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) (G UCA1A) have been associated with autosomal dominant cone dystrophy (COD3). In the present study, a severe disease phenotype in a large white family wa s initially shown to map to chromosome 6p21.1, the location of GUCA1A. Subs equent single-stranded conformation polymorphism analysis and direct sequen cing revealed an A464G transition, causing an E155G substitution within the EF4 domain of GCAP1. Modeling of the protein structure shows that the muta tion eliminates a bidentate amino acid side chain essential for Ca2+ bindin g. This represents the first disease-associated mutation in GCAP1, or any n euron-specific calcium-binding protein within an EF-hand domain, that direc tly 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 fa ils to inactivate at high Ca2+ concentrations. The overall effect of this w ould 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.