A novel mutation in AlphaA-crystallin (CRYAA) caused autosomal dominant congenital cataract in a large Chinese family

Hereditary cataract is a phenotypically and genetically heterogeneous lens disease that is responsible for a significant proportion of the visual impairment and blindness that occurs in children. In a five-generation Chinese family with autosomal dominant inherited congenital cataract, clinical examination showed three cataract phenotypes: punctuate, nuclear, and total cataracts. Linkage analysis was performed and positive two-point LOD scores (with maximum of 4.43 and 4.27 at theta=0) were obtained for markers D21S1411 and D21S1890 on chromosome 21q22.3, flanking the CRYAA (alphaA-crystallin-encoding gene) locus. Sequencing of CRYAA revealed a novel heterozygous G>A transition (c.346G>A) in exon 3 that cosegregated with the disease phenotype and results in a conservative substitution of Arg to His at codon 116 (p.R116H). To understand the molecular basis of cataract formation, mutant and wild-type alphaA-crystallins were expressed in E. coli. RP-HPLC (reverse phase-high-performance liquid chromatography) suggested an increased hydrophobicity of the mutant recombinant protein, compared to that of wild-type alphaA-crystallins. Furthermore, loss of chaperone activity of the mutant was seen in DTT (DL-dithiothreitol)-induced insulin aggregation assay. FPLC (fast protein liquid chromatography) purification showed that the His-116 mutant protein had increased binding affinity to lysozyme. Gain of activated lysozyme binding, elevation of hydrophobicity and loss of chaperone activity of the mutant protein may be some of the molecular mechanisms underlying cataract in this large family.