Identification and Functional Consequences of a New Mutation (E155G) in the Gene for GCAP1 That Causes Autosomal Dominant Cone Dystrophy

Mutations in the gene for guanylate cyclase–activating protein-1 (GCAP1) (GUCA1A) have been associated with autosomal dominant cone dystrophy (COD3). In the present study, a severe disease phenotype in a large white family was initially shown to map to chromosome 6p21.1, the location of GUCA1A. Subsequent single-stranded conformation polymorphism analysis and direct sequencing revealed an A464G transition, causing an E155G substitution within the EF4 domain of GCAP1. Modeling of the protein structure shows that the mutation eliminates a bidentate amino acid side chain essential for Ca²⁺ binding. This represents the first disease-associated mutation in GCAP1, or any neuron-specific calcium-binding protein within an EF-hand domain, that directly coordinates Ca²⁺. The functional consequences of this substitution were investigated in an in vitro assay of retinal guanylate cyclase activation. The mutant protein activates the cyclase at low Ca²⁺ concentrations but fails to inactivate at high Ca²⁺ concentrations. The overall effect of this would be the constitutive activation of guanylate cyclase in photoreceptors, even at the high Ca²⁺ concentrations of the dark-adapted state, which may explain the dominant disease phenotype.