyme causes build up of glycoaminoglycans (mucopolysaccharide) within the cornea, causing it to become opaque. In MLS type B the deficient enzyme is aryl sulphatase B. Recently, mutations in the βig-h3 gene, which codes for βig-h3 protein of the extracellular matrix, have been shown in four autosomal dominant corneal dystrophies. Ahktar et al looked at how proteoglycans, keratan—a byproduct of incomplete breakdown of proteoglycan of the cell envelope and extracellular matrix—and βig-h3 were distributed in diseased corneal tissue from a 16 year old girl with MSL VI type B undergoing a corneal transplant and a normal cornea obtained from necropsy.
In MLS epithelial cells, intercellular spaces, and cell junctions contained proteoglycans and the stroma was heavily loaded. Keratocytes were vacuolated and their lysosomes packed with proteoglycan filaments. βig-h3 protein was localised around electron lucent spaces in the stroma, and keratan sulphate densely located in the posterior stroma and in keratocytes, their vacuoles, and lysosomes. In the normal cornea βig-h3 protein appeared evenly throughout, save in keratocytes and endothelial cells, and keratan sulphate uniformly within the stroma.
Aryl sulphatase deficiency leads to build up of keratan sulphate within cells, causing keratocytes to degenerate and abnormal deposits of proteoglycan to form. This, the authors conclude, increases light scattering and the opacity of the cornea.