Purpose: The aim of the present study was to investigate the molecular basis underlying a nonsyndromic presenile autosomal dominant cataract in a three-generation pedigree. The phenotype was progressive from a peripheral ring-like opacity to a total cataract with advancing age from teenage to adulthood. The visual impairment started as problem in distant vision at the age of 16 years, to diminishing vision by the age of 24.
Methods: Clinical interventions included complete ophthalmological examination, a collection of case history, and pedigree details. Blood samples were collected from available family members irrespective of their clinical status. A functional candidate gene approach was employed for PCR screening and sequencing of the exons and their flanking regions of CRYGC, CRYGD, and CRYAA genes. For structural consequences of the mutated alphaA-crystallin we used the bioinformatics tool of the ExPASy server.
Results: Sequence analysis of CRYGC and CRYGD genes excluded possible causative mutations but identified known polymorphisms. Sequencing of the exons of the CRYAA gene identified a sequence variation in exon 2 (292 G->A) with a substitution of Gly to Arg at position 98. All three affected members revealed this change but it was not observed in the unaffected father or sister. The putative mutation obliterated a restriction site for the enzyme BstDSI. The same was checked in controls representing the general population of the same ethnicity (n=30) and of randomly selected DNA samples from ophthalmologically normal individuals from the population-based KORA S4 study (n=96). Moreover, the Gly at position 98 is highly conserved throughout the animal kingdom. For the mutant protein, the isoelectric point was raised from pH 5.77 to 5.96. Moreover, an extended alpha-helical structure is predicted in this region.
Conclusions: The G98R mutation segregates only in affected family members and is not seen in representative controls. It represents very likely the fourth dominant cataract-causing allele in CRYAA. In all reported alleles the basic amino acid Arg is involved, suggesting the major importance of the net charge of the alphaA-crystallin for functional integrity in the lens.