Congenital
cataract is a leading cause of visual disability in children. Inherited isolated (non-syndromic)
cataract represents a significant proportion of cases and the identification of genes responsible for inherited
cataract will lead to a better understanding of the mechanism of
cataract formation at the molecular level both in congenital and age-related
cataract.
Crystallins are abundantly expressed in the developing human lens and represent excellent candidate genes for inherited
cataract. A genome-wide search of a five-generation family with autosomal dominant lamellar
cataract demonstrated linkage to the 17p12-q11 region. Screening of the CRYBA1/3 gene showed a 3 bp deletion, which resulted in a G91del mutation within the
tyrosine corner, that co-segregated with disease and was not found in 96 normal controls. In order to understand the molecular basis of
cataract formation, the
mutant protein was expressed in vitro and its unfolding and refolding characteristics assessed using far-UV circular dichroism spectroscopy. Defective folding and a reduction in solubility were found. As the wild-type
protein did not refold into the native conformation following unfolding, a corresponding CRYBB2 mutant was genetically engineered and its refolding characteristics analysed and compared with wild-type CRYBB2. Its biophysical properties support the hypothesis that removal of the
glycine residue from the
tyrosine corner impairs the folding and solubility of
beta-crystallin proteins. This study represents the first comprehensive description of the biophysical consequences of a mutant
beta-crystallin protein that is associated with human inherited
cataract.