A novel mutation, Gly86Arg, in a calcium-sensor protein GCAP1 (GUCA1A) alters regulation of the retinal guanylyl cyclase and causes dominant cone-rod degeneration

Abstract

Purpose : Calcium sensor proteins (GCAPs) activate retinal membrane guanylyl cyclase (RetGC) in photoreceptors when light exposure reduces Ca²⁺ concentrations in the outer segment and inhibit the cyclase in the dark, when Ca²⁺ concentrations rise. Mutations in GCAP1 are often associated with retinal degenerations. A novel mutation, G86R, in GCAP1 (GUCA1A) was found in a patient with a dominant retinopathy. We established a link between the mutation and the disease and characterized the biochemical changes in RetGC regulation by the mutant GCAP1.

Methods : The patient had genetic and clinical studies. Regulation of a recombinant RetGC1 isozyme of the cyclase (GUCY2D) by the mutant GCAP1 was assayed using recombinant proteins in vitro. Calcium binding properties of the purified recombinant GCAP1 were assessed using Trp fluorescence spectroscopy and ITC.

Results : The patient carrying a heterozygous G→C transversion in codon 256 of the GUCA1A gene displayed abnormal foveal morphology, reduced cone function and subtle rod disease. The G86R GCAP1 coded by the disease-linked allele activated RetGC1 at low Ca²⁺ concentrations characteristic of light-adapted photoreceptors with even higher efficiency than the wild type, but failed to decelerate the cyclase at the Ca²⁺ concentrations typical for dark-adapted photoreceptors. Ca²⁺-dependent increase in Trp94 fluorescence, indicative of the GCAP1 transition to its RetGC1-inhibiting state, was suppressed and shifted to a higher Ca²⁺ range. Inhibition of the GCAP1:RetGC1 complex by RD3 protein at low Ca²⁺ concentrations also became less effective in case of the G86R GCAP1.

Conclusions : The G86R substitution in a “hinge” region connecting two of the EF-hand domains in GCAP1 strongly interferes with its Ca²⁺-dependent activator-to-inhibitor conformational transition. The changes it causes in the cyclase regulation by GCAP1 indicate that this mutation must lead to abnormally elevated cGMP production and Ca²⁺ influx in the diseased photoreceptors in the dark.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.